Showing papers in "Journal of Sedimentary Research in 2002"

Provenance and Tectonic Setting of the Proterozoic Turbidites in Hunan, South China: Geochemical Evidence

Abstract: The geochemical composition of the Middle-Upper Proterozoic turbidites in Hunan, South China is related to provenance and tectonic setting of the sedimentary basin. The turbidites are characterized by moderate SiO 2 contents (Al 2 O 3 /SiO 2 typically 0.1-0.3) and moderate K 2 O/Na 2 O ratios (generally 0.1-0.3) but relatively high Fe 2 O 3 * + MgO contents (5-10 wt %) and unusually low contents of CaO (typically < 1 wt %). Abundances of ferromagnesian trace elements and incompatible elements are moderate. The REE patterns are uniform and similar to the upper continental crust and typical post-Archean shales, with LREE enrichment, flat HREE, and significant negative Eu-anomalies. In general, the slates show higher contents of Fe 2 O 3 * + MgO and ferromagnesian trace elements, and lower ratios of La/Sc and Th/Sc than the associated graywackes, suggesting that more mafic materials were incorporated into the clay-size fraction. Compared to the Upper Proterozoic Banxi Group, the Middle Proterozoic Lengjiaxi Group has higher concentrations for most ferromagnesian trace elements and lower La/Sc ratios, implying more mafic components in the provenance of the latter. The relationship among alkali and alkaline earth elements, the Chemical Index of Alteration (CIA), and the Th/U ratios indicate that the source area was affected by a moderate weathering history. The main source was old continental crust (the Yangtze craton) dominated by felsic rocks of magmatic origin, with a variable admixture of mafic components from continental island arcs (e.g., the Fangjingshan-Dayong and the Sibao-Yiyang island arc chains). The flysch deposition took place in a back arc basin, rather than a failed intracontinental rift as proposed by some workers. We suggest that suturing between the Yangtze and Cathaysia blocks did not occur until late Proterozoic time and that the final assembly of the supercontinent Rodinia in South China was probably marked by the Jingning orogeny at ∼0.8 Ga.

High-Precision Measurements of Wetland Sediment Elevation: II. The Rod Surface Elevation Table

Abstract: A new high-precision device for measuring sediment elevation in emergent and shallow water wetland systems is described. The rod surface-elevation table (RSET) is a balanced, lightweight mechanical leveling device that attaches to both shallow ( 1 m in order to be stable. The pipe is driven to refusal but typically to a depth shallower than the rod bench mark because of greater surface resistance of the pipe. Thus, the RSET makes it possible to partition change in sediment elevation over shallower (e.g., the root zone) and deeper depths of the sediment profile than is possible with the SET. The confidence intervals for the height of an individual pin measured by two different operators with the RSET under laboratory conditions were ± 1.0 and ± 1.5 mm. Under field conditions, confidence intervals for the measured height of an individual pin ranged from ± 1.3 mm in a mangrove forest up to ± 4.3 mm in a salt marsh.

Aragonite-Calcite Relationships in Speleothems (Grotte De Clamouse, France): Environment, Fabrics, and Carbonate Geochemistry

Abstract: In Grotte de Clamouse (France), aragonite forms in a variety of crystal habits whose properties reflect the conditions of formation. Prolonged degassing and evaporation yield needle aragonite, which is more enriched in 18O and 13C than aragonite ray crystals, which form near isotopic equilibrium. At present, aragonite ray crystals form at the tops of stalagmites at very low discharge (0.00035 ml/ min), and when fluid Mg/Ca ratio is > 1.1. Temperature and evaporation do not seem to have a significant role in their formation. The presence of aragonite in stalagmites should be indicative of a decrease in drip rate related to either dry climate conditions or local hydrology. Fossil aragonite was in part replaced by calcite in a time frame 13C signal and U content), and preserved aragonite relicts (up to 16 weight %). The isotope signal of different aragonite habits may reflect conditions of formation rather than climate parameters. The real extent of aragonite-to-calcite transformation may be underestimated when replacement calcite inherits both textural and chemical properties of the precursor.

High-precision measurements of wetland sediment elevation. I. Recent improvements to the sedimentation--erosion table

Abstract: The sedimentation-erosion table (SET) developed by Boumans and Day (1993) is herein renamed the surface elevation table (SET) to better reflect the conceptual view of the processes being measured. The SET was designed for making high-resolution measurements of small-scale changes in elevation of loose, unconsolidated sediments in shallow water and mudflat habitats. The SET has undergone three major improvements to increase precision and so that it can be used to measure sediment elevation in vegetated wetlands as well as shallow water habitats. The remote-release "sliding plate" mechanism has been replaced with a single plate, collars (first 2.5 cm then 7.5 cm in length) have been attached to the plate to reduce play in the placement of the measuring pins, and the brass measuring pins have been replaced with fiberglass pins to reduce bending and consequent loss of precision. Under ideal laboratory conditions, the 95% confidence limit for individual pin measurements averaged about ± 1.4 mm (range ± 0.7 to ± 1.9 mm). These modifications have resulted in a reduction of error by about 50%.

Wave-Modified Turbidites: Combined-Flow Shoreline and Shelf Deposits, Cambrian, Antarctica

