Showing papers in "Journal of Sedimentary Research in 1998"

Processes controlling the distribution of Ti and Al in weathering profiles, siliciclastic sediments and sedimentary rocks

Abstract: The ratio of Ti to Al varies greatly in primary (igneous) source rocks. These elements are considered to be relatively immobile in most weathering regimes, so that TiO2:Al2O3 ratios have been used to investigate the provenance of sediments and sedimentary rocks. In modern weathering profiles that have undergone moderate weathering, Ti:Al ratios remain fairly constant so that siliciclastic sediments derived from them should have Ti:Al ratios like that of the source materials. Under extreme weathering conditions, however, the upper (most weathered) parts of profiles commonly show a marked increase in Ti:Al ratio that is thought to be due to preferential translocation of Al-rich phases. Under steady-state weathering conditions, one particular level of the profile may act as the main source of detrital material and may thus control the Ti:Al ratio of sediments produced from it. In a glacio-fluvial setting (minimum weathering, moderate sorting) there is considerable variation in Ti content but Al is nearly constant. This is due to concentration of Ti-bearing mafic minerals, such as biotite, in the fine fraction. In modern sediments derived from moderately weathered and sorted material, the Ti:Al distribution reflects mixing of Al- and Ti- enriched fine-grained materials with sands that are depleted in these elements. When a plot of TiO2 vs. Al2O3 is constructed for a suite of such sands and muds, the resultant trend differs significantly from that shown by a similar suite of unweathered glacio-fluvial sediments. Comparison of these results with data from sedimentary rock suites suggests that trends shown by plots of TiO2 vs. Al2O3 may provide insight into weathering and depositional history, in addition to their use as provenance indicators.

A composite reference section for terminal proterozoic strata of southern Namibia.

Abstract: Integrated sequence stratigraphic and chemostratigraphic data yield a framework for correlations of stratigraphic units in the terminal Proterozoic to Cambrian Witvlei and Nama Groups of Namibia. Coupled with precise U-Pb zircon age constraints, these correlations make it possible to construct a composite reference section for use in calibrating terminal Proterozoic chronostratigraphy. The Namibian reference section starts with two distinct glacial horizons and extends up to within 1 million years of the Proterozoic-Cambrian boundary. The two glacial horizons may represent each of two distinct Varanger-age glaciations better known from the North Atlantic region. From the higher of the two glacial horizons up, the composite stratigraphy preserves one of the thickest and most complete available records of carbon-isotope variability in post-Varanger terminal Proterozoic seawater. Four carbon-isotope chemostratigraphic intervals are recognized: (1) a postglacial negative δ^(13)C excursion (Npg interval); (2) a rising interval (Pr interval) of increasing positive delta 13 C values; (3) a falling interval (Pf interval) characterized by decreasing positive δ^(13)C and culminating in near zero or negative values; and (4) an interval of moderately positive, relatively invariant δ^(13)C values (I interval) that extends up to the unconformity that contains the Proterozoic-Cambrian boundary. Each of these chemostratigraphic intervals can be recognized in widely separated correlative sections around the world. By comparing sediment accumulation rate in the radiometrically calibrated Namibian stratigraphy with sediment accumulation rates in correlative sections in Arctic Canada and Oman, a maximum age of 564 Ma is estimated for the end of the younger Varanger glaciation, 25 m.y. younger than previous estimates.

Sedimentary processes at the base of a West Antarctic ice stream; constraints from textural and compositional properties of subglacial debris

Abstract: Samples of sediments from beneath Ice Stream B (at camp UpB), West Antarctica, provide the first opportunity to study the relationship between sediment properties and physical conditions in a sub-ice-stream environment. Piston coring in holes bored by hot-water drilling yielded five 1-3 m long, undisturbed subglacial sediment cores. We analyzed granulometry, composition, and particle morphology in these cores. The UpB cores are composed of a clay-rich, unsorted diamicton containing rare marine diatoms. Sedimentary particles in these cores bear no evidence of the recent crushing or abrasion that is common in other subglacial sedimentary environments. The presence of reworked diatoms and their state of preservation, as well as the relative spatial homogeneity of this diamicton, suggest that the UpB cores sampled a several-meter-thick till layer and not in situ glacimarine sediments. The till does incorporate material recycled from the subjacent poorly indurated Tertiary glacimarine sediments of the Ross Sea sedimentary basin, which extends beneath this part of the West Antarctic Ice Sheet. We propose that the lack of significant comminution in the UpB till is ultimately due to its setting over these easily erodible, clay-rich source sediments. The resulting fine-grained till matrix inhibits glacial comminution, because it facilitates buildup of high pore-water pressures and hinders interparticle stress concentrations. Our observations are consistent with the conjecture that subglacial deformation of weak, fine-grained tills does not produce significant comminution of till debris (Elson 1988). Based on our findings, we hypothesize that extensive layers of weak till may develop preferentially where ice overrides preexisting, poorly indurated, fine-grained sediments. Since such weak till layers create a permissive condition for ice streaming, sub-glacial geology may have an indirect but strong control over the location, extent, and basal mechanics of ice streams.

