Lake Hoare in the Dry Valleys of Antarctica is covered with a perennial ice cover more than 3 m thick, yet there is a complex record of sedimentation and of growth of microbial mats on the lake bottom. Rough topography on the ice covering the lake surface traps sand that is transported by the wind. In late summer, vertical conduits form by melting and fracturing, making the ice permeable to both liquid water and gases. Cross-sections of the ice cover show that sand is able to penetrate into and apparently through it by descending through these conduits. This is the primary sedimentation mechanism in the lake. Sediment traps retrieved from the lake bottom indicate that rates of deposition can vary by large amounts over lateral scales as small as 1 m. This conclusion is supported by cores taken in a 3 x 3 grid with a spacing of 1.5 m. Despite the close spacing of the cores, the poor stratigraphic correlation that is observed indicates substantial lateral variability in sedimentation rate. Apparently, sand descends into the lake from discrete, highly localized sources in the ice that may in some cases deposit a large amount of sand into the lake in a very short time. In some locations on the lake bottom, distinctive sand mounds have been formed by this process. They are primary sedimentary structures and appear unique to the perennially ice-covered lacustrine environment. In some locations they are tens of centimetres high and gently rounded with stable slopes; in others they reach approximately 1 m in height and have a conical shape with slopes at angle of repose. A simple formation model suggests that these differences can be explained by local variations in water depth and sedimentation rate. Rapid colonization of fresh sand surfaces by microbial mats composed of cyanobacteria, eukaryotic algae, and heterotrophic bacteria produces a complex intercalation of organic and sandy layers that are a distinctive form of modern stromatolites.
The thickness frequency distribution of stratigraphic layers intersected by the Apollo 15 deep core suggests that the majority of impact events reworking the lunar soil are small and produce ejecta blankets with an average thickness of less than 1.5 cm. The energy frequency distribution of the meteorites producing the layers may be bimodal. The impacting meteorites produce both normal and reverse graded beds which appear to be the end products of two depositional mechanisms. First, the normally graded beds appear to be produced in base surges as escaping gases fluidize the flowing debris and larger particles move downward in response to Stokes Law. Second, if the gas loss from the base surge is excessive, the fluidization may cease and inertial grain flow dominates. In this situation, the beds are reverse graded as larger particles move under dispersive pressure to the region of minimum shear stress at the upper boundary of the base surge.
Three lines of evidence based on data from more than 400 boreholes and vibrocores have been used to reconstruct the evolution of the barrier islands during the Holocene transgression in southern Long Island, New York: (1) the Holocene transgressive stratigraphic sequence behind the present barriers, (2) the stratigraphic patterns of the inner shelf, and (3) the morphology of the now-buried late Pleistocene coastal features.
The extensive preservation of backbarrier sediments, radiocarbon dated between 7000 and 8000 yr BP, on the inner shelf of southern Long Island suggests that the barriers have not retreated by continuous shoreface erosion alone, but have also undergone discontinuous retreat by in-place ‘drowning’ of barriers and stepwise retreat of the surf zone. Such stepwise retreat of the surf zone has prevented the backbarrier sediments from being reworked.
Based on the presence of submerged barrier sand bodies in seismic records, it is inferred that about 9000 years ago, when the sea stood about 24 m below the present sea level, a chain of barriers developed on the present shelf about 7 km offshore of the present barriers. With continued sea-level rise, the – 24 m barrier built upward until the sea reached about – 15 m MSL, just prior to 7000 yr BP. The barriers were then submerged by the rapidly rising sea, and the surf zone shifted rapidly landward to a position about 2 km from the present shoreline. The surf zone overstepped to the landward margin of the old lagoon, which had become fixed at the steep seaward face of mid-Wisconsinan (?) or Sangamonian coastal barriers. During the past 5000 or 6000 years, the shoreface has retreated continuously by about 2 km.
Evidence from southern Long Island and elsewhere in regions of coastal submergence indicates that rapid sea-level rise and low sand supply seem to favour the stepwise retreat of barriers, whereas slow rates of submergence and a greater supply of sand generally favour continuous shoreface retreat. Stationary upbuilding, or seaward progradation of barriers may occur when supply of sand is great, and/or submergence is slowed or reversed. Morphologic highs on the pretransgression surface (such as old barrier ridges) tend to fix the migrating barrier shoreline during either continuous retreat, or stepwise retreat of barriers.
The effects of source rock composition and climate on the natural abundances of rare elements (REE) in the first leg of the sedimentary cycle are evaluated using a study with Holocene fluvia sands. The medium grained sand fraction of samples collected from first order streams exclusively draining granitic plutons in Montana (semi-arid), Georgia (humid), and South Carolina (humid) are analyzed. It is found that the REE distribution patterns (but not the total absolute abundances) of the daughter sands are very similar, despite compositional differences between parent plutons. Averages of the three areas are determined to have a La/Lu ratio of about 103, showing a depletion of heavy REE with respect to an average granite (La/Lu = 79) or the composition of North American Shales (La/Lu = 55). However, the Eu/Sm ratio in sands from these areas is about 0.22, which is very close to this ratio in North American Shales (0.21), although the overall REE distribution of these sands is not similar to that of the North American Shales in any way. It is concluded that the major rock type, but neither its minor subdivisions nor the climate, controls the REE distribution patterns in first cycle daughter sands, although the total and the parent rock-normalized abundances of REE in sands from humid areas are much lower than those in sands from arid areas.
Bioclastic flow deposits offshore from the Soufrière Hills volcano on Montserrat in the Lesser Antilles were deposited by the largest volume sediment flows near this active volcano in the last 26 kyr. The volume of these deposits exceeds that of the largest historic volcanic dome collapse in the world, which occurred on Montserrat in 2003. These flows were most probably generated by a large submarine slope failure of the carbonate shelf comprising the south-west flank of Antigua or the east flank of Redonda; adjacent islands that are not volcanically active. The bioclastic flow deposits are relatively coarse-grained and either ungraded or poorly graded, and were deposited by non-cohesive debris flow and high density turbidity currents. The bioclastic deposit often comprises multiple sub-units that cannot be correlated between core sites; some located just 2 km apart. Multiple sub-units in the bioclastic deposit result from either flow reflection, stacking of multiple debris flow lobes, and/or multi-stage collapse of the initial landslide. This study provides unusually precise constraints on the age of this mass flow event that occurred at ca 14 ka. Few large submarine landslides have been well dated, but the slope failures that have been dated are commonly associated with periods of rapid sea-level change.