Abstract: Sandstone tempestite beds in the Starshot Formation, cen- tral Transantarctic Mountains, were deposited in a range of shoreline to shelf environments. Detailed sedimentological analysis indicates that these beds were largely deposited by wave-modified turbidity currents. These currents are types of combined flows in which storm-generated waves overprint flows driven by excess-weight forces. The interpreta- tion of the tempestites of the Starshot Formation as wave-dominated turbidites rests on multiple criteria. First, the beds are generally well graded and contain Bouma-like sequences. Like many turbidites, the soles display abundant well-developed flutes. They also contain thick divisions of climbing-ripple lamination. The lamination, however, is dominated by convex-up and sigmoidal foresets, which are geometries identical to those produced experimentally in current-dominated com- bined flows in clear water. Finally, paleocurrent data support a tur- bidity-current component of flow. Asymmetric folds in abundant con- volute bedding reflect liquefaction and gravity-driven movement and hence their orientations indicate the downslope direction at the time of deposition. The vergence direction of these folds parallels paleocur- rent readings of flute marks, combined-flow ripples, and a number of other current-generated features in the Starshot event beds, indicating that the flows were driven down slope by gravity. The wave component of flow in these beds is indicated by the presence of small- to large- scale hummocky cross-stratification and rare small two-dimensional ripples. Wave-modified turbidity currents differ from deep-sea turbidity cur- rents in that they may not be autosuspending and some proportion of the turbulence that maintains these flows comes from storm waves. Such currents are formed in modern shoreline environments by a com- bination of storm waves and downwelling sediment-laden currents. They may also be formed as a result of oceanic floods, events in which intense sediment-laden fluvial discharge creates a hyperpycnal flow. Event beds in the Starshot Formation may have formed from such a mechanism. Oceanic floods are formed in rivers of small to medium size in areas of high relief, commonly on active margins. The Starshot Formation and the coeval Douglas Conglomerate are clastic units that formed in response to uplift associated with active tectonism. Sedi- mentological and stratigraphic data suggest that coarse alluvial fans formed directly adjacent to a marine basin. The geomorphic conditions were therefore likely conducive to rapid fluvial discharge events asso- ciated with storms. The abundance of current-dominated combined- flow ripples at the tops of many Starshot beds indicates that excess- weight forces were dominant throughout deposition of many of these beds.

Depositional Evolution of Confined Turbidite Basins

Abstract: Confined turbidite basins are a common feature of many structurally complex continental slopes, but their depositional history has never been characterized using outcrop data. A synthesis of outcrop data from Tertiary Alpine basins with subsurface data from the Gulf of Mexico indicates that the progressive infill of confined turbidite basins can be characterized by four phases: (1) Flow ponding, where incoming flows are totally trapped, depositing thick, sheet-like sand-mud couplets. (2) Flow stripping, where the finer, more dilute portion of the flow is able to escape over the confining topography to be deposited elsewhere, causing increased sand/mud ratio within the basin. (3) Flow bypass, either by flows traversing over the filled basin or by switching of feeder channels away from the basin; the former resulting in incision, the latter in abandonment. (4) Blanketing, of the basin and surrounding topography due to base-level rise; this usually takes the form of meandering channel-levee complexes with low sand/mud ratios. Confined basin sequences may be stacked during the episodic growth of the confining topography to a basin, and may appear similar to sea-level-induced depositional sequences.

Turbulence Modulation in Clay-Rich Sediment-Laden Flows and Some Implications for Sediment Deposition

Abstract: Clay-rich flows are common in many sedimentary environments, ranging from rivers with high suspended sediment concentrations and fluid muds on the continental shelves to deep-ocean density currents. Such sediment-laden flows have fluid-dynamic characteristics which are radically different from their clear-water counterparts. Laboratory experiments demonstrate that an increasing concentration of kaolinite results in a distinct change in velocity and turbulence structure in flows transitional between Newtonian and non-Newtonian behavior. Such transitional flows develop a lower region of reduced velocity that is separated from the overlying flow by a distinct shear layer. Large-scale Kelvin-Helmholtz instabilities are generated along this shear layer and dominate both turbulence production and fluid mixing. These experimental results are used to interpret the formation of parallel laminae in turbiditic muds and propose a model for sediment sorting in a range of clay-rich flows. Non-Newtonian flows with low concentrations of kaolinite (∼ 4% by volume) are devoid of both turbulent and cohesive strength at the shear velocities investigated. Deposition of coarser, noncohesive particles in such flows would be expected to be controlled entirely by differences in settling velocity, and the deposits would therefore be expected to be graded and massive.

Origin and Significance of Isotope Shifts in Pennsylvanian Carbonates (Asturias, NW Spain)

Abstract: The primary variability of the composition and properties of seawater is much greater in the shallow coastal zones than in the main body of ocean water. An inadequate understanding of this variability, as well as different diagenetic environments, severely limit the interpretation of the stable-isotope record of shoalwater carbonates. In order to investigate this primary and diagenetic variability along a Bashkirian-Moscovian platform-to-basin transect, 13C and 18O analyses have been performed on more than 1000 matrix micrite, carbonate cement, and brachiopod shell samples. In isotope analysis, these different carbonate materials tend to complement each other, inasmuch as they have different advantages and shortcomings. The resulting data reveal spatial trends in 13C and 18O signatures from platform top (lower values) to basin (higher values). In the case of 13C from pristine brachiopods, this trend can be explained by the long residence time (aging) of platform-top water masses. In the case of brachiopod 18O, this variance is interpreted to reflect temperature differences between warm surface and colder bottom water separated by a permanent thermocline at about 150 to 200 m beneath the shelf break. Micrite and marine cement isotopic values from the platform interior were reset (lowered) during pervasive early meteoric diagenesis. In contrast, micrite and marine cement isotopic values from the outer platform, slope, and basin show higher values close to the assumed Pennsylvanian seawater isotopic composition. This implies that isotopic data from shoalwater carbonates (including pristine brachiopod shells) might not necessarily reflect paleoceanographic trends of the open-ocean water masses because of changes in coastal water-mass isotope signature and interaction with early meteoric fluids.

Origin and Timing of the Miocene-Pliocene Unconformity in Southeast Australia

Abstract: An unconformity is present close to the Miocene-Pliocene boundary in the onshore and nearshore portions of the Otway, Port Phillip-Torquay, and Gippsland basins of southeast Australia. The unconformity is angular (generally < 1-5° angularity), with the underlying Miocene units having been deformed (gentle folding and reverse faulting) and eroded prior to deposition of the Pliocene succession. The unconformity also marks a change from Oligocene-Miocene deposition of cool-water carbonate sediments and brown coal-bearing successions to the accumulation of more siliciclastic-rich sediments in Pliocene time. The Miocene-Pliocene boundary therefore represents an interval of significant regional uplift in the southeast Australian basins. The amount of section removed is greatest around the Otway and Strzelecki ranges in Victoria, where up to a kilometer of section may have been removed. In most onshore sections of the Victorian basins hundreds of meters of section have been eroded. In distal offshore locations the boundary becomes conformable. The timing of uplift and erosion is best constrained in the Otway and Port Phillip basins, where late Miocene (N16 ∼ 10 Ma) sediments underlie the unconformity and earliest Pliocene (∼ 5 Ma) sediments overlie it. This timing coincides with a change in the dynamics of the Australian plate, beginning at around 12 Ma. Southeast Australia is currently under a NW-SE compressional regime, and this has probably persisted since the late Miocene. In the basins (as opposed to the basement-dominated highland areas), the late Miocene uplift event is more significant than later Pliocene-Recent uplift.

