Showing papers in "Journal of Sedimentary Research in 1996"
TL;DR: In this article, the authors define a set of discrete classes of shoreline trajectories: accretionary and non-accretionary forced regressions, normal regression, and nonaccretional transgression.
Abstract: Shoreline migration patterns can be described in terms of the shoreline trajectory, which is the shoreline path viewed along a cross-sectional depositional-dip section. Discrete classes of shoreline trajectories can be defined: accretionary and non-accretionary forced regression; normal regression; and accretionary and non-accretionary transgression. "Accretionary" implies that sediment supplied to the shoreline participates in determining the shoreline trajectory, whereas "non-accretionary" implies that existing topography dictates the trajectory. In the latter case, translation of the shoreline takes place without significant deposition. The directions of the shoreline trajectories and the above classes provide a basis for describing variable shoreline behavior for individual shorel ne excursions, for stacked shoreline migration patterns, for systems tracts, and for depositional cycles. A non-accretionary forced regression takes place when little or no sediment is deposited at the shoreline as sea level falls. Accretionary forced regressions occur when sea-level fall is accompanied by coastal sediment accumulation. The architecture of accretionary forced regressions is controlled by the slopes of the alluvial and marine environments, and the shoreline trajectory. A regressive surface of marine erosion is most likely to be formed when the path of the shoreline converges with the fronting depositional foundation. During normal regression, the shoreline trajectory and the depositional foundation usually diverge, with accompanying deepening of water in front of the prograding shoreline. A non-accretionary transgression is defined as a transgression with a shoreline trajectory coinciding with the alluvial depositional surface at the onset of transgression. Accommodation may or may not be present at the landward side of the shoreline at the onset of transgression, but will not be generated there during transgression. An accretionary transgression takes place when the shoreline trajectory diverges relative to the alluvial depositional surface at the onset of transgression. This implies that accommodation is continuously generated and filled behind the retreating shoreline. If a ravinement surface is formed during transgression, its chronostratigraphic significance is maintained only if sediments accumulating during transgression are solely preserved seaward (and not lan ward) of the transgressing shoreline. As the shoreline migrates in various directions, composite stacking patterns, systems tracts, and depositional cycles, as well as surfaces or thin intervals of slow deposition, nondeposition, or erosion are formed. These levels can be used to envelop the depositional cycles. The cycles can be subdivided into two or four systems tracts. The maximum transgressive surface is the most applicable for delineation of cycles. A ravinement surface that merges with or erodes into the subaerial unconformity, in combination with the maximum regressive surface, can also be useful for bracketing cycles.
426 citations
TL;DR: More than 100 offshore mass-movement deposits have been studied in Holocene and Pleistocene sediments, and the processes can be divided into three main types: slides/slumps, plastic flows, and turbidity currents, of which 13 main varieties have been recognized.
Abstract: More than 100 offshore mass-movement deposits have been studied in Holocene and Pleistocene sediments. The processes can be divided into three main types: slides/slumps, plastic flows, and turbidity currents, of which 13 main varieties have been recognized. The three types are differentiated mainly by motion, architecture, and shape of failure surface. For slides, the morphology of deposits can usually be linked to a process, but for plastic flows and turbidity currents, information about the motion is mainly provided by the sedimentary record. A static classification based on these features is given, and is related to a dynamic classification system to try to underline the morphological transformation of an offshore event from initiation to deposition.
413 citations
TL;DR: The concept of high-density turbidity current refers to high-concentration, commonly nonturbulent, flows of fluids in which sediment is supported mainly by matrix strength, dispersive pressure, and buoyant lift as mentioned in this paper.
Abstract: Conventionally, turbidity currents are considered as fluidal flows in which sediment is supported by fluid turbulence, whereas debris flows are plastic flows in which sediment is supported by matrix strength, dispersive pressure, and buoyant lift. The concept of high-density turbidity current refers to high-concentration, commonly nonturbulent, flows of fluids in which sediment is supported mainly by matrix strength, dispersive pressure, and buoyant lift. The conventional wisdom that traction carpets with entrained turbulent clouds on top represent high-density turbidity currents is a misnomer because traction carpets are neither fluidal nor turbulent. Debris flows may also have entrained turbulent clouds on top. The traction carpet/debris flow and the overriding turbulent clouds are two separate entities in terms of flow rheology and sediment-support mechanism. In experimental and theoretical studies, which has linked massive sands and floating clasts to high-density turbidity currents, the term "high-density turbidity current" has actually been used for laminar flows. In alleviating this conceptual problem, sandy debris flow is suggested as a substitute for high-density turbidity current. Sandy debris flows represent a continuous spectrum of processes between cohesive and cohesionless debris flows. Commonly they are rheologically plastic. They may occur with or without entrained turbulent clouds on top. Their sediment-support mechanisms include matrix strength, dispersive pressure, and buoyant lift. They are characterized by laminar flow conditions, a moderate to high grain concentration, and a low to moderate mud content. Although flows evolve and transform during the course of transport in density-stratified flows, the preserved features in a deposit are useful to decipher only the final stages of deposition. At present, there are no established criteria to decipher transport mechanisms from the depositional record.
334 citations
Journal Article•
TL;DR: The concept of high-density turbidity current refers to high-concentration, commonly nonturbulent, flows of fluids in which sediment is supported mainly by matrix strength, dispersive pressure, and buoyant lift.
Abstract: Conventionally, turbidity currents are considered as fluidal flows in which sediment is supported by fluid turbulence, whereas debris flows are plastic flows in which sediment is supported by matrix strength, dispersive pressure, and buoyant lift. The concept of high-density turbidity current refers to high-concentration, commonly nonturbulent, flows of fluids in which sediment is supported mainly by matrix strength, dispersive pressure, and buoyant lift. The conventional wisdom that traction carpets with entrained turbulent clouds on top represent high-density turbidity currents is a misnomer because traction carpets are neither fluidal nor turbulent. Debris flows may also have entrained turbulent clouds on top. The traction carpet/debris flow and the overriding turbulent clouds are wo separate entities in terms of flow rheology and sediment-support mechanism. In experimental and theoretical studies, which has linked massive sands and floating clasts to high-density turbidity currents, the term "high-density turbidity current" has actually been used for laminar flows. In alleviating this conceptual problem, sandy debris flow is suggested as a substitute for high-density turbidity current. Sandy debris flows represent a continuous spectrum of processes between cohesive and cohesionless debris flows. Commonly they are rheologically plastic. They may occur with or without entrained turbulent clouds on top. Their sediment-support mechanisms include matrix strength, dispersive pressure, and buoyant lift. They are characterized by laminar flow conditions, a moderate to hi h grain concentration, and a low to moderate mud content. Although flows evolve and transform during the course of transport in density-stratified flows, the preserved features in a deposit are useful to decipher only the final stages of deposition. At present, there are no established criteria to decipher transport mechanisms from the depositional record.