Erosional offshore sand ridges and lowstand shorefaces; examples from tide- and wave-dominated environments of France

Abstract: Offshore sand bodies are described from many continental shelves in the world, as well as in the stratigraphic record, where they commonly are productive reservoirs. Some of these ancient sand bodies, initially interpreted as sand ridges, are now reinterpreted as lowstand shoreface deposits. Modern sand bodies, in contrast, have received relatively little attention with regard to reinterpretation of their origin, largely because of the lack of information about their internal structure. Improved techniques in acquisition and processing of very high-resolution seismic profiles, along with some shallow cores, allow us to reconstruct the architecture of "offshore sand bodies" from the Celtic Sea (tide-dominated) and the Gulf of Lions (wave-dominated) shelves of France, in water depths of 100-170 m. In both areas, our investigations demonstrate that these particular sand bodies consist mainly of lowstand deposits (estuarine/deltaic systems, sharp-based shorefaces), reworked during transgressions. In the Celtic Sea, intense erosion by combined waves and tidal currents resulted in the shaping of shore-oblique ridges by cannibalization of older lowstand deposits. In the Gulf of Lions, the shore-parallel orientation of the lowstand shorefaces has been preserved, leaving an ancestral sand body with reworked (transgressive) surface deposits. Understanding the architecture and distribution of offshore sand bodies requires taking into consideration not only the effects of relative sea-level changes and sediment supply, but also the role of hydrodynamical processes. The erosional sand bodies we describe represent a new category of outer-shelf sand bodies, a combination of the "autocyclic" examples described by Houbolt (1968) in the southern North Sea and the "allocyclic" lowstand shorefaces mainly described in the stratigraphic record of the Western Interior Seaway of North America. Our findings have applications for predicting the geometry of ancestral sand bodies and their orientation and position with respect to paleo-shorelines. The magnitude of erosional processes also implies that a large amount of shelf sediment (mainly sand) was transferred to the adjacent deep-sea floor during the early transgression.

Controls on Facies Architecture of a Large Triassic Carbonate Platform: the Great Bank of Guizhou, Nanpanjiang Basin, South China

Abstract: The Great Bank of Guizhou (GBG) is an exceptionally well exposed isolated Triassic platform in the Nanpanjiang Basin of South China. The platform is exhumed with its depositional profile preserved and is dissected by a faulted syncline that exposes a complete and uncomplicated cross section providing a unique opportunity to evaluate mechanisms involved in its birth, evolution, and demise. The GBG formed near the southern margin of the Yangtze Platform during a deepening event that expanded the Nanpanjiang Basin and drowned the region surrounding the GBG in the latest Permian. Initial accumulation of the GBG began in the latest Permian with small reef mounds and open-marine skeletal packstones. Following the end-Permian extinction, cyanobacterial boundstones grew over the bank top. During the Early Triassic the GBG developed a low-relief bank profile with mobile oolite shoals at the margin, shallow-subtidal and peritidal deposits in the interior, and gentle slopes dominated by pelagics, debris-flow deposits, and turbidites at the basin margin. In the Middle Triassic (Anisian) the GBG developed a progressively steepening profile rimmed with massive Tubiphytes reefs. The platform was flat topped with tidal-flat deposits across the interior. Basin-margin deposition was dominated by turbidites and debris-flow deposits but eventually shifted to avalanche and rock-fall deposits as the slopes steepened to the angle of repose. In the Middle Triassic (Ladinian) an erosional escarpment up to 1700 m high developed at the margin. Platform-margin strata are bedded packstones similar to interior strata, whereas breccias at the basin margin contain coral-boundstone clasts suggesting erosion of reefs from the escarpment. A restricted subtidal lagoon formed in the interior, producing an atoll-like morphology. Later, a flat-topped profile was restored as tidal flats spread across the interior. In the beginning of the Late Triassic deepening contributed to termination of the GBG before siliciclastic turbidites and shales were deposited over the platform. In contrast with the well known platforms of the Dolomites of northern Italy, the GBG contains abundant muddy carbonates and a progressively steepening bank to reef-rimmed and escarpment architecture. The Italian platforms contain little mud and have angle-of-repose, pinnacle geometries. The GBG's larger size increased mud production and protected it from extensive winnowing of mud, which in turn resulted in off-bank shedding of muddy sediments that were stable on relatively gentle, basin-margin slopes which progressively steepened and ultimately led to avalanche deposits and a high-relief erosional escarpment. In contrast, the lesser mud content of the Dolomites platforms forced avalanche and talus deposition to dominate throughout deposition of basin margins, which in turn produced their angle-of-repose geometries.

Modeling of Erosion and Deposition by Turbidity Currents Generated at River Mouths

Abstract: A numerical approach is developed to simulate sediment transport by turbidity currents, with particular application to hyperpycnal plumes. The model extends the Chezy equation to explicitly include water entrainment, sediment erosion and deposition, and internal grain friction. Water entrainment is shown to be particularly important to the motion of hyperpycnal plumes, wherein internal friction is greatly reduced and the plume can flow even on small reverse slopes. Marine deposits associated with a 28-day flood on the Saguenay River in 1663 A.D. are compared favorably to model simulations on the shape (runout distance, turbidite thickness) and grain-size properties of the deposit. Properties of the turbidite are shown to be strongly linked to the duration and hydrograph of the flood event. During the rising limb of the flood wave, when sediment concentration and flow velocities are on the increase or remain high, deposition of the turbidite shifts seaward. On the falling limb of the flood wave, deposition of the turbidite shifts landward, as sediment concentration and flow velocities decrease. This later phase leads to the formation of a deposit that thickens and then thins seaward, in contrast to turbidites deposited from an ignitive surge, where deposit thickness simply decreases with distance. The deposit of a hyperpycnal flood event is initially inversely graded (finer to coarser particles measured from the base of the deposit), in association with the period of increasing discharge, then normally graded in association with the period of decreasing river flow.