Evolution of the Coastal Depositional Systems of the Changjiang (Yangtze) River in Response to Late Pleistocene-Holocene Sea-Level Changes

Abstract: The paleo-Changjiang (Yangtze) incised-valley fills, approximately 80-90 m thick, provide an opportunity to document the evolution of coastal depositional systems with large sediment supply in response to late Pleistocene-Holocene sea-level fluctuations on time scales of 103 to 104 years. The sedimentary facies of the incised-valley fills record three main depositional systems: fluvial, tide-dominated estuary, and tide-dominated delta. Radiocarbon ages for the incised-valley fills suggest that these depositional systems developed before about 11 ka, between 11 and 8 ka, and after approximately 8 ka, respectively. By applying sequence-stratigraphic concepts, the evolution of the depositional systems can be divided into three systems tracts--a lowstand systems tract (LST), a transgressive systems tract (TST), and a highstand systems tract (HST). Sea-level changes on a 104-year time scale controlled the basic architecture of the sequence of the incised-valley fill. On the other hand, sea-level changes on a millennial time scale affected the stacking pattern of the systems tracts. In particular, the continuous sea-level rise with episodic rapid rises during the last deglaciation affected the stacking pattern of the TST, which is characterized by a combination of aggradation and backstepping. The aggradation of fluvial and estuarine systems was dominant and the shoreline migrated only gradually landward under the relatively slow rise in sea level, and a very rapid sea-level rise around 12 and 10 ka caused the system to migrate abruptly landward. Unlike the transgressive estuarine phase, the stacking pattern of the regressive tide-dominated delta (HST), which developed within the almost filled incised valley and on the surrounding interfluve zones, was characterized by seaward progradation with clinoform architecture. It was initiated with aggradational and progradational stacking about 8 ka during the last phase of decelerated sea-level rise, and was followed by a progradational phase after the highest sea level about 6 ka.

Classification of Australian Clastic Coastal Depositional Environments Based Upon a Quantitative Analysis of Wave, Tidal, and River Power

Abstract: A statistical assessment of wave, tide, and river power was carried out using a database of 721 Australian clastic coastal depositional environments to test whether their geomorphology could be predicted from numerical values. The geomorphic classification of each environment (wave- and tide-dominated deltas, wave- and tide-dominated estuaries, lagoons, strand plains, and tidal flats) was established independently from remotely sensed imagery. To our knowledge, such a systematic numerical analysis has not been previously attempted for any region on earth. The results of our analysis indicate that a relationship exists between the ratio of annual mean wave power to mean tidal power and the geomorphic development of clastic coastal depositional environments. Deltas and estuaries are associated with statistically significant differences in mean wave and tidal power. Statistically significant distinctions between populations of deltas, estuaries, strand plains, and tidal flats are also associated with river discharge and river flow rate (defined as discharge divided by open water area). Our results support the hypotheses of previous workers that wave, tide, and river power exert the principal control over the gross geomorphology and facies distribution patterns in clastic coastal depositional environments. Mean values and confidence limits of wave power, tide power, and river flow for the coastal depositional environments targeted in this study may provide a basis for the comparison of modern environments as well as constraints for paleo-reconstructions.

Process-Response Modeling of Wave-Dominated Coastal Systems: Simulating Evolution and Stratigraphy on Geological Timescales

Abstract: Numerical modeling on a geological timescale is a rapidly expanding tool to investigate controls on formation of the stratigraphic record. Modeling enables us to test existing ideas, but verification of model results is commonly difficult. Many models are based on geometric or diffusion rules, yet neither type of model has much relevance with actual processes that control sedimentary systems. Here we describe a process-response approach to model the evolution and stratigraphy of wave-dominated coastal systems in two dimensions, based on simple approximations of cross-shore erosion and sedimentation. Separating erosion and deposition functions enables us to simulate coastal evolution, stratigraphy, erosion surfaces, and transport of multiple-grain-size classes. The simulated stratigraphic record contains detailed information on grain size and stratal geometry. We calibrated the model with data sets on coastal transgression in the Caspian Sea, Dagestan, and on grain-size distributions at the island of Terschelling, The Netherlands. Furthermore, hypothetical examples are presented to show the effect of changes in sea level and sediment supply, substrate slope, and sediment size distribution. These tests show that the model is capable of reproducing widely accepted conceptual models of coastal evolution on geological timescales (progradation, aggradation, and various modes of retrogradation).

Ichnology and sedimentology reveal depositional characteristics of bay-margin parasequences in the Miocene Amazonian foreland basin

Abstract: Proposed depositional models for Miocene Amazon foreland basin strata (Pebas Formation, Peru) are controversial. Recent depositional models include lacustrine and tidally influenced, brackish-water embayment. This paper presents data that support tidally influenced, brackish-water deposition for at least part of Pebas time (10-14 Ma). Two parasequences are presented (Santa Julia and Tamshiyacu). Both crop out along the Amazon River in Upper Amazonia near Iquitos, Peru. At these locations, abundant evidence of brackish-water, tidally influenced deposition is documented, including marginal marine bioturbation, sedimentary couplets, semidiurnal couplets (preserved in burrows), and pinstripe lamination. The deposits are locally highly bioturbated. At both locations ichnogenera normally associated with marine to brackish-water depositional environments are common. Three normally marginal-marine ichnofabrics are reported: (1) a Chondrites-reburrowed, Planolites ichnofabric resident only in massive-appearing muds; (2) a Scolicia (Laminites), Thalassinoides, Ophiomorpha ichnofabric that is manifested as intensely bioturbated silty sands; and (3) a Thalassinoides-generated ichnofabric that is interpreted to have descended into consolidated substrates and is thus representative of the Glossifungites Ichnofacies. Several trace fossils contain laminated infills organized into distinct sedimentary couplets that are best interpreted as resulting from semidiurnal processes. Six conclusions are arrived at: (1) sedimentological and ichnological data consistently indicate that sediment accumulation dominantly occurred in sporadically dysaerobic, marine to brackish water, under a tidal influence; (2) sediment accumulation occurred in bay-margin environments that prograded into a shallow, quiescent bay; (3) a stratified water column is indicated by the ichnofauna; (4) low accommodation space, repetitive and rapid adjustments of relative sea level, shallow wave base, and a stratified water column combined to generate an atypical parasequence architecture; (5) previously published isotopic data are consistent with sediment accumulation in brackish to marine water; and (6) marine incursion into Amazonia occurred during the Middle Miocene.