317 citations
TL;DR: The Marinoan (Varanger equivalent 600 Ma) cap dolostone units exhibit many of the characteristics displayed by such deposits elsewhere around the globe, including fine-grained dolomitic mineralogy, lateral persistenc at basinal scales, thin laminae, graded beds, intervals with abundant marine cements, crystal fans, and a distinctive negative d13C isotopic signature of up to -5 PDB as discussed by the authors.
Abstract: Thin (d13C excursions in the geologic record, they are interpreted to record a brief postglacial anomaly in ocean chemistry. In Australia, the Marinoan (Varanger equivalent 600 Ma) cap dolostone units exhibit many of the characteristics displayed by such deposits elsewhere around the globe, including fine-grained dolomitic mineralogy, lateral persistenc at basinal scales, thin laminae, graded beds, intervals with abundant marine cements, crystal fans, and a distinctive negative d13C isotopic signature of up to -5 PDB. On the basis of these features, the Australian examples are interpreted here to be deeper-water deposits (below storm wave base) resulting from an anomalous flux of inorganic carbonate to the sea floor during postglacial transgression. Detailed isotopic analysis of Australian cap dolostones indicates d13C values ranging from -1 to -5 and gen rally becoming more depleted in 13C upsection. Trace-element data indicate some diagenetic stabilization, but textural evidence and the presence of similar profiles in different basins argue against a pervasive recrystallization event. The range in d13C values between sections as well as more negative 13C values appear to correlate with greater paleobathymetry within basins. This implies either (1) only partial preservation of the complete oceanic variation of d13C or (2) precipitation of this peculiar facies owes its genesis to basinal-specific oceanographic processes such as proximi y of a given section to a postglacial upwelling zone. In the later case, d13C values would not represent whole ocean values. Stratigraphic constraints and paleoenvironmental interpretations suggest that much of this excursion lies within the geologically brief period of postglacial transgression. This implies that the cause(s) of d13C variation likely operated on time scales significantly less than the residence time of carbon in the oceans (105 yr). The continent-wide and perhaps global nature of cap dolostones indicates that large volumes of carbonate were precipitated during the postglacial transgressive period. Such large-scale carbonate precipitation over the hypothesized short time interval of postglacial transgression must have caused (or have been the product of) profound changes in the carbon cycle and global climate at tha time. Similar evidence for transgression and increased carbonate deposition in the Holocene are attributed to the changing basin shape and pH of the oceans (the Coral Reef Hypothesis of Berger 1982). Cap dolostones may record a Proterozoic equivalent of this process with the substitution of abiotic carbonate precipitation for skeletal precipitation by reef organisms.
296 citations
TL;DR: In this article, the authors used I.C.P. Mass Spectrometry to analyze 130 samples from seven localities from Newfoundland to New York to test the suitability of using shale geochemistry as a provenance indicator of a source with an ultramafic component.
Abstract: To test the suitability of using shale geochemistry as a provenance indicator of a source with an ultramafic component, we used I.C.P. Mass Spectrometry to analyze 130 samples from seven localities from Newfoundland to New York. These samples represent mud from the foreland basin produced during the collision of the Taconic are with North America in the Ordovician. The sandstones in the flysch contain detrital chromite and other detritus that indicate that the source contained ultramafic rocks, presumably from ophiolites emplaced during collision. This setting is an ideal test case for examining the spatial and temporal variation in Cr and Ni geochemistry of shale derived from ophiolite-bearing highlands. Shale samples with high concentrations of Cr and Ni have a Cr/Ni ratio of 1.4, which is approximately the Cr/Ni ratio for ultramafic rocks ( 1.6), suggesting only minor geochemical partitioning, but sandstones have a Cr/Ni ratio of > 3.0, suggesting significant sedimentary fractionation. Analyses of samples taken upsection in a single stratigraphic section suggest that proximity to the source influences Cr and Ni concentrations. A decrease in Cr and Ni concentrations through time suggests that uplift and erosion of the thrust complex (non-ophiolitic) diluted the ultramafic signal. We attribute significant along-strike variation in Cr and Ni concentrations to the relative proportion of ultramafic rocks in the source region. This case study shows that Cr and Ni geoch mistry of shale from basin strata can be used to determine the lateral and temporal variability of ultramafic rocks in an active orogenic setting.
280 citations
TL;DR: In this paper, the authors consider a simple model whereby avulsion takes place only when a minimum, critical relief is developed between a channel bank and the adjacent flood plain, and they provide an alternative, simple geometric model that links changes in subsidence rate to downstream rate of change in stacking pattern as seen in three dimensions within sedimentary basins.
Abstract: Various, but related, models have been proposed to explain the architectural arrangement of channel stacking patterns in avulsion-dominated alluvial sequences. The early models published by Leeder, Allen, and Bridge (LAB) addressed the role of changes in sedimentation rate (a proxy for subsidence rate) as a control on stacking patterns. The models decouple avulsion frequency from sedimentation rates, resulting in an inverse relationship between stacking density (or interconnectedness) and sedimentation rates. A key element missing from these models is the likely dependence of avulsion frequency on local sedimentation rate within the active channel belt. We consider a simple model whereby avulsion takes place only when a minimum, critical, relief is developed between a channel bank and the adjacent flood plain. If avulsion frequency increases at rates slower than the increase in sedimentation rate, then stacking density increases with decreasing sedimentation rate, similar to that predicted by the LAB models. However, if avulsion frequency increases linearly with sedimentation rate, then there is no change in stacking pattern with changes in sedimentation rate. If avulsion frequency increases faster than sedimentation rates, as seen in some data sets, then stacking patterns become more dense with increasing sedimentation rates, a result that is the exact opposite of that predicted by the LAB models. Therefore sensitive dependence on the relationship between avulsion frequency and sedimentation rate calls into question the veracity of some previous interpretations of re ative subsidence made in alluvial architecture studies. We provide an alternative, simple geometric model that links changes in subsidence rate to downstream rate of change in stacking pattern as seen in three dimensions within sedimentary basins. Other controls that are considered include: the geometry of subsidence; whether avulsions take place locally along a river or regionally affect the basin; whether local sedimentation rate or flow depth controls the thickness of sand bodies; and the exact relationship between avulsion frequency and sedimentation rate. The primary result of the model is that subsidence strongly influences the rate at which alluvial architecture changes in the downstream direction, but other controls dictate whether the stacking pattern becomes more dense or less dense downstream. Hence, we suggest that subsidence e erts an influence on stacking patterns not necessarily evident in individual vertical sections, but may be recorded in three dimensions as downstream changes in alluvial architecture. Unfortunately any model of alluvial architecture in avulsion-dominated sequences is limited by our lack of understanding of the processes controlling avulsion. As a result any model of alluvial stacking patterns is at best a working hypothesis that should not be taken as proof of changes in tectonic subsidence rates or sea-level changes.