Microbial Biofacies in Hot-Spring Sinters: A Model Based on Ohaaki Pool, North Island, New Zealand

Abstract: Ohaaki Pool was the main hot spring in the Broadlands-Ohaaki geothermal area before recent anthropogenic modification. The alkaline Na-HCO3-Cl water, which discharged at 95°C with a flow rate of 10 l·s-1, flowed down a broad discharge apron into the Waikato River. The discharge apron was inhabited by thriving microbial communities that included Calothrix, Phormidium, and numerous Synechococcus. These microbes mediated the construction of columnar stromatolites around the edge of Ohaaki Pool, oncoids in the discharge channel, and intercalated stratiform stromatolites, "Conophyton", and coccoid microbial mats on the distal part of the discharge apron. All the microbes were variably replaced and encrusted by amorphous silica while alive or shortly after death. Consequently, fabrics in the siliceous sinters around Ohaaki Pool are controlled by the growth patterns and composition of the microbial community. For example, the Calothrix-dominated community gave rise to stratiform stromatolites that are characterized by alternating erect and prostrate laminae. Conversely, the Synechococcus-dominated communities formed mats that produced dense, white siliceous laminae. The Phormidium-Synechococcus community constructed the "Conophyton". Platy, skeletal, and blocky calcite crystals are found in, around, and between the siliceous stromatolites that formed around Ohaaki Pool, the proximal part of the discharge apron, and in the oncoids. Although minor calcite is found locally in the stratiform stromatolites on the discharge apron, there is no evidence that microbes played any role in calcite precipitation.

Carbonate Recrystallization in Shallow Marine Environments: A Widespread Diagenetic Process Forming Micritized Grains

Abstract: Integrated morphologic, petrographic, and mineralogic analyses of sediment grains of the miliolid foraminifer Archaias and the green alga Halimeda from shallow marine environments in Florida, the Bahamas, and Belize document pervasive syndepositional recrystallization in these grains. Scanning electron microscopy shows that original skeletal rods and needles recrystallize on the sea floor to a variety of equant micritic fabrics. In thin section, this textural alteration corresponds to progressive micritization, with loss of the familiar golden brown color exhibited by fresh miliolids and Halimeda and gradual development of gray cryptocrystalline carbonate. X-ray diffraction and electron microprobe studies show that this textural alteration may be accompanied by mineralogical changes from Mg-calcite to aragonite or from aragonite to Mg-calcite. Our findings support petrographic studies that reported syndepositional recrystallization in a wide variety of carbonate grains about thirty years ago, but which have been largely ignored. Together, these studies indicate that carbonate recrystallization is a widespread process of early diagenesis. Extensive recrystallization in shallow tropical seas challenges basic principles regarding the setting and timing of textural and mineralogic alteration of metastable carbonate grains. In addition, recognition of recrystallization as an important process of micritization contradicts prevailing theories that micritization results solely by infilling of microborings. Finally, our findings raise fundamental questions concerning the nature of carbonate crystal structure, mechanisms of crystal growth, and driving forces for recrystallization. Answering these questions will require sedimentologists to step beyond traditional bounds into fields such as biomineralization and materials science.

Multicomponent analysis of ftir spectra: quantification of amorphous and crystallized mineral phases in synthetic and natural sediments

Abstract: The study of weathering and erosion processes requires the establishment of a mass balance of minerals from the soils towards the sedimentation area. This calls for the development of quantitative tools that can be applied to mixtures of finely divided and sometimes poorly crystallized minerals found in soils and sediments. We present here the use of Fourier transform infrared (FTIR) absorbance spectroscopy in the mid-infrared as a method to calculate mineral modes for such materials. Samples were prepared using the KBr disc method. This ensures that Lambert-Beer's law is valid. A quantitative determination of the mineral content from various blends was performed by making a multicomponent analysis of the experimental spectrum using the spectra of each component in the mixture. To check the validity of the procedure, the multicomponent analysis was performed on spectra from synthetic mixtures of standard minerals (quartz, kaolinite, gibbsite, amorphous silica). Good agreement between actual and computed wt % was obtained in the range 1315-315 cm-1. This application of FTIR spectroscopy was used to quantify the mineral components of a lacustrine sediment cored at Salitre, Minas Gerais, Brazil. The major phases are organic matter, kaolinite, gibbsite, quartz, anatase, and amorphous silica (phytoliths and sponge spicules). Fifty-three samples were analyzed. Quantitative results from FTIR spectroscopy were compared to chemical analyses to assess the validity of the method for the natural sediments. Phases with no sharp diagnostic spectral features, such as anatase or amorphous silica, were quantified using the multicomponent analysis. The variations of the main mineral phases along the core reflect the mechanisms of transfer from the surrounding soils to the sediment area and the role of the vegetal cover over the drainage area.