A numerical model presented here develops a three-dimensional image of alluvial media on an elementary scale significant for groundwater flow modelling. The model was tested on the alluvial plain of the Rhône River (France), on a scale of several kilometres and, from geomorphological observations and dating, reproduced the construction of this alluvial plain from ≈15 000 years BP to the present. The history of the alluvial plain during the Late Glacial and Holocene periods is summarized. Through most of this time, the River Rhône has maintained a braided pattern, with the exception of two incising phases with a meander pattern. The model does not use any physically based equations or water representation. The main processes governing the construction of the plain are modelled by simple rules chosen according to geometrical or empirical laws taken from the literature or as modelling assumptions. Using multi-agent concepts of distribution and interaction of elementary entities, these sedimentary rules are applied to ‘sediment’ entities or to conceptual ‘erosion’ entities that simulate local deposition and erosion of sediments. The sedimentation model reproduces the various climatic periods during which the sediments were deposited by simulating genetic periods and associated modelled processes. For each period, the model was constrained by quantitative field data such as altitude of ancient channels and deposits or thickness of sediments. The general geometry of the alluvial deposits was satisfactorily reproduced. During the simulation, characteristic large-scale features emerge despite the use of local rules. The model results are discussed with reference to other approaches, such as geostatistical or Boolean models, and the applicability of the model to other less documented alluvial plains is outlined.
A deep borehole through Ribbon Reef 5 in the Great Barrier Reef off north-eastern Australia has identified a variety of cements, including epitaxial, radial prismatic and spherular aragonite, together with blocky, prismatic and fibrous calcite. These cements are discontinuously arranged within the sequence that consists predominantly of grainstones but locally includes clotted muddy and filamentous textures that may be of microbial origin. Calcite cements vary in morphology with groups of crystals that include acute scalenohedral, rhombohedral and flattened concordant terminations; these show varying densities of inclusions that locally define growth zones and in some terminations divide in the manner of ‘split crystals’ to form fibrous fringes. Morphological changes in calcite are inferred to reflect changes in water chemistry and crystal growth rates at the time of growth, allied to their relationship to the palaeo-water table, and linked in turn to changes in sea-level. Neomorphism and dissolution are widespread and variations in the severity of both imply response to the degree of undersaturation of pore waters that at times were probably balanced within very narrow limits. A total of 10 depositional units are identified. Those units at the base of the borehole reflect deposition and diagenesis within a marine environment. The influence of meteoric waters, indicated by stable isotopes, is first apparent at the top of Unit 1 and in Unit 2 (184 to 155 m below sea floor). Petrographic evidence of vadose conditions appears at the tops of Unit 3 (131 to 99 m below sea floor). Units 4 to 8, all deposited under marine conditions, provide isotopic evidence of meteoric or mixing-zone waters and petrographic indicators of vadose conditions, typically at the top of the units. Evidence indicates that in Unit 5 the water table was mobile and Units 6a, 6b, 7 and 8, all characterized by ultraviolet fluorescent cements, are capped by sub-aerial erosion surfaces. Unit 9 (the Holocene) reflects the recent re-establishment of marine conditions. The extent of alteration of the entire sequence reflects the substantial and pervasive influence of meteoric waters. This effect is interpreted as a result of a greater rainfall and river flow from the mainland during early and late stages of interstadial periods. The study reflects progress in the ability to recognize the diagenetic signal generated by sea-level change. However, whereas the isotopic results reflect the changing relationships between vadose and phreatic zones in groundwater systems beneath successive emergent surfaces, their correspondence with petrographic features is expressed only weakly and commonly lacks the systematic sequential overprinting implied by the distribution of cathodoluminescent zones of cements in many ancient limestones.
Cyclothemic sedimentary rocks of the Plio-Pleistocene Petane Group outcrop extensively in the Tangoio block. They are products of inner to mid-shelf sedimentation and were deposited during glacio-eustatic sea level fluctuations along the western margin of a shallow, pericontinental seaway located in a forearc setting. The succession consists of five laterally continuous cyclothems, each containing a fine grained interval of silt and a coarse grained interval of siliciclastic sand ± gravel or limestone. Five sedimentary facies assemblages comprising 20 separate facies have been recognized. Coarse grained intervals of cyclothems were deposited mostly during relative sea level lowstands and contain up to four facies assemblages. -Authors
The following dimensionless parameters (two of them well-known and five of them new) are defined for determination of ripple mark geometry: ripple index (RI), ripple symmetry index (RSI), continuity index (CI), bifurcation index (BI), straightness index (SI), and two different parallelism indices (PI1 and PI2).In general, RI = 15 or less indicates wave or water current origin; RI = 17 or more indicates wind or swash origin. RSI = 1.5 or less indicates wave or swash types; RSI = 3 or more indicates wind or water current types. CI = 15 or more suggests wind or wave origin; CI = 10 or less suggests water current origin. BI = 10 or more suggests wave varieties; BI = 1 or less suggests wind varieties. SI = 102 or more indicates wind or deep-water wave types; SI = 15 to 102 indicates wind or wave types; SI =4 or less indicates current types. PI1 = 7 or more suggests wave origin; PI1 = 1 or less suggests water current origin. PI2 = 0.4 or more is probably the result of swash or water current action. PI2 = 0.2 or less is probably the result of wind or wave action. Longitudinal ripple marks (such as rib-and-furrow) and deformed or modified varieties (such as flat-topped tidal-flat ripple marks and nearly- flat-topped intermittent creek ripple marks) have been excluded, inasmuch as (1)they are commonly easy to identify from their appearance, and (2)they are difficult to measure with ordinary methods.Plots of two indices against each other on coordinate paper can be particularly useful; the best combinations are RI vs. RSI, and RI vs. PI1, although several other pairs are almost as good. Where all seven parameters can be obtained, the confidence one can have in the interpretation is close to 98%.The effects of current bias, or depth bias, on wave-type ripple marks, extend to both the symmetry (RSI) and to sediment-transport interpretations. Unless the investigator is reasonably sure that no such bias is present (i.e., RSI = 1.0 instead of some significantly higher value such as 1.5), wave-type ripple marks cannot be used to determine direction of either wave approach or sediment transport. If no such bias is present, wave-type ripple marks still cannot be used to determine precise sediment transport direction. If RSI = 1.0 precisely, it is not even necessary that the ripple crests parallel the waves that formed them. The same restrictions apply to the interpretation of micro-crossbedding (that is, ripple mark internal structure). Despite these seemingly severe limitations, general geometry commonly permits a reliable interpretation, and hence ripple marks can provide a great deal of useful data for paleogeographic interpretations.The swash-zone variety of ripple marks includes two sub-types: those modified by a small but unmistakeable hydraulic jump, and those not so modified. The RI can be used to distinguish between these two, even when they were not observed to form.
