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First find of the Upper Tithonian ammonite genus Blanfordiceras from the Miers Bluff Formation, Livingston Island, South Shetland Islands
Abstract
The first find of a macrofossil, the age-diagnostic Upper Tithonian ammonite Blanfordiceras sp. (aff. wallichi) is reported from the Miers Bluff Formation (MBF). The presence of this genus is indicative of the Antarctica-specific Blanfordiceras ammonite zone from the middle Upper Tithonian. This zone is an approximate time equivalent of the Mediterranean Paraulacosphinctes transitorius zone. The new Late Tithonian age of the MBF (Hurd Peninsula, Livingston Island) allows to suggest that the MBF is partly synchronous with the Anchorage Formation (Byers Peninsula, Livingston Island), the Nordenskjöld and the Latady Formations (Antarctic Peninsula), and thus plays an important role for the Upper Jurassic to Cretaceous depositional history and structural evolution of the South Shetland Islands and of the Antarctic Peninsula.
... The base is unexposed, and the formation is overlain by volcanic deposits (Pallàs et al., 1992). Pimpirev et al. (2002) Pimpirev et al. (2002), the Middle Triassic maximum sedimentation age opens the possibility of a wider time interval, suggesting that it could correspond to a progressive migration of the TPG environment away from the continental margin. ...
... The base is unexposed, and the formation is overlain by volcanic deposits (Pallàs et al., 1992). Pimpirev et al. (2002) Pimpirev et al. (2002), the Middle Triassic maximum sedimentation age opens the possibility of a wider time interval, suggesting that it could correspond to a progressive migration of the TPG environment away from the continental margin. ...
... The results for the sedimentary samples were of Middle Jurassic and Middle Triassic, while for the igneous data wasEarly Cretaceous (Valanginian). Although the results are in agreement with the Late Jurassic fossil age indicated by Pimpirev et al. (2002), the Middle Triassic maximum sedimentation age opens the possibility of a wider time interval, suggesting that a progressive migration of the TPG environment from the continental margin might have taken place. ...
El presente trabajo consiste en dos estudios geológicos desarrollados de forma independiente en
las islas del archipiélago Shetland del Sur, Antártica. El primero está enfocado en la
caracterización y origen de la mineralogía secundaria de muy bajo-grado en la sucesión volcánica
de Punta Hannah en Isla Livingston (ILi). El segundo en la comparación de la proveniencia
sedimentaria de las rocas aflorantes en Cabo Wallace en Isla Low (ILo) con unidades similares en
ILi y Península Antártica (PA).
La sucesión volcánica andesito-basáltica del Cretácico Superior en Punta Hannah, presenta a una
profusa ocurrencia de minerales secundarios de muy bajo-grado. Se encuentran presentes en
amígdalas, vetillas, masa fundamental y fenocristales. Las plagioclasas primarias se encuentran
alteradas a albita, ceolitas y calcita, mientras que los clinopiroxenos a filoslicatos máficos y
menor celadonita. Difractometría de rayos-X y análisis de microsonda electrónica muestran la
ocurrencia de filosilicatos máficos interestratificados entre clorita y esmectita trioctraédrica, así
como también de las ceolitas: laumontita, heulandita, estilbita, clinoptilolita y faujasita. Los
porcentajes de estratificados de clorita varían entre 57-84%. Las temperaturas de equilibrio en los
filosilicatos máficos varían entre 160-190°C, los cuales están en conformidad con la temperatura
estimada de 150-230°C para la asociación paragenética en ceolitas. Se estima que las condiciones
barométricas en la paragénesis se encuentran entre los 600-1800 bars, que sugieren por lo menos
1km de erosión en la sucesión. La albitización de las plagioclasas primarias contribuyó a los
fluidos ricos en Ca−Na, que deberían ser los precursores de las ceolitas ricas en Ca y de la calcita.