Basin-Scale Changes in Floodplain Paleosols: Implications for Interpreting Alluvial Architecture

Abstract: Floodplain paleosols in time-equivalent strata of the Willwood For mation differ in terms of paleodrainage and degree of pedogenic development. Ancient soils were more poorly drained and more strongly developed in one area compared to a second area about 45 km away. This difference in drainage is attributed to parent material. The more poorly drained paleosols formed in an area with less permeable, clay-rich floodplain deposits. In the stratigraphic interval that accumulated more slowly, paleosols are more strongly developed, less numerous, and thicker than in the more rapidly accumulating stratigraphic interval. These differences are attributed not only to different sediment accumulation rates, which are commonly considered to control the degree of ped ogenic development, but also to differences in avulsion frequency. A simple model is offered to show how paleosols change with varying basin subsidence rates and the relationship between avulsion frequency and accumulation rate. When avulsion frequency increases more rapidly than sediment accumulation rate, an increase in subsidence rate causes strongly developed paleosols to be overlain by a stratigraphic interval with paleosols that are less well developed, thinner, and more densely spaced. When avulsion frequency is not related to accumulation rate, as early models of alluvial architecture assumed, an increase in subsidence rate produces a stratigraphic section in which strongly developed paleosols are overlain by paleosols that are less well developed, thicker, and more widely spaced. Study of the thickness and degree of development of floodplain paleosols can be used together with more traditional studies of alluvial architecture to provide a clearer understanding of the factors that influenced the stratigraphic architecture in a particular alluvial basin.

Glacially Driven Cycles in Accumulation Space and Sequence Stratigraphy of a Stream-Dominated Alluvial Fan, San Joaquin Valley, California, U.S.A.

Abstract: High-resolution sequence stratigraphy provides a framework to interpret unconformity-bounded depositional sequences in the stream-dominated Kings River alluvial fan, located near Fresno, California. Depositional units in the fan are analogous to systems tracts described from marine deposits. Fan sequences reflect changes in accumulation space (Blum and Tornqvist 2000) associated with Pleistocene glacial cycles in the Sierra Nevada and preservation space created by tectonic subsidence in the San Joaquin basin. Adjustments in accumulation space are driven by changes in the ratio of sediment supply to discharge during glacial advances and retreats. At the end of glacial periods and the beginning of interglacial periods, declines in the ratio of sediment supply to discharge led to fan incision, a basinward shift in the fan intersection point, and loss of accumulation space. In mid- and upper-fan settings, incised valleys and laterally extensive, moderately mature paleosols formed, marking the unconformable base of the depositional sequence. Throughout the interglacial period, relatively low accumulation space existed and deposition was confined to the distal areas of the fan. Rapid aggradation and, thus, accumulation space increase, in response to increased sediment supply during the next glacial event initially filled the incised valley with a fining-upward succession of relatively coarse-grained channel and overbank deposits that contain rare, immature paleosols. Upon filling of the incised valley, the intersection point stabilized near the fan apex. This led to unconfined, open-fan deposition, indicating that widespread accumulation space was available across most of the fan surface. These high-accumulation-space units consist of fluvial deposits from multiple, large glacial outwash channels that radiated outward from the proximally located intersection point. Sequence boundaries and units associated with accumulation-space cycles can be used to understand and predict facies distributions and stratigraphic packaging within glacially influenced fans similar to the Kings River alluvial fan.

Spatial Patterns of Sediment Accumulation on a Holocene Carbonate Tidal Flat, Northwest Andros Island, Bahamas

Abstract: To characterize spatial patterns of sedimentation and analyze the morphology of part of the modern tidal flats of northwest Andros Island in the Bahamas, this study integrated remote sensing, geographic information systems (GIS), and carbonate sedimentology. The fundamental data are a Landsat TM image that has been classified to create a thematic map of eight subfacies, interpreted to represent a distinct tidal-flat subenvironment such as adjacent marine, exposed levee-beach ridge, pond, and algal marsh. Spatial statistics of the thematic map characterize the patterns of sediment accumulation. Quantitative analysis highlights several interesting results concerning subfacies character and distribution: (1) of the eight mapped subfacies, low algal marsh is most widespread, representing 27.5% of the total area, whereas exposed levee-beach ridge is the least widespread, accounting for 10% of the area; (2) the patches of different subfacies have different shape complexities, with low algal marsh, high algal marsh, and mangrove ponds being the least complex and exposed levee-beach ridge being the most complex; (3) Markov chain analysis suggests that lateral transitions between different subfacies are highly ordered; and (4) frequency distribution of subfacies patch area and lacunarity (gap size distribution) data exhibit power law relationships over several orders of magnitude, consistent with fractal characteristics. The fractal nature of patch size and gaps between facies illustrate that on this tidal flat neither the size nor the spatial distribution of subfacies has a characteristic scale. This statistical behavior is consistent with the presence of self-organization, or emergence of pattern in the absence of a template or external forcing. The statistical self-organization on the tidal flat is the cumulative expression of local processes, but it becomes apparent only through analysis of the whole system. These results are inconsistent with models suggesting that tidal flats include a migrating complex of randomly distributed, randomly sized subenvironments. Ancient successions that include random patterns may reflect the more pronounced influence of forces external to the sedimentary system, instead of an absence of those forces.

