Article

Rhodoliths, uniformitarianism, and Darwin: Pleistocene and Recent carbonate deposits in the Cape Verde and Canary archipelagos

April 2012
Palaeogeography Palaeoclimatology Palaeoecology 329-330:83-100

DOI:10.1016/j.palaeo.2012.02.019

Authors:

Markes E. Johnson

Williams College

B. Gudveig Baarli

Williams College

Mário Cachão

University of Lisbon

Carlos Marques da Silva

University of Lisbon

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Visiting “St. Jago” (Santiago) in the Cape Verde Islands in 1832 and again in 1836 aboard HMS Beagle, Charles Darwin was the !rst to trace and describe the tri-part sequence of white limestone and sandstone beds stratigraphically located between two levels of basalt exposed almost uninterrupted for 10 km along coastal cliffs. The Pleistocene carbonate sediments dominated by rhodoliths and rhodolith debris accumulated on a basalt shelf and subsequently became buried by subaerial and submarine basalt on the southeast coastline of Santiago. The main goal of this contribution is to re-examine Darwin's stratigraphic sequence. The secondary goal is to provide a general taphonomical model based on the observation of Recent rhodolith deposits for evaluation of fossil rhodolith assemblages. Environmental uniformitarianism is employed to understand the depositional history of the southern Santiago rhodolith-bearing strata. The mixed clastic-carbonate sequence includes a basalt-derived basal conglomerate with an intertidal to shallow subtidal fossil assemblage mainly denoted by limpets and oysters. Upper layers typically demonstrate swaley and hummocky cross strati!cation incorporating rhodolith debris further modi!ed by bioturbation. Pillow basalts from 10 to 18 m in thickness succeeded by subaerial "ows imply swift burial of the carbonate succession under equivalent water depths. The calcareous nannofossil assemblage was investigated to more precisely date the deposits. Darwin's paleoshore is reinterpreted to represent two different transgressions occurring between approximately 1.1 and 0.7 Ma. Taphonomic grades from whole rhodoliths to !nely crushed rhodolith debris observed under present-day conditions on Maio (Cape Verde Islands) and Fuerteventura (Canary Islands) were used to model rhodolith preservation and to constrain the depositional settings to which rhodoliths may be transported from the offshore banks where they naturally thrive. Coastward transport of rhodoliths commonly ends with deposition in subtidal storm beds, tidal pools, and platform over-wash deposits, as well as beach, berm, hurricane, tsunami, and coastal dune deposits.

Rhodolith beds and their onshore transport in Fuerteventura Island (Canary Archipelago, Spain)

Article

Full-text available

Oct 2022

Rhodoliths occur extensively around the shores of Fuerteventura Island in the Canary Archipelago, with Lithothamnion cf. corallioides being the most prominent species. A large number of rhodoliths end up washed onshore, the debris from which contributes to the formation of sediments constituting modern beaches. In a previous study by one of the co-authors (MEJ), the northern coast of Fuerteventura was shown to comprise various types of rhodolith deposits such as beach, platform overwash, tidal pools, coastal dunes, and others. An extraordinary example of stranded rhodoliths is located near Caleta del Bajo de Mejillones, approximately 3 km west of Corralejo, on the north coast of the island. The deposit forms a supratidal beach 120 m long and 10 m wide that sits above the landward termination of an extensive wave-cut platform eroded in basalt and exposed at low tide to a width of 130 m perpendicular to shore. Here, rhodoliths are very small (<3 cm) resembling popcorn, and the locality is known as the “Popcorn Beach”. Other examples are berms up to 150 m long and 9 m wide at Caleta del Bajo de Mejillones, or an exposed beach at Playa del Hierro with an area of more than 1500 m² covered entirely of very coarse rhodolith sand. Extensive living rhodolith beds were found at a water depth of 22 m.

Heterozoan carbonate-enriched beach sand and coastal dunes—with particular reference to rhodoliths, Dirk Hartog Island, Shark Bay, Western Australia

Article

Full-text available

Jul 2018

This is a preliminary interdisciplinary study on the enrichment of heterozoan carbonates on Dirk Hartog Island, Shark Bay, Western Australia, with particular reference to rhodolith (free-living non-geniculate) coralline algae. The current study aims to investigate the geological impact of shallow-water rhodoliths in Shark Bay, as well as fill critical information gaps on the biogeographical distribution of rhodoliths in Australia. We analyzed the composition of sand from eight sites (totaling 21 beach and sand dune samples) on the eastern (windward) shore of the island, and investigated the origin of the coralline algal grains. Heterozoan carbonates (shell, geniculate coralline algae grains, and rhodolith grains) together comprised 3–84% of the carbonate-enriched beach and dune sand samples. While shell fragments often comprised the highest percentage (up to 73%), rhodolith grains (up to 27%) were found in 12 of 21 samples, with rhodolith grains also occurring in two dune samples. Geologically, the study has shown that rhodoliths and rhodolith beds are important shallow-marine habitats in Shark Bay, with a proven capacity to enrich beach/dune sands in Shark Bay and potentially other areas along the Australian coast. Biogeographically, the study confirmed the presence of a previously undescribed shallow rhodolith bed in Shark Bay (the first bed documented on the Western shore) with the possibility of a third bed near Sandy Point on Dirk Hartog Island. It also confirmed the presence of rhodolith forming Neogoniolithon brassica-florida and Lithophyllum sp. in Shark Bay, and is the first record of Hydrolithon reinboldii rhodoliths in Australia.

Modern rhodoliths from the insular shelf of Pico in the Azores (Northeast Atlantic Ocean)

Article

May 2018
ESTUAR COAST SHELF S

A seabed sediment-sampling survey conducted on the Pico insular shelf found abundant rhodoliths between −64 and −73 m off the south coast of the island. These were small and mainly ellipsoidal in shape with a maximum diameter of 3.75 cm. Granules and small pebbles of eroded basalt were also a typical component of these samples. Thin algal crusts were secreted on basalt pebbles by the coralline red algae Phymatolithon calcareum which, in turn, were covered by Lithophyllum incrustans. Additional samples were collected by snorkelling at Maré (Lajes do Pico), a shallow lagoon (2–4 m in depth) on the south coast. Here, rhodoliths are mostly spheroidal and bigger (maximum diameter of 4.75 cm), formed by thin thalli of P. calcareum. Based on these results (distribution of shapes and species) and previous work on the characteristics of fossil specimens from Neogene deposits on Santa Maria Island (Azores) and other North Atlantic Archipelagos, an empirical depositional model is proposed for the development of rhodoliths on the Pico Island shelf: (1) Nearshore rhodoliths, formed solely by P. calcareum, are subjected to a wide range of currents and waves resulting in their spheroidal shapes. However, those of Maré lagoon are protected from offshore transport and tend to grow larger than deep-water rhodoliths; (2) Although not sampled, there must be middle shelf rhodoliths formed solely by P. calcareum that tend to form more ellipsoidal shapes due to seafloor oscillatory movements caused by waves crossing the shelf; (3) During storms, these middle shelf rhodoliths are then transported to the outer shelf, where L. incrustans overgrows the initial cover of P. calcareum. Shallow associations are normally larger, reflecting therefore, a longer life span than the deeper associations. Transport by storms appears to be an important factor in the formation of some deep-water rhodoliths around volcanic oceanic islands subjected to high-wave energy. However, their exposure to an energetic environment and likely frequent offshore transport does not allow them to grow as large as those from shallow-water.

Upper Pleistocene trace fossils from Ponta das Bicudas, Santiago, Cape Verde Islands: Systematics, taphonomy and palaeoenvironmetal evolution

Article

Mar 2018
PALAEOGEOGR PALAEOCL

Bioturbation structures from Ponta das Bicudas in Santiago Island, Cape Verde Archipelagoare located in coarse-grained biocalcirudite sandwiched between basaltic lavas of the Upper Pleistocene Assomada Formation. Four ichnospecies have been identified: Rhizocorallium commune irregulare, Rhizocorallium jenense versum, nov ichnosubsp., Alaichnus kabuverdiensis nov. ichnogen. nov. ichnosp. and cf. Dactyloidites. From a taphonomic point of view, most trace fossil specimens are undergoing throughout intense erosion. Nevertheless, the observations made on their taphonomy and state of preservation allowed i) to reconstruct the original, complete structures of the trace fossils, ii) to evaluate the existence and magnitude of erosional and depositional episodes, and iii) to quantify the reach of erosion on the seafloor sediment. In this context, it was possible to quantify the thickness of sediment being eroded at the final erosional episode, which happened right before the second extrusion episode of volcanic pillow lava, which buried all the traces previously formed: the total interval of sediment eroded was estimated in ca. 1 m. From a palaeoenvironmental point of view, in a synthesis, the traces were produced in a soft to stiffground substrate composed of very coarse-grained biocalcirudite with common rhodolith debris, in a shallow setting (4 to10 m of depth) with moderate wave energy, nearby rocky basalt knobs where corals and red coralline algae crusts were growing. Organisms producing Rhizocorallium commune irregular settled in the seafloor during a first phase of sedimentation, which was followed by an erosional episode. Once sedimentation resumed, specimens of R. jenense versum nov. ichnosubsp. were produced and overlapped with the former ones. Later, a new erosional event erased most of the previous traces, being preserved only the basal imprint of the specimens. This was followed by a rapid sedimentation episode where Alaichnus kabuverdiensis nov. ichnogen. nov. ichnosp. recorded intense bioturbation activity of the bivalve Panopea. Subsequently, under low-energy sedimentation, a new episode of intense bioturbation of the seafloor occurred, resulting in the development of abundant cf. Dactyloidites. Finally, a strong erosional event - probably related to the onset of the second volcanic episode - cut all previous structures before being buried by the overlying, well preserved pillow lava.

Taphonomic Range and Sedimentary Dynamics of Modern and Fossil Rhodolith Beds: Macaronesian Realm (North Atlantic Ocean)

Chapter

Oct 2017

Distribution of living rhodoliths in the Macaronesian realm is limited by extensive rocky shores and narrow insular shelves that rapidly drop off beyond the 50-m isobath. Wind and wave erosion is most intense on north and northeast-facing shores due to the prevailing northeasterly trade winds over much of the region. Southern shores offer more sheltered, leeward settings. Rhodolith beds tend to thrive on eastern shores with strong long-shore currents and southeastern shores that benefit from wave refraction. Rhodoliths are not entirely absent off northern shores, but may fail to reach maximum size before being washed ashore to make berms and beaches. Islands considered in greater detail in this survey include Santiago, Maio, and Sal from the Cape Verde Islands, Fuerteventura and the related islet of Lobos in the Canary Islands, Selvagem Grande and Pequena from the Savage Islands, Porto Santo in the Madeira Islands, and Santa Maria in the Azores. This contribution expands on the concept that living rhodoliths enter the fossil record through a range of taphofacies defined by the degree of breakage and corrosion and further characterized by sedimentological criteria regarding the amount of matrix and packing among bioclasts. Rhodolith deposits in Macaronesia seldom reflect settings under natural growth conditions. Rather, rhodoliths are subject to transportation and post-mortem disintegration resulting in the accumulation of rhodolith materials captured by subtidal storm deposits, tidal pools and platform over-wash deposits, as well as beachrock, beach, berm, hurricane, tsunami, and coastal dune deposits. Some of this material is transferred farther offshore, but exposed island strata show a tendency for shoreward migration of taphofacies. Rhodolith beds provide a habitat for some species of marine invertebrates, including epifaunal and infaunal elements directly associated with whole rhodoliths and these features play a role in rhodolith biostratinomy.

Recent Rhodolith Deposits Stranded on the Windward Shores of Maio (Cape Verde Islands): Historical Resource for the Local Economy

Article

Jan 2016
J COASTAL RES

Maio is a volcanic island with an area of 269 km2 in the Cape Verde archipelago off the west coast of Africa. Although considered a leeward island, it absorbs NE trade winds that typically register 5 to 6 on the Beaufort Scale (moderate to fresh breeze). The trade winds produce ocean swells commonly 3.5 m in height that scour the island’s north coast but also generate eastern longshore currents. Outcrops with Pleistocene rhodoliths occur on the SE and south shores and include lithified dunes mainly composed of crushed rhodolith debris. In contrast, the modern beaches and Pleistocene dunes on the more sheltered west coast are practically devoid of rhodoliths. Present-day rhodolith banks off the north coast would seem to be precluded by intense wave action. This study examines rhodoliths from overwash and beach-rock deposits around Ponta Cais in the far north. Lumpy rhodoliths (likely Lithothamnion sp.) are concentrated in a sheltered corner on the bay south of Ponta Branca. A more extensive overwash deposit covers an area of 27,000 m2 that is 1 m above mean sea level with a surface exposure of 450 rhodoliths/m2. A unique specimen nucleated around a ceramic fragment indicates that the deposit is historical in context. Rhodolith beach rock extends all along Praia Real east of Ponta Cais. A northern bank clearly exists, but it does so at a water depth normally adequate to protect larger rhodoliths from all but major storms. Abandoned limekilns behind Praia Real demonstrate that the local economy on a volcanic island used rhodoliths as a source of mortar and whitewash.

Recent Rhodolith Deposits Stranded on the Windward Shores of Maio (Cape Verde Islands): Historical Resource for the Local Economy

Article

Jul 2016
J COASTAL RES

Johnson

Maio is a volcanic island with an area of 269 km² in the Cape Verde archipelago off the west coast of Africa. Although considered a leeward island, it absorbs NE trade winds that typically register 5 to 6 on the Beaufort Scale (moderate to fresh breeze). The trade winds produce ocean swells commonly 3.5 m in height that scour the island's north coast but also generate eastern longshore currents. Outcrops with Pleistocene rhodoliths occur on the SE and south shores and include lithified dunes mainly composed of crushed rhodolith debris. In contrast, the modern beaches and Pleistocene dunes on the more sheltered west coast are practically devoid of rhodoliths. Present-day rhodolith banks off the north coast would seem to be precluded by intense wave action. This study examines rhodoliths from overwash and beach-rock deposits around Ponta Cais in the far north. Lumpy rhodoliths (likely Lithothamnion sp.) are concentrated in a sheltered corner on the bay south of Ponta Branca. A more extensive overwash deposit covers an area of 27,000 m² that is 1 m above mean sea level with a surface exposure of 450 rhodoliths/m². A unique specimen nucleated around a ceramic fragment indicates that the deposit is historical in context. Rhodolith beach rock extends all along Praia Real east of Ponta Cais. A northern bank clearly exists, but it does so at a water depth normally adequate to protect larger rhodoliths from all but major storms. Abandoned limekilns behind Praia Real demonstrate that the local economy on a volcanic island used rhodoliths as a source of mortar and whitewash.