La ocurrencia de los minerales secundarios en anillos periféricos en las amígdalas sugiere
involucramiento de procesos de enterramiento, mientras que la presencia de vetillas cortando
estas ocurrencias sugiere hidrotermalismo. Se reconocieron tres etapas paragenéticas: (1)
celadonita/filosilicatos máficos, (2) ceolitas y (3) calcita. La Etapa (1) se habría formado durante
el metamorfismo de enterramiento, mientras que las Etapas (2) y (3) durante el hidrotermalismo
provocado por los sistemas geotermales en el régimen extensivo del Mioceno medio al Plioceno
registrado en el archipiélago.
La aquí definida Cuenca Wallace-Byers (CBB) consiste principalmente en una secuencia Jurásica
sedimentaria-volcánica en el flanco oeste de la PA que aflora en los Estratos de Cabo Wallace
(ECW) en la ILo y en el Grupo Byers en la ILi. Representa la evolución de un ambiente de
sedimentación marino profundo en el Jurásico Superior a un ambiente volcánico a volcaniclástico
que fue interrumpido por un episodio magmático en el Cretácico Inferior. La comparación de los
diagramas de probabilidad relativa en las edades de los circones detríticos y el registro fósil de
los ECW con la Formación Miers Bluff (FMB) y el Grupo Península Trinidad (GPT) no presenta
correlación concluyente. El peak Pérmico tardío en los circones detríticos de ECW indica un
evento levemente más joven que el registrado en las fuentes Pérmicas de GPT y FMB. Los 100
Ma entre la edad de cristalización Valanginiana de la Granodiorita de Cabo Wallace y el peak
Pérmico tardío, que son respectivamente la edad mínima y máxima de sedimentación de los
ECW, representan un considerable retrabajo sedimentario debido probablemente a alguna barrera
geográfica que no permitió adiciones sedimentarias a los ECW de los plutones Jurásicos ubicados
en la PA. Los ECW indican que en las latitudes de ILo, los complejos turbidíticos habrían
continuado durante el Triásico y parte del Jurásico a lo largo del margen continental activo al
oeste de la PA septentrional.
... Thus, the archipelago is a recently detached block and is comprised of a late Palaeozoic metasedimentary basement (e.g., Castillo et al., 2015) overlain by Jurassic and younger magmatic and sedimentary rocks (e.g., Smellie et al., 1984;Haase et al., 2012;Bastias et al., 2019;Leat and Riley, 2021). The geology of the South Shetland Islands records evidence of Mesozoic to Cenozoic tectonic, global sea-level and climate change, including the emergence of submarine marginal basins as part of a continental island volcanic arc (e.g., Hathway and Lomas, 1998;Riley et al., 2012;Bastias et al., 2019) and the consequent proliferation of Cretaceous plant species (e.g., Philippe et al., 1995;Torres et al., 1997;2015;Falcon-Lang and Cantrill, 2002;Leppe et al., 2007;Warny et al., 2019a). This was followed during the Eocene by the development of a mountainous landscape with stratovolcanoes and lava fields that were covered by Valdivian-type forest (Poole et al., 2001;Hunt and Poole, 2003), which are similar to wetlands and freshwater environments still present today in Chile and Argentina. ...
... These units are cut by widespread mafic dyke swarms (Zheng et al., 2003). The age of this succession was assigned to the Middle to Late Jurassic based on provenance studies and an Early Cretaceous age derived from U-Pb SHRIMP zircon age dating of a granodiorite pluton (138 ± 1 Ma; Hervé et al., 2006) and calcareous nannoplankton fossils (Pimpirev et al., 2002;Pimpirev et al., 2006). This formation has been proposed to be deposited in a multiple source, gravel-rich, deep-sea ramp system and was deposited in a strike-slip setting (Muñoz et al., 1992 The Fort Point exposures crop out on the east coast of Greenwich Island (Figure 8), and are comprised of volcanic rocks (basalts, basaltic andesites, and andesites) and plutonic rocks (granites, tonalites, diorites and gabbros) (e.g., Machado et al., 2005). ...