Sediment Supply-Limited Bedforms in Sand-Gravel Bed Rivers

Abstract: The stability of bedforms in mixtures of gravel and sand is not well understood. Two bedform types are characteristic: flow- parallel sand ribbons and flow-transverse barchans. Flume experi- ments and field data presented here show that gradual transitions exist from sand ribbons to barchans, and from barchans to fully de- veloped dunes. Barchans and sand ribbons occur when not enough transportable sediment is available for the formation of fully devel- oped ripples or dunes. The reason is that a part of the bed sediment is immobile, e.g., with an armor layer, which limits the sediment sup- ply and thus the volume of sediment available for the formation of bedforms. Bedform stability diagrams are shown to be extendable to sedi- ment supply-limited bedforms in sand-gravel sediment, if the par- ticle parameters of the diagrams are derived from the transported sediment instead of the bed sediment. Barchans and forms transi- tional to fully developed dunes plot in the dune stability fields. Sand ribbons, on the other hand, plot in the ripple, lower plane bed, and dune fields. In the case of sediment supply limitation, bedforms are partly or completely related to the characteristics of the sediment supply from upstream. The sediment underlying the bedforms may be a stable armor and the exchange of sediment between this armor and the bedforms may be small or non-existent. Consequently, bedform char- acteristics in sand-gravel mixtures in supply-limited conditions often are not predictable from the local hydraulics and sediment charac- teristics.

Distribution of Diagenetic Alterations in Fluvial, Deltaic, and Shallow Marine Sandstones Within a Sequence Stratigraphic Framework: Evidence from the Mullaghmore Formation (Carboniferous), Nw Ireland

Abstract: The distribution of diagenetic alterations in the fluvial, deltaic and shallow marine, arkosic to subarkosic sandstones (average Q72F26L02) of the Mullaghmore Formation (Carboniferous, NW Ireland) can be predicted within a sequence stratigraphic framework. Eogenetic calcite (18OPDB = -13.3‰ to -6.5‰, 13CPDB = -3.0 to +3.4‰, and 87Sr/86Sr = 0.706721 to 0.709227) and ferron dolomite (FeCO3 = 8-12 mol%; 18OPDB = -14.2‰ to -7.8‰, 13CPDB = -1.4‰ to -1.0‰, and 87Sr/86Sr = 0.709051 to 0.709167) occur in bioclast-rich, transgressive lag deposits at parasequence boundaries and transgressive surfaces, and in wave-influenced, deltaic, highstand systems tract (HST) deposits. Mesogenetic illite, chlorite, baroque dolomite (FeCO3 = 16 mol%; 18OPDB = -14.2‰ to -12.7‰, 13CPDB = -3.8‰ to -1.0‰), quartz, and calcite (18OPDB = -15.7‰ to -12.5‰, 13CPDB = -5.8‰ to -3.7‰, and 87Sr/86Sr = 0.709016 to 0.709122) were formed mainly in the bioclast-poor deposits, which were not pervasively cemented by carbonates during near-surface eodiagenesis. These deposits include fluvial, incised-valley sandstones of lowstand systems tract (LST), and fluvial-dominated, deltaic sandstones of transgressive systems tract (TST) and HST. Illite is the dominant diagenetic clay mineral in the fluvial, incised-valley sandstones of LST, possibly because of simultaneous albitization of K-feldspars. Conversely, chlorite, dominates in the fluvial-dominated, deltaic sandstones of TST and HST, because of the presence of suitable precursor clays. The integration of diagenesis into sequence stratigraphic framework of clastic sequences should improve the ability to predict the spatial and temporal distribution of diagenetic alterations and related reservoir-quality modifications of sandstone deposits.

Sequence Stratigraphy of the Neoproterozoic Infra Krol Formation and Krol Group, Lesser Himalaya, India

Abstract: A BSTRACT: A sequence stratigraphic study of terrigenous and carbonate rocks of the Infra Krol Formation and Krol Group in the Lesser Himalaya fold and thrust belt of northern India was undertaken as part of a broader investigation of the significance of carbon isotope data in Neoproterozoic successions. Eight regional stratigraphic discontinuities were traced over a distance of nearly 300 km, and interpretations were anchored in a series of local studies involving the mapping of key beds and the measurement of closely spaced sections. Three of the regional surfaces are interpreted as sequence boundaries on the basis of (1) locally developed incised valleys , 60 m deep; (2) paleokarstic depressions with , 50 m of mappable relief; (3) subaerial dissolution and weathering products (breccias and calcrete) filling vertical fissures, dikes, cavities, and shallow de- pressions in underlying carbonate rocks; and (4) small-scale evidence for sub- aerial exposure at an erosion surface. The remaining five discontinuities are regional flooding surfaces identified on the basis of either facies changes with an abrupt upward deepening across the surface or transitions in facies stacking patterns, typically from forestepping to backstepping. A glacio-eustatic origin is permitted, although not required, for the three sequence boundaries, but no evidence has been found for marked lowering of sea level at other horizons. A mismatch between the stratigraphic location of sequence boundaries and car- bon isotope minima suggests that local diagenetic alteration or oceanographic phenomena unrelated to glaciation may be in part responsible for observed isotopic variation, and that small ice sheets may have existed during apparently nonglacial times without producing either cap carbonates or negative carbon isotope excursions.

Using Magnetic Resonance Imaging and Petrographic Techniques to Understand the Textural Attributes and Porosity Distribution in Macaronichnus-Burrowed Sandstone

Abstract: Magnetic resonance images are paired with petrographic data to evaluate the textural characteristics of rocks dominated by Macaronichnus segregatis, a trace fossil that is commonly associated with rocks deposited in shallow, marginal marine sedimentary environments. MRI techniques used revealed the three-dimensional geometry of the trace fossil. Burrows are typically horizontal and in plan view range between straight, sinuous, meandering, and spiral geometries. Changes in burrow morphology may be related to population density and patchy resource distribution. The pairing of MRI and petrographic data helped map the distribution of porosity in the burrowed rock. Because MRI images represent complex composites of nuclear spin density and MR relaxation times, each of which is related to pore size, stronger MR signals must be calibrated to known porous zones by integrating petrographic data with MR data. The complex distribution of porosity and its relationship to the matrix show that this fabric represents a dual porosity-permeability system and may affect the resource (reservoir or aquifer) quality of similarly burrowed sedimentary rocks. Future research should elaborate upon the porosity-permeability model for this and similar fabrics.

