Showing papers in "Journal of Sedimentary Research in 2000"

A Process Model for the Evolution, Morphology, and Architecture of Sinuous Submarine Channels

Abstract: Although analogies have been drawn between some types of meandering rivers and medium- to high-sinuosity, aggradational, leveed submarine channels, a number of different or additional processes operate in submarine channels. Analysis of several individual submarine channels suggests that they undergo much slower bend growth than alluvial rivers and may reach a planform equilibrium, in contrast to meandering rivers, in which bends progressively migrate downstream. Sinuous leveed submarine channels should therefore aggrade to produce isolated ribbons of thalweg deposits (of predictable 3D geometry), in contrast to the stacked channel belts characteristic of most alluvial meandering rivers. A simple model of the flow structure and flow evolution of turbidity currents traversing submarine channels is proposed, based on theoretical, experimental, and field-derived concepts. It predicts that submarine channel flows are highly stratified, have significant supra-levee thicknesses, and form broad overbank bodies of low-concentration fluid moving along the entire channel length. The interaction between the broad body of overbank fluid and within-channel flow is controlled by the processes of towing and angular shear, whose possible effects on channel sedimentation and planform stability are explored.

Depositional Facies and Aqueous-Solid Geochemistry of Travertine-Depositing Hot Springs (Angel Terrace, Mammoth Hot Springs, Yellowstone National Park, U.S.A.)

Abstract: Petrographic and geochemical analyses of travertine-depositing hot springs at Angel Terrace, Mammoth Hot Springs, Yellowstone National Park, have been used to define five depositional facies along the spring drainage system. Spring waters are expelled in the vent facies at 71 to 73 degrees C and precipitate mounded travertine composed of aragonite needle botryoids. The apron and channel facies (43-72 degrees C) is floored by hollow tubes composed of aragonite needle botryoids that encrust sulfide-oxidizing Aquificales bacteria. The travertine of the pond facies (30-62 degrees C) varies in composition from aragonite needle shrubs formed at higher temperatures to ridged networks of calcite and aragonite at lower temperatures. Calcite "ice sheets", calcified bubbles, and aggregates of aragonite needles ("fuzzy dumbbells") precipitate at the air-water interface and settle to pond floors. The proximal-slope facies (28-54 degrees C), which forms the margins of terracette pools, is composed of arcuate aragonite needle shrubs that create small microterracettes on the steep slope face. Finally, the distal-slope facies (28-30 degrees C) is composed of calcite spherules and calcite "feather" crystals. Despite the presence of abundant microbial mat communities and their observed role in providing substrates for mineralization, the compositions of spring-water and travertine predominantly reflect abiotic physical and chemical processes. Vigorous CO2 degassing causes a +2 unit increase in spring water pH, as well as Rayleigh-type covariations between the concentration of dissolved inorganic carbon and corresponding delta 13C. Travertine delta 13C and delta 18O are nearly equivalent to aragonite and calcite equilibrium values calculated from spring water in the higher-temperature (approximately 50-73 degrees C) depositional facies. Conversely, travertine precipitating in the lower-temperature (< approximately 50 degrees C) depositional facies exhibits delta 13C and delta 18O values that are as much as 4% less than predicted equilibrium values. This isotopic shift may record microbial respiration as well as downstream transport of travertine crystals. Despite the production of H2S and the abundance of sulfide oxidizing microbes, preliminary delta 34S data do not uniquely define the microbial metabolic pathways present in the spring system. This suggests that the high extent of CO2 degassing and large open-system solute reservoir in these thermal systems overwhelm biological controls on travertine crystal chemistry.

Calcite Fabrics, Growth Mechanisms, and Environments of Formation in Speleothems from the Italian Alps and Southwestern Ireland

Abstract: Five fabrics were identified in Alpine and Irish caves on the basis of morphological and microstructural characteristics, and re- lated to growth mechanisms and growth environment. Columnar and fibrous fabrics grow when speleothems are continuously wet, and from fluids at near-equilibrium conditions (low supersaturation; SIcc , 0.35), through the screw dislocation mechanism. The highly defective microcrystalline fabrics form at the same supersaturation range as co- lumnar fabric but under variable discharge and the presence of growth inhibitors. Dendritic fabrics, which have the highest density of crystal defects, develop in disequilibrium conditions (high supersaturation) un- der periodic very low-flow-regime periods that result in prolonged out- gassing. Cave calcareous tufa forms in disequilibrium conditions. Only the calcite crystals of fabrics formed at low supersaturation seem to precipitate near-isotopic-equilibrium conditions.

Organogenic Dolomitization in Peritidal to Deep-Sea Sediments

Abstract: Presumed barriers to early dolomitization in normal seawater-derived pore fluids at earth-surface temperatures appear to be overcome within some anoxic, organic-rich sediments as a result of bacterial sulfate reduction and methanogenesis. These processes may promote early dolomitization, particularly during methanogenesis and late stages of sulfate reduction, by concurrently raising and sustaining high pH and high total alkalinity and CO 3 2- concentrations in pore fluids, and by simultaneously either decreasing Mg and Ca hydration or by promoting crystal surface reactions with less hydrated Mg-Ca neutral ion pairs. Volumetrically significant quantities of dolomite are associated with sulfate reduction and/or methanogenesis within peritidal, shallow-marine, and deep-sea deposits. Average concentrations of organogenic dolomite are as much as 70% in some Holocene peritidal deposits, and 28% in Mesozoic to Quaternary deep-sea sediments. Organogenic dolomites are mainly cements, and contain relatively low concentrations of Sr and Mn. Sulfate-reduction dolomites generally are Fe-deficient because of concurrent pyrite precipitation, whereas methanogenetic dolomites may be somewhat more ferroan as Fe substitutes for depleting Mg. Sources of Mg and Ca for dolomite are diffusion from overlying seawater and/or dissolution of precursor carbonate sediments. Dolomites are characterized by a wide range in δ 13 C values wherein those of sulfate reduction versus methanogenetic origin typically are 13 C-depleted and 13 C-enriched, respectively. The extent of 13 C depletion or enrichment, however, depends on the extent of organodiagenetic reactions and amount of 13 C contributed by seawater diffusion, and commonly results in overlap of δ 13 C dolomite values. The range of δ 18 O dolomite values is somewhat more restricted, and generally reflects differences in pore-fluid temperature and salinity. Some of the unresolved issues in organogenic dolomitization are: the relative efficiencies of sulfate reduction versus methanogenesis in promoting dolomitization, depths of dolomite formation inferred on the basis of δ 18 O dolomite values and probable sources of Mg and Ca, and the mode of dolomitization with progressive burial into methanogenetic zones.

Basic Types of Submarine Slope Curvature

Abstract: Curve fitting of the first-order morphology of recent slope profiles shows that with only three basic types of equations, a linear, an exponential, and a Gaussian distribution function, over 80% of a random collection of 150 seismic profiles, covering most of the world's continental margins, can be quantified. Profiles from carbonate as well as siliciclastic margins, and from mud- to sand-dominated profiles, are present in each group. A small group of submarine slopes have a linear profile, and are interpreted to rest at the angle of repose. Oversteepening of this critical angle by sedimentation disturbs the entire planar surface, starting mass movements of variable style, number, and size. The exponential trend is attributed to the exponential decay of transport capacity or competence with increasing distance from the sediment source at the shelf break. The fact that the majority of slopes follow a Gaussian curve rather than an exponential one may be due to the disturbing effect of extrinsic processes. We propose as a working hypothesis that they represent regular exponential profiles whose upper parts have been disturbed by the interplay of wave-dominated transport with gravity-driven transport at the shelfbreak during base-level fluctuations. Gaussian profiles are also observed when sediment is eroded and redistributed at the shelfbreak by ocean currents during clinoform progradation. The geometry of slopes gives information on the depositional environment, whereas the shape parameters of the three governing equations offer some promising clues to deducing sediment composition. Mud-dominated slopes have a lower slope angle, curvature of exponential profiles, and peakedness of Gaussian curves than sand-dominated slopes.