Over the last few decades, numerous geological studies have been carried out in the South Shetland Islands, which have greatly contributed to a better understanding of its geological evolution. However, few attempts have been conducted to correlate the geological units throughout this archipelago. We present herein a review of the literature available in the South Shetland Islands, which we use to propose a lithostratigraphical correlation that constitutes a coherent stratigraphy for the main Mesozoic and Cenozoic rocks of the South Shetland Islands along with a new geological map. The lithostratigraphical correlation shows that the geological and environmental evolution comprises three main stages: 1) deep marine sedimentation from ∼164 to 140 Ma, 2) subaerial volcanism and sedimentation with a proliferation of plants and fauna from ∼140 to 35 Ma and 3) glacial and interglacial deposits from ∼35 Ma. The lithostratigraphical correlation also shows a broad geographical trend of decreasing age of volcanism from southwest to northeast, which has been previously suggested. However, this spatial age trend is disrupted by the presence of Eocene magmatism in Livingston Island, located in the centre of the archipelago. We suggest that the migration of volcanism occurred from the Late Cretaceous until the early Eocene. Subsequently, enhanced magmatic activity took place from the mid-Eocene until the Miocene, which we associate with processes related with the waning of subduction. Constraining the protolith age of the metamorphic complex of Smith Island remains challenging, yet holds key implications for the tectonic and accretionary evolution of the Antarctic Peninsula. The rocks recording the glaciation of this sector of Antarctica are well exposed in the northern South Shetland Islands and hold critical information for understanding the timings and processes that lead to the greenhouse to icehouse transition at the end of the Eocene. Finally, contemporaneous rocks to the breakup of Antarctic Peninsula from Patagonia that led to the opening of the Drake Passage and the development of the Scotia Sea are exposed in the centre and north of the South Shetland archipelago. Better constraints on the age and tectonic settings on these units may lead to further understanding the paleobiogeographical evolution of the region, which may have played an important role for speciation as a land bridge between South America and Antarctica. The dataset containing the geological map and associated information is shared as a shapefile or KML file.
... The present study, however, summarizes newly ob- tained macro-and nannofossil data that prove a Tithonian-Maastrichtian age for the MBF ( Pimpirev et al. 2002;Stoykova et al. 2002). ...
... aff. wallichi (Gray 1832), that indicates a late Tithonian age for the lowermost, unexposed part of the formation ( Pimpirev et al. 2002). ...
The age of the sedimentary sequences of Hurd Peninsula (here referred to the Miers Bluff Formation (MBF)), has been considered so far as Triassic, coeval of the Trinity Group. Recently, a Tithonian ammonite species was found in a non-in situ block, coming from the lowermost unexposed part of the Formation. Our micropaleontological study reveals the occurrence of calcareous nannofossils in six sections. The recorded nannofossil association comprises the following species: Micula decussata, Calculites obscurus, Arkhangel-skiella cymbiformis, Prediscosphaera cretacea, Lucianorhabdus coyeuxii, Cyclagelosphaera reinhardtii, Braarudosphaera bigelowii, Ceratolithoides aculeus, Broinsonia cf. parca, Thoracosphaera sp. indet., Nephrolithus sp. indet., Cretorhabdus sp. indet., Watznaueria sp. indet. It determines a Campanian-Maastrichtian age for the middle and upper part of MBF. Two calcareous nanofossil species, Prediscosphaera cretacea and ?Fasciculithus sp. indet., found in the Burdick Peak section suggest a Late Maastrichtian to (?) Paleocene age of the uppermost part of the MBF. The sediments of the MBF are possibly coeval of a part of Marambio Group (James Ross Island and Seymour Island) and Williams Point beds (Livingston Island).
... This Formation crops out at the Hurd Peninsula, Livingston Island, which mainly consists of turbiditic sandstones, mudstones, conglomerates and sedimentary breccias (Figure 1b;Dalziel, 1972b;Hervé, Faúndez, et al., 2006;Hervé, Miller, & Pimpirev, 2006). The age of this succession was assigned to the Late Jurassic-Late Cretaceous based on SHRIMP U-Pb zircon age dating and calcareous nannoplankton fossils (Hervé, Faúndez, et al., 2006;Pimpirev et al., 2002Pimpirev et al., , 2006. This formation has been proposed to be deposited in a multiple source, gravel-rich, deep-sea ramp system and has undergone significant deformation and was emplaced in a strike-slip fault setting (Muñoz et al., 1992). ...