Recent Debris-Flow Processes and Resultant Form and Facies of the Dolomite Alluvial Fan, Owens Valley, California

Abstract: A multiphase debris flow triggered by an afternoon thunderstorm was deposited 22 August 1984 on the Dolomite alluvial fan of Owens Valley, California. Two tracts 10-300 cm thick accumulated during this event, together covering 68,000 m2 (26% of the fan) with ~ 50,000 m3 of sediment. Both tracts consist of numerous paired boulder-rich levees 100-300 cm high on the proximal fan, and multiple attached lobes 20-100 cm thick on the distal fan. Each levee-lobe pathway was built by an individual surge, with flow-path switching caused by clogging of the frontal bouldery snout of the ensuing surge. The levees consist of clast-supported to matrix-supported, muddy, pebbly, cobbly, boulder gravel displaying a well-developed, radially aligned a-axis clast fabric. The lobes mostly consist of matrix-supported, muddy, pebbly cobble gravel. The clast-supported lobe margins are enriched in cobbles displaying a strongly preferred slope-parallel a-axis fabric along the sides that changes to a slope-perpendicular fabric at the snout. The distal ends of most of the clast-rich lobes are overlain and offlapped by clast-poor debris-flow lobes of granular mud 10-30 cm thick. The central zones of the levees and most lobes were surficially washed by recessional-stage water flows that moved winnowed granules, sand, and mud off the fan. The radial change from levees in the proximal fan to lobes distally is morphologically expressed as two intersecting slope segments, the upper one sloping 9-12° and the lower one 3-5°. This direct relationship between the constituent forms and slope values also is characteristic of pre-1984 deposits, suggesting that the 1984 event is typical of the primary processes constructing the Dolomite fan. The pronounced radial change from levees to lobes, and its manifestation as a slope inflection, was caused by the selective loss on the upper fan of most of the boulders from the passing debris-flow surges. Boulders were sorted from the surges because of their greater buoyancy, which caused them to be preferentially concentrated in the part of the flow that subsequently was sheared off as levees. This selective boulder removal depleted the flow of the clasts that provided lateral support, facilitating flow expansion and the onset of lobe development. Thus, the resultant facies assemblage is a function of the specific sediment grain-size suite available in the catchment colluvium, and the transport characteristics of this mix when transformed into a debris flow by the rapid addition of heavy rainfall during infrequent thunderstorms.

Discontinuity surfaces on a shallow-marine carbonate platform (Berriasian, Valanginian, France and Switzerland)

Abstract: Discontinuities in sedimentation are commonly expressed as surfaces in outcrop sections and are due to rapid and substantial environmental changes. On shallow-marine carbonate platforms most such surfaces represent hiatuses below biostratigraphic resolution, and detailed analysis is necessary to identify and evaluate the environmental change involved. Surfaces in nine sections of the Lower Cretaceous of the French and Swiss Jura platform are characterized on the basis of eight universally applicable criteria (geometry, lateral extent, morphology, biological activity, mineralization, facies contrast, diagenetic contrast, and biostratigraphy). Nine different surface types are distinguished by their common features and environment of formation. All of them are related to environmental changes in the form of subaqueous erosion, subaerial exposure, subaqueous omission, or changes in texture and facies. The distribution of surface types in the studied sections shows that condensation and exposure-related surfaces tend to occur repetitively in certain intervals. Calibrated by biostratigraphy, these surface zones can be correlated across the platform from proximal to distal positions. In comparison with the global sequence-stratigraphic framework (Hardenbol et al. 1997) most exposure zones correlate with third-order sequence boundaries; condensation zones fall in between. In the studied sections, third-order eustatic sea-level drops appear to be represented rather by zones of small-scale discontinuities than by widespread and well-marked single sequence boundaries. This is explained by the superposition of high-frequency, low-amplitude sea-level fluctuations on a larger-scale sea-level trend under greenhouse conditions. The lateral extent of the surface zones varies through time and indicates important changes in platform morphology. Changes in local subsidence rate indicated by variable thicknesses of the deposits in comparison with second- and third-order sea-level trends suggest an evolution of the French Jura platform from a ramp-type morphology in the late Middle Berriasian to a flat-topped platform in the Late Berriasian. The Early Valanginian again is characterized by increased differential subsidence and well-marked platform morphology. This study demonstrates that: (1) small-scale and short-lived discontinuities can reflect large-scale variations of relative sea level; (2) on shallow platforms characterized by small topographic variations and lateral facies changes, third-order sequence-stratigraphic surfaces are not necessarily expressed by one widespread single surface, but by zones of surfaces indicating repeated environmental changes; surface zones can serve as an additional tool for correlation and interpretation of platform evolution; and (3) small-scale discontinuities form an integral part of the stratigraphic record and should receive the same attention as the sedimentary deposits they delimit.