Charles Darwin in the Cape Verde and Galápagos archipelagos: The role of serendipity in development of theories on the ups and downs of oceanic islands

Article

Full-text available

Jan 2015

The 1831–1836 voyage of H.M.S. Beagle under Captain Robert FitzRoy launched Charles Darwin's entry into the world of geology with two pioneering publications on oceanic islands to his credit. Best known is Darwin's 1842 contribution on the theory of atoll development from the subsidence of volcanic islands and coeval upward growth of coral reefs. This work can be linked, in part, to the ten days during which the Beagle visited the Keeling (Cocos) Islands. The subsequent and lesser known of Darwin's parallel contributions is his 1844 summary on all the volcanic islands visited during the expedition, including Santiago (Cape Verde Islands), Terceira (Azores), St. Paul's Rocks, Fernando Noronha, Ascension, St. Helena, the Galápagos Islands, Tahiti, and Mauritius. Ostensibly, the centerpiece of the 1844 volume is Darwin's extensive coverage of Ascension based on the five days spent there in 1836. However, Darwin had many more days at his disposal in the Galápagos and 'St. Jago' (Santiago), where the Beagle stopped in the Cape Verde Islands at the outset and again near the end of the voyage. The volcanic islands where Darwin spent the most time were in the Galápagos (thirty-five days) and the Cape Verdes (twenty-nine days). In particular, those island groups make an interesting comparison with respect to the development of Darwin's ideas on tectonic uplift based on basalt flows with inter-bedded limestone formations. Chance played a huge role in what Darwin saw and did not see during his island travels. The initial visit to the Cape Verde islands was instrumental in shaping Darwin's earliest vision of a book on volcanic islands, but his time there was entirely fortuitous due to a forced change in FitzRoy's plan for a stay in the Canary Islands. Although Darwin was on the look out for limestone formations in the Galápagos islands comparable to those on Santiago in the Cape Verdes, he missed finding them due only to the vagaries of FitzRoy's charting schedule in the Galápagos. This overview looks at limestone distribution in the Cape Verde and Galápagos archipelagos as now understood and speculates on how a wider knowledge of both regions may have influenced Darwin's thinking on global patterns of island uplift and subsidence.

Systematics and paleobiogeography of Quaternary corals from Cabo Verde Archipelago

Article

May 2024

Although true coral reefs have seldom been reported from the fossil record of the Cabo Verde Archipelago, many single fossil corals and coral colonies can be found reworked in tsunami deposits and in raised marine terraces onshore on these islands. This study provides the first detailed survey of fossil corals from 7 of the 10 islands of the archipelago, sampled from Pleistocene marine terraces and tsunami deposits. A total of 168 scleractinian corals were analyzed and identified to genus and/or species level. Thirteen taxa from the families Acroporidae, Dendrophylliidae, Faviidae, Pocilloporidae, Poritidae, and Rhizangiidae were identified. The zooxanthellate fossil corals found on the Cape Verde Archipelago likely migrated from the Caribbean to the West African coast, while azooxanthellate species likely originated from the Indo-Pacific. Differences between present-day coral assemblages and fossil assemblages are assumed to result from changing environmental factors. Although reef-building taxa occur (e.g., Porites ), extensive reef frameworks are absent.

Ontogeny of islands associated with mantle-plume hotspots and its implications for biogeographical models

Article

Full-text available

Sep 2023

Mantle-plume hotspot islands are a common focus of biogeographical studies, and models for the growth of their biodiversity often incorporate aspects of their physical evolution. The ontogenetic pathways of such islands have generally been perceived as simple, comprising successive episodes of emergence, growth, peak size, reduction and elimination. In this paper, we improve knowledge of island development by examining key physical data from 60 islands at eight archipelagoes in equatorial to mid-latitude regions of the Atlantic, Indian and Pacific oceans. Such landmasses achieve their maximum sizes within 200-500 kyrs. However, island longevity varies by up to a factor of 5 and is strongly controlled by the speed of the associated tectonic plate as it moves over the narrow, thermally-elevated conduit where volcanism is focused. At moderate to high speeds (40-90 mm/year; e.g., Gal apagos, Hawaii), lifetimes are no more than 4-6 Myrs. In contrast, the oldest landmasses (in the Cabo Verde, Canary, and Mascarene archipelagoes) are built upon slow-travelling plates (<20 mm/year) and date from the Miocene. Notably, Fuerteventura in the Canary Islands, where the rate is c. 2.5 mm/year, has existed since 23 Ma. Two processes likely sustain the sub-aerial elevation of these massifs: heat from the plume expands the underlying lithosphere thus increasing its buoyancy, which in turn inhibits cooling-contraction subsidence; protracted magmatic activity counteracts denudation. Furthermore, the Cabo Verde and the Canary archipelagoes sit within dry climatic regions, which likely reduced erosion and mass-wasting. Consequently, two ontogenetic models are presented, one for the edifices on the intermediate-and fast-moving plates, and a second for the constructions on the slow-moving plates. The development path for the former is similar to the schema that is commonly envisaged (see above) and takes place over c. 5 Myrs, whereas the one for the latter is rather different and involves quasi-continuous surface renewal plus the maintenance of elevation that lasts for c. 10-25 Myrs. The new information should permit a fuller understanding of how a hotspot island's physical development shapes its biota and inform the formulation of related theoretical models.

Insights into the Migration Routes and Historical Dispersion of Species Surviving the Messinian Crisis: The Case of Patella ulyssiponensis and Epizoic Rhodolith Lithophyllum hibernicum

Article

Full-text available

Nov 2021

The genus Patella (Patellogastropoda, Mollusca) is represented by a group of species exclusive to the Northeast Atlantic Ocean (including Macaronesian archipelagos) and Mediterranean Sea. The species Patella ulyssiponensis and Patella aspera are common in European waters, with the first inhabiting continental coast, and the second endemic to Macaronesian archipelagos. However, the acceptance of these two lineages as separate species is still highly debated. The red coralline species algae Lithophyllum hibernicum, distributed from Northeast Atlantic to the Mediterranean, is usually found as epilithic crusts or unattached forms (named rhodolith beds), although it also forms epizoic crusts on other organisms, e.g., shell surfaces. In order to study the historic dispersal and migration routes of the Patella ulyssiponensis-aspera complex, taxonomic, genetic and biogeographic approaches were employed based on haplotype network analyses and estimations for the most common recent ancestor (TMRCA), using Cytochrome Oxydase I. A synonymy for these two species is proposed, with the presence of a shared haplotype between the continental (P. ulyssiponensis) and insular (P. aspera) lineages, and with basis of morphological and nomenclatural data. We propose an evolutionary scenario for its dispersal based on a high haplotype diversity for the Mediterranean regions, indicating its possible survival during the Messinian Salinity Crisis (6–5.3 Mya), followed by a colonization of the Proto-Macaronesian archipelagos. The epizoic association of L. hibernicum on P. ulyssiponensis shell adult surface is recorded in this study, likewise the promotion of settlement conditions provided by these coralline algae to P. ulyssiponensis larvae, may explain the reach of P. ulyssiponensis distribution through rhodolith transportation.

REEVALUATION OF THE INFERRED RELATIONSHIP BETWEEN LIVING RHODOLITH MORPHOLOGIES, THEIR MOVEMENT, AND WATER ENERGY: IMPLICATIONS FOR INTERPRETING PALEOCEANOGRAPHIC CONDITIONS

Article

Full-text available

Dec 2020

Free-living forms of crustose coralline algae (rhodoliths) accumulate on shallow-water marine shelves in many regions worldwide. They form their own benthic habitats termed ‘rhodolith beds' and their deposits are well preserved in the rock record. Characteristics of rhodolith deposits can be used to interpret ancient water depths, light levels, and water energies. Accurate paleoceanographic interpretations rely on large-scale, detailed studies of living rhodolith beds, but these studies are scarce. Spencer Gulf in South Australia has the largest living rhodolith beds in southeastern Australia (∼ 3,000 km2). Documentation of these rhodolith deposits includes facies analysis, coralline algal taxonomy, characterization of growth forms and morphometrics, and integration with oceanographic data. Extensive oceanographic data and hydrodynamic modeling enable comparisons between rhodolith morphologies and bottom water energies with unprecedented accuracy. Rhodolith deposits in the gulf are intermixed with carbonate sands and muds under diverse hydrodynamic conditions, with current speeds up to 1.04 m s−1. At least five coralline algal genera construct rhodoliths in Spencer Gulf. Growth forms include fruticose, lumpy, and warty. Rhodolith morphologies imply that water energy is not the only thing affecting rhodolith movement and growth; rhodoliths can also be shifted by fish and invertebrates. Linear regression models in this study evaluate relationships between rhodolith morphometrics (mean diameter, sphericity, and branch density) and maximum bottom water speeds in Spencer Gulf and their results support the idea that water energy is not a major control on rhodolith morphology under these water energy conditions. Previous studies have revealed that the relationship between rhodolith morphologies and water energy levels is complicated and many authors have cautioned to not rely solely on rhodolith morphologies to interpret paleoceanographic conditions. Results of this study demonstrate that accurate paleoceanographic interpretations cannot be made based on rhodolith morphologies alone, but should also include sedimentology, coralline taxonomy, and associated biota.

Palaeoenvironmental dynamics of Holocene shoreface bryoliths from the southern coast of Brazil

Article

Jan 2019

Beds of free-living coated nodules (coralline algae, bryozoans, acervulinid foraminifera) create shallow-water carbonate biogenic benthic habitats, which are sensitive to human disturbance and slow to recover. Holocene bryoliths, ranging from sub-spheroidal, sub-discoidal to sub-ellipsoidal in shape, were found scattered in the foredunes in ca. 30-km stretch along the Hermenegildo and Concheiros do Albardão beaches on the southernmost coast of Brazil (Santa Vitória do Palmar municipality, Rio Grande do Sul State). The dominating bryozoan species forming the bryolith is Biflustra holocenica Vieira, Spotorno-Oliveira and Tâmega sp. nov. The inner bryolith arrangement, generally asymmetrical, shows multilamellar and circumrotatory growth of colonies that envelop the bivalve Ostrea puelchana. Bryozoans and subordinate corals characterize the outer bryolith surfaces. The ichnogenera Gastrochaenolites (made by the boring bivalve Lithophaga patagonica) and Caulostrepsis occur throughout the bryoliths, from the inner part up to the outer surface. The studied bryoliths, originated in a shoreface setting at ca. 7910–7620 cal. yr BP and during subsequent storm waves, were resedimented onto the foreshore and foredunes (to ca. 5700 cal. yr BP) where the bryoliths were finally fossilized.

Critical bed shear stress and threshold of motion of maerl biogenic gravel

Article

Jun 2017

A determination of the critical bed shear stress of maerl is a prerequisite for quantifying its mobility, rate of erosion and deposition in conservation management. The critical bed shear stress for incipient motion has been determined for the first time for samples from biogenic free-living maerl beds in three contrasting environments (open marine, intertidal and beach) in Galway Bay, west of Ireland. The bed shear stress was determined using two methods, Law of the Wall and Turbulent Kinetic Energy, in a rotating annular flume and in a linear flume. The velocity profile of flowing water above a bed of natural maerl grains was measured in four runs of progressively increasing flow velocity until the flow exceeded the critical shear stress of grains on the bed. The critical Shields parameter and the mobility number are estimated and compared with the equivalent curves for natural quartz sand. The critical Shields parameters for the maerl particles from all three environments fall below the Shields curve. Along with a previously reported correlation between maerl grain shape and settling velocity, these results suggest that the highly irregular shapes also allow maerl grains to be mobilised more easily than quartz grains with the same sieve diameter. The intertidal beds with the roughest particles exhibit the greatest critical shear stress because the particle thalli interlock and resist entrainment. In samples with a high percentage of maerl and low percentage of siliciclastic sand, the lower density, lower settling velocity and lower critical bed shear stress of maerl results in its preferential transport over the siliciclastic sediment. At velocities ∼10 cm s⁻¹ higher than the threshold velocity of grain motion, rarely-documented subaqueous maerl dunes formed in the annular flume.

Temporal variation in seaweed and invertebrate assemblages in shallow rhodolith beds of Baja California Sur, México

Article

Feb 2017
AQUAT BOT

We assessed temporal variation (2013-2015) in rhodolith (non-geniculate corallines) size and cover, as well as variability in associated macroalgae and invertebrates in one rhodolith bed from the Gulf of California and two on the Pacific Coast of Baja California Sur, México. While rhodoliths persisted at all sites, those in the Gulf were less abundant (Lithothamnion muelleri: 10-14% cover) compared to Pacific sites (Lithophyllum margaritae: 40-64% cover). Rhodoliths from Pacific sites were influenced by storms and tidal currents that shifted beds and redistributed fine sediments. At the Gulf site, 30 macroalgal taxa were encountered and abundance peaked during spring (76-78% cover) and early summer (73-97% cover), then declined following extreme summer heat (22-25% cover) suggesting algae cycle in response to seasonal temperature changes. At Pacific sites, 14 macroalgal taxa were encountered with highest covers observed in winter 2014 (44%) and spring 2015 (26%). Seasonality was potentially masked by algal blooms or inter-annual temperature variation. Sessile invertebrate cover was low at the Gulf site (typically <7%) and ranged from 3-21% at Pacific sites, while mobile invertebrates were dominated by arthropods (Gulf: 1-3 ind m⁻², Pacific: 1-91 ind m⁻²) and molluscs (Gulf: 0-2 ind m⁻², Pacific: 9-470 ind m⁻²). Invertebrate assemblages did not vary seasonally at any site and were likely impacted by episodic recruitment as well as other biotic and abiotic factors. Detailed life history studies and more complete environmental data are needed to better connect relationships between biological patterns observed in rhodolith beds worldwide and associated environmental factors.

Sediment generation on a volcanic island with arid tropical climate: A perspective based on geochemical maps of topsoils and stream sediments from Santiago Island, Cape Verde

Article

Oct 2016
APPL GEOCHEM

The present research tests the application of geochemical atlas of soils and stream sediments in the investigation of weathering and erosion processes on volcanic islands. The composition of surface soils collected in six catchments from Santiago Island (Cape Verde) are compared with bedload stream deposits sourced by these catchment areas in order to evaluate the spatial variability of these exogenous processes. The geochemistry of bedload stream deposits is between that of the fresh rocks and the topsoils of their source areas. Relative to average soil composition, bedload deposits are depleted in most of less-mobile elements (e.g., Al, Fe, La, Sc) and strongly enriched in Na and, usually, Ca. When the topsoil weathering intensity in the catchment areas is highly variable and the composition of bedload deposits is substantially different from the average soil composition, bedload deposits should incorporate significant amounts of poorly-weathered rocks and sectors from erosion occur within the drainage basin. Ratios of non-mobile elements allow the identification of highly vulnerable and erosion-protected sectors within the catchments. It is proposed that the catchments of the rivers in the SW flanking side of Santiago Island include sectors where lava shields formed during the post-erosional eruptive phases are capable of an effective protection to erosion. Conversely, the NE-facing part of the island is highly dissected and any younger post-erosional cover was either completely eroded away, or never existed in the first place. Simple compositional parameters derived from the databases of geochemical maps of soil and stream sediments provide important information for the analyses of weathering, erosion and denudation processes at the catchment scale.