Understanding the tectonic framework of the Antarctic Peninsula is hindered by a paucity of paleomagnetic data from key locations. In this study, we present paleomagnetic data from the South Shetland Islands, to the northwest of the Antarctic Peninsula, which provides valuable paleoposition constraints on the Western domain of the Antarctic Peninsula. We report a key reliable paleopole (58.1°S, 354.3°E, A95 = 6.3°) from Livingston Island in the South Shetland Islands at ∼102 Ma. Plate reconstruction models from the Early Cretaceous attach the South Shetland Islands to the Pacific margin of southern Patagonia‐Fuegian Andes at ∼140 Ma. The South Shetland Islands then experienced southward translation to its current position to the northwest of the Antarctic Peninsula following counterclockwise rotation during ∼100–90 Ma. A similar counterclockwise rotation has also been identified from southern Patagonia‐Fuegian Andes but is absent in the Antarctic Peninsula, suggesting a direct affinity between the South Shetland Islands and southern Patagonia‐Fuegian Andes. However, the consistent, almost northward Cretaceous paleomagnetic declination in the Antarctic Peninsula, and the near‐synchronous tectonic‐magmatic history between the Antarctic Peninsula and the southern Patagonia‐Fuegian Andes support an autochthonous continental subduction model for most of the Antarctic Peninsula.
... The Miers Bluff Formation (MBF) crops out exclusively at Hurd Peninsula on Livingston Island (South Shetland Islands, Fig. 1). It is a turbiditic deposit originally correlated with the TPG (Trouw et al., 1997), but now considered to be a younger sedimentary succession, on the basis of the occurrence of a Tithonian ammonite (Pimpirev et al., 2002) and the youngest detrital zircon population of ca. 170 Ma . ...
Metasedimentary rocks in the Antarctic Peninsula and south-western Patagonia record detrital zircon evidence for significant Permian magmatic events along the Palaeo-Pacific margin of south West Gondwana. However, it is unclear where and how this magmatism formed due to the lack of outcropping Permian igneous sources at similar latitudes. Combined U-Pb, O, and Lu-Hf isotope analyses of detrital zircon grains in Permo-Triassic metasedimentary rocks indicate that the Permian magmatism resulted from the interaction of crust- and mantle-derived sources in an active continental margin. Permian detrital zircons from the Trinity Peninsula Group in the Antarctic Peninsula range from crustal signatures in the northern part (δ18O of ~8‰, initial εHf of ~-6) to mantle-like values in the south (δ18O of ~5‰, initial εHf of ~+3). Zircons from the northern domain have isotopic features similar to those from the Patagonian Duque de York Complex. They also share a secondary Ordovician component of ca. 470Ma. The Middle Jurassic Cape Wallace Beds in Low Island record a ca. 250Ma igneous source, with stronger crustal signatures (δ18O and initial εHf values of 7.5 to 10.8‰ and -3.2 to -14.2, respectively). In contrast, zircons from the upper Jurassic Miers Bluff Formation on Livingston Island and Cretaceous sediments on James Ross Island have similar Permian U-Pb ages, O and Hf trends to their Trinity Peninsula Group counterparts, suggesting reworking after the late Jurassic. Our results provide evidence for a Permian subduction-related magmatic arc, partly located in Patagonia and extending to West Antarctica. The southerly decrease in δ18O coupled with increasing initial εHf indicate fewer sedimentary components in the magma source and is consistent with a glaciated cold and dry climate. These conditions are comparable with West Antarctica climate settings, located close to the South Pole during the Carboniferous and Permian.