ABSTRACT During Leg 177 of the Ocean Drilling Program (ODP), well-preserved Middle Miocene to Pleistocene carbonate-rich sediment records were recovered on a north–south transect through the south-eastern Atlantic sector of the Southern Ocean at Site 1088 on the Agulhas Ridge and Site 1092 on Meteor Rise. Both sites were dominated by the deposition of calcareous nannofossil oozes through the Miocene, indicating low biological productivity in warm to temperate surface waters. A continuous increase in the proportions of foraminifera since the latest Miocene (6·5 Ma) points to enhanced nutrient supply, possibly related to the global ‘biogenic bloom’ event across the Miocene–Pliocene boundary. Since the Late Pliocene, different styles of biological productivity developed between the sites. Enhanced deposition of biosiliceous constituents at the southern Site 1092, particularly in the Early Pleistocene, is consistent with the formation of the Circum-Antarctic Opal Belt since 2·5 Ma in a setting near the Polar Front, whereas carbonate deposition still prevailed at the northern Site 1088 situated near the Subtropical Front. Clay-mineral tracers of water-mass advection together with the pattern of sedimentation rates and hiatuses reflect distinct pulses in the development of regional ocean circulation between 14 and 12 Ma, around 8 Ma and since 2·8 Ma. These pulses can be related to Antarctic ice-sheet extension that mediates the production and flow of southern source water, and stepwise increases in North Atlantic Deep Water production that drives global conveyor circulation. At Site 1088, illite chemistry and silt/clay ratios of the terrigenous sediment fraction reflect the history of terrestrial climate in southern Africa, with humid conditions prior to the Early Late Miocene (9·7 Ma), followed by a dry episode until 7·7 Ma. The latest Miocene and Early Pliocene were characterized by a humid episode until modern aridity was established in the Late Pliocene between 4·0 and 2·8 Ma. These climate changes were related to the latitudinal migration of climate belts in response to tectonically caused reorganizations in atmospheric and ocean circulation.
Limestone–marl alternations are usually directly interpreted to reflect cyclic palaeoenvironmental signals. However, uncertainty in such interpretations stems from the differential diagenesis that most limestone–marl alternations have undergone. Differential diagenesis results in markedly different alterations between limestones and marls and in the loss of comparability of many measurable parameters. For an unequivocal interpretation of the origin of rhythmic alternations, diagenetically robust parameters or parameters that clearly indicate the degree of diagenetic bias are required. The present study uses a multiproxy approach (independent biotic, sedimentary and geochemical parameters) in order to unravel the palaeoenvironmental signal recorded in Valanginian (Early Cretaceous) limestone–marl alternations from the Blake-Bahama Basin (DSDP site 391). Using this approach, terrestrial and marine influences can be distinguished, changes in nutrient levels estimated and prediagenetic differences in the non-carbonate fraction constrained. Surprisingly, no systematic variations in any of these parameters were observed between limestone and marl layers, implying that none of these was directly responsible for the formation of the rhythmic alternation. Hence, none of the current models of sedimentary formation of limestone–marl rhythmites is applicable here. Calcareous nannofossils are equally well preserved in limestone and marl layers, ruling out their dissolution in marl layers as a source of the calcite cement in the limestone beds. Sr values of 700–900 p.p.m. indicate that aragonite may have been present in the original, pelagic sediment. The assumption of fine-grained sedimentary aragonite imported from nearby carbonate platforms as the source of the cement would explain a number of otherwise enigmatic features in these rhythmites, including the source of the calcite cement observed in the limestones, the equally good preservation of calcareous nannofossils in limestones and marls and the higher concentration of calcareous nannofossils in marl layers. The study demonstrates that examination of diagenetically inert parameters or parameters in which diagenetic effects can be filtered can yield unexpected results. Clearly, careful analysis of such parameters needs to be undertaken in order to make reliable palaeoenvironmental interpretations from rhythmite successions.
Photographs and descriptions of quartz sand grains from soils, paleosols and silcretes show that grain shape can be modified by solution in situ. Dissolution is attributed to solutions rich in organic molecules present in weathering profiles, and commonly results in rounding of protuberances and re-entrants on grains. The incidence of this process may vary with climate. It is postulated that solution rounding during weathering plays an important role in shaping quartz sand grains in general, its significance varying with climate. This postulate, which remains unconfirmed, has considerable implications for the interpretation of textural maturity of sandy sediments. If true, super-mature sands may be as much a climatic indicator as an indicator of tectonic quiesence.
Syntectonic carbonate veins were found along the décollement zone between the Atlantic and Caribbean plates and in two sites 12 and 17 km landward of the deformation front of the Barbados accretionary prism (ODP Leg 110). Both calcite and lesser amounts of rhodochrosite (MnCO3) occur in microcrystalline, fibrous, and blocky mosaic forms in veins deposited along scaly shear zones. Descriptions of the occurrences, textures, mineralogy, and stable isotope geochemistry of these veins presented herein help trace the hydrological evolution of the Barbados accretionary prism, which has seen the impact of more deeply derived fluids on elevated sediment temperatures and altered geochemistry of interstitial fluids. At one site, for example, vein-forming fluids have been as much as 10°C warmer and 1‰ more enriched in 18O than expected.Carbon isotope ratios of veins indicate a mixture of carbon derived from seawater and from oxidized organic matter during sulphate reduction, although dissolution of calcareous skeletons may be an additional component. The presence of methane is recorded in one vein sample in the accretionary prism, indicating that either the plumbing of the prism has changed with time, allowing more fluid to leak into the prism in the past, or that materials currently 1 km above the décollement zone were once deformed in the décollement.Using oxygen isotope analyses of rhodochrosite samples from two sites in conjunction with measurements of fluid temperature and δ18O values of interstitial fluids, it is proposed that the fractionation of oxygen isotopes between rhodochrosite and water (1000 1n)∼38‰ at 25°C, 10‰ greater than the fractionation between calcite and water. However, it appears that substitution of 10-20 mol.% Ca into rhodochrosite reduces the fractionation factor by 2-9‰ so impure diagenetic rhodochrosite samples from the deep ocean will rarely be this enriched in 18O.