Can Dispersive Pressure Cause Inverse Grading in Grain Flows

Abstract: Grain flows in the collisional regime are self-regulated in such a way that dispersive pressure at every level tends to balance static pressure, and so cannot promote the upward segregation of large particles in a steady flow. Le Roux adds new insights to this analysis by pointing out that dispersive and static pressures would not balance in unsteady flows. When shear strain rate increases in a grain flow, dispersive pressure, which is positively correlated to shear strain rate, increases too, and the gradient of dispersive pressure becomes higher than the gradient of static pressure. As a consequence, the grain flow expands, thus lowering the dispersive pressure, until the two gradients reequilibrate. During the …

Tertiary Provenance History of the Northern and Central Altiplano (Central Andes, Bolivia): A Detrital Record of Plateau-Margin Tectonics

Abstract: The internally drained Altiplano plateau in the high-elevation hinterland of the central Andes contains an extremely thick (up to 12 km) succession of Tertiary nonmarine strata that recorded the topographic evolution of eastern and western plateau margins. In this study, temporal and spatial variations in detrital composition (based on 113 sandstone thin sections and 31 conglomerate clast counts), combined with sediment dispersal patterns and regional stratigraphic correlations, help delineate the Tertiary history of Altiplano basin development in relationship to thrust deformation and arc magmatism along plateau margins. Stratigraphic and provenance data suggest limited subsidence and west-directed sediment dispersal during the early Tertiary history of the Altiplano region. By late Eocene time, development of a rapidly subsiding basin (probable foredeep) was underway, as recorded by extensive east-directed fluvial systems of the 3-6.5-km-thick Potoco Formation. Potoco compositional data are compatible with sediment sources in the incipient Western Cordillera and regions farther west, including (1) a probable thrust-belt source terrane composed of Paleozoic-Mesozoic strata and Precambrian granitic/gneissic basement and (2) a subordinate magmatic-arc source terrane composed of Tertiary volcanic rocks. Increased abundances of plagioclase grains and volcanic lithic fragments upsection indicate a progressively greater arc influence, due to either enhanced volcanism or eastward arc migration. By late Oligocene time, west-directed fluvial and alluvial-fan systems of the upper Potoco Formation and the 3-km-thick Coniri Formation reached the Altiplano basin, providing a minimum uplift age for the backthrust belt along the western flank of the Eastern Cordillera. Paleozoic-clast conglomerate and relatively quartz-rich sandstone containing sedimentary and metasedimentary lithic fragments show unroofing of predominantly Paleozoic strata in the backthrust belt. Continued late Oligocene to Quaternary accumulation in the Altiplano reveals that the basin persisted as a depositional region during a well-documented period of Neogene shortening and uplift in the Andean thrust belt farther east. This intra-orogenic (hinterland) deposition between eastern and western highs during ongoing shortening suggests that the Altiplano basin existed as a crustal-scale piggyback basin, possibly in an internally drained plateau-like geomorphic setting, since 25 Ma. Provenance data and independent structural and thermochronological evidence for spatially and temporally varying tectonic histories along the eastern and western plateau margins imply that several modes of basin development were active during growth of the Altiplano. Although flexural subsidence due to thrust loading along both plateau margins, initially in the west and subsequently in the east, may explain most Tertiary subsidence, possible closed-basin conditions due to structural damming could have promoted additional sediment accumulation from late Oligocene to Quaternary time.

Role of Turbidity Currents in Setting the Foreset Slope of Clinoforms Prograding into Standing Fresh Water

Abstract: Clinoforms produced where sand-bed rivers flow into lakes and reservoirs often do not form Gilbert deltas prograding at or near the angle of repose. The maximum slope of the sandy foreset in Lake Mead, for example, is slightly below 1°. Most sand-bed rivers also carry copious amounts of mud as wash load. The muddy water often plunges over the sandy foreset and then overrides it as a muddy turbidity current. It is hypothesized here that a muddy turbidity current overriding a sandy foreset can substantially reduce the foreset angle. An experiment reveals a reduction of foreset angle of 20 percent due to an overriding turbidity current. Scale-up to field dimensions using densimetric Froude similarity indicates that the angle can be reduced to as low as 1° by this mechanism. The process of angle reduction is self-limiting in that a successively lower foreset angle pushes the plunge point successively farther out, so mitigating further reduction in foreset angle.

Is Quartz Cathodoluminescence Color a Reliable Provenance Tool? A Quantitative Examination

Abstract: We examined cathodoluminescence (CL) colors of quartz by using red (590-780 nm), green (515-590 nm), and blue (380-515 nm) optical filters interfaced with a cathodoluminescence (CL) detector attached to a scanning electron microscope (SEM). SEM/CL images taken through these filters were captured digitally and transferred to a computer. Luminescence intensities (luminosities) of the images were measured by using available commercial software. Measured luminosities of these CL images are directly related to relative intensities of red, green, and blue CL emissions. Luminosity data were then used to construct plots that display relative luminosities of the CL images acquired through the red, green, and blue filters. An unfiltered CL image of each quartz grain, generated by photons with wavelengths ranging from 200-700 nm, was also acquired. By subtracting the numerical luminosity values of the images acquired through the red, green, and blue filters from the luminosity value of the unfiltered image, the contribution to total luminosity provided by CL emission in the near ultraviolet (UV) was calculated. The CL colors of quartz from a variety of volcanic, plutonic, and metamorphic rocks and hydrothermal deposits were examined. Volcanic quartz phenocrysts have the most restricted CL color range, with strongest emission intensity in the blue wavelength band. CL colors of plutonic quartz overlap those of volcanic phenocrysts but extend over a broader range to include quartz that displays higher intensity of red emission. CL emission in hydrothermal (vein) quartz is similar to that in plutonic quartz, although some hydrothermal quartz exhibits stronger green-CL emission than does plutonic quartz. The CL colors of metamorphic quartz exhibit the widest variation, overlapping the color fields of both volcanic and plutonic quartz and extending further into the red. CL emission in the near UV makes a significant contribution ( 5-85 percent) to the total luminosity of SEM/CL images, particularly images of plutonic quartz. Because of overlap in the CL color ranges of volcanic, plutonic, metamorphic, and hydrothermal quartz, unambiguous identification of quartz provenance on the basis of CL color alone is problematic. It is difficult to distinguish between volcanic and some plutonic quartz, and between some plutonic and hydrothermal quartz, or to distinguish magmatic quartz from metamorphic quartz that exhibits blue CL color. Only metamorphic quartz that exhibits moderately strong red emission appears distinguishable (on the basis of color) from quartz of other origins. Our work thus suggests that CL color is not a reliable indicator of quartz provenance.