The Lower Cretaceous to Miocene South Shetlands Islands’ volcanic arc is a result of the subduction of the Phoenix Oceanic Microplate (part of the Pacific Plate) beneath the Antarctic Peninsula. The magmatic activity on Livingston Island has been going on for a long time: since the Early Cretaceous to the present. The evolution of the volcanic arc resulted in the formation of various sedimentary basins and paleoenvironments (turbiditic to shallow marine and continental) that formed varied sedimentary successions and tholeiitic to potassium calc-alkaline subduction-related magmatic rocks. Considering the magmatic activity on Livingston Island, a general trend of rejuvenation can be observed from the WNW (Byers Peninsula) to the ESE (to Hannah Point, Point Williams and further to the Hurd Peninsula and the Tangra Mountain). The rejuvenation of the magmatic rocks in the direction of the thrust plate can be explained as an effect of a flattening subduction that was active from the Early Cretaceous (135 Ma) up to around 90–70 Ma. The mixed ages of the dikes and small intrusions in the Hurd Peninsula are most probably due to periods of rotation and probably beginning of а slab-roll back. A regional Paleocene–Eocene (65–47 Ma) compressional episode that caused a low temperature to high-pressure metamorphism is registered on Smith and Elephant islands. The regional metamorphism was followed by a vast extension between 50–30 Ma that culminated with the opening of the Drake Passage around 34–30 Ma. The magmatic response of that regional scale extension was the intrusion of the Eocene Barnard Point Pluton (46–40 Ma) and dikes with youngest ages of around 30 Ma. The major uplift phase and the exhumation of the Tangra Mountain happened between 22–16 Ma as indicated by the Ap FT thermochronology. The last magmatic event on the island produced the contrastingly different in composition Quaternary alkaline mafic rocks that occupy the area between Innot Point and Burdick Peak. This episode is interpreted as related to a process of crustal extension that culminated in asthenospheric upwelling and rifting along the Bransfield Strait. The latter is due to a slab roll-back that led to the break-up of the South Shetlands from the Antarctic Peninsula and consequent subduction termination.
The connections between the Antarctic Peninsula and Patagonia, here referred to as South America south of 44 degrees S,are analysed in the light of new geological information and hypotheses. The previously supposed existence of a continuous belt of Late Paleozoic accretionary complexes in the western margin of both Patagonia and the Antarctic Peninsula has been repudiated in recent years, since these complexes are mainly Mesozoic in terms of deposition and metamorphism. This is consistent with paleogeographic models in which the Antarctic Peninsula lays west of Patagonia in pre-break-up times. This disposition is favoured by similarities in provenance between the late Early Permian to ?Late Triassic Duque de York Complex and Trinity Peninsula Group,which share sedimentological characteristics and U-Pb detrital zircon age patterns. After the Early Jurassic Chonide orogeny, the Antarctic Peninsula started to drift southwards, as indicated by paleomagnetic reconstructions of Weddell Sea ocean floor spreading, allowing the present-day margin of Patagonia to become progressively, from north to south, an actively subducting margin. In the Late Jurassic and Late Cretaceous, while the Antarctic Peninsula was drifting to its present position, turbiditic sedimentation took place at Hurd Peninsula, Livingston Island, previously considered to be Triassic in age,which has no equivalent in the Patagonian margin. The new data presented here, combined with recent data and models from other authors, reveal two major geological problems to be resolved in the future: to identify a late Early Permian magmatic arc which shed detritus to both the Duque de York Complex and the Trinity Peninsula Group, and to resolve the apparent contradiction between left lateral post break-up movements in the Patagonian Andes, and right lateral displacements in the tectonic configuration of the Antarctic Peninsula.
Knowledge of the Early Cretaceous ammonoids of the NW-Himalayas was poor until recent discoveries. Intense sampling from the Giumal Formation exposed near the village of Chikkim (Spiti Valley, Himachal Pradesh, India) led to the recognition of a new Early Cretaceous ammonoid fauna. The succession consists of arenitic sandstone interbedded with shale that was deposited by turbidity currents on an unstable shelf in the Early Cretaceous. Ammonoids have been obtained only from sandstone beds in the lower one-third and close to the top of the c. 350-m-thick section. Eight new ammonoid taxa (1 genus and 7 species) are described: Sinzovia franki sp. nov. (rare), Giumaliceras giumaliense gen. et sp. nov. (abundant), Giumaliceras bhargavai gen. et sp. nov. (rare), Neocomites (Eristavites) platycostatiformis sp. nov. (rare), Cleoniceras oberhauseri sp. nov. (abundant), Australiceras himalayense sp. nov. (rare) and Deshayesites fuchsi sp. nov. (rare). Sinzovia and Deshayesites are reported for the first time from the Tethyan Himalaya. According to the biostratigraphic relevance of some ammonoid taxa described here, the age of the Giumal Formation can be constrained from Berriasian (Giumaliceras assemblage) to Aptian (Cleoniceras assemblage). The discovery of the new fauna substantiates the significance of the Giumal Formation around Chikkim and facilitates comparison with faunal assemblages from other regions in the Tethys Ocean and beyond.