Geometry, Lithofacies, and Spatial Distribution of Cretaceous Fluvial Sandstone Bodies, San Jorge Basin, Argentina: Outcrop Analog for the Hydrocarbon-Bearing Chubut Group

Abstract: Fluvial deposits of the Cretaceous Chubut Group, San Jorge Basin, Argentina, were studied in outcrop to provide analogs for adjacent subsurface hydrocarbon-bearing strata. Outcrops were described using photomosaics and detailed sedimentological logs. Particular attention was paid to describing the geometry (e.g., width, thickness), lithofacies, and spatial distribution of sandstone bodies. Sediment accumulation rates were calculated using radiometric ages obtained from the tuffs and ignimbrites that are an important component of these strata. Interpretation of depositional environment included quantitative reconstruction of the geometry, hydraulics, and mode of migration of paleochannels. The proportion, connectedness, and spatial distribution of channel-belt sandstone bodies were interpreted using alluvial stratigraphy models. Sandstone bodies are generally meters thick and tens to hundreds of meters across (normal to paleocurrent direction). Channel-form sandstone bodies represent channel bars and fills within channel belts, whereas sandstone sheets, wedges, and lenses represent the deposits of overbank sheet floods, levees, and crevasse splays, respectively. Most of the rivers were single-channel and sinuous (sinuosity less than 1.2), but there were also braided rivers. The rivers flowed eastward and were perennial. Individual channel widths were on the order of tens of meters (mainly 35 to 65 m) and maximum channel depths were on the order of meters (mainly 2 to 6 m). The thickest and widest sandstone bodies (up to 16 m thick and in excess of 1 km wide) represent the largest channel belts or superimposed channel belts. Inasmuch as the proportion of channel-belt deposits is generally less than 0.5, most channel belts are unconnected. Channel-deposit proportion varies laterally and vertically on a 100-m scale. These variations are related to changes in the dimensions of channel belts, but they may also be related to variations in the deposition rate, floodplain width, and the timing and location of avulsions. These factors may in turn be related to intrinsic fluvial processes, tectonic tilting of the floodplain, or variations in sediment supply related to climate, tectonism, and igneous activity. Thickness and orientation of the sandstone bodies are similar to those interpreted from adjacent subsurface data. However, the width of subsurface sandstone bodies estimated from well-to-well correlation is greater than measured in outcrop. This discrepancy is because: (1) subsurface sandstone-body width less than the well spacing (typically 300 m) cannot be resolved; (2) the width of some of the subsurface sandstone bodies may be overestimated in well-to-well correlation; and (3) the full extent of the widest sandstone bodies cannot be observed in the smaller outcrops.

Evolution of Mesozoic Sandstone Compositions, Southern Junggar, Northern Tarim, and Western Turpan Basins, Northwest China: A Detrital Record of the Ancestral Tian Shan

Abstract: Sandstone compositional data can be a powerful tool in the interpretation of tectonic and climatologic influences on sedimentary basin fill, in addition to yielding important information about porosity in sandstone petroleum reservoirs. In order to explore these relationships, a modified Gazzi-Dickinson point-counting technique was used to analyze the composition of 143 Mesozoic sandstone samples from the southern Junggar, northern Tarim, and western Turpan basins of northwestern China. Results indicate that a Mesozoic, ancestral version of the Tian Shan physiographically separated the Junggar and Tarim basins and provided sand of very different composition to each basin. Mesozoic sandstone from the northern Tarim basin is diverse in composition and lithic-rich (Qm41F14L45; Qp42Lvm26Lsm32), is locally micaceous, and contains common radiolarian-chert grains and few dense accessory minerals. Inferred source rocks include upper Paleozoic alkali granite and metamorphic complexes, thick Silurian bedded-chert sequences, and lower Paleozoic strata of a passive continental margin. In contrast, sandstone from the southern Junggar and western Turpan basins is uniformly volcanic-rich (Qm21F21Lt58; Qp13Lvm68Lsm19), and contains abundant dense accessory minerals and only local radiolarian chert and mica. Inferred principal source rocks are Devonian-Carboniferous andesitic arc volcanics. The effect of sampling scale on sandstone composition outweighs that of plate-tectonic setting. Samples were derived mostly from medium- and coarse-grained fluvial systems that likely drained only portions of the ancestral Tian Shan and hence preserve local source-rock signatures, rather than an integrated compositional signal that can be directly compared to plate-tectonic petrofacies models. In addition, though Mesozoic basins of western China were most akin to broken foreland basins, Mesozoic sandstone is considerably more compositionally diverse and lithic-rich than that of modern or ancient broken foreland basins because of the variety of accreted terranes constituting the ancestral Tian Shan. Temporal changes in sandstone composition are consistent with episodes of Mesozoic deformation in the Tian Shan. Each deformational episode increased physiographic relief of the ancestral range, produced renewed downcutting and erosion of source rocks, and resulted in the deposition of compositionally very immature sandstone in adjacent basins. Although a regional early Mesozoic megamonsoon and an Early Cretaceous rain shadow cast across the northern Tarim basin are interpreted from regional facies and paleontologic data, neither paleoclimatic phenomenon appears to have significantly modified sandstone composition in the study area. Calculations of intergranular volume (% porosity + % cement) indicate that porosity in sandstone from the Tarim and Junggar basin depocenters was reduced principally by burial compaction and that the rate of porosity reduction was highest for lithic-rich samples.

Discontinuity Surfaces, Clinoforms, and Facies Architecture in a Wave-Dominated, Shoreface-Shelf Parasequence

Abstract: Detailed outcrop analysis of a wave-dominated, shoreface-shelf parasequence exposed in the Book Cliffs, Utah reveals minor stratigraphic discontinuities, which define clinoforms. Two types of discontinuity are recognized, each with a distinctive lithologic character and geometry. Nondepositional discontinuities are marked by an abrupt decrease in the thickness and amalgamation of storm-generated event beds, and are interpreted to record hiatuses in sedimentation. These discontinuities define clinoforms with a concave-upward geometry that dip gently (0.02-0.58°) over distances of 800-6000 m down depositional dip. Erosional discontinuities are marked by an abrupt increase in event-bed amalgamation, grain size, and sand content, and are interpreted as enhanced storm-wave scours. These discontinuities define more steeply dipping (0.22-0.95°), concave-upward clinoforms that extend over 100-1600 m down depositional dip and 500-1500 m along depositional strike. The distribution and amalgamation of minor stratigraphic discontinuities defines linear zones of distinctive facies architecture that are oriented parallel to the paleoshoreline trend. Using the simple assumption that the shoreface-shelf equilibrium profile remained approximately constant for each type of discontinuity throughout shoreface migration (the "Bruun rule"), intra-parasequence facies architecture can be speculatively interpreted in terms of shoreline trajectory, which reflects the balance between sediment supply and accommodation. The resulting interpretations support the notion that shoreline trajectory exerts a strong control on intra-parasequence facies architecture and preservation of the shoreface-shelf profile.