We are pleased to have the opportunity to reply to the critical discussion of our paper by Gatt (2009). However, it appears to be based on several misunderstandings regarding the theme and thrust of our article. Our article does not set out to redefine the Lower Coralline Limestone Formation. On the contrary, it seeks to expand
information on the Attard Member and its implications for the development of modern reefs. This member lies at the junction between the middle and inner ramps (junction between coralline
‘algal rhodolith platform’ facies and ‘coralgal patch-reef belt’ of Buxton & Pedley, 1989). Of course, there are other members comprising the
formation and we also accept that there is much of the Attard Member which lies down-ramp of our study site. However, these matters are beyond the remit of our article. Consequently, we present our comments (Table 1) in the order raised by Gatt (2009).
Much of our understanding of submarine sediment-laden density flows that transport very large volumes (ca 1 to 100 km3) of sediment into the deep ocean comes from careful analysis of their deposits. Direct monitoring of these destructive and relatively inaccessible and infrequent flows is problematic. In order to understand how submarine sediment-laden density flows evolve in space and time, lateral changes within individual flow deposits need to be documented. The geometry of beds and lithofacies intervals can be used to test existing depositional models and to assess the validity of experimental and numerical modelling of submarine flow events. This study of the Miocene Marnoso Arenacea Formation (Italy) provides the most extensive correlation of individual turbidity current and submarine debris flow deposits yet achieved in any ancient sequence. One hundred and nine sections were logged through a ca 30 m thick interval of time-equivalent strata, between the Contessa Mega Bed and an overlying ‘columbine’ marker bed. Correlations extend for 120 km along the axis of the foreland basin, in a direction parallel to flow, and for 30 km across the foredeep outcrop. As a result of post-depositional thrust faulting and shortening, this represents an across-flow distance of over 60 km at the time of deposition. The correlation of beds containing thick (> 40 cm) sandstone intervals are documented. Almost all thick beds extend across the entire outcrop area, most becoming thinly bedded (< 40 cm) in distal sections. Palaeocurrent directions for flow deposits are sub-parallel and indicate confinement by the lateral margins of the elongate foredeep. Flows were able to traverse the basin in opposing directions, suggesting a basin plain with a very low gradient. Small fractional changes in stratal thickness define several depocentres on either side of the Verghereto (high) area. The extensive bed continuity and limited evidence for flow defection suggest that intrabasinal bathymetric relief was subtle, substantially less than the thickness of flows. Thick beds contain two distinct types of sandstone. Ungraded mud-rich sandstone intervals record evidence of en masse (debrite) deposition. Graded mud-poor sandstone intervals are inferred to result from progressive grain-by-grain (turbidite) deposition. Clast-rich muddy sandstone intervals pinch-out abruptly in downflow and crossflow directions, in a fashion consistent with en masse (debrite) deposition. The tapered shape of mud-poor sandstone intervals is consistent with an origin through progressive grain-by-grain (turbidite) deposition. Most correlated beds comprise both turbidite and debrite sandstone intervals. Intrabed transitions from exclusive turbidite sandstone, to turbidite sandstone overlain by debrite sandstone, are common in the downflow and crossflow directions. This spatial arrangement suggests either: (i) bypass of an initial debris flow past proximal sections, (ii) localized input of debris flows away from available sections, or (iii) generation of debris flows by transformation of turbidity currents on the basin plain because of seafloor erosion and/or abrupt flow deceleration. A single submarine flow event can comprise multiple flow phases and deposit a bed with complex lateral changes between mud-rich and mud-poor sandstone.
The occurrence of early diagenetic Ca-rhodochrosite [(Mn,Ca)CO3] is reported in association with ‘griotte’-type nodular limestones from basinal settings in the geological record; however, without the comparison of analogous modern examples, the controls on precipitation remain speculative. Here the findings of four layers of primary Ca-rich rhodochrosite recovered from a modern deep-sea setting in the Eastern Equatorial Pacific, from bioturbated sediments 300 m below sea floor, are reported (Ocean Drilling Program, Leg 201, Site 1226). The mineralogy is similar to cements in burrows recovered during Deep Sea Drilling Project Leg 68 at Eastern Equatorial Pacific Site 503 and from Ca-rhodochrosite laminae in sediments of the central Baltic Sea. Petrographic relationships and constant oxygen isotopic compositions in the Ca-rhodochrosite around 5‰ at all depths indicate a shallow burial depth of formation. The onset of 1‰ heavier oxygen isotope composition of Ca-rhodochrosite at Site 503, about 30 m below the Pliocene/Pleistocene boundary, further suggests that precipitation occurs in the range of 30 m below sea floor. The approximate depth of formation allowed an approximate empirical fractionation factor for marine Ca-rhodochrosite to be constrained that strongly differs from previously published theoretical values. Based on the approximate precipitation depth, authigenic Ca-rhodochrosite forms within the SO42−-reduction zone. Moderately negative δ13C values (around −3‰) and total organic carbon lower than 2 wt% indicate a relatively low contribution of CO32− from organic C mineralization within the expanded redox zonation in the Eastern Equatorial Pacific. It is suggested that the alkalinity is increased by a rise in pH at focused sites of Mn-reduction coupled with S2− oxidation. High concentrations of Mn-oxide can accumulate in layers or burrows because of Mn-cycling in suboxic sediments as suggested for the Baltic Sea Ca-rhodochrosites. This study demonstrates how early diagenetic precipitates document biogeochemical processes from past diagenetic systems.
Fifty-three samples from D.S.D.P. Sites 127, 128 (Hellenic Trench), 130 (Mediterranean Ridge) and 131 (Nile Cone) ranging in size from clayey silt to sand were submitted to grain-size and compositional analyses. The former confirmed the evidences obtained from the visual observation of the sedimentary features as to the presence of turbidites (Sites 127, 128, 131) and fluxoturbidites (Site 131). The cumulative frequency distributions of the latter are closely similar to those obtained from ancient deposits of the same type.