265 citations
TL;DR: In this paper, the authors studied the growth of calcite and aragonite under controlled supersaturation conditions by the counter diffusion of Ca2+ and CO32- ions through a column of silica gel.
Abstract: Morphological development of calcite crystals is related to supersaturation conditions during growth. Crystallization of calcium carbonate (calcite and aragonite) as well as Mg-calcite was studied under controlled supersaturation conditions by the counter diffusion of Ca2+ and CO32- ions through a porous transport medium (a column of silica gel). Under our experimental conditions, where ion transport is constrained to be diffusion controlled, nucleation and growth take place under conditions of high supersaturation, the actual threshold value of the supersaturation depending on the supersaturation gradient. In the pure CaCO3 system, calcite grows at lower supersaturation than aragonite. The calcite develops relatively simple rhombohedra whil the aragonite grows as spherulites. Presence of Mg2+ in the interstitial fluid inhibits nucleation, increasing the threshold supersaturation at which crystallization begins. The resulting Mg-calcite crystals show a range of morphologies depending on the Mg content and the supersaturation at the point of crystallization. At high values of supersaturation, up to 15 mol % MgCO3 is incorporated into the calcite and the crystals form spheres. At lower supersaturations, Mg content decreases and morphologies change progressively through a well-defined and reproducible sequence from spheres to dumbbell-like forms to wheat-sheaf-like bundles and eventually single crystals with steep rhombohedral faces. The crystals are compositionally zoned, showing both sector and oscillatory zoning. The compositional evol tion is related to the supersaturation and interface roughness during crystal growth.
247 citations
TL;DR: Oelkers et al. as discussed by the authors showed that mica grains have penetrated into quartz grains without being significantly deformed, and that quartz dissolution takes place at pressures less than 10 bars, i.e., at pressures only a fraction of the overburden load.
Abstract: Textural observation of quartz dissolution at mica-quartz interfaces in sandstones from the Norwegian continental shelf show that mica grains have penetrated into quartz grains without being significantly deformed. Theoretical calculations of the mechanical properties of the mica suggest that quartz dissolution takes place at pressures less than 10 bars, i.e., at pressures only a fraction of the overburden load. Cathodoluminescence and element mapping show that K-and Al-bearing material (probably illitic and/or micaceous clay) is present at all interfaces where quartz-to-quartz dissolution and interpenetration is observed. Because illitic and micaceous clay is likely to have an effect upon quartz dissolution similar to that of mica, this suggests that what has been considered to be a ressure-solution process may instead be a clay-induced dissolution process. Because quartz-to-quartz dissolution is not observed in the absence of illitic or micaceous clay, this suggests that quartz dissolution is not mainly controlled by pressure. In a quartz-mica or quartz-illite system the rate of quartz precipitation, which is strongly controlled by the temperature, may therefore be the main control on quartz cementation (Oelkers et al. 1992, and in press).
242 citations
TL;DR: In this article, the number of laminae with the local history of the site for modern stalagmites and with radioisotopic ages for Late Glacial and Holocene stalagmite was compared.
Abstract: Fifteen stalagmites from four caves and one sealed tunnel in southern Belgium are composed of alternations of annually deposited white-porous and dark-compact laminae. This is demonstrated by comparing the number of laminae with the local history of the site for modern stalagmites and with radioisotopic ages for Late Glacial and Holocene stalagmites. Annual cyclicity in the internal structure of these speleothems is explained by the highly seasonal variations of the water excess, which influences underground water flow. Comparison between climatic data and modern stalagmites of a closed tunnel shows that growth laminae can record climatic variations: (1) there is a good correlation (R = 0.84) between lamina thickness in a stalagmite and water excess; (2) during years with a high water excess, dark-compact laminae are more developed, which makes the speleothem darker. Vertical successions of several laminae represent microsequences that may have recorded climatic variations with a time resolution of 1/2 year. In a Late Glacial stalagmite, successive laminae microsequences form very regular cycles of 11 years separated by a thick darkcompact lamina. It is supposed that, as for modern stalagmites, the thick dark-compact lamina corresponds to a period of high water excess. Hence, this 11-year cycle may reflect a climatic cycle.
208 citations
TL;DR: The role of the dead organic constituents of the stromatolites is not only confirmed in micrometer-size crystal precipitation but extended to the post mortem internal mineralization of cyanobacterial filaments, and to the formation of peloids that evolve into spherulites as mentioned in this paper.
Abstract: Textures of modern lacustrine stromatolites on Kiritimati (Line Islands, Central Pacific Ocean), and of buried layers in the stromatolitic carbonate sediments from French Polynesian atoll lakes (kopara), have been studied using cryo-scanning electron microscopy (SEM equipped with a freeze-drying sample preparation device). This study confirms that microscopic three-dimensional organic networks built through reorganization of polysaccharide fibers inherited from sheaths of dead cyanobacteria, and from other extracellular polymer secretions, are common components of microbial sediments, of which they may form the framework, In addition to this role in sediment cohesion and formation of microstructure, the organic framework appears to be involved in carbonate precipitation with n the stromatolites, through chemical (nucleating), steric, and hydrodynamical controls. The role of the dead organic constituents of the stromatolites is not only confirmed in micrometer-size crystal precipitation but extended to the post mortem internal mineralization of cyanobacterial filaments, and to the formation of peloids that evolve into spherulites. Bacterial, including nannobacterial, carbonate bodies, and carbonate-impregnated cyanobacterial sheaths are also shown to form in the stromatolites studied. All these carbonate precipitation processes may cooperate in lithification of a given sediment.
TL;DR: In this article, the relative precision of the estimated percentiles to distribution shape is examined using synthetic skewed and bimodal grain size distributions, and it is shown that the absolute precision of percentile estimates varies with the grain size distribution of a particular sediment.
Abstract: Grid-by-number ("Wolman") sampling is commonly used to characterize the texture of surficial fluvial sediments. Assessments of sampling precision have concentrated on the median grain size, but adequate representation of other grain size percentiles is paramount in a number of sedimentological contexts. The field effort required to construct percentile sampling distributions by replication is prohibitive, and the absence of any general size distribution for fluvial sediments precludes theoretical calculation of percentile standard errors. Bootstrapping is used in this paper to determine percentile standard errors for large samples from two gravel bar surfaces. The improvement of percentile precision with sample size follows the theoretical expectation, hence improvement in percentile recision is achieved only at the expense of much greater sampling effort once sample sizes reach about 400 stones. However, absolute percentile standard errors differ significantly from theoretically derived values. Of particular interest is the stability of coarser percentile estimates in coarse-skewed distributions; D95 estimates may be as precise as D50 estimates for a given sample size. The sensitivity of the precision of estimated percentiles to distribution shape is examined using synthetic skewed and bimodal grain size distributions. The absolute precision of percentile estimates is shown to vary with the grain size distribution of a particular sediment. Our field results provide a reference for further work.