The Influence of Rock Fabric and Mineralogy on Thermochemical Sulfate Reduction: Khuff Formation, Abu Dhabi

Abstract: Thermochemical sulfate reduction (TSR) is the reaction between anhydrite and petroleum fluids at elevated temperatures to produce H2S and calcite. In this study of the dolomite-hosted hydrocarbon gas reservoirs in the Permo-Triassic Khuff Formation, Abu Dhabi, a geochemically well constrained rock-gas system, we demonstrate for the first time a clear influence of rock texture and mineralogy on the rate and extent of TSR reactions and thus on H2S concentration in the gas phase. The controls on the rate on H2S accumulation were: TSR reaction kinetics. TSR became significant as temperature exceeded 140°C, a critical threshold temperature for chemical reaction between aqueous sulfate and aqueous methane. Anhydrite dissolution rate. Once initiated, the subsequent reaction rate was controlled by the rate of supply of aqueous sulfate to the reaction site. Sulfate limitation is indicated by the lack of fractionation of sulfur isotopes between sulfate and sulfide (i.e., suggesting total reaction for each unit of sulfate that dissolves). Also, finely crystalline anhydrite began reacting at a lower temperature than coarse crystalline anhydrite (likely a result of relative surface area), suggesting that anhydrite dissolution was the rate-limiting step. Transport rates are unlikely to have been rate-limiting at the earliest stage of reaction because anhydrite was replaced in situ by calcite on the very edges of anhydrite nodules and crystals. Transport rate. Later, as TSR proceeded, it became transport controlled, as calcite, growing on the surface of anhydrite crystals, began to isolate them from dissolved methane. TSR ceased once calcite had effectively armor-plated (totally isolated) the remaining anhydrite. Finely crystalline anhydrite underwent more extensive and more rapid TSR than coarser anhydrite crystals because these had a greater ratio of surface area to volume, allowing more and faster dissolution and requiring more calcite to isolate them from methane. Localized loss of H2S occurred in the reservoir by reaction with indigenous Fe-bearing clays. Consequently, reservoirs with a relatively high siliciclastic content have less H2S than would be expected from the advanced state of anhydrite replacement by calcite. In order to predict TSR-related H2S concentration in hydrocarbon gases it is thus important to understand the diagenetic and textural characteristics of the reservoir as well as the thermal and petroleum-emplacement history.

Depositional Processes of Submarine Debris Flows in the Miocene Fan Deltas, Pohang Basin, SE Korea with Special Reference to Flow Transformation

Abstract: Subaqueous debris flows undergo various flow transformations, involving dilution and stripping of surface materials, penetration of ambient water into the flow interior, and detachment and disintegration of hydroplaning flow fronts. The surface transformation is a self-limiting process because the products of the process, such as an overriding suspended-sediment cloud or an armor of gravel at the flow front, inhibit effective working of the process. The degree of flow transformation therefore depends largely on whether a debris flow hydroplanes or not. For a subaqueous debris flow to hydroplane, its densiometric Froude number should be larger than 0.4, and the time scale of pore-pressure decay should be larger than the duration of a debris flow. In addition, a debris flow should be devoid of an extremely permeable girth of openwork gravel around the flow head because high pressures cannot be sustained underneath the gravelly material. Detailed sedimentological measurements and estimation of flow properties for three debris-flow beds in the Miocene fan deltas in SE Korea suggests that only a pebbly debris flow with a muddy (impermeable) matrix hydroplaned. On the other hand, bouldery debris flows are interpreted to have not hydroplaned irrespective of the nature of matrix. Nonhydroplaning debris flows were subject mainly to surface transformation and were outrun by surface-transformed suspended-sediment flows and debris-fall blocks after the flows entered a base-of-slope setting. Deposits of nonhydroplaning debris flows therefore overlie deposits of turbidity currents and debris falls. In the case of a hydroplaning debris flow, large chunks of debris could be detached from the flow front repetitively to form a series of small-volume flows that proceeded in front of the host debris flow. The preceding flows were promptly diluted to produce voluminous suspended-sediment clouds and were outrun by the faster-moving host debris flow. A deposit from a hydroplaning debris flow is therefore associated with thick and extensive turbiditic deposits that may either underlie or overlie the host debris-flow deposit. The turbiditic deposits associated with a hydroplaning debris flow are distinguished from those of a nonhydroplaning debris flow in that the former contain abundant gravel clasts and chunks of poorly sorted and clast-rich debris that cannot be suspended by the surface transformation process but were more likely derived from the detached fronts of a hydroplaning debris flow. These differences in sediment volume and grain size of turbiditic deposits and the stacking pattern of related debrites and turbidites provide a clue to the behavior of subaqueous debris flows.

Influence of Provenance, Weathering, and Sedimentary Processes on the Elemental Ratios of the Fine-Grained Fraction of the Bedload Sediments from the Vembanad Lake and the Adjoining Continental Shelf, Southwest Coast of India

Abstract: Geochemical studies, comprising twelve major elements and twenty-four trace elements, including the rare earth elements (REE), have been carried out on the Concentrations normalized to the average upper continental crust (UCC) show that the sediments studied are depleted in Rb, K, Ba, Ta, Hf, Na, and Ca and enriched in Cs, U, Th, REEs, Sc, Fe, Co, and Cr. UCC-normalized patterns are strikingly similar for all the three sediment types, probably because of efficient mixing of sedimentary material during weathering, transport, and deposition. The depletion of Na, K, Ca, and Ba in sediments relative to UCC could be attributed to their mobility during weathering. The depletion of Ta and Hf and enrichment of Th, REE, and Sc are probably related to provenance characteristics, and enrichment of U is attributed to oxidation-reduction processes. Shale-normalized REE values reveal steep HREE patterns [(Tbsample/Tbshale)/(Ybsample/Ybshale)] >> [(Lasample/Lashale)/(Smsample /Smshale)] and flat LREE patterns. The shale-normalized patterns for all the three types of sediments are remarkably similar, with variations in the magnitude of LREE enrichment (Lan/Ybn varies between 1.72 to 2.57 times that of PAAS). LREE enrichment and steep HREE patterns are attributed to felsic source-rock characteristics. Other geochemical characteristics such as high La/Th and low La/Sc and Th/Sc ratios compared to UCC also support a felsic provenance. Eu/Eu* values for all the three types are little more than 1, which is also a characteristic feature of Archean crust. Furthermore, data plotted on discrimination diagrams of La/Th vs. Hf and La-Th-Sc cluster closer to granite and away from basalt and komatiite and are consistent with source-rock geology. Geochemical data have also helped in ascertaining the weathering trends. The chemical index of alteration (CIA) has been used to quantify the degree of weathering. CIA values range between 81 and 94 on a scale of 40-100, indicating a high degree of alteration (fluvial sediments seem to be more altered). On an A-CN-K diagram, the data fall closer to the compositional fields of highly weathered minerals kaolinite, gibbsite, and chlorite. A high degree of weathering is also indicated by other plots such as K2O-Fe2O3-Al2O3 and log K vs. log Rb. The geochemical data are consistent with the studies on clay mineralogy (higher kaolinite) carried out by other authors on the sediments of the same area. Secondary processes seem to affect other geochemical discriminants such as Co, Cr, Ni, and U, as revealed by plots of (1) La/Sc vs. Co/Th and (2) Th-Hf-Co and Co/Th, Cr/Th ratios and makes them unsuitable for determining provenance and tectonic setting.