Upper Jurassic-Lower Cretaceous Morrison and Cedar Mountain formations, NE Utah-NW Colorado; relationships between nonmarine deposition and early Cordilleran foreland-basin development

Abstract: Sedimentologic, stratigraphic, and petrologic analysis of the Upper Jurassic-Lower Cretaceous Morrison and Cedar Mountain formations of Utah and Colorado provides information on the timing and nature of early Cordilleran foreland-basin development. The Morrison Formation can be subdivided into three depositional facies assemblages: (1) lower progradational shallow marine, lacustrine, fluvial, and eolian facies deposited during Oxfordian-Kimmeridgian retreat of the Stump-Sundance sea; (2) a middle assemblage containing sandy-gravelly braided fluvial deposits, which are overlain by meandering fluvial channel and overbank facies; and (3) an upper assemblage of laterally stable, low-sinuosity, fluvial channel facies deposited during Tithonian-early Neocomian (?) time. The upper part of this assemblage shows evidence of alteration and early diagenesis related to development of an Early Cretaceous unconformity. The overlying Cedar Mountain Formation is subdivided into two facies assemblages: (1) the Neocomian Buckhorn Conglomerate was deposited by northeast-trending, sandy-gravelly braided rivers that were incised into the underlying Morrison Formation; (2) an upper assemblage containing laterally stable, low-sinuosity, fluvial channel facies deposited during late Neocomian-Albian time. The base of the unit contains a thick calcrete zone that formed during an unconformity following Buckhorn deposition. Morrison and Cedar Mountain formation sandstones contain three petrofacies: a feldspar-rich lower Morrison petrofacies (%QmFLt = 70, 19, 11), and chert-rich upper Morrison and Buckhorn/Cedar Mountain petrofacies (%QmFLt = 54, 5, 41 and 69, 4, 27, respectively). Sandstone composition and paleocurrent data indicate a Cordilleran source area composed of Proterozoic, Paleozoic, and Mesozoic sedimentary rocks. The Morrison Formation and Buckhorn Conglomerate were deposited in the back-bulge depozone of the Late Jurassic Cordilleran foreland-basin system and onlapped a flexural forebulge located in central Utah. Late Neocomian eastward migration of the forebulge uplifted areas in eastern Utah, producing an unconformity, while the foredeep in central Utah underwent flexural subsidence. The upper part of the Cedar Mountain Formation represents overfilling of the foredeep and deposition above the forebulge.

Shelf-To-Basin Facies Distributions and Sequence Stratigraphy of a Steep-Rimmed Carbonate Margin: Capitan Depositional System, Mckittrick Canyon, New Mexico and Texas

Abstract: Shelf-to-basin outcrop studies in steep-rimmed, shelf-margin settings are uncommon because continuous shelf-to-basin transects are rarely exposed in a single outcrop. Discontinuous or absent stratigraphic marker beds across the shelf margin further complicate outcrop studies in the shelf-margin setting. This paper discusses the results of a high-resolution investigation of the shelf-to-basin profile along the north wall of North McKittrick Canyon, New Mexico and Texas. In McKittrick Canyon, carbonate-dominated sedimentary rocks associated with the steep-rimmed, Upper Permian Capitan depositional system are exposed along a continuous 5-km outcrop face. Measured sections, lateral transects, scintillometer readings, and geochemical data were synthesized into a digital database and interpreted in conjunction with a digital photomosaic of the entire canyon wall. Results of this work include a shelf-to-basin facies map and sedimentologic interpretation of the north wall of North McKittrick Canyon, and indicate that the dominant bathymetric profile during Capitan deposition was a marginal mound. In this model, the Capitan reef facies was deposited at the shelf-slope break in water depths ranging from 15 to 75 m, but always downdip from the topographically higher shelf crest. This model is supported by the following observations and interpretations: (1) a facies progression from the shelf crest to the shelf margin interpreted to represent a shallow-to-deeper-water succession; (2) proportional expansion of beds in a downdip direction; (3) presence of oriented (transported) fusulinid grainstones downdip from in situ fusulinid wackestones and packstones updip; (4) siltstones that thin and pinch out towards the shelf margin; (5) a decrease in dolomite from the shelf crest to the shelf margin; and (6) the absence of true toplap stratal geometries. In reality, a static paleobathymetric model cannot characterize the depositional system, because the facies distributions, facies proportions, stratal geometries, and quantified depositional parameters vary systematically from the Seven Rivers through the Tansill. In order to understand the observed variations, emphasis was placed on quantifying key depositional parameters such as progradation, aggradation, offlap angle, outer-shelf dip, water depth, distance to the shelf margin and toe of slope, and facies-tract width. The systematic variations in these parameters, in conjunction with the facies distribution map and stratal geometries, helped to define the sequence-stratigraphic framework, and allowed for comparative evaluation of such things as sediment accumulation rates and sites, and stratigraphic evolution. The Capitan depositional system is represented by three composite sequences, each containing four high-frequency sequences. Two and one half of these composite sequences are exposed in McKittrick Canyon. The overall depositional system is interpreted to have evolved predictably from a deeper-water margin in the Seven Rivers composite sequence, to a shallow-water margin in the Tansill composite sequence. The subtidal outer-shelf and shelf-margin facies tracts were sites of major sediment production. Accumulation rates across the shelf margin indicate a relatively continuous growth history, with periods of nondeposition or erosion limited to the terminal phase of each composite sequence. As a result, the preserved sedimentary record of high-frequency and composite sequences in the outer-shelf to upper-slope position is equally proportioned between transgressive and highstand systems tracts. This symmetric outer-shelf to upper-slope record of carbonate accumulation is significantly different from the asymmetric, highstand-dominated middle-shelf accumulation record reported previously for this and many other carbonate shelves.