New biostratigraphic data from co-occurring radiolarians and ammonites in Upper Jurassic sequences of the Antarctic Peninsula (Byers Peninsula on Livingston Island and Longing Gap, Graham Land), permit a revised and more refined regional stratigraphy. The new data also allow a revision of the chronostratigraphic assignment of some American radiolarian zones established by Pessagno and collaborators:
The boundary of Zone 3-4 is assigned to the latest Kimmeridgian, contrasting the former assignment to the early/late Tithonian boundary. The boundary between Subzone 4 beta and 4 alpha is assigned to the early Tithonian, but was usually correlated with the early late Tithonian/late late Tithonian boundary.
The new chronostratigraphic data from Antarctica are used together with recent results of Baumgartner and collaborators to revise the age assignment of the North American Late Jurassic radiolarian zones.
The sedimentary sequences outcropping in Hurd Peninsula, Livingston Island, are formally defined as members of the Miers Bluff Fonna tion (MBF): Johnsons Dock and Napier Peak Members and Moores Peak breccias. The lowermost strata (Johnsons Dock Member) are studied in detail, while the investigations of the upper part of the sedimentary sequences crop ping out at Napier Peak and Cerro Mirador are of a reconnaissance nature only. The sediments in castern Hurd Peninsula are studied in three sedimentological and geological scctions (Napier Peak, Cerro Mirador and Moores Peak). Meas urcments of the orientation of thc axes of small-scale folds represented in the sections suggest that phases 3a and 3b of MUNoz et al, (1992) can be unificd in one final phase. Their orientations are in the frame of normal deviations usu ally within the developmcnt of a fold system. Thc Moores Peak breccias are a component and an inseparable part of the whole sedimentary sequence in Hurd Peninsula. They are the upper lithostratigraphical division of the MBF and rep resent the final stage of sedimentation in the Carboniferous(?)-Triassic Miers Bluff depositional basin. The dominant transport of the Moores Peak breccias has been by dcbris flows.
The Byers Group, exposed on Byers Peninsula, western Livingston Island, Antarctica, comprises a mudstone dominated sequence at least 1 km thick which accumulated in a marginal fore-are environment. The basal, 105 m thick Anchorage Formation consists of radiolarian mudstones and tuff-rich interbeds of Kimmeridgian-Tithonian age; it correlates with Upper Jurassic organic-rich mudstone units throughout the proto-South Atlantic region. The succeeding 244 m thick Devils Point Formation marks the first major pulse of coarse volcaniclastic material into the basin. It is in turn followed by the extensive President Beaches Formation, comprising several hundred metres of finely laminated mudstones with at least two major sandstone intercalations. Molluscan and dinoflagellate cyst taxa indicate a Berriasian age and comparatively nearshore depositional environment for this unit. An unconformity of late Berriasian or early Valanginian age separates the three lowest formations from the Chester Cone Formation. The fine-grained Sealer Hill Member at the base of the latter is dated as Valanginian, and grades up into several hundred metres of pebbly sandstones and pebble-granule conglomerates. These mark the second major volcaniclastic pulse and may be of Hauterivian or even younger age. Definition of this major new group will facilitate more precise Upper Jurassic-Lower Cretaceous stratigraphical correlations within the southern South America-Scotia arc-Antarctic Peninsula region. It will also aid our understanding of the critical palaeogeographical transition in the northern Antarctic Peninsula from anoxic basin to active magmatic are.