Factors Controlling the Holocene Avulsion History of the Rhine-Meuse Delta (The Netherlands)

Abstract: The avulsion history of the Holocene Rhine-Meuse delta (The Netherlands) was reconstructed, on a timescale of millenia, using detailed paleogeographic maps based on approximately 200,000 lithological borehole descriptions, over 1150 14C ages, and 36,000 archaeological artifacts. Gradient lines were constructed for all channel belts. These allowed determination of gradients, paleo-flow direction, relative age of channel belts, and time correlation of undated channel-belt fragments. Avulsion sites were inferred from the paleogeographic reconstruction. At least 91 avulsions occurred over the past 10,000 years, of which 82 could be dated with an accuracy of ± 200 14C years. The location and shifting of Holocene avulsion sites in the Rhine-Meuse delta in space and time are related to: (1) Relative sealevel rise. In the Early Holocene, avulsions could not take place, because rivers were still incised. Around 7500 yr BP, avulsions occurred in the western part of the present delta as a result of backfilling of the Late Weichselian valley. Between 7500 and 3700 yr BP, the zone where avulsions occurred shifted inland as a result of relative sealevel rise. (2) Neotectonics. Between 4900 and 1700 yr BP, the location of avulsion sites seems to have been influenced by neotectonic movements of the upthrown Peel Horst. Four out of six avulsion nodes in the Rhine-Meuse delta were located in the Peel Horst fault zones. (3) Increased discharge and/or within-channel sedimentation. From 2800 until about 1500 yr BP, avulsion sites were located all over the delta. During this time, the number of channels was high, and avulsion frequency reached a maximum, at a time when aggradation rate decreased with a reduction in the rate of sealevel rise. After 2000 yr BP meander wavelength of alluvial channels increased considerably. The increased meander wavelength and the high avulsion frequency are attributed to increased bankfull discharge or within-channel sedimentation (leading to channel widening), or both. (4) Human influence. Between 1000 and 650 yr BP, all the rivers were embanked, and avulsions could no longer take place. The few that occurred were induced by humans.

Carbonate Cementation in a Sequence-Stratigraphic Framework: Upper Cretaceous Sandstones, Book Cliffs, Utah-Colorado

Abstract: Three macroscopic diagenetic features can be recognized in the sandstones of the Upper Cretaceous Desert Member of the Blackhawk Formation and Castlegate Sandstone of the Mesaverde Group exposed in the Book Cliffs, Utah, each of which have distinctive form, geometry, and stratigraphic distribution. Diagenetic alterations are: (1) leached zones ("whitecaps"), up to 10 m thick, beneath coal beds; (2) large (up to 8 m) concretionary carbonate-cemented bodies in amalgamated shoreface and thin fluvial sandstones; and (3) thin (up to 2 m), laterally extensive carbonate-cemented horizons beneath major marine flooding surfaces. Each feature has distinct petrographic and geochemical signatures, and formed through discrete diagenetic processes. Large isolated carbonate-cemented bodies are composed of ferroan dolomite, most of which precipitated during early diagenesis. Field and petrographic data, coupled with stable-isotope data (early cements, 13C = -2.5 to +3.4o/oo VPDB; 18O = -7.8 to -12.0o/oo VPDB; 87Sr/86Sr = 0.7078; later cements, 13C = -3.1 to -5.7o/oo VPDB; 18O = -12.0 to -15.1o/oo VPDB; 87Sr/86Sr = 0.7093) suggest precipitation from meteoric fluids, input into sediments during times of relative sea-level fall. The source of carbonate for the dolomite cement was dissolution of detrital dolomite from beneath coals by organic acids and subsequent mobilization by meteoric fluids. Carbonate precipitation in laterally extensive cement horizons appears to have started as a result of hiatus in sediment accumulation during marine flooding events (relative sea-level rise). Cement precipitation in these horizons continued through sediment burial as a result of organic-matter oxidation reactions in overlying organic-rich mudstones. The results of this study show a link between sedimentation (related to changes in relative sea level) and diagenesis, leading to the potential for the development of process-based, predictive models of early diagenesis in depositional successions.

Calcite Moonmilk: Crystal Morphology and Environment of Formation in Caves in the Italian Alps

Abstract: Calcite moonmilk, which is a cave deposit formed of calcite crystals and water, is found in many caves in the Italian Alps. These modern and ancient deposits are formed of fiber calcite crystals, 50-500 nm wide and 1 to > 10 µm long, and polycrystalline chains that have few crystal defects. Radiocarbon dating indicates that most moonmilk deposits in these caves are fossil and that for most precipitation ceased 6400 cal years BP, at the end of the mid-Holocene Hypsithermal. In the caves of the Italian Alps, the optimal conditions for formation of calcite moonmilk are: (1) a temperature range of 3.5-5.5°C, (2) low discharge volumes of seepage waters that are slightly supersaturated (SICAL = 0.0 to 0.2), and (3) relative humidity that is at or close to 100%. Microbial activity apparently did not play an active role in the formation of the calcite moonmilk. Conditions for moonmilk formation are typically found in caves that are located beneath land surfaces, which are soil covered and support a conifer forest. Precipitation of the fiber calcite crystals apparently involved very slow flow of slightly supersaturated fluids. The fact that moonmilk appears to form under a narrow range of environmental conditions means that this cave deposit has potential as a paleoclimatic indicator in high alpine karst areas.

Quantitative Outcrop Data for Flow Simulation

Abstract: An architectural analysis documents variations in bedding geometry and rock properties within a tide-influenced deltaic sandstone exposed in the Cretaceous Frontier Formation of central Wyoming, USA. Digital maps of bedding, lithofacies, and diagenetic cements, as well as vertical logs of grain size, lithofacies, and permeability, describe rock properties that potentially influence fluid flow behavior. These records are used to construct simulation models that assess the relative importance of different types of geologic variability on prediction of subsurface fluid flow. Two 25-meter-thick tide-influenced deltaic sandstone bodies coarsen upward and contain inclined beds that reflect episodic delta-front progradation. Decimeters- to meters-thick beds within bodies alternate between cross-stratified sandstones formed during rapid flows and shales deposited during more quiescent conditions. Down depositional dip, bed-draping shales are more continuous and lithofacies within sandstone beds become finer-grained and increasingly heterolithic. As sandstone beds fine down dip, mean permeability values decrease and coefficients of variation increase, permeability values change from nearly normal distributions to highly right-skewed, and permeability values become more strongly spatially correlated. Nodular cements also affect permeability. All of these variations were modeled using stratigraphic cornerpoint grids that preserve stratal geometry and gridblocks with properties assigned using a combination of rock property maps and statistical models based on rock property logs. Simulations predict effects on fluid flow of geologic heterogeneity at different scales, the influences of process variables, and the effects of different methods of grid construction and rock property assignment. Flow simulations of water flooding through a 22 m thick by 360 m long segment of a deltaic sandstone oil reservoir predict that: (1) rapid flow through coarser-grained deposits at the top of the sandstone body tends to draw water upward; (2) thin shales draping sandstone beds shunt downdip-directed flow downward and updip-directed flow upward; (3) cement nodules cause more tortuous flow patterns but have little effect on recovery efficiency; (4) Methods of predicting intrafacies correlation of permeability have little effect on flow behavior at this scale. A simulation model constructed using a high-resolution Cartesian grid did not resolve the effects of inclined shales, demonstrating the usefulness of stratigraphic cornerpoint grids for modeling flow through complex geologic deposits. Flow simulations of tracer flow through a meter-thick cross-stratified bed within the deltaic sandstone body showed that at this scale shale drapes and models of the intrafacies distribution of permeability have statistically significant effects.