A nova carta geológica da ilha da Madeira na escala 1:50.000

Conference Paper

Full-text available

Jun 2012

The emergence of volcanic oceanic islands on a slow-moving plate: The example of Madeira Island, NE Atlantic

Article

Full-text available

Feb 2015
GEOCHEM GEOPHY GEOSY

The transition from seamount to oceanic island typically involves surtseyan volcanism. However, the geological record at many islands in the NE Atlantic - all located within the slow-moving Nubian plate - does not exhibit evidence for an emergent surtseyan phase but rather an erosive unconformity between the submarine basement and the overlying subaerial shield sequences. This suggests that the transition between seamount and island may frequently occur by a relative fall of sea level through uplift, eustatic changes, or a combination of both, and may not involve summit volcanism. In this study we explore the consequences for island evolutionary models using Madeira Island (Portugal) as a case-study. We have examined the geologic record at Madeira using a combination of detailed fieldwork, biostratigraphy, and 40Ar/39Ar geochronology in order to document the mode, timing, and duration of edifice emergence above sea level. Our study confirms that Madeira's subaerial shield volcano was built upon the eroded remains of an uplifted seamount, with shallow marine sediments found between the two eruptive sequences and presently located at 320–430 m above sea level. This study reveals that Madeira emerged around 7.0–5.6 Ma essentially through an uplift process and before volcanic activity resumed to form the subaerial shield volcano. Basal intrusions are a likely uplift mechanism, and their emplacement is possibly enhanced by the slow-motion of the Nubian plate relative to the source of partial melting. Alternating uplift and subsidence episodes suggest that island edifice growth may be governed by competing dominantly volcanic and dominantly intrusive processes.

Numerical terrain analysis of fluvial-marine watersheds on the Isle of Santiago, Cape Verde, based on satellite imagery, ground-truthing and landform indices - A preparatory study in search of Nb -Ta - REE deposits related to hotspot islands

Article

Full-text available

Jan 2025
J AFR EARTH SCI

Trough cross-bedded rhodolith limestones in the Atlantic-linked Ronda Basin (Messinian, Southern Spain)

Article

Full-text available

Sep 2022

Rhodolith limestones occur in the upper part of the Miocene infill of the Ronda Basin in southern Spain. This basin was an embayment at the southern margin of the Atlantic-linked Guadalquivir Basin, the foreland basin of the Betic Cordillera. Messinian rhodolith limestones crop out in the mesa of the Roman settlement Acinipo. They mostly consist of trough cross-bedded rhodolith rudstones, which change basinward to large-scale planar cross-bedded rhodolith rudstones, which in turn pass laterally to planar cross-bedded and flat-bedded bryozoan rudstones. Rhodoliths in rudstones are generally broken, exhibiting several phases of breakage and restarted growth of coralline algae. Many rhodoliths also show asymmetrical growth. The rudstone matrix is a packstone with fragments of coralline algae, bryozoans, calcitic bivalves, echinoids, and foraminifers. Large lithoclasts from the basement, heavily bored by bivalves, are common in the rhodolith rudstone, especially in the most massive type. Rhodolith characteristics and sedimentary structures suggest that trough cross-bedded rhodolith rudstones accumulated in submarine dunes moved by storm surges in a littoral wedge at the western side of a small bay (the Ruinas de Acinipo bay) in the Ronda Basin. Large-scale planar cross-bedded coralline algal and bryozoan rudstones formed in the foresets of the wedge progradation below the storm-wave base. The dominance of Lithophyllaceae and Hapalidiales, with scarce representatives of Corallinaceae in the coralline algal assemblages, reflects that Ronda and Guadalquivir basins opened to the Atlantic Ocean.

OS FÓSSEIS DE SANTA MARIA (AÇORES). 2. PEDRA-QUE-PICA:: UMA HISTÓRIA COM 5 MILHÕES DE ANOS

Book

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Jan 2022

Os Fósseis de Santa Maria (Açores). 2. Pedra-que-pica: uma história com 5 milhões de anos

Book

Full-text available

Jul 2022

ÁVILA S.P., R.S. RAMALHO, C.M. DA SILVA, M.E. JOHNSON, A. UCHMAN, B. BERNING, R. QUARTAU, P. MADEIRA, C.S. MELO, A.C. REBELO, L. BAPTISTA, S. ARRUDA, E. GONZÁLEZ, M.W. RASSER, A. HIPÓLITO, R. CORDEIRO, R. MEIRELES, V. RAPOSO, J. POMBO, R. CÂMARA, M.X. KIRBY, J. TITSCHACK, J.M. HABERMANN, R. VULLO, A. KROH, J.H. LIPPS, M. CACHÃO & J. MADEIRA, 2022. Os fósseis de Santa Maria (Açores). 2. Pedra-que-pica: uma história com 5 milhões de anos, 160 pp. OVGA, Lagoa. Índice: 1. Localização geográfica 2. Enquadramento geodinâmico e paleoclimático 3. Enquadramento geológico dos Açores 4. História Geológica de Santa Maria 5. Os fósseis de Santa Maria 6. A jazida da “Pedra-que-pica” 7. A fauna da “Pedra-que-pica” 8. Reconstrução paleoambiental 9. Possíveis rotas de colonização das ilhas dos Açores 10. Os fósseis de Santa Maria à luz da legislação portuguesa 11. O potencial de visitação turística e de uso educacional da Pedra-que-pica 12. Agradecimentos 13. Fotografias dos participantes nos workshops “Paleontologia em Ilhas Atlânticas” 14. Bibliografia 15. Glossário

“Pink round stones”—rhodolith beds: an overlooked habitat in Madeira Archipelago

Article

Full-text available

Oct 2021
BIODIVERS CONSERV

Despite its worldwide distribution in sedimentary infralittoral and circalittoral bottoms, rhodolith beds have been the subject of fewer studies than other nearshore communities, like kelp forests and coral reefs. This is also the case in Madeira archipelago (Eastern Atlantic), where until recently our knowledge on rhodolith beds was limited to a few references to its occurrence and species composition. In the course of an ongoing habitat-mapping project developed in Madeira, observations revealed that rhodolith beds are more common and extensive than previously supposed. The habitat maps for these beds in Madeira archipelago here presented are the frst ever produced for the region. They reveal a total of 46 rhodolith beds at eleven diferent locations spread across three islands (Madeira, Desertas and Porto Santo), with areas ranging from 776 to 101,081 m² and at depths between 12 and 35 m. Author’s observations, as well as the results suggest that more rhodolith beds are likely to exist in the archipelago, particularly at greater depths and unexplored locations. The application of molecular systematic tools for the identifcation of rhodolith-forming species revealed the occurrence of four species belonging to the genera Lithothamnion and Phymatolithon. This latter genus is represented by a single species which is commonly found in rhodolith beds of the archipelago. Genetically, our results show similarities both with the rhodolith communities from the Canary islands and the Algarve (south of Portugal) and highlight the singularities of the archipelago’s marine flora. The new array of data here presented is deemed essential for an efective management and conservation of these important and sensitive habitats.

Diverse bioerosion structures in lower Pliocene deposits from a volcanic oceanic island: Baía de Nossa Senhora section, Santa Maria Island, Azores (Central Atlantic Ocean)

Article

Feb 2021
PALAEOGEOGR PALAEOCL

Pliocene body fossils from Santa Maria Island, Azores, have been studied for decades, but only more recently have ichnofossils received their due attention. Calcareous Pliocene deposits from the Baía de Nossa Senhora section contain numerous, diverse, well-preserved natural casts of invertebrate borings. The study of this type of fossils adds to knowledge on the dispersal of benthic faunas across oceans to volcanic oceanic islands. The borings belong to seven ichnogenera and twenty-two ichnotaxa at the ichnospecies level with more than half pertaining to Entobia, which is produced by clionaid sponges. Other borings found were produced by bivalves (Gastrochaenolites), polychaete worms (Caulostrepsis and Maeandropolydora), sipunculid worms (Trypanites), phoronid worms (Talpina) and ctenostome bryozoans (Iramena). The occurrence, ichnogeny, distribution and preservational state of the borings suggest that the bearing bioclasts have been exposed for several years on the sea floor. The borings derive from different bathymetric zones on the shelf, and their formation took place during several bioerosional phases. The association of borings belongs to the Entobia ichnofacies, which is typical of carbonate rocky shores, and shows close similarity to those described from the Paratethys, Mediterranean and partly the eastern Atlantic regions. This fits the idea that most of the Neogene shallow-water marine fauna in the Azores is biogeographically related to the eastern Atlantic shores.

Detrital record of the denudation of volcanic islands under sub-tropical climate (Cape Verde)

Article

Full-text available

Feb 2019
CHEM ERDE-GEOCHEM

This research is focused on the composition of the sediments produced in volcanic islands when the climate does not favour weathering. The XRD mineralogy (bulk sample and fraction finer than 63 µm), petrography and geochemistry of a set of bedload stream and beach samples collected in the “old” Maio and the “young” Fogo islands of Cape Verde archipelago are used to investigate the compositional transformations promoted by exogenous processes during island denudation. The main factor responsible for the variability in sediment composition is the incorporation of biogenic material derived from the evolving shelves; it largely exceeds the effects of the exhumation of different volcanic and basement units. Given the arid climate (and steep land surface in Fogo), only the most labile components of basaltic rocks, such as volcanic glass, are decomposed. The incipient weathering and sorting processes are responsible for the depletion of Al in bedload deposits. The same happens with other elements usually regarded as non-mobile (namely, Nb, Th, REE, etc.), while Mg is concentrated. Thus, weathering indices grounded on the premise that “mobile” elements are lost and “non-mobile” elements are enriched via weathering are useless in Cape Verde bedload sediments. With time, weathering is able to promote Na leaching and the formation of secondary minerals, which tend to retain non-mobile elements released in the earlier stages of alteration (e.g., LREE, Th, Y, Nb, Ta etc.). Sorting processes are responsible for the selective removal of less-dense grains, explaining local differences between beach and stream deposits. Beach placers are enriched in augitic clinopyroxene (occasionally also in olivine in the Fogo island), and Sc, Cr and Co. Niobium and Ta must be hosted in fine-grained particles that are easily windblown and their abundances in dusts may reveal Cape Verde as a source of airborne material crossing the Atlantic Ocean.

Rhyolite Domes and Subsequent Offlap of Pliocene Carbonates on Volcanic Islets at San Basilio (Baja California Sur, Mexico)

Article

Full-text available

Feb 2019

San Basilio basin in Baja California Sur (Mexico) exhibits distinct styles of volcanism that interrupted phases of normal sedimentation correlated with the Zanclean Stage (Lower Pliocene). Sea cliffs around a 4-km² bay opening onto the Gulf of California are dominated by rhyolite, mudstone, sandstone, and limestone. Volcanism associated with re-sedimented hyaloclastite is regionally uncommon and the goal was to investigate interactions between volcanic events and intervals of stability represented by fossil-rich strata. Methods of study involved a combination of microfossil and macrofossil analyses. Relating the basin’s faults to Pliocene development in the greater Gulf of California was a secondary goal. Microfossils Bolivina bicostata and B. interjuncta recovered from mudstone indicate an initial water column of 150 m. An abrupt hydromagmatic explosion ruptured the mudstone cover, followed by banded rhyolite flows inter-bedded with sandstone. Outlying limestone beds with the index fossil Clypeaster bowersi are separated from rhyolite by conglomerate eroded under intertidal conditions. A renewed phase of activity saw eruption of smaller volcanoes in the basin center semi-contemporaneous with pecten limestone deposited on unstable slopes. Normal faults conform to a pattern of extensional rifting in the proto-gulf, followed by cross-cutting faults indicating the onset of transtensional tectonics beginning about 3.5 Ma.

The role of climate, marine influence and sedimentation rates in late-Holocene estuarine evolution (SW Portugal)

Article

Full-text available

Jan 2019

Estuaries are sensitive to changes in global to regional sea level, to climate-driven variation in rainfall and to fluvial discharge. In this study, we use source and environmentally sensitive proxies together with radiocarbon dating to examine a 7-m-thick sedimentary record from the Sado estuary accumulated throughout the last 3.6 kyr. The lithofacies, geochemistry and diatom assemblages in the sediments accumulated between 3570 and 3240 cal. BP indicate a mixture between terrestrial and marine sources. The relative contribution of each source varied through time as sedimentation progressed in a low intertidal to high subtidal and low-energy accreting tidal flat. The sedimentation proceeded under a general pattern of drier and higher aridity conditions, punctuated by century-long changes of the rainfall regime that mirror an increase in storminess that affected SW Portugal and Europe. The sediment sequence contains evidence of two periods characterized by downstream displacement of the estuarine/freshwater transitional boundary, dated to 3570–3400 cal. BP and 3300–3240 cal. BP. These are intercalated by one episode where marine influence shifted upstream. All sedimentation episodes developed under high terrestrial sediment delivery to this transitional region, leading to exceptionally high sedimentation rates, independently of the relative expression of terrestrial/marine influences in sediment facies. Our data show that these disturbances are mainly climate-driven and related to variations in rainfall and only secondarily with regional sea-level oscillations. From 3240 cal. BP onwards, an abrupt change in sediment facies is noted, in which the silting estuarine bottom reaches mean sea level and continued accreting until present under prevailing freshwater conditions, the tidal flat changing to an alluvial plain. The environmental modification is accompanied by a pronounced change in sedimentation rate that decreased by two orders of magnitude, reflecting the loss of accommodation space rather than the influence of climate or regional sea-level drivers.

Re-sedimented Rhodoliths in Channelized Depositional Systems

Chapter

Full-text available

Oct 2017

The knowledge of re-sedimented rhodolith deposits has always lagged behind that of in situ deposits, which can be formed in shallow and deeper water carbonate and mixed siliciclastic-carbonate depositional settings. A combination of detailed outcrop analyses from three published case studies reveals a series of palaeobiological and taphonomic signals that are used to identify fossil re-sedimented rhodoliths. The re-sedimented rhodolith deposits of the middle Eocene carbonates in the Venetian area (northeast Italy), the lower Miocene carbonates from southern Sardinia (Italy), and the lower–middle Miocene carbonates from Southern Apennines (southern Italy) are described in terms of rhodolith morphology, coralline algal assemblages, inner arrangement, outer growth-forms, and taphonomic signatures. In all the cases, shallow water rhodolith beds were redeposited to feed offshore deposits through submarine channel systems. The sedimentological features, rhodolith characteristics and taphonomic signatures of the rhodolith deposits are compared from the carbonate factory, through the shelf-margin to the proximal and distal parts of the tributary belt. Within submarine channelized carbonate settings, complex relationship patterns of autochthonous/parautochthonous and allochthonous rhodolith deposits were governed by the interplay of changes in environmental factors such as water energy, light irradiance, substrate characteristics, and residence time on the sediment-water interface.