Strontium geochemistry of Miocene primary gypsum; Messinian of southeastern Spain and Sicily and Badenian of Poland

Abstract: The strontium contents of primary marine gypsum facies were studied in Messinian (Upper Miocene) basins of SE Spain and Sicily, and in the Badenian (Middle Miocene) Carpathian foreland basin of Poland. The results obtained were interpreted on the basis of the findings reported for the gypsum facies of recent salinas in the western Mediterranean coast. The strontium profile obtained from the lower part of the gypsum deposit of the Sorbas basin (SE Spain) has homogeneous strontium contents, reflecting deposition under conditions of hydrological stability and relatively deep waters, but the upper part has a more variable strontium content related, in accordance with other sedimentological criteria, to a shallowing of the basin. This is consistent with data reported from other Messinian basins of SE Spain (San Miguel de Salinas and Palma de Mallorca). In the Caltanissetta basin (Sicily), the section studied in Eraclea Minoa (Upper Evaporite) is made up of 6-8 gypsum cycles that have a facies evolution from fine-grained laminar gypsum at the base of each cycle to selenites at the top, accompanied by increasing strontium values. This evolution is attributed to increasing salinity conditions and is consistent with geochemical data reported in the literature. In the Badenian basin the six gypsum sections studied are from different subbasins. The gypsum deposits comprise two members. In the lower member, which is mainly selenitic, two parts can be differentiated and a significant strontium increase (attributed to a rise in salinity) was recorded in the transition from the lower to the upper part. In the upper member, which is mainly clastic gypsum, a general strontium decrease indicates a fall in salinity linked to a deepening or freshening of the platform. In the Badenian selenitic layers a lateral variation in the strontium contents exists: higher and less homogeneous strontium contents were recorded in the marginal zones, suggesting variable salinity conditions, whereas in the deeper areas the gypsum layers show more homogeneous strontium values, expected from more stable and less concentrated brines. This distribution is in agreement with data reported from the Messinian basin of San Miguel de Salinas (SE Spain). The strontium contents in both the Messinian and Badenian selenitic samples are lower than those in the selenitic crusts of the recent salinas. These lower contents may be a result of a lower salinity of the mother brines at the steady state achieved during the precipitation of the thick selenitic sequences, with respect to the highly concentrated (with no reflux) brines of the recent salinas. Our findings highlight the relevance of studying strontium contents in primary gypsum facies of marine formations as a tool for interpreting the paleogeography and the paleosalinity of the evaporitic enviroents and for correlating cyclic selenite formations. This method could be especially helpful when combined with other geochemical indicators (i.e., isotopic and/or fluid inclusions composition).

A Late Cambrian positive carbon-isotope excursion in the Southern Appalachians; relation to biostratigraphy, sequence stratigraphy, environments of deposition, and diagenesis

Abstract: A positive carbon-isotope excursion is recorded within the Upper Cambrian sedimentary succession in the southern Appalachians that consists of the Nolichucky Shale, the Maynardville Formation, and the Copper Ridge Dolomite. The lower part of the succession contains Aphelaspis zone fauna (Early Steptoean). The extensively dolomitized and poorly fossiliferous nature of the upper part of the succession precludes any detailed biostratigraphic determinations. Correlation with similar positive carbon-isotope excursions in coeval successions elsewhere suggests that this excursion represents a perturbation in the global cycling of carbon. Comparison of excursions at different localities in North America provides a means for the application of carbon-isotope stratigraphy. In the southern Appalachians the excursion started during deposition of the upper Nolichucky Shale. Maximum 13C values (4 to 5o/oo PDB) are associated with the conformable interval at the Maynardville/Copper Ridge Dolomite transition, which has been interpreted as a correlative conformity in sequence-stratigraphic terms. The excursion ended during deposition of the lower Copper Ridge Dolomite. In western North America the excursion started at the base of the Pterocephaliid Biomere (near the base of the Aphelaspis Zone). This well-documented excursion ended prior to the end of the Pterocephaliid Biomere, with the maximum excursion at the Sauk II/Sauk III unconformity. This supports the correlation between Late Steptoean (Dresbachian/Franconian) sea-level fall and the sequence boundary at the end of Cambrian Grand Cycle deposition in the southern Appalachians. The cause of this carbon-isotope excursion remains unclear. The excursion most likely reflects the enhanced burial of organic carbon promoted by ocean stratification, a warm nonglacial climate, and a sea-level maximum during the early Late Cambrian. The onset of regression may have contributed to the maximum carbon-isotope excursion by enhancing sedimentation rates, and by increasing organic productivity because of increased nutrient availability. The removal of carbon from the ocean surface may have caused a decrease in pCO2 of the atmosphere. The resulting cooling episode could have triggered an oceanic overturn bringing 12C-enriched bottom waters to the surface, which in conjunction with oxidation of organic matter during the sea-level fall, ended the carbon-isotope excursion. Comparison of 13C and 18O values of matrix samples to the associated cement phases provides insights into the relationship between isotope variations and depositional and diagenetic environments. 13C values of peritidal dolomicrite define a rather smooth stratigraphic variation curve, whereas the values for subtidal micrite have significant scatter resulting from involvement of organic matter in diagenesis. Fibrous to bladed calcite cement from the subtidal deposits has 13C and 18O values comparable to the associated micrite, suggesting precipitation from marine water and similar diagenetic modifications. Meteoric diagenesis may be responsible for the depletion of 13C and 18O in equant calcite cement relative to the micrite. For saddle dolomite cement, the depletion of 18O and 13C values similar to those for the peritidal dolomicrite, are consistent with formation during burial at elevated temperatures in a rock-dominated system. This study demonstrates the potential of applying carbon-isotope stratigraphy, developed in highly fossiliferous successions, to stratigraphic intervals with poorly constrained biostratigraphy. Such studies require evaluation of the effects of depositional environments and diagenesis upon the preservation of marine isotope signatures.