Gravity-Driven Consolidation of Granular Slurries--Implications for Debris-Flow Deposition and Deposit Characteristics

Abstract: Fresh debris-flow deposits consolidate under their own weight. How quickly they consolidate (dissipate excess pore-fluid pressure and compact) affects their resistance to remobilization as well as their sedimentologic and stratigraphic characteristics. Here, analysis of small-volume (0.05 m3) noncohesive debris-flow slurries and larger (10 m3) experimental debris-flow deposits reveals the nature, rate, and magnitude of consolidation of typical debris-flow deposits. A simple, linear, one-dimensional model describing the diffusion of excess pore-fluid pressure satisfactorily approximates the overall timing and magnitude of consolidation of noncohesive debris-flow deposits. The model and measurements of pore-fluid pressure demonstrate that changes in fluid pressure and effective stress evolve upward from the base of a deposit, and show that hydraulic diffusivities of muddy slurries containing about 5 to 50 wt% mud are remarkably similar, about 10-6-10-7 m2/s. By comparison, sandy-gravel debris-flow deposits containing <2 wt% mud have higher hydraulic diffusivities, 10-4 m2/s. Pore-fluid seepage across a permeable basal boundary accelerates consolidation response time in the lower stratum compared to that over a no-flow boundary. However, changes in sediment fabric resulting from porosity changes alter hydraulic properties of basal debris and retard expected decay of fluid pressure immediately above the bed. This result suggests that fluid infiltration to the substrate does not contribute significantly toward debris-flow deposition. Low hydraulic diffusivities promote high and persistent pore-fluid pressure in debris flows, key factors enhancing mobilization. Elsewhere, pore-fluid pressures nearly sufficient to liquefy debris have been shown to persist through transit and deposition. Here, I show that significant dissipation of such fluid pressure is restricted to postdepositional consolidation. Therefore, neither uniform decay of excess pore-fluid pressure nor intrinsic viscoplastic yield strength explain debris-flow deposition. Instead, debris-flow deposition results from friction concentrated along flow margins where high pore-fluid pressures are absent. Sustained high pore-fluid pressure following deposition fosters deposit remobilization, which can mute or obliterate stratigraphic evidence for multiple events. A thick deposit of homogeneous, poorly sorted debris can result from mingling of soft deposits and recurrent surges rather than from a single flow wave if deposit consolidation time greatly exceeds typical sediment emplacement times.

The Origin of Dolomites in Tertiary Sediments from the Margin of Great Bahama Bank

Abstract: Based on an integrated geochemical and petrographic investigation of dolomites from two cores drilled on Great Bahama Bank, we have determined three different environments of formation for the dolomites that are common throughout the Pliocene and Miocene parts of these cores. The first environment of dolomitization occurs in association with development of nondepositional surfaces. Dolomite typically forms below each of these surfaces, the concentration and extent of which is governed by the length of the period of nondeposition. These dolomites are recognized by their association with the nondepositional surfaces, characteristic positive 18O values indicative of formation from cold bottom waters, and 18O and Sr profiles with depth that suggest formation in the presence of diffusive temperature and Sr gradients. The second environment of dolomitization occurs in pore fluids where the cation and anion profiles are governed by diffusive processes. The dolomite forming here is termed background dolomite. This is a microsucrosic dolomite and forms both by recrystallization of the existing sediment and precipitation directly into void space. Dolomitization by this mechanism uses a local source of Mg2+, and consequently the dolomite never constitutes more than between 5 and 10% of the sediment. This type of dolomite is characterized by extremely high Sr concentrations, which reflect concentrations of Sr2+ in the pore fluids from which it formed. The high concentrations of Sr2+ in the pore fluids arise through the continued recrystallization of metastable aragonite and high-Mg calcite to dolomite and low-magnesium calcite driven by oxidation of organic material by sulfate. Not only does sulfate reduction provide an additional thermodynamic drive for recrystallization, but because the absolute concentration of Sr2+ in the pore fluids is governed by the solubility of celestite (SrSO4), removal of sulfate allows the Sr2+/Ca2+ ratio of the interstitial fluid to become much higher than normally encountered. The final type of environment of dolomitization is associated with coarse-grained reefal sediments. The pervasive nature of the dolomitization and the relatively normal Sr concentrations suggest the circulation of normal marine water in a relatively open system.

Microboring Versus Recrystallization: Further Insight into the Micritization Process

Abstract: SEM observations of lightly etched thin sections of Bahamian sediments reveal an unusual process of micritization that involves carbonate precipitation in microborings concurrent with endolithic activity. A coccoid cyanobacterium, tentatively identified as Solentia sp., bores tunnels, which initially penetrate just beneath grain surfaces and eventually extend throughout the entire grain. These tunnels are filled by radial fibrous aragonite, which is precipitated as the microorganism advances. Extensive multicyclic repetitions of this process result in obliteration of original grain textures with almost complete preservation of grain margins and rare empty bore holes. The rapidly filled tunnels cannot be detected by resin cast embedding techniques that are commonly used to study microboring. This type of multicyclic boring and concurrent filling of bore holes forms micritized grains that can be difficult or impossible to distinguish from micritized grains formed by recrystallization.

Facies and Evolution of the Modern Brazos Delta, Texas: Wave Versus Flood Influence

Abstract: In order to better define the facies architecture and processes controlling the evolution of wave-dominated deltas, a detailed sedimentary and geomorphologic study was undertaken on the Brazos Delta, Texas. The delta has formed since 1929, when the Brazos River was diverted by the U.S. Army Corps of Engineers. The Brazos Delta is composed primarily of fine-grained sediments. Prodelta clay constitutes more than half of the sediment volume. Thick sands are restricted to the narrow delta-front environment, whereas back-bar lagoonal clays are a significant component of the delta-plain sequence. The facies architecture is not representative of the classic strandplain model for wave-dominated deltas. This is due to the strong influence of floods on deltaic evolution. In early 1992, statewide flooding gave rise to a major constructional phase of the delta. Significant quantities of fine-grained sediments were deposited in the prodelta. One year after the onset of flooding, a channel mouth bar emerged offshore of the river mouth and enabled progradation of the delta. Similar flood events that occurred during 1941, 1957, and 1965 were recorded as ridge-trough pairs in the delta headland. The Brazos delta is therefore fluvially influenced during floods and wave influenced during intervening periods.