TL;DR: In this paper, the authors show that a relatively high-standing seamount or ocean island probably resided on the subducting oceanic plate in the early to middle Paleozoic subduction system of the Culbertson Lake allochthon, which contains the pre-Upper Devonian Quartz Hill, Toms Creek, and McMurray Lake cherts.
Abstract: The Shoo Fly Complex, the remnants of an early to middle Paleozoic subduction system, contains the Culbertson Lake allochthon. Included in the Culbertson Lake allochthon are the post-Cambrian and pre-Upper Devonian Quartz Hill, Toms Creek, and McMurray Lake cherts. Geochemical data derived from these three units plot on the Fe2O3/TiO2 versus Al2O3/(Al2O3 + Fe2O3) and Lan/Cen versus Al2O3/(Al2O3 + Fe2O3) discrimination diagrams within either the continental margin-island arc field, or that part of the pelagic field overlapping the continental margin-island arc field. These relationships a e consistent with the presence of argillaceous turbidites interstratified with radiolarite, and suggest a relatively distal continental-island arc setting where muddy turbidity currents episodically interrupted pelagic deposition. Specimens from the Quartz Hill and Toms Creek cherts with Al2O3/TiO2 values > 20 make up Group I samples, whereas specimens with Al2O3/TiO2 values < 10 are classified as Group II samples. The Al2O3/TiO2 ratios of Group II specimens suggest derivation from mafic rock. Mean Th/Sc and Th/U values, as well as LREE-enriched patterns, are consistent with this interpretation, and indicate a source area dominated by alkalic seamount or ocean island material. The Al2O3/TiO2 ratios of Group I samples indicate a source area with an average andesitic to rhyodacitic composition. Chondrite-normalized REE distribution patterns, as well as mean Th/Sc and Th/U value , support such an interpretation. Samples from the McMurray Lake chert have Al2O3/TiO2 values ranging between 19 and 28, and REE patterns characterized by LREE enrichment, negative Eu anomalies, and slightly fractionated HREE patterns. These features, along with mean Th/Sc and Th/U values, indicate a source area dominated by old upper continental crust. The location of the source area(s) supplying material to cherts and argillites of the Culbertson Lake allochthon is unknown. However, existing tectonic models commonly portray the source of old upper continental crustal material in the Shoo Fly Complex as being located somewhere along the western North American continental margin, whereas the source of island-arc debris is portrayed as being located in a fringing arc system. Our data suggest that a relatively high-standing seamount or ocean island probably resided on the subducting oceanic plate. Thus, our work supports the idea that Al, Fe, Ti, Th, Sc, and the REEs in chert and argillite deposited in proximal continental-island arc settings can be used to assess the characteristics of sources supplying particulate matter to ancient t ench systems.
TL;DR: The diagenetic growth of francolite during this type of bone fossilization implies that oxygen isotopic ratios measured in dinosaur bones, even those that appear fresh and unaltered, may dominantly reflect groundwater temperature rather than dinosaur body temperature.
Abstract: The fossil-bone mineral in dinosaurs from the Morrison Formation is well-crystallized, stoichiometric francolite. The closely packed, subparallel francolite crystals have equidimensional cross sections mostly 10-40 nm wide. In contrast, crystallites in unaltered mammal bone tissue are poorly crystallized, nonstoichiometric carbonate hydroxyapatite, a few nanometers thick, and plate-, rod- or needle-shaped. The diagenetic francolite crystals are elongated in the c axis direction, as are bone-tissue crystallites. Along permeable cracks, francolite crystals grew to 250 nm wide. Compared to unaltered crystallites in mammal bones, the francolite has higher concentrations of Ba, Ce, Cr, F, La, Mn, Ni, Pb, Rb, Sr, Th, U, V, and Y, and perhaps Zr. Na and Mg are lower in concentration. Diagene ic francolite grew from groundwater enriched in ions from silicic ashes in the Brushy Basin Member and from dissolution of bone-tissue crystallites. After burial, francolite grew on crystallite seeds, filling the space formerly occupied by collagen, perhaps half the tissue volume, and dissolved crystallites. The micron-scale structures such as Haversian canals and laminae are preserved because the diagenetic francolite retains the orientations of the seeds. The laminae, 1-10 µm thick, commonly preserve an orthogonal plywood structure. The diagenetic growth of francolite during this type of bone fossilization implies that oxygen isotopic ratios measured in dinosaur bones, even those that appear fresh and unaltered, may dominantly reflect groundwater temperature rather than dinosaur body temperature. Trace-element concentrations in the francolite may reflect groundwater composition rather than dinosaur diet. Pore-filling cements precipitated in the sands and bones at shallow burial depths, as evidenced by the 35-52% minus-cement porosities of the sandstones and numerous uncompacted oversize pores filled by cements. Gypsum and calcite were precipitated from shallow alkaline groundwater. As the groundwater became increasingly dominated by ions from silicic ash, argentite, chalcopyrite, native gold, native silver, tiemannite, and uraninite replaced areas of bone a few microns to tens of microns in size along cracks. A thin chlorite crust commonly mantled francolite crystals along bone cracks and lined openings in bones and sandstones. Pervasive precipitation of a chalcedony crust followed, with pseudomorphs of chalcedony after gypsum. In larger cancellous openings, the remaining space was fi led by the various forms of silica. The bones are brown due to goethite-filled cracks that cut pore-filling minerals.
TL;DR: In this paper, a two-axis diagram that compares fabric modality to fabric isotropy (S3/S1) is presented, which addresses the problem of eigenvectors falling between the modes of some well-developed till fabrics with low eigenvalues.
Abstract: The modal distribution of stone long-axis fabrics and their respective eigenvalues can be used to infer the genesis of subglacial till. In this paper we offer a two-axis diagram that compares fabric modality to fabric isotropy (S3/S1) and addresses the problem of eigenvectors falling between the modes of some well-developed till fabrics with low eigenvalues. Our simple five-fold scheme of modality categories includes: (1) unimodal clusters, (2) spread unimodal, (3) bimodal clusters, (4) spread bimodal, and (5) polymodal to girdle-like fabrics, and requires the analyst to study equal-area, lower-hemisphere (Schmidt) plots of the fabric data. After assigning the fabric to a morality category, isotropy is calculated and both results are plotted on the graph, whic helps to separate two main fields of subglacial till: (1) lodgement and subglacial meltout tills, and (2) deformation fill. On the basis of selected published fabrics from tills at modern glaciers, as well as our own Pleistocene till data, lodgement and subglacial meltout tills tend to have unimodal or bimodal fabrics. In contrast, deformation tills and tills that experienced multiple processes tend to have polymodal to girdle-like fabrics. Some overlap occurs between fields be. cause of the complex nature of till formation (i.e., because pure end-member till facies are rare and most tills are hybrids). We strongly recommend that Schmidt plots be visually analyzed and used in conjunction with eigenvalues when studying till. However, fabric data alone is not enough. Multiple criteria including structural, lithologic, and stone morphologic data from the till must also be considered before drawing conclusions on till genesis. Furthermore, if eigenvectors fall between fabric modes, then they cannot be used to indicate former ice movement directions. Finally, our new modality-isotropy diagram may have wider applications.