Rhodoliths and Rhodolith Beds in the Rock Record

Chapter

Oct 2017

Calcareous coralline algae (Rhodophyta; Corallinales, Hapalidiales, and Sporolithales; corallines hereafter) constitute one of the most widespread and successful groups of marine macrophytes. They occur as crusts partially coating hard or soft substrates, as laminar thalli growing directly on the seabed, or forming structures rolling freely on the substrate with an inner nucleus or without it. These latter structures are called rhodoliths. They can be one of the most abundant components in carbonate platform deposits, forming the so-called rhodalgal facies. In assessments of the rhodoliths, internal and external algal growth morphology, rhodolith external form, rhodolith inner arrangement, and assemblages of organisms forming the rhodoliths can provide valuable information for reconstructing palaeoenvironmental and palaeoclimatic conditions. Rhodoliths can occur massively concentrated in beds several meters thick. These concentrations are referred as rhodolith beds. These rhodolith beds may be the result of biotic (autochthonous rhodolith beds), abiotic (allochthonous rhodolith beds) concentrations or due to a mixture of processes (paraautochthonous rhodolith beds). Taphonomic and facies analyses, as well as faunal assemblages, can provide the information needed to confidently differentiate among them. The rock record offers unique information to envisage the founding conditions and the long-term maintenance of the rhodolith beds. In this chapter, we review and update the information on fossil rhodoliths and rhodolith beds, and discuss their value for palaeoenvironmental and palaeoclimatic reconstructions. Also, we discuss the sedimentary and the sequence stratigraphy contexts in which rhodolith beds are preferentially formed and developed.

Rocking around a volcanic island shelf: Pliocene Rhodolith beds from Malbusca, Santa Maria Island (Azores, NE Atlantic)

Article

Jun 2016

Rhodoliths are a common producer of carbonates on modern and ancient shelves worldwide, and there is growing evidence that they thrive on volcanic insular shelves. However, little is still known on how rhodoliths cope with the demands of this particularly dynamic environment. In this study, the focus is placed on fossil rhodoliths from a Pliocene sequence at Santa Maria Island, Azores, in order to gain further insight into the life cycle (and death) of rhodoliths living within a mid-ocean active volcanic setting. These rhodoliths occur as a massive accumulation within a larger submarine volcano-sedimentary sequence that was studied from the macro- to the microscale in order to reconstruct the paleoenvironmental conditions under which the rhodolith accumulation was deposited and buried. All fossil rhodoliths from this setting are multispecific and demonstrate robust growth forms with a lumpy morphology. Moreover, taphonomical analyses show the succession of several destructive events during rhodolith growth, suggesting life under a highly dynamic system prior to stabilization and burial. The rhodoliths therefore tell a story of an eventful life, with multiple transport and growth stages, owing to the environment in which they lived. Transport and deposition to their final resting place was storm-associated, as supported by the general sedimentary sequence. In particular, the sequence features an amalgamation of tempestites deposited under increasing water depths, sediment aggradation, and before burial by volcanic activity. This transgressive trend is also attested by the overall characteristics of the volcano-sedimentary succession, which exhibits the transition to subaerial environment in excess of 100 m above the rhodolith bed.

Rhodolith/Maerl beds: A Global Perspective

Book

Feb 2016

Palaeoecology, taphonomy, and preservation of a lower Pliocene Shell bed (coquina) from a volcanic oceanic island (Santa Maria Island, Azores)

Article

Full-text available

Apr 2015
PALAEOGEOGR PALAEOCL

Massive fossil shell accumulations require particular conditions to be formed and may provide valuable insights into the sedimentary environments favouring such concentrations. Shallow-water shell beds appear to be particularly rare on reefless volcanic oceanic islands on account of narrow, steep and highly-energetic insular shelves where the potential for preservation is limited. The occurrence of an exceptional coquina (Pedra-que-pica) within the Miocene–Pliocene deposits of Santa Maria Island (Azores), therefore provides a rare opportunity to understand the conditions that led to the formation and preservation of a massive shell bed at mid-ocean insular setting. This study provides a detailed analysis regarding a 10–11 m-thick bivalve-dominated fossil assemblage exposed at Pedra-que-pica on Santa Maria Island in the Azores. Integration of taphonomical, palaeoecological and sedimentological observations are used to reconstruct the genesis of the coquina bed and related events, and to discuss why such exceptional sedimentary bodies are so rare on shelves around reefless volcanic oceanic islands. The sequence at Pedra-que-pica demonstrates a complex succession of sedimentary environments in response to the drowning of an existing coastline during a period of rapid sea-level rise. The Pedra-que-pica shell bed incorporates storm-related materials and possible debris falls that originated nearby in a shallow and highly productive carbonate factory. Deposition took place below fair-weather wave base, at around 50 m depth, as inferred from the overlying volcanic succession. The preservation of this coquina was favoured by deposition on a platform laterally protected by a rocky spur, combined with rapid burial by water-settled volcanic tuffs and subsequent volcanic effusive sequences. The recent exhumation of the deposit is the result of island uplift and subsequent erosion.

Palaeoecology, taphonomy, and preservation of a lower Pliocene Shell bed (coquina) froma volcanic oceanic island (Santa Maria Island, Azores)

Article

Apr 2015
PALAEOGEOGR PALAEOCL

Rhodolith forming coralline algae in the Upper Miocene of Santa Maria Island (Azores, NE Atlantic): a critical evaluation

Article

Dec 2014

The Late Miocene Malbusca outcrop is located in the southeastern coast of Santa Maria Island (Azores, NE Atlantic), interspersed in volcanic formations. At ~20 meters above present sea level, a prominent discontinuous layer of rhodoliths seizes with an extension of ~250 meters. This paper presents the first taxonomic record of fossil rhodolith forming coralline algae for the Miocene of the Azores. The preserved taxonomic features used were the following: (1) arrangement of basal filaments, (2) epithallial cells (when observable), (3) presence of cell fusions, (4) conceptacle type, (5) number of cells layers which conceptacle chamber floors are situated below the surrounding thallus surface and (6) for the sporangial pores, the orientation of the filaments around the conceptacle pores. Based on these characters, six taxa were identified encompassing three Corallinaceae (Lithophyllum prototypum, Lithophyllum sp., Spongites sp., Hydrolithon sp.) and one Hapalidaceae (Phymatolithon calcareum and cf. Phymatolithon sp.). An unidentified coaxial thallus was also present, the coaxial construction ascribing the specimens to the genus Mesophyllum or Neogoniolithon. Taxonomic accounts for the identified taxa are described, illustrated and an identification key is provided. The report of L prototypum represents the first Miocene record and the preservation of the specimens is very good. Miocene coralline algae seem very consistent among deposits but some species are relevant for particular areas, like in the Azores.

Associated fauna in a mesophotic rhodolith bed in the Campos Basin, Brazil, southwestern Atlantic

Article

Apr 2024

Rhodolith beds are predominantly formed by calcareous red algae with a rich benthic fauna. Nonetheless, little is known about the biota relationships with their structural variability. Rhodolith shape, size and vitality were examined from 22 sampling stations in dense and sparse areas of a mesophotic rhodolith bed in the Campos Basin, Brazil located within a major oil production area. Nine taxonomic benthic faunal groups were identified: Mollusca (58 taxa), Polychaeta (51), Echinodermata (27), Crustacea (24), Bryozoa (20), Porifera (19), Cnidaria (6), Ascidiacea (2) and Brachiopoda (1). Cnidaria, Mollusca, Bryozoa and Brachiopoda were abundant in areas with gravelly/rhodolith bottoms, while Polychaeta were associated with sandy/muddy bottoms. The faunal composition in the Peregrino oil field showed greater species richness than observed in other rhodolith beds worldwide. The present study contributes to the knowledge of the structural, taxonomical and functional characteristics of a rhodolith bed present along the Brazilian continental shelf.

References

Chapter

Sep 2023

A. B. Watts

Isostasy is a simple concept, yet it has long perplexed students of geology and geophysics. This fully updated edition provides the tools to better understand this concept using a simplified mathematical treatment, numerous geological examples, and an extensive bibliography. It starts by tracing the ideas behind local and regional models of isostasy before describing the theoretical background, the observational evidence. It now also includes an exploration of the role of flexure in landscape evolution and dynamic topography and discussions of lithosphere memory, inheritance, and new NASA mission topography and gravity data. The book concludes with a discussion of flexure's role in understanding the evolution of the surface features of the Earth and its neighboring planets. Intended for advanced undergraduate and graduate students of geology and geophysics, it will also be of interest to researchers in gravity, geodesy, sedimentary basin formation, mountain building and planetary geology.

EL REGISTRO FÓSIL MARINO DE MACARONESIA: INTERPRETANDO EVENTOS DE SU HISTORIA GEOLÓGICA

Conference Paper

Full-text available

May 2023

Esther Martín González

A marine bird (sulidae, Aves) from the Langhian (middle Miocene) of Penedo beach (Setúbal Peninsula-SW Portugal) and its paleoenvironmental context

Article

Full-text available

Jan 2023
J IBER GEOL

The fossil remains of birds from the Miocene of Portugal are scarce, encompassing a total of twelve specimens, from nine paleon-tological outcrops located in Leiria (central Portugal), southern sector of Setúbal Peninsula and along the lower Tagus Basin. This study focuses on a new specimen found in the Praia do Penedo Norte (Sesimbra) coastal cliff corresponding to a coracoid bone, attributed to Morus sp., a sulid bird, biostratigraphically framed by calcareous nannofossils within the middle Miocene (Langhian).

Crustose coralline red algae frameworks and rhodoliths: Past and present

Book

Jan 2023

Development and preservation of transgressive sandy versus rocky shorelines: Observations from the SE African shelf

Article

Oct 2022
GEOMORPHOLOGY

Relict rocky shorelines are seemingly rarely preserved in the geological record, often underrepresented in the literature and the reasons for their scarcity, as well as the conditions needed for preservation are not well understood. On the “Wild Coast” shelf along the subtropical east coast of South Africa a series of submerged shorelines are present. High-resolution multibeam bathymetry, side scan sonar, ultra-high-resolution seismic profiling and legacy seismic reflection data show several fluvial channels incised into the shelf, associated with the Last Glacial Maximum (LGM) lowstand. Palaeo-shorelines are preserved at −85, −75, −60 and −40 m. The −85 and −75 m shorelines reflect sediment abundance and depositional features preserved as lagoonal depressions, lithified barrier-spits and coastal barrier dunes with limited bedrock influence. The shallower shorelines at −60 and −40 m occur on a steeper shelf gradient and reflect the development of rocky headlands, palaeo-cliffs and rock shore platforms. Their occurrence implies increasing bedrock control, erosion and sediment bypass during transgression. The preservation of rocky shoreline signatures on the inner shelf is aided by competency of the bedrock lithologies, in addition to their depth relative to sea-level stillstands and rapid rises in sea level associated with meltwater pulses (MWPs) (notably the −60 m shorelines and MWP1B). MWPs may have helped facilitate the overstepping and eventual preservation of these shoreline landforms on the seabed. Preservation of the −40 m shoreline is considered to correlate with accelerated rates in sea- level rise prior to MWP1C. The preservation of the −85 and −75 m aeolianite shorelines are not linked to meltwater pulses, however, their substantial sediment volume precluded total loss through transgressive erosion and this, coupled with early cementation, promoted their partial preservation. Such deposits rarely survive subaerial exposure but could be preserved if covered by marine sediments. Conversely, the existence of well-preserved, Pleistocene-age rocky shorelines on the shelf points to their high potential for inclusion into the longer-term geological record under subsequent high or low sedimentation. The formation of distinctive rocky shorelines, however, may reflect alternating slow and fast rates of sea-level rise.

THE CORALLINE ALGAE (RHODOPHYTA) FACED TO THE OCEAN ACIDIFICATION WITH SPECIAL REFERENCE TO THE CANARY ISLANDS

Chapter

Full-text available

Mar 2022

Julio Afonso-Carrillo

Las algas coralinas (Rhodophyta) ante la acidificación del océano con especial referencia a las Islas Canarias

Article

Full-text available

Dec 2021

Julio Afonso-Carrillo

La futura acidificación de los océanos tendrá un impacto generalizado en las especies marinas y los ecosistemas. Las algas coralinas (Corallinophycidae, Rhodophyta) son elementos conspicuos de las comunidades litorales en todas las latitudes. En la pared celular de las algas coralinas interviene la forma más soluble de calcita, lo que las hace particularmente vulnerables a variaciones en el pH del mar. En esta síntesis discutimos el actual conocimiento sobre las algas coralinas de Canarias en el contexto de los avances producidos en las últimas décadas tanto en la taxonomía (aplicación de diferentes criterios taxonómicos) como en la ecología (un grupo de organismos que prestan unos servicios ecosistémicos muy valiosos). En Canarias las algas coralinas están actualmente representadas por 56 especies, pero existen fundadas razones para creer que es una subestimación del número real de especies. Participan en la mayoría de las comunidades litorales y su abundancia permite considerarlas como elementos característicos de numerosos hábitats, algunos de ellos escasamente investigados.

Natural History of Rhodolith/Maërl Beds: Their Role in Near-Shore Biodiversity and Management

Chapter

Oct 2017

Rafael Riosmena-Rodríguez

Rhodolith/maërl beds are living and dead aggregations of free-living non-geniculate coralline algae that cover extensive benthic areas in recent oceans and are common in fossil deposits. They are slow growing organisms and can be long-lived (>100 years), distributed over a wide depth range from intertidal sites to 270 m. Rhodolith/maërl beds are a common feature of modern and ancient carbonate shelves worldwide that represent a sedimentary transition from sandy/muddy areas to the rocky substrate. They are bioengineers and provide a three-dimensional habitat for associated species. It has been demonstrated that rhodolith/maërl grounds are a suitable habitat for multispecies recruitment and provide refuge for juvenile life stages of commercially important shellfish species. Rhodoliths are resilient to a variety of environmental disturbances, but can be severely impacted by harvesting these commercial species, ocean acidification or global warming. The value of rhodoliths as a unique biotope around the world is under threat from different kinds of human activities. Despite the importance of rhodolith/maërl beds in the marine environment, a major limitation for protection is the lack of a clear definition of an ecosystem. A thorough review of the literature revealed a total of 12 vernacular/scientific terms that have been applied to free-living coralline red algae and these should be treated as synonyms. The Challenger Expedition (1872–1876) was one of the first voyages that promoted the understanding of the rich flora and fauna associated with coralline deposits. During the nineteenth century additional surveys in other areas of the world have confirmed the value of this ecosystem. During twentieth and twenty-first centuries many researchers have produced a vast scientific literature, documenting the importance of rhodolith/maërl, to understand their relevance regarding biodiversity in nearshore habitats. The relevance includes the description of new species or where the distribution of poorly known species has been extended, but more importantly the high number of associated species which includes species under protection, species ecologically relevant or species which are part of a formal fisheries. As a consequence of the concern about the state of the ecosystems in Europe at the end of the twentieth century, the EU developed a network of protected areas known as Natura 2000 sites. A series of publications on the conservation status of the maërl/rhodoliths in Atlantic and Mediterranean waters, Brittany, Gulf of California, and their relationship with fisheries, stated clearly that the health of rhodolith habitats in some areas of the world is decreasing, and there is an urgent need for management strategies. The combination of the interest in developing rhodolith/maërl conservation in other countries, the decline of the French Atlantic maërl deposits, and the correlation of rhodolith/maërl presence in or near oil deposits has motivated the exploration of rhodoliths in other areas such as Brazil, México, Australia and New Zealand. Understanding is increasing about the ecological role of rhodoliths in nearshore environments worldwide, the biodiversity associated with rhodoliths, and how human activities are having an increasing impact. The recognition of the importance of rhodolith beds as biodiversity centers has increased with the number of published papers and the growth in knowledge about the taxonomic status of the associated species.