Molar-tooth carbonates: shallow subtidal facies of the mid- to late Proterozoic

Abstract: Molar tooth is a series of peculiar, ptygmatically folded and spar-filled cracks in fine-grained carbonates of Precambrian age. This sedimentary structure appears to be limited in time and space. Our field studies in northern Canada and a survey of literature on Precambrian carbonates worldwide indicates that such features are localized to mid/inner ramp and shallow platform paleoenvironments of Mesoproterozoic and Neoproterozoic age (ca. 1500-650 Ma) with only sporadic occurrences in older and younger strata. As such they are equivalent in terms of texture, location, and abundance to the ubiquitous shallow subtidal, fossiliferous limestones of the Phanerozoic. The amount of synsedimentary calcite spar in molar tooth, in excess of 50% of individual beds and 5-25% of entire formations, is further roughly equivalent to the amount of calcite in calcareous benthic invertebrates in these younger rocks. Thus, there is an evolving trend in such facies through geologic time; Archean-Paleoproterozoic = massive seafloor precipitates; Mesoproterozoic-Neoproterozoic = molar-tooth mudstones and grainstones; Phanerozoic = burrowed and fossiliferous limestones.

Early Devonian Kess-Kess carbonate mud mounds of the eastern Anti-Atlas (Morocco), and their relation to submarine hydrothermal venting

Abstract: Spectacular conical Early Devonian carbonate buildups up to 55 m high that crop out in the eastern Anti-Atlas of southern Morocco are microbially mediated carbonate mud mounds that were surfaced by soft-bottom communities dominated by small tabulate corals. They formed on the Hamar Laghdad elevation, which was created by a submarine volcanic eruption, and were associated with a network of synsedimentary radial and tangential faults that originated by uplift of the intrusive laccolithic body underlying the Kess-Kess Formation. These faults served as conduits for the migration of hydrothermal fluids to the sea floor. Most mounds developed over cross-points of radial and tangential faults. Vents were episodically active until the Famennian, but extensive vent carbonate production occurred only during the Emsian. Preliminary geochemical results document that mud-mound carbonates and calcite cements in neptunian dikes precipitated from brines comprising a mixture of hydrothermal fluids and seawater. In addition, carbon isotope compositions (13C as low as -18o/oo PDB) suggest a contribution from thermogenic methane derived presumably from underlying basaltic intrusives. Aerobic bacterial oxidation of methane is favored as the main process driving carbonate precipitation in, and rapid lithification of, the mounds.

Evaluation of the Global Oxygen Isotope Curve as a Proxy for Quaternary Sea Level by Modeling of Delta Progradation

Abstract: Patterns of delta progradation in rapidly subsiding basins have been used previously as a proxy for Quaternary sea-level change. Major transgression surfaces and maximum lowstand positions are readily recognizable in seismic reflection profiles. Quaternary sea-level change can be approximated by stacked and tuned oxygen isotope records, but significant departures from this proxy result from variation in isotopic composition of ice sheets, temperature variations in the deep sea, and the effect of bioturbation in degrading the isotopic record. We analyze the influence of rapid Quaternary sea-level change on delta progradation in the eastern Mediterranean using DELTA, a multi-process sediment transport model. Subsidence rates and sediment inputs may vary through time, but compared to Quaternary sea-level change, such variations were minor and had little effect on model results. For a variety of geologically reasonable sediment inputs and subsidence rates, the delta architecture observed in seismic profiles could not be reproduced when the sea-level estimates were taken directly from a scaled global oxygen isotopic curve. However, when this sea-level curve was corrected to include independent estimates of sea level from geological data elsewhere in the world, previously used sediment inputs and subsidence rates invariably predict the general form of the observed architecture. The difference between isotopic and geological estimates of sea level for the last 200 ka follows a linear trend that was applied to previous glacial-interglacial cycles, producing a corrected sea-level history for the last 540 ka. Using DELTA over this time span, the sea-level history based directly on the oxygen isotope curve was again less able to reproduce the observed delta architecture compared with a corrected sea-level history. This study shows the importance of modeling delta progradation as an avenue for testing and exploring Quaternary sea-level history.