TL;DR: In this paper, the authors divided the Eocene alluvial rocks in the northern Bighorn Basin into three major kinds: fine-grained deposits on which cumulative paleosols developed, lithologically heterogeneous packages associated with immature paleosol, and major sheet sandstone bodies.
Abstract: Eocene alluvial rocks in the northern Bighorn Basin can be subdivided into three major kinds: fine-grained deposits on which cumulative paleosols developed, lithologically heterogeneous packages associated with immature paleosols, and major sheet sandstone bodies. The heterogeneous packages, which alternate vertically with the cumulative paleosols, are dominated by fine-grained deposits and also contain ribbon sandstone bodies and thin (< 1 m) sheet sandstone bodies. Most ribbon sandstones are less than 3 m thick and are more common in the middle and upper parts of the packages. Cut-and-fill deposits with fine-grained fills are locally important. Because the lithologic packages are readily apparent in the field because of the immature or simple paleosols developed on the fine-grain d facies, they are referred to as "simple paleosol packages". The major sheet sandstones were deposited by meandering trunk rivers and locally overlie and truncate the other deposits. Contrary to classical models of meandering streams, only a portion of the fine-grained rocks in the study area were deposited by overbank flooding of a trunk channel. On the basis of their stratigraphic position and few ribbon sandstones, the upper parts of some simple paleosol packages are interpreted as ancient levee deposits associated with a trunk channel. Farther from the trunk channel, overbank floods deposited alluvium on which the cumulative paleosols formed. The greater parts of the simple paleosol packages, including their fine-grained deposits, were deposited rapidly during channel avulsion. Ribbon sandstone bodies represent ancient splay channels on the avulsion belt. Simple paleosol packages in the Willwood Formation support the contention of Smith et al. (1989) that, n many meandering river systems, a significant portion of the fine-grained deposits were deposited as the trunk channel avulsed. Deposits that resemble the simple paleosol packages may provide a means of recognizing avulsion episodes in other alluvial sequences.
TL;DR: In this paper, the diagenetic evolution of clay minerals in the Rotliegend sandstone reservoir under contrasting burial histories in the Broad Fourteens basin (Dutch sector, Southern North Sea).
Abstract: This paper describes the diagenetic evolution of clay minerals in the Rotliegend sandstone reservoir under contrasting burial histories in the Broad Fourteens basin (Dutch sector, Southern North Sea). The diagenetic modifications affecting the crystal structure of clay minerals (both kaolin-group and illitic minerals) were studied using X-ray diffraction (XRD). The XRD study includes oriented and random mounts of various size fractions, numerical processing (decomposition) of XRD profiles, and simulation of one-dimensional and three-dimensional XRD patterns. Petrographic observations, differential thermal analysis, K/Ar geochronology, and geochemical considerations complement the XRD study and allow determination of the sequence of mineral crystallization and the morphological evoluti n of clay minerals and place further constraints on the absolute timing of diagenetic events and on the nature of the fluids responsible for clay-mineral crystallization. From deposition time ( 275 Ma) to the Kimmerian orogeny ( 155 Ma), crystallization of kaolinite at the expense of K-feldspars was favored by acid fluids from the underlying Carboniferous Coal Measures source rocks; kaolinite crystallization is followed by a steady kaolinite-to-dickite transformation affecting both the structure and the morphology of kaolin-group minerals. The structural characteristics of kaolin-group minerals are related to the burial history of the sediments prior to the Kimmerian orogeny. During the Kimmerian orogeny, rapid illitization of kaolin-group minerals was favored both by increased heat flow in the sedimentary pile and by widespread presence of faults, which permitted significant fluid flow probably from the Zechstein Formation. The morphological and structural characteristics of illitic minerals, i.e., illite content of illite/smectite mixed layer (I/S), ratio of illite to I/S, and three-dimensional structure of illitic minerals, do not represent the progress of a smectite-to-illite transformation, but these characteristics clearly reflect the temperature during illitization of kaolin.
TL;DR: The Castlegate Sandstone at its type section, Price Canyon, near Price, Utah, encompasses a single stratigraphic sequence spanning approximately 5 m.y. as mentioned in this paper, which includes a sandstone member corresponding to a lowstand systems tract, consisting of braided-fluvial sheet sandstones, and a mudstone member, in which shales are more abundant and some evidence of tidal influence.
Abstract: The Castlegate Sandstone at its type section, Price Canyon, near Price, Utah, encompasses a single stratigraphic sequence spanning approximately 5 m.y. It includes a sandstone member corresponding to a lowstand systems tract, consisting of braided-fluvial sheet sandstones, and a mudstone member, in which shales are more abundant and some evidence of tidal influence is present. This is the transgressive to highstand systems tract. From near Trail Canyon eastward the mudstone member passes laterally into the Sego Sandstone and Neslen Formation, a succession of at least six higher-frequency sequences of fluvial-estuarine origin. The Buck Tongue, a marine shale unit separating the Castlegate Sandstone and the Sego Sandstone east of Green River, is erosionally truncated below the Sego Sand tone northwest of Trail Canyon. We suggest that the origin of the sequences is related to flexural loading and intraplate stress on two time scales. Eustasy cannot be ruled out, but there is no independent evidence for this process. The main 5 m.y. sequence reflects regional tectonism, with the sandstone member developing at a time of slow subsidence, and the mudstone member reflecting a higher long-term subsidence rate. The higher-order sequences nested within the third-order sequence east of Trail Canyon are interpreted as a basinal response to episodes of crustal shortening on a 105 yr time scale. This study amplifies the model of Posamentier and Allen (1993a), in which ramp-type foreland basins are divided into areas of rapid and slow subsidence (Zones A and B). We postulate that these zones migrated asinward and landward in response to variations in long-term subsidence rate (an effect not predicted in the original model), and can be mapped by reference to the distribution of Type 1 sequence boundaries in the higher-order sequences. Differences in sequence architecture east and west of Trail Canyon may have been amplified by differences in crustal rheology. The sequence architecture changes at the boundary of the underlying Paleozoic Paradox Basin, a zone of NW-SE-oriented folds, faults, and salt diapirs, which we suspect were reactivated by Cretaceous tectonism. The high-frequency sequences are within the area of the Paradox Basin, an area that may have been more prone to vertical structural movements in response to intraplate stresses. Incipient uplift of Laramide structures may also have modified fluvial patterns and controlled the orientation of incised valleys on several of the sequence boundaries.
TL;DR: In this paper, the formation of shrub travertine is attributed to both abiotic and microbial processes, and the shrub carbon isotope values are between 0.5 per thousand and 6.0 per thousand larger than values for associated abiotic precipitates, and this difference is probably caused by microbial activity.