Mid-Miocene (Badenian) transgression on Mesozoic basement rocks in the Mt. Medvednica area of northern Croatia

Article

Full-text available

Apr 2016
FACIES

Rocky shorelines in the geological record, which represent major transgressive surfaces and provide us with crucial information about palaeoshorelines and ancient sea-levels, often involve a basal conglomerate associated with an unconformity. The unconformity between the Mesozoic basement and the overlying Middle Miocene (Badenian) deposits (which belong palaeo-geographically to the south-western margins of the Central Paratethys, and geotectonically to the Pan¬nonian Basin System) in the Gornje Orešje section (NE Mt. Medvednica, N Croatia) is marked by basal Badenian conglomerates. The Upper Cretaceous limestone lithoclasts occurring in basal conglomerates show abundant truncated Gastrochaenolites and Entobia borings (represented by an in situ rocky substrate community of bivalves and sponges, respectively), with Gastrochaenolites being the dominant ichnogenus. Gastrochaenolites-Entobia ichnofossil assemblage related to the Entobia subichnofacies and in turn assignable to the Trypanites Ichnofacies, is very typical of Neogene rocky shores. This association characterizes littoral rockground environments indicating wave-cut platforms and marine flooding surfaces (transgressive surfaces) with a low or null rate of sedimentation. Erosion of a pre-existing Mesozoic basement rocky shore during a marine transgressive phase in these high-energy littoral conditions, which formed basal conglomerates analysed here, is also evidenced by truncation and the occurrence of Gastrochaenolites borings on all sides of limestone clasts. Calcareous red algae encrusting bored lithoclasts and often forming rhodoliths, also imply that the basal conglomerates occur at the start of a deepening upwards succession of a transgressive systems tract deposits. The biostratigraphic correlation of the transgressive deepening upward sequence from the Gornje Orešje section (ascribed to a single Badenian transgressive event that affected the Mesozoic basement) is based on the presence of the orbulinas in the uppermost outer shelf marl interval. The co-occurrence of Praeorbulina glomerosa circularis and Orbulina suturalis points to the short interval between FO (first occurrence) of Orbulina suturalis and LO (last occurence) of Praeorbulina glomerosa circularis. These samples belong to the plankton zone M6 (Orbulina suturalis Zone) or the base of NN5 zone, marking the 3rd order sequence TB 2.4, which represents the second, main Badenian transgression in the Central Paratethys.

The emergence of volcanic oceanic islands on a slow-moving plate: The example of Madeira Island, NE Atlantic

Research

Full-text available

Jan 2016

Madalena Calado Santos Sobral Fonseca

The transition from seamount to oceanic island typically involves surtseyan volcanism. However, the geological record at many islands in the NE Atlantic—all located within the slow-moving Nubian plate—does not exhibit evidence for an emergent surtseyan phase but rather an erosive unconformity between the submarine basement and the overlying subaerial shield sequences. This suggests that the transition between seamount and island may frequently occur by a relative fall of sea level through uplift, eustatic changes, or a combination of both, and may not involve summit volcanism. In this study, we explore the consequences for island evolutionary models using Madeira Island (Portugal) as a case study. We have examined the geologic record at Madeira using a combination of detailed fieldwork, biostratigraphy, and 40Ar=39Ar geochronology in order to document the mode, timing, and duration of edifice emergence above sea level. Our study confirms that Madeira’s subaerial shield volcano was built upon the eroded remains of an uplifted seamount, with shallow marine sediments found between the two eruptive sequences and presently located at 320–430 m above sea level. This study reveals that Madeira emerged around 7.0–5.6 Ma essentially through an uplift process and before volcanic activity resumed to form the subaerial shield volcano. Basal intrusions are a likely uplift mechanism, and their emplacement is possibly enhanced by the slow motion of the Nubian plate relative to the source of partial melting. Alternating uplift and subsidence episodes suggest that island edifice growth may be governed by competing dominantly volcanic and dominantly intrusive processes.

Darwin and palaeontology: a re-evaluation of his interpretation of the fossil record

Article

May 2015

Warren D. Allmon

Charles Darwin's empirical research in palaeontology, especially on fossil invertebrates, has been relatively neglected as a source of insight into his thinking, other than to note that he viewed the fossil record as very incomplete. During the Beagle voyage, Darwin gained extensive experience with a wide diversity of fossil taxa, and he thought deeply about the nature of the fossil record. That record was, for him, a major source of evidence for large-scale transmutation, but much less so for natural selection or single lineages. Darwin's interpretation of the fossil record has been criticised for its focus on incompleteness, but the record as he knew it was extremely incomplete. He was compelled to address this in arguing for descent with modification, which was likely his primary goal. Darwin's gradualism has been both misrepresented and exaggerated, and has distracted us from the importance of the fossil record in his thinking, which should be viewed in the context of the multiple, sometimes competing demands of the multifaceted argument he presented in the Origin of Species.

Deep-water rhodolith bed from central Brazilian continental shelf, Campos Basin: coralline algal and faunal taxonomic composition

Article

Full-text available

Dec 2014

Rhodolith beds are distributed from the north eastern to the southeastern Brazilian continental shelf, constituting the largest extension of coralline algal de posits in the world. Little is known about the deep rho dolith beds within the Campos Basin: the largest oil pro duction area in the country and a priority area for marine life conservation. This study illustrates a deep rhodolith bed covering about 15 km 2 of a 40 km 2 area in the Peregrino oil field sampled at 100-106 m water depth. Coralline algae are the dominant components on the living rhodolith surfaces associated with subordinate bryozoans, cnidarids, brachiopods and porifers. In some inner parts of the coralline algal nodules, encrusting acervulinid fora minifera are the main nodule contributors. Through accel erator mass spectrometric analysis, radiocarbon age esti mates show that the range in ages between the living outer rhodolith parts and within 3 mm from it the rhodoliths is ca. 4,700 years. This suggests that a proportion of fossil rhodoliths had been recolonized after periods of burial and/or erosion. The presentday Peregrino rhodolith bed played a fundamental ecological role in the Brazilian con tinental shelf's benthic habitats for thousands of years.

Temperate carbonate production: biodiversity of calcareous epiliths from intertidal to bathyal depths (Azores)

Article

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Jun 2014

The diversity of calcareous epiliths along a bathymetrical transect spanning the intertidal to bathyal (0–500 m) was investigated during a two-year carbonate cycling experiment deployed in the Faial Channel, Azores. The epilith inventory comprises 47 taxa of bryozoans, 9 serpulids, 3 bivalves, 3 cirripeds, 3 foraminiferans, 2 vermetid gastropods, and 1 scleractinian coral, along with 6 rhodophyte morphotypes. Colonised surface area, carbonate accretion rates, and biodiversity peak in the deeper euphotic zone (15 and 60 m), where mature biocoenoses were established after 2 years exposure, whereas colonisation was retarded at dysphotic and aphotic depths. Particularly in the photic zone, colonised surface area, accretion rates, as well as species richness and abundance were higher on down-facing compared to up-facing surfaces. Analyses of similarity (ANOSIM) and non-metrical multidimensional scaling (NMDS) of species abundance data revealed that water depth and substrate orientation were the dominant factors controlling the community structure as a result of direct (photosynthesis) and indirect (bioerosion pressure; nutrient supply) effects of the light regime, while exposure time and substrate type had little influence. The same hierarchy applies for the ichnodiversity of bioerosion traces, but with an inverse pattern in case of substrate orientation, reflecting the interaction of encrustation and bioerosion. Positive net carbonate production rates support the evelopment of oyster bioherms and heterozoan-dominated carbonate/volcaniclastic sediments accumulating in the Faial Channel and adjacent slope. A comparison with biogenic sediments from other Macaronesian archipelagos and seamounts demonstrates the abundance and diversity of nontropical heterozoan carbonates in oceanic islands.

A Middle Miocene carbonate embankment on an active volcanic slope: Ilhéu de Baixo, Madeira Archipelago, Eastern Atlantic

Article

Jan 2014
GEOL J

Carbonate factories on insular oceanic islands in active volcanic settings are poorly explored. This case study illuminates marginal limestone deposits on a steep volcanic flank and their recurring interruption by deposits linked to volcaniclastic processes. Historically known as Ilhéu da Cal (Lime Island), Ilhéu de Baixo was separated from Porto Santo, in the Madeira Archipelago, during the course of the Quaternary. Here, extensive mines were tunnelled in the Miocene carbonate strata for the production of slaked lime. Approximately 10 000m3 of calcarenite (�1 to 1ø) was removed by hand labour from the Blandy Brothers mine at the south end of the islet. Investigations of two stratigraphic sections at opposite ends of the mine reveal that the quarried material represents an incipient carbonate ramp developed from east to west and embanked against the flank of a volcanic island. A petrographic analysis of limestones from the mine shows that coralline red algae from crushed rhodoliths account for 51% of all identifiable bioclasts. This materialwas transported shoreward and deposited on the ramp between normal wave base and stormwave base at moderate depths. The mine’s roof rocks are formed by Surtseyan deposits from a subsequent volcanic eruption. Volcaniclastic density flows also are a prevalent factor interrupting renewed carbonate deposition. These flows arrived downslope from the north and gradually steepened the debris apron westwards. Slope instability is further shown by a coral rudstone density flow that followed from growth of a coral reef dominated by Pocillopora madreporacea (Lamarck), partial reef collapse, and transport from a more easterly direction into a fore-reef setting. The uppermost facies represents a soft bottom at moderate depths in a quiet, but shore-proximal setting. Application of this study to a broader understanding of the relationship between carbonate and volcaniclastic deposition on oceanic islands emphasizes the susceptibility of carbonates to dilution and complete removal by density flows of various kinds, in contrast to the potential for preservation beneath less-disruptive Surtseyan deposits.

Miocene–Pliocene rocky shores on São Nicolau (Cape Verde Islands): Contrasting windward and leeward biofacies on a volcanically active oceanic island

Article

Feb 2014
PALAEOGEOGR PALAEOCL

Trace fossils and ichnofabrics from Cretaceous carbonate deposits, Las Cuevas, High Andes, Mendoza

Article

Full-text available

Jan 1994

An upper Neocomian carbonate succession, assigned to the Agrio Formation (Mendoza Group), crops out at Quebrada de La Avalancha, Las Cuevas (Mendoza, Argentina). The whole section shows a general regressive trend. It is mainly composed of stacked shallowing-upward cycles. Each cycle consists, from base to top, of three facies: A) fossiliferous limestones deposited in an offshore setting; B) nodular limestones formed in an offshore-shoreface transition and C) massive limestones interpreted as lower shoreface deposits. Facies B shows pervasive bioturbation evidenced by a Thalassinoides paradoxicus ichnofabric of bioturbation index 4. Skolithos linearis, Schaubcylindrichnus isp. and Phycodes aff. P. palmatus are present in facies C. This facies displays lower ichnofabric indices (B1 2-3). The ichnogenus Schaubcylindrichnus is only known from the Cretaceous of United States and the Neogene of Taiwan. -from English summary

Rhodolith-bearing limestones as transgressive marker beds: Fossil and modern examples from North Island

Article

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Stratigraphic applications of the Glossifungites ichnofacies: Delineating discontinuities in the rock record

Article

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Jan 1992

The Glossifungites ichnofacies is a substrate-controlled assemblage of trace fossils, characterized by robust, sharp-walled (locally scratch-marked), unlined, passively filled, vertical to sub-vertical domichnia, excavated into semiconsolidated (firmground) substrates. The assemblage is dominated by the ichnogenera Diplocraterion, Skolithos, Arenicolites, firmground Thalassinoides/Spongeliomorpha?, firmground Gastrochaenolites, Psilonichnus, and Rhizocorallium, which typically cross-cut the softground, resident trace fossil suite. The Glossifungites assemblage demarcates discontinuity surfaces that reflect pauses in sedimentation, generally accompanied by erosion. Colonization of erosionally exhumed substrates and excavation of domiciles occur overwhelmingly within marine or marginal marine settings. Many of these discontinuity surfaces correspond to boundaries of stratigraphic significance. Lowstand surfaces of erosion marked by Glossifungites assemblages reflect settings lying in marine or marginal marine positions immediately succeeding erosion related to sea level lowstand conditions. These settings are largely limited to seaward margins of some incised valley-fills, incised submarine canyons, and lowstand (forced-regression) shorefaces. More favourable to firmground development and colonization are marine flooding surfaces (parasequence boundaries) with accompanying transgressive erosion. Under such conditions, generation of the discontinuity surface occurs within a marine setting, permitting almost immediate colonization by opportunistic organisms. Examples of amalgamated (co-planar) lowstand erosion and transgressive erosion surfaces have also been recognized. These are produced where lowstand conditions expose or exhume the substrate but the marine conditions permitting colonization do not occur until the following transgressive phase.

Rhodolith detritus impounded by a coastal dune on Isla Coronados, Gulf of Califomia

Article

Full-text available

Dec 2007

Sand from a small dune on the southwest side of Isla Coronados with an estimated volume of 30,000 m3 is enriched as much as 86.5% by calcium-carbonate detritus from beached rhodoliths. Elsewhere in the Gulf of California, coastal sand dunes often occur on the north sides of islands or north-facing peninsular shores, and calcium-carbonate input is more commonly linked to bivalve mollusks from adjacent sand flats. The local calcium-carbonate budget for the west-facing shore of Isla Coronados must take into account how many rhodoliths of a given size are required to build a sand dune with a known composition and volume. To this end, 135 whole rhodoliths were collected from above the tide line at Punta El Bajo, across from Isla Coronados, on the peninsular mainland. One cubic meter is calculated to accommodate 8640 whole rhodoliths with an average diameter of 5 cm and an average sphericity of 0.86. The age of a rhodolith this size could be several decades. Through stages, the sample rhodoliths were crushed to a maximum grain size ≤ 2.38 mm in diameter (-1.25ø equivalent), and the product was used to estimate the proportion required to generate 1 m3 of pure carbonate sand. Accounting for 2% loss throughout the reduction process, about 16,265 crushed rhodoliths are needed to produce 1 m3 of carbonate sand. Thus, a 30,000-m3 dune requires approximately 488,000,000 rhodoliths to generate 86% of the dune's volume. Dunes of this kind may be rare, but the computation is applicable to other rhodolith-derived dunes in the Gulf of California.