The Moores Peak Formation occurs on eastern Hurd Peninsula, Livingston Island, part of the South Shetland Island volcanic arc, which was active during Mesozoic and Cenozoic times. The rocks were formerly described as deformed Miers Bluff Formation (MBF) turbidites of early Triassic age. More recent work identified lithogically distinctive sedimentary and volcanic breccias in an ambiguous stratigraphic position, possibly belonging to the underlying turbiditic accretionary complex, or to the overlying volcanic sequence of ?Cretaceous age (Antarctic Peninsula Volcanic Group — APVG), or representing a formation in between the MBF and APVG.New mapping reinterprets the Moores Peak Formation as a c. 150 m thick megabreccia unit unconformably overlying the MBF, and forming pan of the mid- to late Cretaceous APVG. The megabreccia consists of sheared angular clasts and large blocks of lithified, deformed and hydrothermally altered sandstone, distal turbidite, mudstone and conglomerate (derived from the MBF), and volcanic breccia and tuff and lava clasts (typical of the APVG), supported in a matrix of pulverized sedimentary and volcanic material, with relatively abundant epidote and clinozoisite. Jigsaw-breccias, irregular colour domains and intra-block shears indicate that the breccias formed as subaerial debris-avalanches from an area of uplifted pre-volcanic basement (MBF) overlain by volcanic rocks. Hydrothermal alteration contributed to weakening of the uplifted rocks.Collapse of large volcanic edifices results in sudden decompression of underlying basement and contained hydrothermal systems, resulting in solution boiling, hydraulic brecciation, and mineral deposition. The MBF hosts a laterally extensive quartz vein and breccia system 3 km west of Moores Peak, which may have formed as a result of a stratovolcano collapse event similar to that represented by the Moores Peak Formation.
The Nordenskjöld Formation is a sequence of thinly interbedded ash beds and black, radiolarian-rich mudstones which is exposed on the eastern coast of the Antarctic Peninsula. As a result of recent field work, the Nordenskjöld Formation has been re-examined and redefined with three new members recognized — the Longing, Ameghino, and Larsen members. The age of the formation, based on its macrofossil content, is Kimmeridgian to Berriasian, although reworked clasts from James Ross Island suggest it might range down into the Oxfordian.RésuméLa Formación Nordenskjöld se compone de una fina intercalación de cenizoas volcánicas y pelitas negras con abundantes radiolarios, la cual está expuesta en la costa este de la península Antárctica. Como resultado de trabajos de campo recientes, la Formación Nordenskjöld ha sido re-examinada y se han definido tres nuevos miembros — Longing, Ameghino y Larsen. La edad de la formación de acuerdo a los macrofósiles es Kimmeridgiana a Berrisiana, aunque clastos retrabajados de la isla James Ross sugieren que puede llegar hasta el Oxfordiano.
New data obtained in Nepal and Spiti (India) on stratigraphy and Jurassic ammonite faunas form the basis for a new biogeographical interpretation of the peri-Gondwanan faunas. Faunas of the same ages from New Guinea, New Zealand, Antarctica and South America are also considered.From the upper Bathonian up to the Tithonian-Berriasian, six main successive assemblages are distinguished in Nepal and are compared to the faunas described in the above-mentioned areas, the latter often without accurate stratigraphic support. The faunas of Nepal show: (1) low taxonomic and generic diversity of the ammonites; and (2) dominance of one genus or a small number of genera in the same family and the subordinate place of the associated taxa.With a few exceptions, the classical biogeographical schemes do not recognize the existence of an austral ammonite fauna and/or Austral Province. Bipolar distributions of several benthic or nekto-benthic invertebrates as well as intertropical distribution of calpionellids express a latitudinal pattern of marine faunal distribution. Austral and Tethyan ammonite faunas show less marked contrast than the Tethyan and Boreal Realms and transitional or mixed faunas of subaustral type, of relatively high diversity, are developed in the Indo-Malagasian trough (or seaway) and the Andean region. The austral fauna of low diversity was spread around East and South Gondwanaland, from the Himalaya to Patagonia.The lack of a geographical trap such as the Arctic basin almost completely enclosed by land explains why the austral faunas were never as distinct as the Tethyan faunas from the Boreal. In addition to the role in the differentiation of the boreal faunas, the land-locked geography of the Arctic basin magnified the role of the other environmental factors, especially those linked with a high-latitude situation. Seasonal fluctuations of illumination in high latitude resulted in environmental instability which promoted eurytopic taxa with high-density and low-diversity populations.