Abstract: Recent and older (early Holocene to Pleistocene) hot-spring travertine carbonates from central Italy have two distinctive macrofabrics, crystalline crusts and shrubs. Crystalline crusts are laminated slope deposits that formed abiotically following CO 2 degassing from spring water. The formation of shrub travertine--irregular, dendritic precipitates from pool environments--is controversial and has been attributed to both abiotic and microbial processes. Oxygen isotope variation in our travertines can be explained by abiotic processes, mainly CO 2 degassing. In contrast, our carbon isotope data cannot be wholly explained by abiotic CO 2 degassing invoked in earlier studies. Because photosynthesis is known to preferentially remove 12 C, leaving ambient waters enriched in 13 C, this fractionation should be recorded in delta 13 C values of microbially influenced travertine. Our shrub carbon isotope values are between 0.5 per thousand and 6.0 per thousand larger than values for associated abiotic precipitates, and this difference is probably caused by microbial activity. These isotope data support previous visual evidence that some shrub fabrics are microbially influenced.
TL;DR: In this article, electron probe microanalysis on more than 100 Late Pennsylvanian brachiopod shells from Texas, Kansas, Missouri, and New Mexico was performed to improve the ability to use minor and trace element (MTE) variation in biotic carbonates as diagenetic and paleoenvironmental indicators.
Abstract: To improve our ability to use minor and trace element (MTE) variation in biotic carbonates as diagenetic and paleoenvironmental indicators, we performed electron probe microanalysis on more than 100 Late Pennsylvanian brachiopod shells from Texas, Kansas, Missouri, and New Mexico. Texturally preserved specimens of the genera Crurithyris, Composita, and Neospirifer from all three regions were analyzed, as were Eridmatus specimens from Texas. Twenty measurements were made in two transects across each shell. Shell microstructure and cathodoluminescence were described for each spot analyzed. Three modern shells were analyzed for comparison. Diagenesis, as indicated by cathodoluminescence and/or absence of microstructure, tends to enrich shells in Fe and Mn (X/Ca >= 0.7 mmol/mol) and deplete shells in Na and S. Mg content shows no consistent trend with diagenesis. In fabric-retentive, nonluminescent shell areas, Mg, Na, and S contents vary twofold to sevenfold depending on taxonomy, microstructure, and season. Overall, taxonomy is the dominant factor controlling MTE composition. Na and S concentrations are consistently highest in Crurithyris and Eridmatus, intermediate in Neospirifer, and lowest in Composita. In taxa with mixed microstructure (Composita, Neospirifer), secondary fibrous layer calcite contains 1.5 to 2 times more Na than does interlayer prismatic calcite. Thus whole-shell Na contents of these taxa depend on the proportion of fibrous and prismatic shell. Seasonal cycles are revealed in MTE transects across growth lines. Mg, Na, and S contents commonly vary by more than a factor of two between maxima (presumably summer) and minima (winter) within the same shell. Retention of taxonomic, microstructural, and seasonal effects in shell chemistry argues for preservation of original chemistry in fabric-retentive, nonluminescent Paleozoic brachiopod shells.
TL;DR: In this article, the authors show that the transgressive system tract in the Gulf of Papua is thin and patchy and it is confined to regions within and north of an incised, east-west-trending shelf-valley system, and an observed northward offset in transgressive sediments in relation to the axis of the shelf valleys suggests that such a northward.
Abstract: The rivers that drain the wet, mountainous island of New Guinea discharge about 1.5 billion tonnes/yr of sediments into the adjacent seas, including the foreland basin between New Guinea and Australia. Despite this huge sediment input, there appears to have been only limited deposition in the Gulf of Papua during the (Holocene) postglacial rise in sea level. Seismic and core data indicate that the transgressive systems tract in the Gulf of Papua is thin and patchy. It is confined to regions within and north of an incised, east-west-trending shelf-valley system. Of the possible explanations for the absence of a significant transgressive systems tract, inland storage and along- and off-shelf transport of the sediment are of greatest significance. Reef growth up to the latitude of the east-west-trending incised-valley system in the southern Gulf of Papua is considered to have been facilitated by a northward-flowing coastal boundary current, the Coral Sea Coastal Current. This current now sweeps turbid, brackish waters and terrigenous sediments discharged by the rivers northwards away from the reefs. An observed northward offset in transgressive sediments in relation to the axis of the shelf valleys suggests that such a northward.flowing shelf current operated during the late Pleistocene and early Holocene. The northern limit of the Great Barrier Reef could thus be controlled by the balance between fluvial sediment supply and northward advection of suspended sediment by the Coral Sea Coastal Current. This current may also be imp rtant in maintaining a supply of clear water to the eastern Gulf of Papua, thus enabling photosynthesis and the flourishing of calcareous-algae (Halimeda) bioherms or biostromes at depths of up to 100 m over much of the middle and outer shelf, directly offshore of the modern Fly Delta. These carbonate sediments represent the exposed maximum flooding surface and condensed section. Modern highstand delta deposits have begun to prograde over this layer on the inner shelf.
TL;DR: In this article, the chemistry of the Cretaceous system was apparently controlled by chemosynthetic bacterial activity, resulting in high aHCO3-, that promoted precipitation of carbonates.
Abstract: Detailed petrography and geochemistry of carbonate precipitates in Cretaceous cold seep mounds from the Canadian Arctic show spectacular early diagenetic products: some still-preserved splays and isopachous layers of fine, acicular aragonite, and large botryoids and crusts of low-magnesium calcite showing unusual entanglement of former fibrous calcite and aragonite. The latter mineralogy is suggested by clear, flat-terminated cathodoluminescence patterns interpreted as ancient crystal growth steps, and the former by rhombohedral terminations. The early cement phases very likely precipitated in cold Arctic water dominated by bicarbonates derived from bacterially oxidized methane: these cements have d13C values around -44. and d18O values of 1.8 to 0.1 PDB. Coexistence of calcite and aragonite early cements in the Cretaceous seep mounds is unusual, because precipitation occurred in high-latitude, cold-water settings, and during a so-called calcite sea mode. As in modern marine hydrocarbon seeps, the chemistry of the Cretaceous system was apparently controlled by chemosynthetic bacterial activity, resulting in high aHCO3-, that promoted precipitation of carbonates. We suggest that, locally, fluctuations in aHCO3-/aSO42- resulted in oscillating aragonite or calcite supersaturation, and hence, controlled the mineralogy of the early precipitates.
TL;DR: In this article, it is suggested that the lamination reflects temporal and spatial variations in bed shear stress and lift forces acting on sediment particles in motion over the bed and related to burst/sweep cycles.