Phycodes templus isp. nov. from the Lower Devonian of northwestern New Brunswick, eastern Canada

Article

Full-text available

Mar 1994

Phycodes templus isp. nov. is described from siliciclastic turbidites of the Wapske Formation of the Tobique Group (Lower Devonian) of northwestern New Brunswick, eastern Canada. The new ichnospecies is characterized by a minimum of two, more typically several, bundled sets of essentially horizontal, broomlike or flabellate, internally structureless, lined burrows. Burrow sets within an individual specimen are interconnected by a single tunnel to form an overall inverted pagoda-shaped structure. The ichnotaxon is believed to have been produced by a vagile deposit feeder, most likely a worm-like organism. RÉSUMÉ Phycodes templus isp. nov. est décrit à partir d'échantillons provenant de turbidites siliciclastiqucs de la Formation de Wapske du Groupe de Tobique (Dévonien inférieur) du nord-ouest du Nouveau-Brunswick, dans L'Est du Canada. La nouvelle espèce d'ichnofossile est caractérisée par un minimum de deux, plus typiquement plusieurs ensembles groupés de terriers doublés, essentiellement horizontaux, en forme de balai ou d'éventail et sans structures internes. Les ensembles de terriers dans un spécimen donné sont interconnectés par un tunnel simple pour former une structure d'ensemble en forme de pagode inversée. On croit que L'ichnofossile a été produit par un animal mobile se nourissant de dépôts et ressemblant probablement à un ver. [Traduit par la rédaction]

Dichotomous Facies on a Late Cretaceous Rocky Island as Related to Wind and Wave Patterns (Baja California, Mexico)

Article

Full-text available

Aug 1993

Local facies forming the basal Rosario Formation of the Upper Cretaceous in the Erendira area of northern Baja California (Mexico) were deposited in distinctive windward and leeward settings. The windward side of the island is dominated by a thick, homogeneous wedge of andesite clasts reducted to small cobbles and pebbles in a limestone matrix. A thinner but crudely graded wedge of andesite clasts ranging from boulder to pebble size is present on the leeward side of the island. -from Authors

Tracers of uplift and subsidence in the Cape Verde archipelago

Article

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Apr 2010

Ocean islands are subject to vertical movements through their lifetime and these movements' origins are poorly understood. Tracking island freeboard provides insight into the mechanisms that cause these movements. The Cape Verde archipelago is the result of hotspot volcanism ranging from 26 Ma to the present, and constitutes a structural pile that has preserved a record of the palaeo-positions of mean sea level. These palaeo-markers, both sedimentary and volcanic, are datable and can be used to track the island's history of vertical movements when used in conjunction with a eustatic curve. We have studied the volcanostratigraphy of Cape Verde and mapped the palaeo-marker positions and features, and we provide the sea-level height information that can be estimated through them. Evidence exists for differential uplift histories throughout the archipelago, with substantial uplift in some of the islands synchronous with volcanism. Other islands, in contrast, exhibit evidence for stability or complex uplift–subsidence histories. Supplementary material Detailed island geology information is available at http://www.geolsoc.org.uk/SUP18390 .

Chapter 4. The Ichnofacies Paradigm

Chapter

Full-text available

Jan 2012

Modern depositional systems are characterized by a complex interaction of numerous physical, biological, and chemical processes, including wave energy, tidal flux, storm influence, fluvial-sediment input, subaqueous vs. subaerial exposure, salinity, temperature, substrate consistency, water turbidity, oxygenation, and other physicochemical factors. These factors can be very difficult to discern and apply to paleoenvironmental interpretations of the rock record. Ichnology constitutes a valuable tool in diagnosing many of these physico-chemical parameters in ancient systems, particularly when integrated with sedimentological and stratigraphic analysis. Trace fossils are unique because they are not merely paleontologic entities but also biogenic sedimentary structures, and they must be evaluated in this sense. The integration of both sedimentological and ichnological data constitutes a powerful tool in the interpretation of ancient depositional systems. The ichnofacies paradigm offers critical information about the conditions operating during deposition or during colonization of stratigraphic discontinuities. One of the strengths of the ichnofacies concept lies in its ease of integration with classical physical sedimentologic facies analysis, and its adherence to Walther's Law.

Coralline Algae Depositional Environments on the Brazilian Central–South-Eastern Shelf

Article

Full-text available

Jan 2012

The Brazilian central shelf along the State of Espirito Santo features several palaeochannels eroded into deposits of coralline algae. Seismic profiles and jet-probe cores have shown a layer of calcareous sediments over 10 m thick that is formed mainly by the fragmentation of coralline algae. Video and scuba-diving observations demonstrate that dense aggregations of rhodoliths often change position on the sea bottom as a result of being dragged ("sail" effect) by foliose algae such as Sargassum, Dictyopteris, and Zonaria. The benthic fauna associated with rhodoliths or folióse macroalgae (Hydrozoa and Amphipoda) is dominated by polychaetes, ophiuroids, and molluscs. The sessile fauna is poorly represented, consisting of some sponge species. It was possible to verify the following dynamics: (i) in summer, folióse macroalgae and zoobenthos (phytal) are established upon the rhodoliths; (ii) in early and midwinter, epiphytic macroalgae are senescent; (iii) in late winter, storms disturb sediments on the sea bottom, burying living rhodoliths and removing the folióse algae; and (iv) in spring, a bloom of the bivalvia Pinctata pectinata occurs, overgrowing the bottom. The rhodoliths tend to remain on the seafloor (with 10% to 20% estimated buried and becoming permanently incorporated to the sediment mass). The sediment dynamics are strongly dominated by erosive phases related to storms.

Ichnology of Carboniferous tide-influenced environments and tidal flat variability in the North American Midcontinent

Article

Full-text available

Jun 2004

Trace fossils are sensitive indicators of environmental fluctuations and, accordingly, ichnological studies have the potential to improve facies characterization of marginal-marine systems. Carboniferous intertidal deposits in eastern Kansas and western Missouri accumulated under contrasting palaeoenvironmental conditions, ranging from the open shoreline to fluvio-estuarine transitions. Comparative analysis of these exposures illustrates lateral variations in trace-fossil content and allows characterization of the intertidal ichnofaunas developed in three sub-environments: open marine, restricted bays and fluvio-estuarine transitions. Open-marine tidal flat ichnofaunas are characterized by (1) high ichnodiversity, (2) marine ichnotaxa produced by both euryhaline and stenohaline forms, (3) the presence of both infaunal and epifaunal traces, (4) the presence of simple and complex structures produced by presumed trophic generalists and specialists respectively, (5) dominance of horizontal trace fossils of the Cruziana ichnofacies, (6) presence of multispecific associations, (7) high density, and (8) wide size range. This ichnofauna is present in heterolithic deposits and reflects the activity of a biota that inhabited tidal flats dominated by normal-marine salinities and connected directly to the open sea. Restricted-bay ichnofaunas display (1) low ichnodiversity, (2) ichnotaxa commonly found in marine environments, but produced by euryhaline organisms, (3) dominance of infaunal traces rather than epifaunal trails, (4) simple structures produced by opportunistic trophic generalists, (5) a combination of vertical and horizontal trace fossils from the Skolithos and Cruziana ichnofacies, (6) the presence of monospecific associations, (7) variable density, and (8) small size. This assemblage occurs in heterolithic facies and records the activity of a brackish-water benthic fauna inhabiting intertidal areas of estuarine basins and embayments. Fluvio-estuarine ichnofaunas are characterized by (1) moderate to relatively high diversity, (2) forms typically present in continental environments, (3) the dominance of surface trails and absence of burrows, (4) temporary structures produced by a mobile deposit-feeding fauna, (5) a mixture of trace fossils belonging to the Scoyenia and Mermia ichnofacies, (6) moderate density of individual ichnotaxa, (7) absence of monospecific suites, and (8) small size. This assemblage occurs in tidal rhythmites and records the activity of a typical freshwater/terrestrial benthos inhabiting tidal flats that were developed in the most proximal zone of the inner estuary under freshwater conditions. Through integration of ichnological and sedimentological data, conventional sedimentological interpretations of marginal-marine depositional systems can be refined and enhanced.

Firmness Profiles Associated with Tidal-Creek Deposits: The Temporal Significance of Glossifungites Assemblages

Article

Full-text available

Sep 2000

Indentation tests are used to detail firmness profiles from intertidal creek deposits and wave exhumed substrates at Willapa Bay. Both of these sedimentological settings are characterized by occurrences of modern Glossifungites assemblages. Firm substrates associated with the intertidal creek deposits are derived from dewatered modern sediments, whereas firmgrounds associated with wave erosion consist of dewatered and compacted Pleistocene strata, The Pleistocene firmgrounds are notably firmer than those derived from modern deposits. A strong correlation between sediment firmness and burrowing behavior is evident in these deposits. In tidal creek systems, the comparatively firm cutbank is characterized by unlined, large-diameter, open burrows that form a Glossifungites assemblage, Intertidal point bar deposits contain a softground suite consisting of mucous-lined, small diameter, dominantly vertical traces. Finally, a softground suite of robust, mucous- or mud-lined, vertical and horizontal traces are observed in intertidal-flat deposits. In contrast, Pleistocene firmgrounds generally contain large- and small-diameter traces, with dominantly vertical architectures (Thalassinoides, Gastrochaenolites-, Diplocraterion, or Arenicolites-like burrows), depending upon the firmness of the substrate. Glossifungites occurrences on modern firmgrounds are temporally insignificant, whereas similar occurrences in Pleistocene substrates are temporally significant, Contrasting these two databases suggests that the stratigraphic significance of a Glossifungites demarcated discontinuity can be assessed in the rock record. Several criteria that are useful for identifying temporally significant surfaces are suggested, including: absence of compaction of the Glossifungites assemblage; presence of well-preserved burrow sculptings; and planar to gently undulatory erosional surfaces as opposed to surfaces with notable topographic relief. Conceptual models demonstrate that muddy substrates potentially require several millennia to compact and dewater, Sandy deposits, on the other hand, have indeterminate significance.

Global dominance of coralline red-algal facies: A response to Miocene oceanographic events

Article

Full-text available

Jun 2005
GEOLOGY

Rhodoliths (free-living coralline red algae) can thrive under a wide range of temperatures, reduced light, and increased nutrient levels, and often form a distinct so-called rhodalgal lithofacies that is an important component of Cenozoic shallow-water carbonates. Global distributions illustrate that from the late-early to early-late Miocene (Burdigalian early Tortonian), rhodalgal facies reached peak abundances and commonly replaced coral-reef environments, accompanied by a decline in other carbonate-producing phototrophs. We argue that the dominance of red algae over coral reefs was triggered in the Burdigalian by enhanced trophic resources associated with a global increase in productivity, as evidenced by a long-term shift toward higher carbon isotope values. Rhodalgal lithofacies expanded further in the middle Miocene when strengthened thermal gradients associated with the establishment of the East Antarctic Ice Sheet led to enhanced upwelling while climate change generated increased weathering rates, introducing land-derived nutrients into the oceans. Globally cooler temperatures following a climatic optimum in the early-middle Miocene contributed to sustain the dominance of red algae and prevented the recovery of coral reefs. The global shift in nearshore shallow-water carbonate producers to groups tolerant of higher levels of trophic resources provides further evidence for increased nutrient levels during that time interval and shows the sensitivity of shallow-water carbonate facies as indicators of past oceanographic conditions.

Trace Fossils of the Upper Eocene-Lower Oligocene Transition of the Manipur, Indo-Myanmar Ranges (Northeast India)

Article

Full-text available

Jan 2008

A detailed ichnological analysis, for the first time, has been preformed on Upper Eocene-Lower Oligocene Transition of Manipur, Indo-Myanmar Ranges (Northeast India). Previous trace fossil analyses in India are scarce and usually poorly detailed, especially with respect to Cenozoic materials. Sediments from the Disang and Barail groups contain a relatively abundant and moderately diverse trace fossil assemblage that has been characterized at the ichnogenus and ichnospecies level. ?Arenicolites Salter 1857, Helminthopsis tenuis Ksi˛a . zkiewicz 1968, Ophiomorpha nodosa Lundgren 1891, Phycodes palmatus (Hall 1852), Planolites montanus Richter 1937, Rhizocorallium jenense Zenker 1836, Thalassinoides paradoxicus (Woodward 1830) and Skolithos linearis (Haldeman 1840) have been described therein in detail, most of them for the first time in the Manipur state. This ichno-assemblage represents the record of classical Skolithos and/or Cruziana ichnofacies, being characteristic of a shallow-marine environment, with occasional high-energy

Mixed carbonate-siliciclastic record on the North African margin (Malta) - Coupling of weathering processes and mid Miocene climate

Article

Full-text available

Feb 2003

We investigated rock outcrops spanning the middle Miocene, global climate-cooling step on the Maltese Islands in order to reconstruct continental weathering rates and terrigenous fluxes, as well as to explore the coupling between these later, regional climate and carbonate accumulations. Sedimentation at this location was dominated during the Oligocene and early Miocene by a transitional platform to slope carbonates but progressively switched to a clay-rich carbonate slope system in the middle Miocene. Around 13 Ma, an abrupt change toward clay-dominated marls occurred, and marl deposition persisted until the Tortonian (ca. 12 Ma), when a shallow-water carbonate ramp was reestablished. Clay mineralogy and bulk-rock oxygen isotope analyses suggest that the deposition of the Blue Clay formation was mainly caused by global climate change and related change in the rate of continental weathering. A significant negative correlation (R2 = 0.65) exists between the carbonate content and the δ18O record. This, combined with the variation of mass accumulation rate of terrigenous material, suggests that shorter- term periods of globally cooler climate (Mi events) were associated with higher rates of accumulation in continental-derived material. Since during the Miocene Malta was attached to the North African Margin, we propose that the observed trends were due to a regional increase in rainfall during cooler periods, which consequently increased continental weathering and runoff. We further suggest that this pattern was linked to the perturbation of atmospheric fronts due to an increased thermal gradient during the Miocene. Thus, regional increase in rainfall might have been linked to the northward migration of the Intertropical Convergence Zone (ITCZ).

Geochemical Stratigraphy of Submarine Lavas (3-5 Ma) from the Flamengos Valley, Santiago, Southern Cape Verde Islands

Article

Full-text available

Jan 2009

New high-precision Pb-Sr-Nd isotope, major and trace element and mineral chemistry data are presented for the submarine stage of ocean island volcanism on Santiago, one of the southern islands of the Cape Verde archipelago. Pillow basalts and hyaloclastites in the Flamengos Valley are divided into three petrographic and compositional groups; the Flamengos Formation lavas (∼4.6 Ma) dominate the sequence, with the younger Low Si and Coastal groups (∼2.8 Ma) found near the shoreline. Olivine and clinopyroxene compositions and isotopic data for minerals and their host melts indicate disequilibrium between some crystals and the melt. Intra-sample disequilibrium suggests homogenisation of liquids but eruption before complete equilibration between crystals and melt preserves the heterogeneity. Pressures of crystallization for clinopyroxene (0.4-1.1 GPa) indicate stalling and crystallization of the magmas over a range of depths in the lithosphere. Major element compositions indicate melting of a carbonated eclogite source. Sr-Nd-Pb isotope data suggest the involvement of FOZO-like and EM1-like components in the mantle source, which are simultaneously available at all depths in the melting column. The Flamengos Valley lavas display large compositional variations, often between stratigraphically adjacent flows; these frequent abrupt changes of magma composition suggest stalling and crystallization of discrete magma batches on transport through the lithosphere. © The Author 2009. Published by Oxford University Press. All rights reserved.