The 87Sr/86Sr ratios of evaporitic carbonates and sulphates from Miocene sediment cored in the Mediterranean Sea show a depletion of 87Sr when compared to the isotopic composition of the Miocene contemporaneous marine strontium: 0.70803 versus 0.70936. The arrival into the evaporitic environment of strontium brought by continental waters can explain this difference.The variation of the 87Sr/86Sr ratios is, nevertheless, noticeable only when the influence of the continental waters is already well marked. This is proved when one compares the results obtained with strontium, to the results of isotopic analysis made on oxygen, carbon, sulphur and hydrogen taken from the same samples.
Positive shifts in global seawater d13CDIC are related to changes in the ratio of organic relative to inorganic carbon burial in oceanic basins, whereas factors such as climatic cooling and the accumulation of polar ice are known to cause positive shifts in d18O. Here, an alternative model is proposed for the formation of local positive isotope shifts in shallow-marine settings. The model involves geochemically altered platform-top water masses and the effects of early meteoric diagenesis on carbonate isotopic composition. Both mechanisms are active on modern (sub)tropical carbonate platforms and result in low carbonate d13C and d18O relative to typical oceanic values. During high-amplitude
transgressive events, the impact of isotopically light meteoric fluids on the carbonate geochemistry is much reduced, and 13C-depleted platform-top wáter mixes with open oceanic water masses having higher isotope values. Both factors are recorded as a transient increase in carbonate 13C and 18O relative to low background values. These processes must be taken into consideration when interpreting the geochemical record of ancient epeiric seas.
Abstract Large-scale explosive eruptions from silicic caldera volcanoes can generate huge volumes of pyroclastic material in terrestrial and marine environments. On land, erosion, remobilization and redeposition of this debris is predominantly carried out by running water in the form of precipitation run-off. Conversely, in the submarine realm, both primary emplacement and subsequent remobilization are influenced by the presence of water as a transporting medium. Despite this, and the number of studies devoted to volcaniclastic sedimentation, relatively little attention has been paid to the hydrodynamic behaviour of the particles themselves, which ought to underpin any assessment of transport or depositional process. This is crucial, as many volcanic particles exhibit variable density: according to composition and as functions of differing degrees of vesiculation and the extent to which pore space is filled by water and/or gaseous phases during transport and deposition. Investigation of the physical and hydrodynamic properties of Taupo 1800a pumice, with reference to sedimentary facies developed during the eruption aftermath, shows that, although buoyant when dry, when sufficiently waterlogged, cool pumice clasts will sink and behave more like quartzo-feldspathic material. Saturation is apparently achieved by a combination of rapid capillary flooding of large interconnected vesicles and slower diffusional air–water exchange in smaller pores. Low saturated pumice densities result in lower settling velocities and easier entrainment by tractional currents than those for equivalent-sized quartzo-feldspathic or crystal/lithic particles. Fine-grained pumice is conversely harder to entrain because of the frictional interlocking of angular particles. These unusual properties of temporary buoyancy, variable saturation, low density and size-dependent cohesion complicate interpretations of the depositional setting and energy of pumiceous sediments and give rise to several unique facies. These findings have implications not only for the analysis of remobilized pyroclastic facies in terrestrial and marine environments, but also for primary depositional processes during subaqueous explosive volcanism.
Calcite septarian concretions from the Permian Beaufort Group in the Maniamba Graben (NW Mozambique) allow controls on the composition and nature of diagenetic fluids to be investigated. The concretions formed in lacustrine siltstones, where they occur in spherical (1 to 70 cm in diameter) and columnar (up to 50 cm long) forms within three closely spaced, discrete beds totalling 2·5 m in thickness. Cementation began at an early stage of diagenesis and entrapped non-compacted burrows and calcified plant roots. The cylindrical concretions overgrew calcified vertical plant roots, which experienced shrinkage cracking after entrapment. Two generations of concretionary body cement and two generations of septarian crack infill are distinguished. The early generation in both cases is a low-Mn, Mg-rich calcite, whereas the later generation is a low-Mg, Mn-rich calcite. The change in chemistry is broadly consistent with a time (burial)-related transition from oxic to sub-oxic/anoxic conditions close to the sediment–water interface. Geochemical features of all types of cement were controlled by the sulphate-poor environment and by the absence of bacterial sulphate reduction. All types of cement present have δ13C ranging between 0‰ and −15‰(Vienna Peedee Belemnite, V-PDB), and highly variable and highly depleted δ18O (down to 14‰ Vienna Standard Mean Ocean Water, V-SMOW). The late generation of cement is most depleted in both 13C and 18O. The geochemical and isotopic patterns are best explained by interaction between surface oxic waters, pore waters and underground, 18O-depleted, reducing, ice-meltwaters accumulated in the underlying coal-bearing sediments during the Permian deglaciation. The invariant δ13C distribution across core-to-rim transects for each individual concretion is consistent with rapid lithification and involvement of a limited range of carbon sources derived via oxidation of buried plant material and from dissolved clastic carbonates. Syneresis of the cement during an advanced stage of lithification at early diagenesis is considered to be the cause of development of the septarian cracks. After cracking, the concretions retained a small volume of porosity, allowing infiltration of anoxic, Ba-bearing fluids, resulting in the formation of barite. The results obtained contribute to a better understanding of diagenetic processes at the shallow burial depths occurring in rift-bound, lacustrine depositional systems.
Dark grey, bituminous dolostones interbedded with marine-derived anhydrite horizons occur in the Triassic Reichenhall Formation of western Austria. Fossils are rare and indicate a hostile, hypersaline depositional environment. The dolomites are finely crystalline, fairly stoichiometric, well ordered and non-ferroan. Closely spaced samples (94 in total) of individual dolomite units have been analysed for their carbon and oxygen isotopic composition. The data indicate surprisingly low δ18O values (-5.7 to -2.1%0 PDB), whereas the δ13C values are comparable to the contemporary Triassic seawater (+0.2 to +2.6%0 PDB). Sedimentological evidence, including (i) lack of any evidence for extensive dissolution, (ii) distinct oxygen and carbon isotope ratios of individual dolomite units, (iii) covariance of carbon and oxygen isotopes within some dolomite layers and (iv) inclusions of celestite in dolomite, indicates a nearly closed system after early diagenesis. Combining this information with water-rock interaction calculations suggests that the lightest oxygen isotope compositions are the result of freshwater influx into the basin during very early dolomite formation. A secondary factor may be dolomite recrystallization at elevated temperatures during burial.