Abstract: Gravelly Gilbert-type progradational wedges 30-90 m thick developed during the Pleistocene in the Bradanic area (foredeep of southern Apennines). Facies within foresets and toesets are essentially included in two major groups: (1) massive or graded beds, and (2) beds with widespread internal lamination, including units with sheet-like geometry and planar lamination, lensing layers with broadly convex-up or wavy upper boundaries (with internal lamination ranging from planar or low-angle, boundary-unconformable, to curved, boundary-conformable), and scour-filling "backsets" (sets of upslope-dipping cross-laminae). It is suggested that the lamination reflects temporal and spatial variations in bed shear stress and lift forces acting on sediment particles in motion over the bed and related to burst/sweep cycles. The lenticular facies is inferred to be generated by migration and aggradation of long-wavelength antidunes. The backsets were probably produced by rapid upstream migration of submerged hydraulic jumps and related erosional pools, accompanied by sediment accretion on the downstream flanks of the pools. As the scours are infilled, a standing-wave condition may be established, generating wavy bedforms with long trochoidal profiles at the tops of backsets. In complete sequences a waning-flow trend is recorded by the transition from backset and standing-wave laminae to planar laminae, and the ripple cross-laminae. The presence of backsets on the foreset slope may reflect the spontaneous formation of hydraulic jumps without the need of break in slope or obstructions to flow. Although the various facies may individually be present everywhere on the foreset and toeset, with common short-distance mutual transitions suggesting high intrinsic instability of flows, a general pattern of downslope change exists, from planar sheetlike stratification and lamination to lenticular/undular units; the transition may reflect a sequence of upper-flow-regime bed phases with increasing densiometric Froude number down the foreset slope. Although typically developed on the toe, sets of backset cross-laminae are nearly ubiquitous, and their presence near the tip of the foreset slope suggests that flows responsible for sediment dispersal were commonly generated on the topset platform and were already turbulent and supercritical before spreading onto the foreset slope. The nearly ubiquitous presence of wave-worked topset gravels supports the hypothesis of the importance of storm-driven flows in promoting gravity resedimentation on the foreset slope. The source was probably a wave-dominated coastal belt with open-fetch, short-headed braid deltas highly affected by longshore drift.
TL;DR: In this article, it was shown that the silica deposition in algal cysts may provide a significant component of intrabasinal quartz sand in shale sequences, which is important to the reconstruction of the depositional history of shale sequences.
Abstract: Devonian black shales deposited on the North American craton contain abundant Tasmanites cysts. Although these are typically flattened because of compaction, a small proportion are filled with diagenetic silica. The latter are spherical to ellipsoidal (0.1-0.5 mm), and filled with chalcedony, microquartz and megaquartz, and single quartz crystals. Chalcedonic cyst fillings are preserved best in cheat and phosphate nodules, whereas megaquartz and single quartz crystals are most common in shale matrix. Together with colloform textures, this suggests that the various silica types originated from recrystallization of early diagenetic silica deposits. Thin sandstone beds in the Chattanooga Shale (e.g., Bransford Sandstone) contain abundant quartz sand that is much coarser than the detrital quartz component of the underlying black shales. Because of this, their quartz component is thought to have been transported over considerable distances from the basin margin. However, because certain shale horizons contain as much as 10% silicified cysts that upon reworking could have yielded quartz grains of fine to coarse sand size, the quartz component of these sandstone beds may actually have formed in situ. Indeed, petrographic examination of the sandstone beds shows them to contain quartz grains with morphological and textural features of "cyst" quartz (e.g., rounding, sphericity, chalcedony, pyrite inclusions, lobate grain margins). Thus, silica deposition in algal cysts may provide a significant component of intrabasinal quartz sand in shale sequences. Distinction of this type of quartz from extrabasinal detrital quartz is important to the reconstruction of the depositional history of shale sequences.
TL;DR: A sidecan-sonar image and cores from the distal reaches of a depositional lobe on the Mississippi Fan showed that channelized mass flow was the dominant mechanism for transport of silt and sand during the formation of the fan as discussed by the authors.
Abstract: SeaMARC 1A sidescan-sonar imagery and cores from the distal reaches of a depositional lobe on the Mississippi Fan show that channelized mass flow was the dominant mechanism for transport of silt and sand during the formation of this part of the fan. Sediments in these flows were rapidly deposited once outside of their confining channels. The depositional lobe is formed of a series of long, narrow sublobes composed of thin-bedded turbidites (normally graded siliciclastic sand and silt, 20 cm thick on average), debris-flow deposits (soft clay clasts up to 5 cm in diameter in a siliciclastic silt matrix, 48 cm thick on average), and background-sedimentation hemipelagic muds. The mass flows most likely originated from slope failure at the head of the Mississippi Canyon or on the outer ontinental shelf and flowed approximately 500 km to the distal reaches of the fan, with debris flow being the dominant flow type. An analysis that uses the geometry of the confining channels and strength properties of the debris-flow material shows that these thin debris flows could have traveled hundreds of kilometers on extremely small sea-floor slopes at low velocities if the flowing medium behaved as Bingham fluids and were steady-state phenomena.
TL;DR: Lower Permian sedimentary cycles of the North American Midcontinent consist predominantly of very shallow marine and paralic facies and well-developed stacked paleosol profiles as mentioned in this paper.
Abstract: Lower Permian sedimentary cycles of the North American Midcontinent consist predominantly of very shallow marine and paralic facies and well-developed stacked paleosol profiles. Although recording glacio-eustatic sea level fluctuations, these cycles also contain evidence of cyclic climate change. This evidence includes the repeated carbonate-to-clastic facies pattern observed for meter-scale cycles, and the regionally consistent change from calcic to vertic paleosols within the variegated mudstones of most cyclothems. Climates are interpreted to have fluctuated from arid or semiarid conditions to seasonally wet/dry conditions during the course of a single cyclothem. Furthermore, within the Midcontinent, drier conditions appear to have characterized times of sea-level rise and highstand, whereas wetter more seasonal conditions characterized times of sea-level fall and lowstand. This relationship is interpreted to have resulted from variations in the intensity of a Pangean monsoon generated by glacial-interglacial cycles.
TL;DR: Herringbone calcite is a previously undescribed carbonate cement and sea-floor precipitate that is common in Archean carbonates but rare in Proterozoic and Phanerozoic rocks as mentioned in this paper.