Rhodolith Stranding Event on a Pliocene Rocky Shore from Isla Cerralvo in the Lower Gulf of California (Mexico)

Article

Full-text available

Jan 2012
J COASTAL RES

Controls on present-day sedimentation around Isla Cerralvo in the southern Gulf of California provide a model for restricted Pliocene limestone distribution. The 10.46-km2 island is elongate and roughly parallel to the direction of prevailing north winds. Debris washed from deeply dissected valleys build fan deltas of sand-to-boulder size igneous clasts. These are transported south by long-shore currents, but the fans also create leeward zones with less agitated water. Remnants of a large Pliocene fan are exposed during low tide at Los Carillos on the SE side of the island. Adjacent is an unconformity between granite and granite-derived conglomerate with Nerita scabricosta. This extant gastropod is typical of the high intertidal rocky shore. The conglomerate is capped by a sandstone ramp with Argopecten abietus as an offshore facies. Basalt dikes exhumed from the granite formed natural groins that captured sediments and shells in the sand ramp. Whole rhodoliths, mostly 3.5 cm in diameter and 15-cm deep, covered 150 m2 within the ramp, now partly exposed among boulders in the basal conglomerate. Many rhodoliths encrust pea-sized rock cores. Accretion occurred in shallow water protected from extreme agitation by the nearby fan delta. Stranding of rhodoliths on the rocky shore was a storm-induced event.

Basso D., Nalin R. & Nelson C. S. — Shallow-water Sporolithon rhodoliths from North Island (New Zealand). Palaios

Article

Full-text available

Feb 2009

Shallow-water Sporolithon rhodoliths from New Zealand are de-scribed here on the basis of shape, size, composition, internal struc-ture, and major taphonomic attributes, with the aim of discussing their significance in the framework of current ecological and paleo-ecological models of rhodolith formation and accumulation. The very shallow water environment (2 m) of the Whangaparaoa Peninsula undergoes daily tidal currents and seasonal storm conditions. These factors, along with the availability of cobbles and pebbles as suitable substrate, lead to the accumulation of spherical, fruticose, monospe-cific, nucleated, and internally compact rhodoliths. The observed taphonomic features include apical abrasion, intercalary growth of protuberances after mechanical breakage, and multiple growth stages with distinctive bioerosion (by bivalves, annelids, and cyanobacteria), which are identified by internal abrasion surfaces and dark layers. A review of the pattern of global distribution of the coralline genera comprising very shallow-water rhodoliths (2 m) identifies five ma-jor categories: (A) unattached protuberances, commonly monospe-cific, made up of one or more genera (Phymatolithon, Lithothamnion, Lithophyllum) in cool to cold waters at middle to high latitudes; (B) rhodoliths composed of dominant Hydrolithon and other mastopho-roids (Neogoniolithon or Spongites) with subordinate Lithophyllum in the tropics; (C) unattached protuberances of Neogoniolithon associ-ated with seagrass meadows, from middle latitudes, in warm to warm-temperate waters; (D) mainly fruticose, monospecific, rhodo-liths composed of Mesophyllum, Lithothamnion, Hydrolithon, and Neogoniolithon under tidal currents in cool waters in the Southern Hemisphere; and (E) Sporolithon rhodoliths of cool waters in the Southern Hemisphere, which suggest an austral polar emergence of the genus.

The occurrence and ecology of recent rhodoliths-a review

Article

Jan 1983

D. W. J. Bosence

Trazas Fósiles: Señales de Comportamiento en el Registro Estratigráfico

Article

Jan 2002

Sedimentological and biological aspects in the formation of branched rhodoliths in northern Norway

Article

Jan 1995

Andre Freiwald

Rhodoliths are very commonly distributed in Cenozoic deposits. However, processes leading to the development of rhodolith accumulations are poorly documented. This study reveals insight into the relationship between reefal coralline algal frameworks, framework-derived rhodoliths and maerl-type sediments from coastal platforms in northern Norway. The rhodolith factories are reefal build-ups which generate branched, non-nucleated rhodoliths by fragmentation of hemispheroidal branched algal heads. The major driving forces which enhance widely spread dislocation of algal heads are storm events. Considerations on phycological and geological aspects in rhodolith formation offer some interesting perspectives in coralline algal life histories. -from Author

A geologia da Ilha de Santiago (Cabo Verde)

Article

Jan 1976

A. Serralheiro

Evaluation Of Paleo-Wave Conditions From Oscillatory Structures Preserved In Cretaceous Sediments Of Baja California

Article

Aug 1993

Measurements were obtained of spacing, dip and height of oscillatory ripples (hummocky/swaley cross-stratification) preserved within the sections of Punta Banda and Penasco La Lobera in the Rosario Formation. Estimates the hydrodynamic characteristics of paleo-waves that produced the oscillation ripples: near-bottom orbital diameter of the wave motion, near-bottom orbital velocity, wave period and wave height. A clear dependence is shown through the use of ripple spacing and five equations to obtain these paleo-wave parameters. -Authors

Contribution of rhodoliths to the generation of Pliocene-Pleistocene limestone in the Gulf of California

Article

Jan 2009

The ichnofacies concept: a fifty-year retrospective

Article

Jan 2007

Redescription of the trace fossil Gyrolithes and taxonomic evaluation of Thalassinoides, Ophiomorpha and Spongeliomorpha

Article

Jan 1974
B GEOL SOC DENMARK

Ecological studies on two unattached coralline algae from western Ireland

Article

Jan 1976

D. W. J. Bosence

Paleoecology and Paleogeography of an Extensive Rhodolith Facies from the Lower Oligocene of South Georgia and North Florida

Article

Jan 1987

An extensive areal occurrence of a Lower Oligocene rhodolith limestone has been observed in cores and outcrops extending nearly 290 km along a NE-SW trend from south-central Georgia to the Florida Panhandle. Maximum observed thickness of the limestone is 30 m. Such a tremendous accumulation of fossil rhodoliths has not been previously described in the literature. Most of the rhodoliths are Archaeolithothamnium with a mean diameter of 5 cm and most are spherical in shape. Rhodolith shape plus their internal laminar growth pattern indicate exposure to high-energy environments, resulting in frequent movement of the algae. The occurrence of Archaeolithothamnium, along with the red alga Lithoporella and a dasycladacean alga, suggest moderate to shallow water depth. Species that lived within the rhodolith facies had specific adaptations for survival on such a substrate including the ability to encrust (corals and ectoprocts), bore (Lithophaga), live interstitially (Lima), and ease of mobility (scallops, echinoids).-Authors

Reefs That Rock and Roll:: Biology and Conservation of Rhodolith Beds in the Gulf of California

Chapter

Apr 2010

Living Rhodolith Bed Ecosystems in the Gulf of California

Chapter

Jan 2009

Pebbles in the Lower Colorado River, Texas a Study in Particle Morphogenesis

Article

Mar 1958

pdf , The following values have no corresponding Zotero field: ID - 552

A semiquantitative classification of ichnofabric

Article

Jul 1986

Rhodolith transport and immobilization on a volcanically active rocky shore: Middle Miocene at Cabeço das Laranjas on Ilhéu de Cima (Madeira Archipelago, Portugal)

Article

Feb 2011
PALAEOGEOGR PALAEOCL

Extraordinary deposits of fossil rhodoliths occur at the Cabeço das Laranjas (Portuguese~Hill of the Oranges) in a small fault block at the northwest end of Ilhéu de Cima off Porto Santo in the Madeira Archipelago. Stratigraphic repetitions of densely packed rhodolith beds up to 2.6 m thick are associated with a receding rocky shoreline, and are interpreted as the result of hurricanes. The initial storm deposit sits unconformably on basalt and eroded basalt boulders associated with tuff and volcaniclastic breccia. Approximately 90,000 rhodoliths of Middle Miocene age (14–15 Ma) are exposed on the upper surface of the initial deposit over a 450-m2 shelf exhumed from the hill's southeast side. Ranging in diameter from ≤3 cm to 20 cm, many of the rhodoliths generated by crustose coralline red algae are now iron stained and resemble a mass of oranges in gross appearance. Sea stacks and large boulders rise through the thick basal rhodolith bed to form small catchment areas that held the deposit in place after the storm's passage. The succeeding rhodolith deposits are variably separated by layers with mixed carbonate and volcanic sand, pure volcanic lapilli, and volcaniclastic tephra mixed with tuff showing swaley cross-stratification. Three out of four rhodolith beds are truncated against the flank of the adjoining rocky shore. Only the youngest (fourth) rhodolith layer is fully exposed around the perimeter of the hill and can be shown to cross a basalt barrier that is traceable for 70 m in cross section as an erosional ramp dipping from 6° to 8° southeast. The entire fossil-rich sequence is capped by a basalt flow showing columnar disjunction. Based on thin-section analysis, three genera of coralline red algae are recognized in the basal rhodolith deposit: Sporolithon, Lithothamnion, and Neogoniolithon. Associated biodiversity is low, represented by 16 kinds of marine invertebrates dominated by encrustations and borings on the rhodoliths and very few free body fossils. The Madeira region of the North Atlantic may have been susceptible to major cyclonic storms immediately after the Middle Miocene Climate Optimum, when a northward shift of the Inter-tropical Convergence Zone was stimulated by a steeper temperature gradient in the southern hemisphere related to expansion of continental glaciers on Antarctica

Paleomagnetic results from a reconnaissance study of Santiago (Cape Verde Islands): Identification of cryptochron C2r.2r-1

Article

Apr 2009
PHYS EARTH PLANET IN

New paleomagnetic data from three lava sequences on Santiago, Cape Verde Islands, are presented. The paleomagnetic data are coupled with Ar40/Ar39-age determinations, allowing detailed correlations between the three profiles and the Geomagnetic Polarity Time Scale (GPTS). The younger of the profiles, the Ribeira da Barca profile, recorded flows of normal polarity that correlate with the Brunhes Chron, whereas the Porto Formosa profile of reverse polarity correlates with the lower Matuyama Chron. The São Gonçalo profile consists of both reverse- and normal-polarity flows that belong to the Matuyama and Gauss Chron, respectively. Virtual geomagnetic poles (VGPs) from a sequence of flow units in the São Gonçalo profile classify as transitional, and we interpret it as a geomagnetic event (SG-I). Well-defined Ar40/Ar39 ages bound the occurrence of event SG-I to the interval 2.37-2.43 Ma (±2sigma), which is synchronous with the timing of cryptochron C2r.2r-1 in the marine magnetic anomaly record. This study therefore confirms the existence of cryptochron C2r.2r-1 and indicates that it was a global-scale phenomenon. The VGPs indicate that the field was in a meta-stable transitional state during cryptochron C2r.2r-1, and that it spent some time in or around the paleomagnetic transitional VGP-cluster patch close to Australia.

Paleodepth and Paleoenvironment of Dactyloidites ottoi (Geinitz, 1849) from Lower Cretaceous Deltaic Deposits (Basque-Cantabrian Basin, West Pyrenees)

Article

Jun 2004

The rosetted trace fossil Dactyloidites ottoi (Geinitz) commonly has been associated with shallow-water, high-energy marine environments, although detailed sedimentological analyses of the host deposits are unfortunately few. The Aptian–Albian Otoio Formation (Basque-Cantabrian Basin) provides an excellent opportunity to test the paleobathymetric and paleoenvironmental controls on the restricted distribution of D. ottoi. The Otoio Formation records a great variety of shallow-water marine environments, but D. ottoi occurs only in intervals interpreted as fluvial-dominated deltas (Gilbert-type and mouth bar-type deltas). Moreover, detailed facies analysis and geometric relationships of the deltaic deposits suggest the following main controls on the distribution of D. ottoi: (1) paleodepth between 0 and 3 m, quantified from the foreset heights of Gilbert-type deltas; (2) siliciclastic and organic matter-rich sandy substrate; (3) high and discontinuous sedimentation rates; and (4) reduced salinity, responsible for the small size of the trace from the Otoio Formation. These observations are in agreement with published data from other localities.

'Marks of extreme violence': Charles Darwin's geological observations at St Jago (Sao Tiago), Cape Verde islands

Article

Sep 2007

The first stop on Charles Darwin's famous voyage around the world in HMS Beagle was at Porto Praya (Praia), the principal town on the island of St Jago (São Tiago) in the Cape Verde archipelago. From 16 January to 8 February 1832, Darwin enjoyed his first substantive opportunity to study the natural history of an exotic place. Darwin himself regarded this occasion as a significant turning point in his life because, according to his autobiography, it was here that he decided to research and publish a book on the geology of the places visited on the voyage. He also recalled that it was here, the very first port call, that convinced him of the 'wonderful superiority' of Charles Lyell's uniformitarian geology over the doctrine of successive cataclysms that he had been taught in England. Later commentators have generally accepted this account, which is significant for understanding the intellectual background to the Origin of Species, at face value. In this paper we reconstruct some of Darwin's observations at St Jago based on his contemporaneous notes and diary, and in the light of our own visit made in January 2002. We find little evidence to substantiate the claim that he interpreted the geology in Lyellian terms at the time. Instead, he formulated a theory involving a great cataclysm to explain the dramatic scenery in the island's interior. He speculated that a torrent of water had carved the main valleys of the island, leaving deposits of diluvium in their beds. It is indisputable that Darwin came to embrace gradualist thinking enthusiastically during the voyage. Some of his observations made on St Jago, especially relating to uplift of the coast, were instrumental in this change of view, but the conversion was gradual, not sudden. His later published works make no mention of his original catastrophist interpretations.

The submarine volcanic succession of the basal complex of Fuerteventura, Canary Islands: A model of submarine growth and emergence of tectonic volcanic islands

Article

Jun 2006

Three lithostratigraphic units have been distinguished in the volcanic succession of the basal complex of Fuerteventura Island. These units are, from bottom to top: the submarine volcanic group, the transitional volcanic group, and the subaerial volcanic group. These three groups record the submarine growth and emergence of the island. The volcanism is represented by ultra-alkaline and strongly alkaline igneous series. The igneous activity was due to the presence of an anomalous zone in the sublithospheric mantle, the low density of which also caused uplift of the Mesozoic oceanic crust. Two extensional phases and an intervening contractional phase developed coeval to the generation of the volcanic succession. The submarine volcanic group was deposited in the hanging wall basin of a large listric extensional detachment directed toward the SSW. The transitional volcanic group was syntectonic with respect to a late inversion of the listric detachment. Finally, the subaerial volcanic group resulted from a second episode of WNW extension. This study of the evolution of the basal complex of Fuerteventura serves as the basis for a tectonic model of submarine growth and emergence of volcanic islands.

Mesozoic deep-water and Tertiary volcaniclastic deposition of Maio, Cape Verde Islands: Implications for Atlantic paleoenvironments and ocean island volcanism

Article

Jan 1984

Alastair Robertson

Maio Island provides a clue to both Mesozoic paleoenvironments in the E and central Atlantic and to Tertiary island building related to 'hot-spot' activity.-from Author

Does Antarctic glaciation force migration of the tropical rain belt?