Meteoric sphaerosiderite lines (MSLs), defined by invariant δ18O and variable δ13C values, are obtained from ancient wetland palaeosol sphaerosiderites (millimetre-scale FeCO3 nodules), and are a stable isotope proxy record of terrestrial meteoric isotopic compositions. The palaeoclimatic utility of sphaerosiderite has been well tested; however, diagenetically altered horizons that do not yield simple MSLs have been encountered. Well-preserved sphaerosiderites typically exhibit smooth exteriors, spherulitic crystalline microstructures and relatively pure (> 95 mol% FeCO3) compositions. Diagenetically altered sphaerosiderites typically exhibit corroded margins, replacement textures and increased crystal lattice substitution of Ca2+, Mg2+ and Mn2+ for Fe2+. Examples of diagenetically altered Cretaceous sphaerosiderite-bearing palaeosols from the Dakota Formation (Kansas), the Swan River Formation (Saskatchewan) and the Success S2 Formation (Saskatchewan) were examined in this study to determine the extent to which original, early diagenetic δ18O and δ13C values are preserved. All three units contain poikilotopic calcite cements with significantly different δ18O and δ13C values from the co-occurring sphaerosiderites. The complete isolation of all carbonate phases is necessary to ensure that inadvertent physical mixing does not affect the isotopic analyses. The Dakota and Swan River samples ultimately yield distinct MSLs for the sphaerosiderites, and MCLs (meteoric calcite lines) for the calcite cements. The Success S2 sample yields a covariant δ18O vs. δ13C trend resulting from precipitation in pore fluids that were mixtures between meteoric and modified marine phreatic waters. The calcite cements in the Success S2 Formation yield meteoric δ18O and δ13C values. A stable isotope mass balance model was used to produce hyperbolic fluid mixing trends between meteoric and modified marine end-member compositions. Modelled hyperbolic fluid mixing curves for the Success S2 Formation suggest precipitation from fluids that were < 25% sea water.
New observations concerning the degree of current-induced erosion and deposition in the path of the 1929 Grand Banks turbidity current are presented. Most of the observations are available from Eastern Valley, Laurentian Fan. Seabeam and SeaMARC I data reveal widespread current erosion along the valley over a distance of 200 km from the shelfbreak. Erosional valley-floor channels are preferentially developed adjacent to the valley margins and the flanks of intravalley highs. Asymmetric transverse bedforms (herein termed gravel waves) are moulded in a deflationary pebble and cobble lag that overlies the eroded valley floor. In contrast, at the distal limit of Eastern Valley, thick deposits of massive granule gravel indicate deposition beneath a decelerating turbidity current. Symmetrical transverse bedforms (herein termed macrodunes) are developed within these granule gravel sediments.
The spatial distribution of both bedforms and the areas of erosive excavation suggest that the turbidity current in 1929 was accelerating over the first 100 km from the shelfbreak and was eroding and entraining sediment from the valley floor over a distance of at least 200 km. With the loss of lateral constraint at the distal limit of Eastern Valley the turbidity current spread laterally and started depositing sediment as it decelerated. Current-induced erosion of the valley floor represented a potential source of between 50 and 100 km3 of sediment for incorporation into the resulting turbidite.
Erosional features on the floor of Eastern Valley of the Laurentian Fan, in 2800 m water depth, have been mapped with SeaMARC I side-scan sonar images and Seabeam multi-beam echo-soundings, and were directly observed during a dive with the deep submersible Alvin. The most spectacular feature is a 100-m-deep flute-shaped scour, more than 1 km long. The surrounding valley is floored by an unconsolidated coarse conglomerate, which was moulded into transverse bedforms by the turbidity current that was triggered by the 1929 Grand Banks earthquake. Direct observations and seismic-reflection profiles show that the flute-shaped scour cuts through this conglomerate and into Plio-Pleistocene valley-floor sediments, thereby exposing a section through the 1929 deposit. Application of the Allen defect theory suggests that the flute is unusually deep because general channel-floor erosion was inhibited by the conglomerate veneer.
Valley-floor channels typically 1 km wide and 10m deep contain series of closed depressions that occasionally deepen to 30 m. These are also interpreted as erosional scours, analogous to pools cut on the beds of bedrock rivers. The large flute was probably formed by detached flow enlarging an initial scour depression. Such scours probably play an important role in channel-floor erosion, increasing the volume of sediment transported by large turbidity currents.
This paper reports a series of 700 porosity–permeability analyses and supporting petrographic and sedimentologic descriptions from Early to Late Miocene carbonate strata cored on the Marion Plateau, offshore from north-eastern Australia, during Ocean Drilling Program (ODP) Leg 194. The samples analysed are not only mainly coarse bioclastic limestones and dolomitized equivalents from platform-top facies, but also include 79 plugs from deeper-water slope to hemipelagic drift facies. Outstanding characteristics of this data set are the wide ranges of porosity and permeability in both limestones and dolostones, the large degree of short-range heterogeneity typical of these strata, and the better porosity–permeability correlation of dolostones than limestones. The platforms have experienced widely varying calcite cementation, dolomitization and dissolution but show little clear evidence of meteoric diagenesis, suggesting that subaerial exposure may have played little role in porosity–permeability evolution. Permeability-for-given-porosity is controlled by grain size and calcite cement content in grainstones and by occurrence of larger shelter pores and vugs in mud-rich samples. Dolomitization tends to reduce the variation of permeability-for-given-porosity by recrystallizing mud matrix to form intercrystalline macroporosity that connects vugs and moulds to become integrated with the effective pore system. As a result, there are no differences in permeability–porosity trends for different dolostone textures, whether dominated by intercrystalline, vuggy, or preserved intergranular pore types. Two platform-top sites separated by only 5 km display a major lateral variation in dolostone porosity–permeability characteristics within the youngest dolostone units. This difference is interpreted as reflecting a relatively ‘windward’ (current-facing) setting of the site with the overall higher permeability-for-given-porosity (Site 1199) that led to less muddy depositional facies, greater cementation, and lesser grain dissolution. Pore-geometry parameters measured by petrographic image analysis confirm that the ‘windward’ dolostones have pores that are both larger and less intricate than dolostones comprising the more current-protected location.