Abstract: Herringbone calcite is a previously undescribed carbonate cement and sea-floor precipitate that is common in Archean carbonates but rare in Proterozoic and Phanerozoic rocks. It is abundant in the 2520 Ma Campbellrand-Malmani platform, South Africa, where field relationships, such as erosional truncation of layers of herringbone calcite and interbedding of herringbone calcite with grainstones, demonstrate that it precipitated from ambient marine water. This interpretation is supported by depositional relationships in the >= 2.6 Ga Huntsman Limestone of the Bulawayo greenstone belt, Zimbabwe; the 2.6 Ga Carawine Dolomite, Australia; the 1.90 Ga Rocknest Formation and the 1.8-1.2 Ga Dismal Lakes Group, Canada; the Ordovician Porterfi ld carbonate buildup, Virginia; and various Silurian carbonate buildups in the Midcontinent, United States. Each of these occurrences is associated with anaerobic depositional environments or organic-rich sediments. Herringbone calcite consists of alternating light and dark crenulated bands; each light-dark pair is 0.5-1.0 mm thick. Microscopically, each pair of bands consists of a row of elongate crystals with their long axes aligned perpendicular to banding and along the growth direction of the cement. The bases of the crystals are optically unoriented, but upwards in each crystal, the optical c axis rotates until it is perpendicular to crystal elongation. The tops of the elongate crystal are thus optically aligned and length slow. The light bands of herringbone calcite correspond to the optically oriented parts of the elongate crystals, whereas the dark bands correspond to the optically unoriented, lower parts of the elongate crystals. Microspar crystals are also present in some dark ba ds. A Mg-calcite precursor for herringbone calcite, now preserved as low-Mg calcite or dolomite, is supported by the presence of micro-dolomite inclusions and textural differences between herringbone calcite and textures interpreted as neomorphosed former aragonite or low-Mg calcite. Precipitation of herringbone calcite may be consistent with a diffusionally controlled growth model involving branching growth of fibrous crystals and diffusion of a precipitation inhibitor away from the crystallization surface. Since herringbone calcite is associated with anaerobic depositional environments, the inhibitor promoting precipitation of herringbone calcite may be present only in poorly oxygenated sea water. Thus, the stratigraphic distribution of herringbone calcite may be an important indicator of the abundance of oxygen in carbonate depositional environments through time.
TL;DR: Hardbottoms associated with each of the different gently dipping Tertiary and Pleistocene depositional sequences have distinctive morphologies, ranging from smooth, flat surfaces to scarped surfaces with up to 10 m of relief as discussed by the authors.
Abstract: High-energy, sediment-starved continental shelves of the mid-Atlantic region have abundant hardbottoms that extend from the shoreface to the shelf edge. Because of the thin and irregularly distributed Holocene sand sheet, shelf morphology is determined mainly by outcropping Tertiary and Pleistocene stratigraphic units. Each unit and combination of units produces different hardbottom morphologies that depend upon the geometry and spatial relationships of the units, lithology and patterns of stratification, and subsequent weathering and erosion. Hardbottoms vary in surface relief from smooth, flat surfaces to scarped surfaces with up to 10 m of relief. The morphology ranges from sloping and stepped erosional ramps to vertical and undercut scarps with associated broad rubble ramps. Hardbottoms associated with each of the different gently dipping Tertiary depositional sequences have distinctive morphologies. Hardbottoms developed on Pleistocene units unconformably overlie the Tertiary sequences as flat-lying marine carbonates, or cut into them as channel systems backfilled with fluvial and estuarine sediments. Initial dissection of hardbottoms produced highly convoluted surfaces that resulted from subaerial weathering, stream erosion, and karst formation during sea-level lowstands. During subsequent sea-level highstands, these rimary morphologies were greatly modified through the interaction of bioerosion and storms. Understanding continental shelf hardbottoms is critical for interpreting the sedimentology and stratigraphy of the Atlantic Coastal Plain and for reconstructing paleoceanographic conditions, for the following reasons. (1) They are an extensive part of the stratigraphic record on shelves that are not actively subsiding and have small volumes of terrigenous input with low sediment accumulation rates. (2) They are important stages in the formation of major stratigraphic unconformities, condensed sections, and sequence boundaries. (3) They support diverse biological communities that produce primary carbonate sediments and are rapidly degraded and modified by bioerosion and physical processes supplying abundant "new sediment" to the continental shelf.
TL;DR: A Late Mississippian paleosol satisfying all of the morphological criteria required for classification of Holocene Vertisols provides quantitative paleoclimate information, in addition to the now commonplace interpretation of precipitation seasonality based on the presence of vertic features as discussed by the authors.
Abstract: A Late Mississippian paleosol satisfying all of the morphological criteria required for classification of Holocene Vertisols provides quantitative paleoclimate information, in addition to the now commonplace interpretation of precipitation seasonality based on the presence of vertic features. Paleoprecipitation was estimated using the empirical relationship between depth to pedogenic carbonate horizon in Quaternary soils. Burial compaction, erosional truncation, and high paleoatmospheric CO2 concentration, all factors that complicate paleoprecipitation estimates, are unusually well constrained for this paleosol. Allowing for 10% compaction, the paleosol had a pre-burial depth of 100 cm for the pedogenic carbonate horizon, yielding a mean annual paleo-precipitation estimate f 648 ± 141 mm. This is comparable to the mean annual precipitation for Brownsville, Texas, where similar soils are found today. A dolomicrite crust, developed in gilgai microlows, is well preserved in the paleo-Vertisol. Higher Late Mississippian paleotemperatures and rates of evapotranspiration associated with a lower-latitude paleogeography for central Tennessee during the Late Mississippian may explain in part why Holocene coastal Vertisols in the Brownsville region lack surficial crusts similar to that of the paleo-Vertisol. We qualitatively define the Late Mississippian climate of central Tennessee as semiarid.
TL;DR: Salda Lake in SW Turkey is a highly alkaline (pH > 9) water body with waters enriched in magnesium as mentioned in this paper, and the majority of the water entering the lake is meteoric, fed via extensive gravelly alluvial fan deltas with predominantly serpentinite pebbles.
Abstract: Salda Lake in SW Turkey is a highly alkaline (pH > 9) water body with waters enriched in magnesium. Microbial stromatolites along shorelines contain a microflora of diatoms and cyanobacteria with extensive associated biofilms. Together these are responsible for precipitation of hydromagnesite, although distribution is not congruent with either cells or films. Deposition has continued over several thousand years and is independent of both lake volume and the general concentration of lake waters. The lake is rimmed on three sides by serpentinites and on the fourth by dolomite. The bulk of the water entering the lake is meteoric, fed via extensive gravelly alluvial fan deltas (with predominantly serpentinite pebbles) whose total areas and catchments exceed that of the lake. These waters are significantly cooler than surface waters of the lake, and there is no evidence for derivation from hot springs. The magnesium is thought to have been leached from the gravels. Extensive areas of recent sediments accumulating on shorelines have apparently formed by mechanical breakdown of microbialites. Similar deposits form conspicuous terraces in two areas and point to the long-term effectiveness of these processes. Terrace deposits are modified by a variety of subaerial processes, including growth of hydromagnesite cements. No microbial structures are preserved in these, and it is presumed that they are destroyed during reworking. Fine-grained hydromagnesite sediment derived from shorelines and from terraces is suspended in the water column during storms and is probably redeposited as turbidites in the deeper parts of the lake. Sediments of this kind may be a useful analogue for some ancient and economically important stratiform magnesite deposits.