Article

Aug 2010
GEOLOGY

High-resolution (similar to 3-6 k.y.) upper ocean temperature and salinity estimates derived from planktic foraminiferal delta O-18 and Mg/Ca in Ocean Drilling Program (ODP) Site 1146 reveal stepwise changes in the precipitation-evaporation balance of the subtropical northwestern Pacific during the Middle Miocene (15.7 to 12.7 Ma). We attribute the punctuated pattern of surface warming and freshening following Antarctic ice growth episodes at 14.6, 14.2, 13.9, and 13.1 Ma to successive northward movements of the Intertropical Convergence Zone, implying high sensitivity of tropical rain belts to the interhemispheric temperature gradient driven by high-latitude climate. This dynamic interaction has implications for future warmer climate regimes with differential warming of the Northern Hemisphere, as it may lead to changes in the latitudinal penetration of tropical Pacific moisture over Southeast Asia.

Origin of hummocky and swaley cross-stratification--- The controlling influence of unidirectional current strength and aggradation rate

Article

Dec 2006
GEOLOGY

A series of wave-tunnel experiments was conducted to investigate the conditions under which hummocky and swaley cross-stratification form. Isotropic 3-dimensional (3-D) hummocky bed forms were generated under long wave periods (˜8 10 s) and moderate oscillatory velocities (Uo ˜50 90 cm/s) with very weak (< 5 cm/s) to no unidirectional flow. Hummocks became anisotropic with the addition of only a small unidirectional current (5 10 cm/s), and began to resemble unidirectional dunes when the unidirectional current was increased above 10 cm/s. Synthetic aggradation of the hummocky bed forms at high (4.2 mm/min) and low (1 mm/min) rates generated stratification resembling hummocky and swaley cross-stratification, respectively. Based on these findings, we suggest that hummocky cross-stratification optimally forms above (but near) storm wave base where aggradation rates during storms are high enough to preserve hummocks but unidirectional current speeds are sufficiently low to generate low-angle, isotropic cross-stratification. Swaley cross-stratification is also hypothesized to be deposited by an aggrading hummocky bed between fair-weather and storm wave base, but in shallower water where aggradation rates are low enough to cause preferential preservation of swales.

Charles Darwin, Geologist

Article

Dec 2006
ISIS

Dennis R. Dean

An 40Ar39Ar study of the Cape Verde hot spot: Temporal evolution in a semistationary plate environment

Article

Aug 2008

The 40Ar-39Ar analyses of 28 groundmass separates from volcanic rocks from the islands of Santiago, Sal, and São Vicente, Cape Verde archipelago, are presented. The new age data record the volcanic evolution for Santiago from 4.6 to 0.7 Ma, for Sal from around 15 to 1.1 Ma, and for São Vicente from 6.6 to 0.3 Ma. The major submarine constructional phase of Santiago was erupted within a few hundred thousand years interval around 4.6 Ma. Most of the subaerial Santiago volcanic rocks were erupted in a second episode from 3.3 to 2.2 Ma and late volcanism occurred at 1.1-0.7 Ma. Volcanism on Sal evolved in five stages: (1) poorly constrained early Miocene activity, (2) 16-14 Ma, (3) 12-8 Ma, (4) around 5.4 Ma, and (5) 1.1-0.6 Ma. São Vicente was constructed during three active periods: (1) >6.6-5.9 Ma, (2) 4.7-4.5 Ma, and (3) ~0.3 Ma. Sr isotope analyses of carbonates from Maio confirm an Early Cretaceous age for limestones deposited on the seafloor and later uplifted. The Cape Verde Rise is indicated to have fully formed in the early Miocene around 22 Ma, accompanied by the initial alkaline volcanism. Considerable volcanism on Sal, Boa Vista, and Maio took place in the Miocene and Pliocene and extended over much larger areas than the present islands, whereas volcanism of the southwestern and northwestern island groups developed mainly during the Pliocene and Pleistocene and was mostly confined to the present island areas. The periods of volcanic activity may be broadly correlated between the northwestern and southwestern groups of islands. Young volcanism (0.3-0.1 Ma) throughout the northwestern group extends along a 150 km long NW-SE trending lineament. A relatively moderate average melting rate for the hot spot over the 22 Ma period is estimated at ~0.026 km3/a, corresponding to a total volume of 570 × 103 km3 of magma emplaced in the crust and a mantle volume flux of 28 m3/s, much lower than Iceland or Hawaii. The archipelago is situated to the south and SW of the center of the mantle plume anomaly and ahead of its relative movement. The timing and location of volcanism suggest that mantle melting takes place in three channels, an eastern one that has been active for 22 Ma and in southwestern and northwestern channels since late Miocene.

Vertical movements of ocean island volcanoes: Insights from a stationary plate environment

Article

Sep 2010
MAR GEOL

Uplift reconstructions based on the Cape Verde's geological record provide a unique opportunity to study the long-term isostatic movements associated with hotspot activity on a stationary plate environment. The archipelago is considered stationary with respect to its melting source so the hotspot-driven isostatic effects affecting the ocean islands are expected to be enhanced. In this study, Ar–Ar and U–Th geochronology techniques were used to date a set of palaeo-markers of sea-level from Santiago's and São Nicolau's edifices, two of the main Cape Verde Islands. A comparison between relative sea-level and eustatic sea-level (from a modern eustatic curve) was established to extract the vertical displacement undergone by the markers, and to reconstruct the uplift/subsidence history of each island. The resulting uplift reconstructions confirm that both Santiago and São Nicolau experienced a general uplift trend over the last 6Ma, seemingly synchronous with the vigorous volcanic activity that built their exposed edifices. These islands, however, exhibit different uplift histories despite their common uplift trend. Several uplift mechanisms were tested and a local rather than regional mechanism is proposed as the main cause of uplift, generally unrelated with far-field effects of surface loading. This mechanism is probably associated with magmatic additions at crustal level.

Primary production, respiration and calcification of the temperate free-living coralline alga Lithothamnion corallioides

Article

Aug 2006
AQUAT BOT

Calcification and primary production responses to irradiance in the temperate coralline alga Lithothamnion corallioides were measured in summer 2004 and winter 2005 in the Bay of Brest. Coralline algae were incubated in dark and clear bottles exposed to different irradiances. Net primary production reached 1.5μmolCg−1drywth−1 in August and was twice as high as in January–February. Dark respiration showed significant seasonal variations, being three-fold higher in summer. Maximum calcification varied from 0.6μmolg−1drywth−1 in summer 2004 to 0.4μmolg−1drywth−1 in winter 2005. According to P–E curves and the daily course of irradiance, estimated daily net production and calcification reached 131μgCg−1drywt and 970μgCaCO3g−1drywt in summer 2004, and 36μgCg−1drywt and 336μgCaCO3g−1drywt in winter 2005. The net primary production of natural L. corallioides populations in shallow waters was estimated at 10–600gCm−2y−1, depending on depth and algal biomass. The mean annual calcification of L. corallioides populations varied from 300 to 3000gCaCO3m−2. These results are similar to those reported for tropical coralline algae in terms of carbon and carbonate productivity. Therefore, L. corallioides can be considered as a key element of carbon and carbonate cycles in the shallow coastal waters where they live.

Diversity of coralline red algae: Origination and extinction patterns from the Early Cretaceous to the Pleistocene

Article

Dec 2000
PALEOBIOLOGY

Data from a comprehensive literature survey for the first time provide stage-level resolution of Early Cretaceous through Pleistocene species diversity for nongeniculate coralline algae. Distributions of a total of 655 species in 23 genera were compiled from 222 publications. These represent three family-subfamily groupings each with distinctive present-day distributions: (1) Sporolithaceae, low latitude, mainly deep water; (2) Melobesioid corallinaceans, high latitude, shallow water, to low latitude, deep water; (3) Lithophylloid/mastophoroid corallinaceans, mid- to low latitude, shallow water. Raw data show overall Early Cretaceous–early Miocene increase to 245 species in the Aquitanian, followed by collapse to only 43 species in the late Pliocene. Rarefaction analysis confirms the pattern of increase but suggests that scarcity of publications exaggerates Neogene decline, which was actually relatively slight. Throughout the history of coralline species, species richness broadly correlates with published global paleotemperatures based on benthic foraminifer δ18O values. The warm-water Sporolithaceae were most species-abundant during the Cretaceous, but they declined and were rapidly overtaken by the Corallinaceae as Cenozoic temperatures declined. Trends within the Corallinaceae during the Cenozoic appear to reflect environmental change and disturbance. Cool- and deep-water melobesioids rapidly expanded during the latest Cretaceous and Paleocene. Warmer-water lithophylloid/mastophoroid species increased slowly during the same period but more quickly in the early Oligocene, possibly reflecting habitat partitioning as climatic belts differentiated and scleractinian reef development expanded near the Eocene/Oligocene boundary. Melobesioids abruptly declined in the late Pliocene–Pleistocene, while lithophylloid/mastophoroids increased again. Possibly, onset of glaciation in the Northern Hemisphere (∼2.4 Ma) sustained or accentuated latitudinal differentiation and global climatic deterioration, disrupting high-latitude melobesioid habitats. Simultaneously, this could have caused moderate environmental disturbance in mid- to low-latitude ecosystems, promoting diversification of lithophylloids/mastophoroids through the “fission effect.” Extinction events that eliminated >20% of coralline species were most severe (58–67% of species) during the Late Cretaceous and late Miocene–Pliocene. Each extinction was followed by substantial episodes of origination, particularly in the Danian and Pleistocene.

Rhodolith detritus impounded by a coastal dune on Isla Coronados, Gulf of California Detrito de rodolitos capturados por la duna costera en Isla Coronados, Golfo de California

Article

Sand from a small dune on the southwest side of Isla Coronados with an estimated volume of 30,000 m 3 is enriched as much as 86.5% by calcium-carbonate detritus from beached rhodoliths. Elsewhere in the Gulf of California, coastal sand dunes often occur on the north sides of islands or north-facing peninsular shores, and calcium-carbonate input is more commonly linked to bivalve mollusks from adjacent sand flats. The local calcium-carbonate budget for the west-facing shore of Isla Coronados must take into account how many rhodoliths of a given size are required to build a sand dune with a known composition and volume. To this end, 135 whole rhodoliths were collected from above the tide line at Punta El Bajo, across from Isla Coronados, on the peninsular mainland. One cubic meter is calculated to accommodate 8640 whole rhodoliths with an average diameter of 5 cm and an average sphericity of 0.86. The age of a rhodolith this size could be several decades. Through stages, the sample rhodoliths were crushed to a maximum grain size ≤ 2.38 mm in diameter (-1.25ø equivalent), and the product was used to estimate the proportion required to generate 1 m 3 of pure carbonate sand. Accounting for 2% loss throughout the reduction process, about 16,265 crushed rhodoliths are needed to produce 1 m 3 of carbonate sand. Thus, a 30,000-m 3 dune requires approximately 488,000,000 rhodoliths to generate 86% of the dune's volume. Dunes of this kind may be rare, but the computation is applicable to other rhodolith-derived dunes in the Gulf of California.

Fossil and recent shallow water corals from the Atlantic Islands off Western Africa CANCAP-contribution no. 56

Article

Jan 1988

Miocene hermatypic corals are listed from Madeira and Porto Santo. Pleistocene and recent shallow water corals are described from the Cape Verde archipelago. The Miocene fauna was part of the Western Tethyan reef association, which went nearly completely extinct by the development of a cool water current in late Miocene times. The Cape Verde fauna immigrated from the Carribean, presumably by way of a compressed subtropical gyre during a mid Pleistocene glacial phase. The fauna was found in a beach terrace, formerly considered Tertiary, on the island of São Tiago. A lava flow directly above this deposit was radiometrically dated 700.000 ± 200.000 years B.P.

Rhodoliths from deep fore-reef to shelf areas around Okinawa-jima, Ryukyu Islands, Japan

Article

Apr 2011
MAR GEOL

The distribution, abundance, composition, and growth history of rhodoliths were investigated based on 222 grab samples and 202 submarine photographs taken from 223 sites arranged at regular intervals and on 13 additional samples (5 dredge and 8 grab samples). These samples were collected at water depths ranging from 15 to 970m around Okinawa-jima, Ryukyu Islands, Japan. The rhodoliths grow in deep fore-reef to shelf areas at water depths of 50 to 135m. Where rhodoliths occur, they cover 45% of the sea bottom. The rhodoliths are primarily spheroidal to ellipsoidal in shape (with mean diameters usually less than 8cm); internally they are primarily composed of nongeniculate coralline algae and an encrusting foraminifer Acervulina inhaerens. The rhodoliths have envelopes of well-preserved, concentric to irregular laminations or, much more commonly, are bored and display various degrees of bioerosion. Constructional voids (primary spaces between encrusters) and borings range from empty to completely filled with unlithified and lithified mixtures of micrites and bioclasts. The bioerosion is more extensive with increasing water depth and is progressively much more pervasive at water depths greater than 90m. The rhodoliths are covered with nongeniculate coralline algae and A. inhaerens associated with other epilithic skeletal and nonskeletal organisms. The living biotic cover on rhodoliths is relatively great down to water depths ~100m; below this, the cover decreases rapidly with increasing water depth. Rhodoliths with similar size, shape, and composing organisms to those in the Ryukyu Islands are commonly found on deep fore-reef to shelf areas or on the banks and seamounts of tropical reef regions, likely as the combined result of ecological degradation (=decreased number and coverage) of hermatypic corals and the relative predominance of nongeniculate coralline algae and encrusting foraminifers in such areas. The slow accretion rates of rhodoliths (

Major middle Miocene global climate change: Evidence from East Antarctica and the Transantarctic Mountains

Article

Jan 2006

We present a glacial record from the western Olympus Range, East Antarctica, that documents a permanent shift in the thermal regime of local glaciers, from wet- to cold-based regimes, more than 13.94 m.y. ago. This glacial record provides the first terrestrial evidence linking middle Miocene global climate cooling to a permanent reorganization of the Antarctic cryosphere and to subsequent growth of the polar East Antarctic Ice Sheet. The composite stratigraphic record constructed from field mapping and analyses of 281 soil excavations shows a classic wet-based till (Circe till, including an extensive melt-out facies), overlain by a weathered colluvial deposit (Electra colluvium), and then a series of stacked tills deposited from cold-based ice (Dido drift). Chronologic control comes from 40Ar/39Ar analyses of concentrated ash-fall deposits interbedded within glacial deposits. The shift from wet-to cold-based glaciation reflects a drop in mean annual temperature of 25-30 °C and is shown to precede one or more major episodes of ice-sheet expansion across the region, the youngest of which occurred between 13.62 and 12.44 Ma. One implication is that atmospheric cooling, following a relatively warm mid-Miocene climatic optimum ca. 17 to 15 Ma, may have led to, and thus triggered, maximum ice-sheet overriding.

Supplementary modification and introduction of code numbers to the low-latitude coccolith biostratigraphic zonation (BUKRY, 1973; 1975)

Article

Dec 1980
Mar Micropaleontol

The Structure and Distribution of Coral Reefs ; being the first Part of the Geology of the Voyage of the Beagle under the Command of Capt. Fitzroy, R.N. during the Years 1832 to 1836: Naturalist to the Expedition. London, pp. 214. 1842

Article

Jan 1845

C.R. Darwin

Rhodoliths, uniformitarianism, and Darwin: Pleistocene and Recent carbonate deposits in the Cape Verde and Canary archipelagos

Abstract

No full-text available

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