Delta-Fed Turbidites Infilling Topographically Complex Basins: A New Depositional Model for the Annot Sandstones, SE France

Abstract: The Eocene-Oligocene Annot Sandstones of southeast France record the predominantly deep-water, siliciclastic infill of the early French Alpine foreland basin. They accumulated in a topographically complex basin as recorded by variable onlap relationships with the underlying Globigerina Marls. Onlap configurations and paleo-bathymetric data from the western outcrops of the Annot Sandstones enables a contour map of the pre-Annot Sandstone basin-floor topography to be reconstructed. The basin comprised a northern and a southern sub-basin, which were separated by a ridge, through which a trough was cut (the Coyer Trough) linking the two sub-basins during the latter stages of infilling. Data on provenance and paleocurrents suggest that the sandstones were sourced from two main feeder systems: an eastern source from the Alps fed the northern sub-basin, and a southern source fed the southern sub-basin. There is evidence of mixing of provenance in the lower parts of the southern sub-basin. Integration of the reconstructed basin-floor topography, sediment-dispersal data, and facies and stratal architectures suggests a depositional model comprising sand-rich, delta-fed submarine ramps and/or aprons whose development was strongly influenced by the basin-floor topography. The southern sub-basin initially had a steeper delta front, characterized by debris flows; this contrasted with the lower-gradient northern sub-basin. The southern sub-basin comprises approximately 800 m of thick-bedded sandstones interpreted as the deposits of sustained and possibly ponded turbidity currents. The upper part of the southern sub-basin records increased channeling and scour related to bypass of material after the basin was filled; these channels transported material northwards through the Coyer Trough, and capped the northern sub-basin. This development of the southern sub-basin is comparable to the "fill and spill" development of perched, intra-slope basins in the Gulf of Mexico.

Paleopedology and Paleohydrology of a Volcaniclastic Paleosol Interval: Implications for Early Pleistocene Stratigraphy and Paleoclimate Record, Olduvai Gorge, Tanzania

Abstract: A cumulative red paleosol interval developed on volcaniclastic parent material under semiarid conditions in Olduvai Gorge, Tanzania. It contains a complex history of pedogenesis that was affected by: (1) episodic pyroclastic and debris-fan processes and (2) episodic expansions and contractions of an adjacent alkaline lake and the associated fluctuations in the water table. The paleosol interval is 130-320 cm thick, represents 25 ka of the 50 kyr duration of lowermost Bed II, and is defined by early Pleistocene ( 1.75 Ma) Tuffs IF and IIA. The paleosol interval records a paleocatena related to both landscape and drainage--the slope position on a pyroclastic fan relative to an alkaline lake, the proximity to freshwater wetlands, and the position of water table. Biogenic paleosol structures include grass and sedge root traces, zeolite rhizocretions, and soil fauna (termite and ant) traces. Abundant pedogenic features sensitive to soil moisture conditions, including redoximorphic mottles in the paleosol matrix, Fe oxide glaebules, grain and pore coatings, illuviated clay grain and pore coatings, and vadose siliciclastic and zeolite crystal silt, record episodic water-table fluctuations. The geochemistry of whole-rock samples distinguishes two parent materials (early low Ti/Zr, weathered volcaniclastic sediment; and late high Ti/Zr, tuffaceous sediment), which represent two distinct pedogenic phases. The Lower Paleosol developed at both sites, whereas the Upper Paleosol developed only at the upslope site. Mass-balance calculations indicate greater weathering, higher Eh and pH, and greater zeolite precipitation at the upslope site than at the downslope site. These relationships are compatible with the upslope site having had a lower overall water table and better-drained conditions than the downslope site, which had a higher water table and poorly drained conditions. The Lower Paleosol provides evidence of a fluctuating water table consistent with a wetter climate followed by a prolonged arid period. The Upper Paleosol began to form after a return to wetter conditions and ended under arid conditions. The position of the Olduvai Subchron, C2n (1.942-1.785 Ma) in Bed I, directly beneath the paleosol interval, is used to make a tentative correlation at 1.75 Ma with global climate (dust) records (wet/dry cycles).

Depositional Sequences in Deep-Shelf Environments: A Response to Sea-Level Changes and Shallow-Platform Carbonate Productivity (Oxfordian, Germany and Spain)

Abstract: Oxfordian deep-shelf deposits of southern Germany and southern Spain are characterized by marl-limestone alternations that are stacked into small-scale, medium-scale, and large-scale depositional sequences. The German sections contain autochthonous sponge reefs and associated fragments of microbialites, whereas the sections studied in Spain display tempestites composed of autochthonous deeper-water and allochthonous shallow-water particles. The facies of the German limestones and marl layers has been analyzed in detail: condensed intervals (implied by glauconitization of particles, abundant cephalopods, intense bioturbation, and generally more marls) are also enriched in sponge reefs and associated particles, and in nannofossils. Limestone-rich intervals, however, contain fewer sponges and fewer nannofossils. Neither bioerosion of sponge reefs nor nannofossil blooms can thus explain the abundance of carbonate mud that forms the limestones. Consequently, it is suggested that most of the carbonate mud is exported from shallow platform areas where carbonate productivity is high. The clay fraction was derived from weathering of massifs in the hinterland. The observed depositional sequences can be correlated between the studied sections in Germany and Spain (situated in different paleotectonic and paleoclimatic domains), and also between deeper-water sections and platform sections. This suggests that they formed through allocyclical processes. Comparision with published time scales implies that the small-scale and medium-scale sequences formed in tune with the 100 kyr and 400 kyr orbital eccentricity cycles, respectively. However, the number of marl-limestone alternations is not always consistent with the expected number of 20 kyr precessional cycles (5 per 100 kyr cycle). The large-scale sequences reflect long-term ("third order") sea-level changes. The observed marl-limestone alternations are interpreted to have formed through cyclically varying export of carbonate mud from the platform towards the deep shelf, the variations being controlled by climatically induced high-frequency sea-level fluctuations. Enhanced marl deposition on the deep shelf can be related to sea-level fall causing exposure of the shallow platform and reducing the area of carbonate production, or by rapid sea-level rise (maximum flooding) leading to partial or total drowning of the platform, and/or to retrogradation of facies belts. Enhanced carbonate deposition occurs during transgression, when large production areas are created on the platform, or during late highstands, when progradation is forced and carbonate-mud export enhanced. Depending on the long-term trend of sea-level change on which the high-frequency fluctuations are superimposed, one high-frequency (20 kyr) sea-level cycle can thus create one or two marl-limestone alternations, or only marly deposits. Consequently, one 100 kyr eccentricity cycle may be formed of a variable number of marl-limestone alternations (commonly 2 to 8). The studied deeper-water depositional sequences thus are strongly linked to the history of the adjacent shallow carbonate platforms.

Evaporitic Subtidal Stromatolites Produced by In Situ Precipitation: Textures, Facies Associations, and Temporal Significance

Abstract: The transition between carbonate platforms or isolated carbonate buildups and overlying evaporites commonly is marked by assemblages of stromatolites and interlaminated carbonates and evaporites. Stromatolites display lamination textures that vary from peloidal and discontinuous on a scale of a millimeter to a few centimeters, to isopachous and continuously laminated on a scale of a centimeter to a few meters. The isopachous lamination texture may be composed of either: (1) micritic or radial-fibrous calcite, or (2) dolomite. Isopachous stromatolitic laminae are remarkably uniform, varying little in thickness over a given lateral distance, in contrast to stromatolites formed of peloidal laminae, which show marked variation in thickness over an equivalent lateral distance. These isopachous textures are uncommon on most open-marine carbonate platforms and apparently developed in transitional carbonate-to-evaporite settings because of increasing temperature, salinity, and anoxia related to water stratification, which would have created ecologic restriction and an opportunity for stromatolite growth. Stromatolites with isopachous lamination are here interpreted to have formed as a result of in situ precipitation of sea-floor-encrusting calcite and possibly dolomite, whereas the stromatolites composed of peloidal, discontinuous lamination are inferred to have formed by trapping and binding of loose carbonate sediment in microbial mats. While the presence of microbes in almost all near-surface environments nullifies use of the term "abiotic" to describe most precipitated minerals, we interpret growth of the isopachous stromatolites to have been dominated by chemogenic precipitation in the absence of microbial mats, and the growth of peloidal stromatolites to have been controlled by sedimentation in the presence of microbial mats. These transitional stromatolite facies are best developed atop Proterozoic and Paleozoic carbonate platforms that underlie major evaporite successions. However, inspection of Jurassic and younger evaporite basins, such as the Messinian of the Mediterranean region, shows that stromatolites with thin, isopachous lamination and radial-fibrous textures, though present, are rare. Instead, these facies may have been replaced by stromatolites with peloidal, clastic textures and by low-diversity diatomaceous and coccolith mudstones. Accumulation of the mudstones would have imposed two important effects: (1) Production of coccoliths would have helped extract calcium carbonate from seawater, thus lowering the growth potential for precipitation of sea-floor-encrusting stromatolites. (2) Settling of both coccoliths and diatoms would have created a sediment flux to the sea floor, which would have served to impede growth of precipitated stromatolites because of smothering of growing crystals.