Maar eruptions form small initially steep-walled basins that contain important archives for the climatic and palaeoenvironmental history in continental areas. The two Ukinrek Maars in south-western Alaska erupted between 30 March and 9 April 1977 and are the best-documented maars that have erupted in historical time. This study presents a preliminary analysis of geomorphology, hydrology, magnetic susceptibility, geochemistry and sedimentology data of a field study in August 2004. These results, photographs and topographic surveys are combined for reconstructing the post-eruptive evolution of Ukinrek East Maar. Within less than 30 years the initially polygonal shape of the crater with nearly vertical crater walls has developed into an almost elliptic form with slopes of 35° inclination on debris fans between a few escarpments. The water table and the crater floor have risen significantly and the crater diameter:depth ratio increased from 3·4 to 5·7, whereas the average height of the crater rim remained almost constant. The main sub-aerial resedimentation process is formation of rock falls, rock slumps and scree resulting in debris flows and turbidites within the lake that is ice-covered throughout about half of the year. Distal lake sediments consist of laminated minerogenic clayey-sandy silts that document frequent turbidity currents. From the linear sedimentation rate of only ca 5 mm year−1 in 17 cm long cores it may be concluded that the largest portion of the crater sediments formed within the first few months of the maar history, however, this has to be confirmed by future studies.
The 1979 Nice turbidity current is modelled using a visco-plastic analysis of flow velocity because the initial flow concentrations are expected to have been very high. The complete history of the failed sediment from debris flow to turbidity current plume is therefore addressed. The turbidity current portion is considered as a steady state flow divided into a dense bottom flow and an upper plume. Model results show that a dense flow can be generated from the debris flow by the disaggregation of the initial slide. The dense flow would be strongly erosive and able to create and maintain a low-density plume at its surface. The depth of erosion of the channel floor by the dense flow is predicted to reach 6–11 m in overconsolidated sediments, with the main erosion taking place in Var Canyon and the Upper Fan Valley. The eroded volume (108 m3) provides additional material to the sediment mass of the initial failure. The dense flow appears able to inject fine sand and silt into the overlying plume during 90 km, and would disintegrate before being able to deposit sediment. The extensive sand layer along the travel path of the turbidity current may have been deposited from the tail of the trailing plume: a result of the velocity difference between the plume and the dense flow. Observations on sedimentary structures, erosion features and distribution of the sand deposit are quite in agreement with our modelling approach. For example, gravel waves can be generated when loose deposits are reworked by the supercritical dense flow. The methodology and equations presented here provide a good estimate of the geological consequences of a high-velocity gravity flow undergoing rheological transition.
Following the eruption of Mount Pinatubo on 15 June 1991, volcanic ash was transported westward to the South China Sea in an atmospheric plume, falling out and settling to the sea floor within days and forming an up to 10 cm thick layer on an area >400 000 km2. Immediately after deposition, surviving deep-burrowing animals re-opened their connection to the sea floor to obtain water for respiration and/or food take-up. Later, small-sized meiofauna and then macrofauna re-colonized the sea floor, mixing newly deposited organic fluff with the underlying ash. Consequently, ash deposits thinner than 1 mm have not often been observed as a continuous layer when cored six years after the eruption, while ash about 2 mm thick is now patchily bioturbated. In areas covered by ash thicker than 5 mm, mixing by benthic animals is controlled mainly by the adaptation of the burrowing fauna to variations in grain-size, the rate of background sedimentation, the availability of benthic food on and within the sediment and pore water oxygen levels. With respect to these factors, four provinces can be distinguished: (i) Along the Philippines margin run-off from land fuels primary production that, in turn, leads to a high benthic food content. The benthic fauna is adapted to a variable grain-size and rapid sedimentation. Therefore, mixing is intense and the preservation potential of the ash layer is low. (ii) In areas affected by deposition of hyperpycnites and turbidites, i.e. in canyons in front of river mouths and in the Manila Trench, the ash layer is preserved due to rapid burial. (iii) The area to the west to about 116° E receives low amounts of benthic food, benthic mixing is less intense and the preservation potential of the ash is high. (iv) The central South China Sea, where the ash is thinner than 3 cm, is affected by intense wind mixing and upwelling and the benthic food content is high; thus, the chance that the ash will be preserved as a sharp-based layer is low. Consequently, the style of ash preservation has palaeo-environmental significance. Older buried and burrowed event layers provide further information to elucidate the fate of the 1991 Pinatubo ash layer; in general their appearance fits with observations in the Recent.
Two regionally significant microbial-foraminiferal episodes (∼150 kyr each) occur within the Early Aptian shallow marine platform in Oman and throughout eastern Arabia. The stratigraphically lower of these two intervals is characterized by isolated or coalescent domes that share similarities with modern, open-marine stromatolites from the Exuma Cays, Bahamas. The upper interval is predominantly built by a problematic Lithocodium/Bacinella consortium in buildup and massive boundstone facies. Based on high-resolution chemostratigraphy, these shoalwater intervals are coeval with oceanic anoxic event 1a (OAE1a; Livello Selli). Field evidence demonstrates that the buildup episodes alternate with stratigraphic intervals dominated by rudist bivalves. This biotic pattern is also recognized in other coeval Tethyan sections and is perhaps a characteristic shoalwater expression of the OAE1a. The short-lived regional expansions of this microbial-foraminiferal out-of-balance facies cannot be explained by local environmental factors (salinity and oxygen level) alone and the buildup consortia do not occupy stressed refugia in the absence of grazing metazoans. Judging from recent analogues, the main fossil groups, i.e. microbial assemblages, macroalgae, larger sessile foraminifera, and rudist bivalves, all favoured elevated trophic levels but with different tolerance limits. The implication of this is that the influence of palaeofertility events, possibly related to OAE1a, on carbonate platform community structures must be investigated. The observations made in these coastal sections are a significant first step for the improved understanding of the Early Aptian period of biotic, oceanic and climatic change.