Paleogeographic Setting and Depositional Architecture of a Sand-Dominated Shelf Depositional System, Miocene Utsira Formation, North Sea Basin

Abstract: The middle-late Miocene Utsira sandstone of the North Sea Basin contains a fully preserved, regional marine sand deposit that records a stable paleogeographic setting of shelf sand transport and accumulation within an epeiric shelf sea which persisted for ca. 8 Myr. The sediment dispersal system was defined by (1) sediment input through a prograding strandplain platform, coast-to-shelf bypass, and regional basin-centered transport and deposition within an elongate seaway; (2) a high-energy marine regime; (3) very low rates of sediment supply and accumulation in the accommodation-dominated seaway; (4) high sediment reworking ratio and consequent abundance of autochthonous sediment; (5) regional along-strike sediment transport; and (6) local to subregional landward sediment transport. Shelf systems were primarily nourished by sediment reworked from the Shetland strandplain, which prograded eastward into the central Viking Trough. The Viking strait lay between the constructional platform and the nondepositional Horda Platform. Southward longshore transport on the high-energy shoreface and shallow shelf platform constructed the mounded south Viking shelf shoal. Southward-building sandy banner banks that formed along the recurved arc of the platform margin nourished the shoal system. Low-angle sigmoid clinoforms with downstepping, aggradational tops are distinctive architectures of the strike-fed sand bodies. The combination of strong marine currents and slow but continual sand supply from the Shetland strandplain created regional, sandy shelf depositional systems that individually covered 3500 to 6000 sq. km. of basin floor. Defining elements of the shelf shoal systems include their location within the basin axis, mounded morphology, stratal architecture, and construction by amalgamated marine sand facies.

Upper Oligocene to Lowermost Miocene Strata of King George Island, South Shetland Islands, Antarctica: Stratigraphy, Facies Analysis, and Implications for the Glacial History of the Antarctic Peninsula

Abstract: The Cape Melville Formation (CMF), exposed on southeastern King George Island, South Shetland Islands, provides rare evidence of extensive earliest Miocene glaciation in the Antarctic Peninsula region. The formation records the presence of regional marine-based grounded ice on the continental shelf. It overlies disconformably the upper Oligocene Destruction Bay Formation, which consists of sandstones recording nonglacial shallow marine conditions. Four units have been identified within the approximately 150 m thickness of the CMF. The basal unit (A) consists of coarse glacigenic debris-flow facies interbedded with glaciomarine mudstone and sandstone. The overlying unit (B) is mainly fine-grained. This succession may represent relatively ice-proximal deposition followed by glacial retreat and/or relative sea-level rise. The upper CMF (units C and D) was deposited in an ice-distal marine environment, with intermittent input of coarse glacigenic debris, mainly from ice rafting. Thin beds of pelagic carbonate ooze within unit C indicate periods of low terrigenous sediment input and high productivity. Lithologically diverse glacigenic gravel clasts (mainly ice-rafted debris) in the CMF had a wide regional source area, suggesting that ice cover was widespread regionally and included calving ice margins. For a small proportion of clasts the nearest known source is the mountains fringing the southern Weddell Sea. Such clasts were presumably transported north in debris-laden icebergs by a strong, cold Weddell Sea surface current. A temperate glacial setting is tentatively inferred from the CMF. Palynological results confirm and enhance the paleoenvironmental interpretation from the sedimentology, and include the first early Miocene dinoflagellate cyst assemblages recorded on the Antarctic Peninsula. This reappraisal of the glacial record from the CMF provides valuable constraints on the Antarctic cryosphere and regional paleoenvironments in the mid-Cenozoic.

Morphology and Stable and Radiogenic Isotope Composition of Pedogenic Carbonates in Late Quaternary Relict Soils, New Mexico, U.S.A.: An Integrated Record of Pedogenic Overprinting

Abstract: This petrographic and stable (13C and 18O) and radiogenic (14C) isotope study of pedogenic carbonates from late Quaternary soils in the Rio Grande Rift region of New Mexico documents that carbonate formation in semiarid to arid soils (Aridisols) occurs principally by overprinting and can result in a complex petrographic and isotopic record. With increasing duration of exposure (<7 to 900 ky) of the four studied profiles, pedogenic carbonates exhibit: (1) an increase in the degree of intraprofile 13C and 18O heterogeneity, (2) increased development of down-profile 13C and 18O trends, (3) an increase in the degree of intermorphologic and intramorphologic stable-isotope variation, commonly in samples distributed laterally within narrow depth intervals, and (4) development of covariation between 13C and 18O values. Radiocarbon ages of the carbonates permit assessment of the degree of isotope heterogeneity in contemporaneous carbonates and recognition of temporal isotopic trends. The range of exposure times recorded by the studied soils are analogous to those associated with the formation of fossil soils. This makes the studied soils ideal for evaluating the origin of stable-isotope heterogeneity in paleosol carbonates and the implications of those heterogeneities for the use of stable isotopes as paleoenvironmental proxies. Specific implications of this study include the following: (1) Pedogenic carbonates that form by overprinting are a cumulative record of the soil conditions present during their accumulation; thus, isotope analysis of overprinted carbonates can result in "time averaging" of changing climatic, paleoenvironmental, and paleoecologic conditions. (2) Carbonates may record only a fraction of the duration of development of their host profile. (3) There is a high level of uncertainty associated with using the sequence of morphologic development of carbonate as a function of time or maturity in paleosols. (4) An exponential decrease in carbonate 13C values with depth in a paleosol does not imply that the carbonates are contemporaneous or accumulated under invariant soil conditions. (5) Shifts in soil and paleoclimatic conditions may be recorded by the 13C and 18O values of carbonates distributed within narrow depth intervals of a given soil in addition to those distributed down-profile, or even from within individual carbonate samples.