Responses of Stable Bay-Margin and Barrier-Island Systems to Holocene Sea-Level Highstands, Western Gulf of Mexico

Abstract: The microtidal, wave-dominated coast of the western Gulf of Mexico displays a variety of Holocene geomorphic features indicating higher-than-present water levels that were previously attributed to storm processes while geoidal sea level was at its present position. Field and aerial-photograph examinations of bay margins, barrier islands, and beach-ridge plains following major hurricanes show that the elevated features are inundated periodically by high storm surge. Despite their inundation, these highstand features are not modified by modern storm processes. Instead, storm-related erosion and deposition are always seaward of and lower than the highstand features and are always limited to the extant shorezone, where elevations typically are less than 1.5 m above present sea level. Bay-margin and lagoonal highstand indicators include raised marshes and subtidal flats, wave-cut benches, abandoned wave-cut scarps with fringing marshes and/or beach ridges, and accretionary islands and recurved spits. Other emergent marine features include abandoned compound flood-tidal delta and washover fan complexes attached to barrier islands and anomalously high beach ridges within both the barrier-island complexes and beach-ridge plains. The highest beach ridges, raised marshes and flats, and erosional scarps and benches are manifestations of one or more rising phases and highstands in sea level, whereas the lower marshes and accretionary topography are mainly products of the falling phases and shoreface adjustment to present sea level. Different elevations of beach-ridge sets, discordant truncation of beach ridges, and elevated marine- and brackish-water faunal assemblages preserved in beach ridges, raised marshes and flats, and natural levees are compelling evidence of sea-level fluctuations of ±1 to 1.5 m from about 5500 to 1200 cal yr BP. Independent evidence from studies of geodynamic, climatic, and glacio-eustatic processes can explain the mid-Holocene highstands and late Holocene lowering of sea level that is observed in tectonically stable coastal regions far from former centers of glaciation.

The Paleohydrology of Lower Cretaceous Seasonal Wetlands, Isle of Wight, Southern England

Abstract: The floodplain deposits of the Wealden Group (Lower Cretaceous) of the Isle of Wight, southern England, were formed in a seasonal wetland setting, a type of environment widespread today along higher-order tropical and subtropical river systems but rarely identified in the geological record The unit consists of four main lithofacies: sheet sandstones with dinosaur footprint casts; green-gray mudstones with vertebrate remains, abundant lignite, pyrite, and siderite; spectacularly color-mottled mudstones with goethite and locally pseudo-anticlines; and red mudstones with pseudo-anticlines, hematite, and carbonate nodules The sheet sandstones are interpreted as crevasse deposits; the green-gray mudstones were deposited in shallow ponds on the floodplain, which acted as sinks for debris released by local floods following wildfires; the mottled mudstones represent surface-water gley soils formed in seasonally waterlogged areas; and the red mudstones resemble present-day Vertisols that formed on topographically elevated areas only intermittently flooded These mudstones show vertical transitions from one to another, and although they could be interpreted as components of simple catenas, the absence of associated facies changes implies that topographic differences were not the only control It is proposed that these three mudstone types formed as seasonal wetland catenas, in which differences in soil drainage conditions resulted from variations in the flooding hydroperiod affecting areas with minor relief differences, rather than drainage variability simply reflecting static topographic differences Such seasonal wetland systems are rarely documented in the stratigraphic record despite being a widespread environment in present-day tropical regions, and the Wealden deposits are used to identify criteria for the recognition of this important environment in the rock record These southern English wetlands are compared with other Lower Cretaceous wetlands from northern Spain, enabling hydrological factors which controlled deposition to be recognized

Morphology and Acoustic Character of the Antarctic Wilkes Land Turbidite Systems: Ice-Sheet-Sourced Versus River-Sourced Fans

Abstract: The Wilkes Land continental slope contains an intricate network of submarine canyons that on the continental rise develop into a series of channel and overbank deposits of turbidite systems. We can define upper-fan, middle-fan, and lower-fan provinces. The Wilkes Land upper fans are characterized by large channels with relief up to 900 m, distances between levee crests up to 18 km, and channel-floor widths up to 6 km. Middle-fan channels also have high relief (∼300 m), and locally, interchannel areas exhibit mounded contourite-style deposits with high relief (up to 490 m). Within middle-fan sediment mounds there are acoustic facies of channel-overbank deposits from turbidity currents, and of sediment waves from contour-current sedimentation. The lower rise is characterized by small, shallow channels (50-75 m relief) and by interchannel areas of low relief, both characteristic of a lower-fan environment. The Wilkes Land turbidite systems show the following significant morphological differences compared to most river-sourced fans: (1) multiple large tributary channels across the upper and middle fan, (2) channel relief several times greater (900 m) than typical relief (100-200 m) for channels on fans less than 300 km in diameter, and (3) steep middle-fan and lower-fan gradients. We interpret the differences in channel network patterns, channel size, and middle-fan and lower-fan gradients between the Wilkes Land fans and other fans to result from the continental ice sheet feeding glacial ice streams that reached the outer continental shelf at times of glacial maxima. The Wilkes Land canyon-channel network patterns are comparable with the high-latitude Laurentian Fan and Labrador Sea channels. The Laurentian Fan also has large upper-fan channels with larger relief (800 m) than typical relief (200-300 m) expected for fans that are 600 km in diameter. Both the Labrador Sea and Laurentian Fan are fed by continental ice sheets at the shelf edge.