Maar lake Laguna Potrok Aike is located north of the Strait of Magellan (south-eastern Patagonia). Seismic reflection profiles revealed a highly dynamic palaeoclimate history. Dunes were identified in the eastern part of the lake at approximately 30 to 80 m below the lake floor, overlying older lacustrine strata, and suggest that the region experienced dry conditions probably combined with strong westerly winds. It is quite likely that this can be linked to a major dust event recorded in the Antarctic ice cores during Marine Isotope Stage 4. The dunes are overlain by a series of palaeo-shorelines indicating a stepwise water-level evolution of a new lake established after this dry period, and thus a change towards wetter conditions. After the initial, rapid and stepwise lake-level rise, the basin became deeper and wider, and sediments deposited on the lake shoulder at approximately 33 m below present-day lake level point towards a long period of lake-level highstand between roughly 53·5 ka cal. bp and 30 ka cal. bp with a maximum lake level some 200 m higher than the desiccation horizon. This highstand was then followed by a regressional phase of uncertain age, although it must have happened some time between approximately 30 ka cal. bp and 6750 yrs cal. bp. Dryer conditions during the Mid-Holocene are evidenced by a dropping lake level, resulting in a basin-wide erosional unconformity on the lake shoulder. A second stepwise transgression between ca 5·8 to 5·4 ka cal. bp and ca 4·7 to 4 ka cal. bp with palaeo-shorelines deposited on the lake shoulder unconformity again indicates a change towards wetter conditions.
The 12 to 13 July 2003 andesite lava dome collapse at the Soufrière Hills volcano, Montserrat, provides the first opportunity to document comprehensively both the sub-aerial and submarine sequence of events for an eruption. Numerous pyroclastic flows entered the ocean during the collapse, depositing approximately 90% of the total material into the submarine environment. During peak collapse conditions, as the main flow penetrated the air–ocean interface, phreatic explosions were observed and a surge cloud decoupled from the main flow body to travel 2 to 3 km over the ocean surface before settling. The bulk of the flow was submerged and rapidly mixed with sea water forming a water-saturated mass flow. Efficient sorting and physical differentiation occurred within the flow before initial deposition at 500 m water depth. The coarsest components (∼60% of the total volume) were deposited proximally from a dense granular flow, while the finer components (∼40%) were efficiently elutriated into the overlying part of the flow, which evolved into a far-reaching turbidity current.
Observations of carbonate sand sheets deposited by the December 2004 Indian Ocean tsunami across island surfaces in the Maldives archipelago provide an opportunity to investigate tsunami flow behaviour across relatively simple coastal forms. This paper presents data on tsunami sand sheet deposits for five islands located along an east–west transect through the central atoll of South Baa. Sedimentary data include: field measurement of sand sheet planform, thickness and volume; descriptions of sedimentary structure and stratigraphic relationships with underlying surfaces; laboratory measurement of grain-size as a proxy of settling velocity; and observations of post-depositional changes to sand sheet characteristics two years following the Indian Ocean tsunami. Results show cross-atoll trends in volume and form of sand sheets that are interpreted as a proxy of waning and scattering of energy, as the wave train interacted with the shallow waters and numerous islands of the atoll. On individual islands, the basin topography resulted in tsunami overwash but no return flow, so that sand sheets are the product of unidirectional flow, with evidence in grain-size trends for minor acceleration across landward slopes. A key finding is the evidence for rapid post-depositional changes to the form and extent of tsunami deposits. Within two years of deposition, reworking of the seaward edge of sand sheets has destroyed the thickest part of the deposit and the thinner landward part of the sand sheets has been reworked by bioturbation. This study concludes that the preservation potential of these tsunami deposits is low to moderate, with the only probable recognizable signature being local variation in the micro-topography of island surfaces.
The internal architecture of the 2006 block-and-ash flow deposits of Merapi volcano (Java, Indonesia) was investigated using data collected from 27 stratigraphic sections measured immediately after flow emplacement, and after one and two rainy seasons of erosion. Identification of different depositional units and their longitudinal and lateral facies variations provide detailed information about: (i) the distribution, volumes and sedimentological characteristics of the different units; (ii) flow types and mobility as inferred from associated deposits; and (iii) changes in the dynamics of the different flows and their material during emplacement. Two main types of block-and-ash flows (short-runout to medium-runout block-and-ash flows and long-runout block-and-ash flows) are defined based on flow generation mechanism, flow volume, travel distance, deposit morphology, distribution, lithology and grain-size distribution. Conceptual models for the transport and depositional mechanisms of these two types of block-and-ash flows are presented. Variations in the runout distances observed for short-runout to medium-runout block-and-ash flows are linked directly to different initial flow volumes, degree of fragmentation and material properties of the moving mass during transport, with the largest and finer grained flows having the greatest mobility. Deposition occurs only over a narrow range of basal inclinations close to the angle of repose for pyroclastic material, indicating that such flows behave in a similar way to granular-free surface flows on unconfined planes. The flow mechanisms of long-runout block-and-ash flows at Merapi are interpreted to be similar, in many respects, to unsteady, cohesionless grain flows with an inertial flow regime where collisional forces largely overcome frictional forces. Flow unsteadiness causes the main body to be segmented into different pulses that run closer to each other as the flow moves downslope. Deposition occurs stepwise, with rapid aggradation of stacked sub-units from different parts of the major flow pulses. In such a model, the arrival of each flow pulse front at selected sites in the main river valley controls the generation and development of highly mobile, unconfined pyroclastic flows outside valley regions and their associated overbank deposits.
All core evidence supports the conclusion that abyssal plains are sites of graded-bed sequences deposited from turbidity flows. The deposits appear to be present-day equivalents of thin- to medium-bedded, graded, flysch-type sands and silts of many ancient geosynclines and sedimentary basins.
Euhedral, tetragonal, dipyramidal crystals of weddellite, an authigenic calcium oxalate, occur in samples from the topmost core of Deep Sea Drilling Project Site 210 in the Coral Sea Basin. Opinions in the literature differ as to whether weddellite is formed in situ or during sample storage. Comparison of smear-slides made of samples after a 4 year storage period with shipboard smear-slides made immediately after core retrieval, indicates that weddellite was formed in situ. Possible further growth during storage is not obvious. The presence of many corroded crystals, and the absence of weddellite in deeper cores, suggest that weddellite is formed soon after deposition, but dissolves again in time and with increased burial, due to changing geochemical parameters.