Shallow burial dolomitization and dedolomitization of cenozoic cool-water limestones, southern australia: geochemistry and origin

Abstract: The Gambier Limestone, a bryozoan-rich, cool-water carbonate of Late Eocene to Early Miocene age in the Otway basin of southeastern Australia, is about one-third variably dolomitized limestone and dolostone. Individual dolomite crystals range from 10 to 500 µm and are of three types: (1) silt-size crystals (4-64 µm diameters) having a brightly luminescent core (10-20 µm) of nearly stoichiometric dolomite with 1 mol% MnCO3 and SiO2 but no Fe, surrounded by a thin, dull or alternating dark and bright CL cortex that is nonstoichiometric with 44-46 mol % MgCO3 but sharp to diffuse zoning in Mn and Fe, (2) sand-size crystals, which also have a brightly luminescent and stoichiometric core and dark and bright non-stoichiometric thin inner cortex, but are mostly a thick outer cortex (at ca. > 50 µm diameter) that is nonluminescent and nonstoichiometric with some Fe but no Mn, and (3) identical sand-size crystals with a variably leached core and inner cortex that is either a void or a void filled with late calcite cement. These three crystal populations represent two distinct dolomites and their alteration product. The smallest of the silt-size crystals, which are rarely preserved, are also the cores of the larger crystals upon which grew an inner and outer cortex of somewhat different composition. The 18O values of dolomite rhombs (-1.6 to 2.9 ) correlate positively with 13C values (0.7 to 3.0), with the lowest values from silt-size or small sand-size crystals and the highest values from large sand-size crystals with leached, unfilled cores. Low and variable 18O and 13C values in conjunction with high and variable Mn and Fe contents in both the core and inner cortex of all crystals are consistent with formation from a mixture of brackish water and seawater. Higher values for the outer cortex in sand-size crystals are typical of those expected for precipitation from normal or near-normal seawater. Low Sr, Mn, and Fe contents in the largest crystals, in conjunction with fluid inclusions of seawater salinity in the outer cortex, are also consistent with the bulk of the large crystals having precipitated from seawater. Using the Sr-isotope evolution curve for Cenozoic seawater, Sr-isotope ratios of the dolomites can be divided into four distinct groups, which correspond to times of major transgressions. This Cenozoic example, in which dolomite petrography and crystal chemistry can be explained in terms of simple transgression-regression, offers a simple sedimentological explanation for many similar dolomites where processes of formation cannot be so easily constrained.

The Relative Roles of Compaction and Early Cementation in the Destruction of Permeability in Carbonate Grainstones: A Case Study from the Paleogene of West-Central Florida, U.S.A.

Abstract: How compaction can yield a progressive decrease in the permeability and pore structure of carbonate grainstones is not well documented. In this study, the destruction of permeability by compaction relative to early cementation is examined in forty-five shallow-buried (30-440 m) grainstones with a range in permeability of 12 to 5180 md. Compaction and cementation indices calculated from thin-section point-count data indicate a group of grainstones dominated by cementation and another by compaction. Log-linear cross plots of permeability versus these indices suggest that permeability loss in these two groups occurs by different processes. The nature of those processes is elucidated by capillary pressure data and the distribution of cement and compaction fabrics as a function of permeability. As cement abundance increases in the cement-dominated samples, a continuous distribution of cement develops along grain surfaces, which is followed by the development of partial and then complete pore-filling mosaics. Permeability is progressively reduced with this succession of cement fabrics and an increasingly greater percentage of pore space is positioned behind progressively smaller throats. Permeability loss results from porosity reduction, progressive constriction of pore throats, and increased tortuosity of flow paths. Once permeability is reduced to tens of millidarcies, effective pore radii are all less than 1 µm and many are less than 0.1 µm, but there is no evidence for complete blocking of all pore throats in this suite of samples. In absolute terms, the pore-lining cements reduce permeability by thousands of millidarcies whereas the pore-filling cements reduce permeability only by hundreds of millidarcies. Mechanical compaction is the dominant cause of permeability reduction between 5000 and about 500 md in the compaction-dominated samples. Linear grain contacts and embayed grains become more abundant and close to overly close packing textures become more homogeneously distributed as permeability decreases. However, there is no concurrent change in the amount of pore space accessed by large or small throats, indicating that mechanical compaction does not uniformly constrict or seal pore throats. Instead, the data suggest that mechanical compaction reduces permeability by reducing porosity, lengthening pore throats, and increasing tortuosity. At about 500 md the compaction-dominated trend steepens on the log-linear plot of permeability versus porosity, a change that corresponds to the development of grain-to-grain pressure solution. Unlike mechanical compaction, grain-to-grain pressure solution constricts and eliminates pore throats and isolates pore space, resulting in a loss of large throats and an increase in the amount of pore volume accessed by small throats. In contrast to cement-dominated samples, the most effective pore radii are still greater than 1 µm at tens of millidarcies of permeability. Such low amounts of permeability can be reached even when pressure solution is not pervasively distributed throughout a sample.

A Preliminary Experimental Study of Turbidite Fan Deposits

Abstract: We present preliminary laboratory experimental results that demonstrate lobe switching in a turbidite fan. After a significant number of individual event beds (more than eight) in both experimental fans made in this study, the flow began to focus itself into a lobe, at the center of which appeared a subtle channel-form. Initial lobe switching occurred after nearly twenty event beds. Subsequent switching appeared to scale with lobe size and sedimentation rate. The densest, coarsest runs tended to initiate the switching of the depocenter. Instabilities within the turbidity current itself appear to be responsible for initial flow confinement to a lobe and ultimate switching events. Currently untreated scale effects and measurement difficulties limit quantitative extension of the results to natural settings, though some general relationships are proposed.