Bypass Margins, Basin-Restricted Wedges, and Platform-to-Basin Correlation, Upper Devonian, Canadian Rocky Mountains: Implications for Sequence Stratigraphy of Carbonate Platform Systems

Abstract: Carbonate platforms can commonly keep up with relative sea-level rise because of high rates of sediment accumulation and platform aggradation. Surrounding basinal environments are commonly starved but can receive variable extrabasinal siliciclastic input and episodically deposited carbonate sediment. If accumulation rates in basinal settings lag behind those of the platform, a bypass or erosional margin can develop. Under these circumstances platform and basin depositional sequences become physically detached and direct correlation of basinal and platform sequences is hindered. We report here the results of high-resolution stratigraphic analyses of two Upper Devonian isolated carbonate platforms in western Alberta that provide insight into the sequence stratigraphy of bypass margins and criteria for accurate correlation of platform and basinal sequences. The slope and basin sequences surrounding the Miette and Ancient Wall platforms consist of basin-restricted, onlapping wedges of fine-grained background sediment deposited dominantly from suspension and coarse-grained platform-derived sediment redeposited by a variety of gravity-flow mechanisms. Sequence boundaries are identified within the redeposited carbonate intervals. Identification of sequence boundaries and differentiation of highstand and lowstand slope and basinal facies was based on the geometry, mineralogy, and clast content of redeposited carbonate units. Highstand carbonates contain sheet-like debris flows and turbidites with abundant slope-derived clasts and background facies with high total carbonate content. Lowstand carbonates contain sheet-like and channelized debris flows and turbidites with abundant platform-derived clasts and background facies with low carbonate content and locally high amounts of organic carbon. Transgressive facies are dominated by initially carbonate-poor and organic-rich background sediments that display a progressive increase in carbonate content and decrease in organic carbon content. These patterns are interpreted to record abundant background carbonate sedimentation during late transgression and highstand when the carbonate factory was robust. Highstand redeposited carbonates record slope erosion due to oversteepening and slope readjustment processes. Lowstand redeposited carbonates indicate platform and platform-margin erosion and low background carbonate sedimentation when the platform was either exposed or under very shallow peritidal conditions. High siliciclastic and organic contents during lowstand and early transgression may partly be the result of reciprocal sedimentation but alternatively may represent continuous siliciclastic supply during times with little dilution by fine-grained carbonate sediment. Successive stages of platform development at Miette and Ancient Wall were controlled by accommodation changes driven by relative sea-level fluctuations. Backstripping analyses of strata from both platforms confirm that significant differential subsidence was a major control on variations in platform thickness and patterns of slope development. Greater subsidence at Ancient Wall fostered the development of a steeper bypass margin and different slope evolution compared to Miette. Slope oversteepening also initiated a process of slope readjustment that eventually reduced the platform-to-basin gradient and facilitated regressive platform progradation. In conventional siliciclastic sequence stratigraphy, basin-restricted wedges are interpreted as lowstand deposits on the basis of their geometry and position relative to an updip margin. Wedge-shaped basinal units along the Miette and Ancient Wall bypass margins contain both highstand and lowstand facies that straddle sequence boundaries. The results of this study provide objective criteria for differentiating systems tracts in carbonate slope and basin environments through mineralogic and compositional analyses providing more accurate correlation of detached platform and basin sequences. Interpretation of carbonate basin-restricted wedges as purely highstand or lowstand deposits may lead to erroneous conclusions regarding sequence stratigraphy, platform-to-basin correlation, and the volumetric partitioning of sediments deposited in different systems tracts.

Palustrine-Lacustrine and Alluvial Facies of the (Norian) Owl Rock Formation (Chinle Group), Four Corners Region, Southwestern U.S.A: Implications for Late Triassic Paleoclimate

Abstract: The Upper Triassic (Norian) Owl Rock Formation was deposited in a low-gradient floodbasin at a subtropical paleolatitude. The lower part of the formation consists predominantly of fine-grained siliciclastic lithofacies deposited by sheetflood and sinuous streams on a muddy floodplain during a period of continuous basin aggradation. Nodular calcretes are increasingly mature higher in the formation, suggesting increasingly episodic depositional conditions. The upper part of the formation consists mostly of interbedded fine-grained siliciclastic facies and laterally continuous ledges of limestone and sandstone. The predominant limestone facies has brecciated to peloidal fabrics, spar-filled circumgranular cracks, and root channeling. The subordinate limestone facies displays wavy to irregular argillaceous lamination, desiccation cracks, and oscillation ripples, and is vertically and laterally gradational with the brecciated facies. The upper Owl Rock Formation records deposition of aggrading sequences of alluvial sediments deposited during base-level rise, capped by highstand carbonates deposited in small perennial and ephemeral carbonate lakes and ponds. Base-level lowstand in an overall semiarid climate resulted in extensive pedogenesis of the limestone and laterally equivalent alluvial facies. Basin wide variations in base level are interpreted as resulting from climatic fluctuations. This depositional model is consistent with an interpreted trend towards aridification on the Colorado Plateau during the Late Triassic as Pangea drifted northward from one climate zone to another.

High-Resolution Seismic Study as a Tool for Sequence Stratigraphic Evidence of High-Frequency Sea-Level Changes: Latest Pleistocene-Holocene Example from the Korea Strait

Abstract: Sequence stratigraphic analysis of high-resolution seismic reflection profiles and sediment data reveals that the latest Pleistocene-Holocene deposits in the Korea Strait shelf off the southeastern Korean Peninsula form a high-frequency sequence consisting of a set of lowstand, transgressive, and highstand systems tracts that corresponds to a fifth-order (20 kyr) sea-level cycle. Eight depositional systems, each with different seismic facies, constitute the systems tracts. The lowstand systems tract, consisting of sandy mud, forms a deltaic wedge that pinches out updip near or at the relict shelf edge. The transgressive systems tract, consisting mainly of sands, includes six depositional systems: (1) inner-shelf transgressive layer, (2) transgressive estuarine-deltaic complex, (3) transgressive sand ridge, (4) mid-shelf transgressive layer, (5) incised-channel fill, and (6) beach-shoreface complex. Although the transgressive systems tract is widely distributed, it is much thinner than the lowstand and highstand systems tracts. The highstand systems tract is composed of a prodelta-shelf complex consisting almost exclusively of Holocene muds. The distribution and geometry of the systems tracts in the latest Pleistocene-Holocene sequence in the Korea Strait shelf is different from that of Vail's model in that: (1) the lowstand systems tract consists only of the lowstand deltaic wedge, which forms an elongated sediment body along the paleoshelf edge, and (2) the highstand systems tract is completely confined to the inner shelf and forms a nearshore belt parallel to the coastline. High-frequency (20 kyr), high-amplitude (magnitude of about 130 m) sea-level change, together with strong currents flowing northeastward along the southeastern coast of the Korean Peninsula and varying sediment supply, have resulted in this rather unusual stratigraphic architecture.

Differential Diagenesis of Rhythmic Limestone Alternations Supported by Palynological Evidence

Abstract: Alternating cemented and uncemented, fine-grained layers from Pliocene periplatform carbonates of the Great Bahama Bank have fundamentally different diagenetic features. The cemented layers consist predominantly of microspar, interpreted as an early marine, shallow-burial cement. The intercalated, uncemented, softer layers are devoid of microspar cement and exhibit signs of mechanical compac- tion. Precursor sediments of both cemented and uncemented layers apparently consisted largely of aragonite needles. The needles in the compacted layers show signs of dissolution, suggesting that the calcium carbonate required for cementation of the uncompacted layers was provided by aragonite dissolution within the compacted layers. The lack of compaction in the cemented layers shows that cementation, and hence aragonite dissolution in the adjacent compacted layers, took place in the shallow-burial realm. The dissolved carbonate was trans- ported by diffusion to adjacent layers and reprecipitated as calcite cement, thereby preventing significant compaction of these layers. These processes are not yet complete in the material examined, and some aragonite remains in the compacted layers. The sedimentary composition of the two rock types (compacted and uncompacted) is similar, indicating a similar precursor sediment for both. Between 1.5 and 7 times the concentration of palynomorphs oc- curs in the compacted layers, apparently the result of passive diage- netic enrichment. Although the trigger for diagenetic differentiation has not been determined, the pure limestone succession studied here appears to serve as a clay-free analog to limestone-marl alternations.