Unroofing History of the Eastern Himalaya and the Indo-Burman Ranges: Heavy-Mineral Study of Cenozoic Sediments from the Bengal Basin, Bangladesh

Abstract: Upper Eocene to Neogene fill of the Bengal basin provides an earlier unroofing history of the eastern Himalaya and Indo-Burman ranges than that provided by drilling of the Bengal fan, and analysis of heavy minerals in these sequences provides useful provenance constraints. Quartzose sandstones of the Eocene Kopili and Oligocene Barail Formations contain only 0.2% heavy minerals, comprising abundant opaque minerals and stable minerals such as tourmaline, garnet, rutile, and zircon. This assemblage suggests sediment sources dominated by low- to intermediate-grade metamorphic, silicic igneous, and metasedimentary rocks; the low heavy-mineral content and abundance of stable minerals suggest intense chemical weathering. These sediments may have been derived in part from incipient uplifts of the proto-Himalaya and/or the Indo-Burman ranges, but a more likely source is the Indian craton immediately to the west. Miocene sandstones of the Surma Group contain more abundant and more diverse heavy-mineral assemblages than Oligocene sandstones. These include abundant opaque minerals and garnet, mostly almandine, and moderate to minor amounts of tourmaline, kyanite, zircon, calcic amphibole, rutile, chlorite, zoisite, staurolite, epidote, sillimanite, and clinopyroxene, indicating a broad range of mostly metamorphic source rocks. Upper strata of the Surma Group also contain abundant blue-green amphibole, orthopyroxene, and sparse chromite, suggesting exhumation of arc and ophiolitic rocks. Sands and sandstones from the upper Miocene to Pliocene Tipam Group and the Pliocene to Pleistocene Dupi Tila Formation contain assemblages similar to those of the underlying Miocene sandstones, along with more abundant orthopyroxene and sillimanite in Dupi Tila sands. Temporal variations in heavy-mineral assemblages in the Bengal basin suggest that orogenic detritus first appears in the eastern Himalayan foredeep much later (early Miocene) than in the western part of the foredeep (Eocene). The appearance of high-pressure phases in upper Miocene strata reflects continued unroofing of deeper-crustal metamorphic and silicic to ultramafic plutonic rocks.

Sequence analysis of the upper cretaceous two medicine and judith river formations, montana: nonmarine response to the claggett and bearpaw marine cycles

Abstract: Superb outcrop exposures and abundant subcrop data allow the accurate tracing of two stratigraphic discontinuities updip into fully nonmarine strata of the Campanian Two Medicine and Judith River formations (Western Interior foreland basin, Montana). These throughgoing discontinuities delimit "regressive" and "transgressive" alluvial equivalents of two third-order sea-level cycles, and provide ground truth for recent conceptual models of alluvial sequence stratigraphy. An erosional disconformity interpreted to mark the boundary between regressive and transgressive alluvial deposits crops out in nonmarine strata of the Two Medicine Formation in northwestern Montana. It is embedded in relatively flat-based fluvial sandstone sheets dominated by downstream accretion elements, and is marked by several meters of internal erosional scour, a thick and laterally persistent intraclast lag facies, pervasive oxidation, and a shift from fine- to medium/coarse-grained sandstone. Physical stratigraphic and geochronometric evidence indicate that this fluvial disconformity, which can be traced throughout the outcrop belt, correlates with the widespread 80 Ma sequence boundary developed in distal parts of the Western Interior Basin. The erosional disconformity in the Two Medicine Formation reflects a negative base-level adjustment that occurred during the Telegraph Creek-Eagle regression (R7), and conforms to the standard definition of a sequence boundary. Identification of the 80 Ma sequence boundary in alluvial facies of the Two Medicine Formation is significant in that it is one of very few well-documented examples of a nonmarine sequence boundary, but unlike most others, it is not characterized by a readily apparent facies tract dislocation reflecting a basinward shift in facies (e.g., braided-stream deposits sharply juxtaposed over coastal coal-bearing facies). A second throughgoing discontinuity embedded within fully nonmarine deposits of the Judith River Formation in central Montana is interpreted to separate regressive and transgressive alluvial deposits that accumulated during the Claggett regression (R8) and subsequent Bearpaw transgression (T9). This discontinuity correlates with the erosional base of a backstepping composite sequence set of shoreface strata, and can be traced inland to the western limit of Judith River strata preserved in central Montana (~ 50 km). The Judith River discontinuity is not erosional, but rather reflects a very abrupt change in alluvial architecture, most notably an abrupt shift from a sand- to a mud-dominated section that can be traced in outcrop and subcrop throughout north-central Montana and into southern Alberta. The throughgoing discontinuity in the Judith River record does not conform to conventional definitions of a sequence boundary, and it apparently did not form in response to a fall in relative sea level. This discontinuity instead appears to record an abrupt increase in the rate of generation of accommodation in the Montana portion of the foreland basin (presumably related to flexural subsidence), and it is provisionally interpreted as the nonmarine equivalent of a third-order transgressive surface coincident with the updip correlative conformity.

Gas bubble and expansion crack origin of molar-tooth calcite structures in the middle Proterozoic Belt Supergroup, western Montana

Abstract: "Molar-tooth" structures in the Belt Supergroup are complexes of interconnecting, thin sheets and small spheroids, composed of uniform, blocky 5-15 mm calcite crystals that cut fine-grained clayey carbonate layers. Bauerman (1885) likened the network of sheets to the corrugated surface of an elephant molar tooth. "Molar-tooth" structure described here comes from the Chamberlain and Helena formations of the Middle Proterozoic Belt Supergroup, but the structure occurs in other silty, clayey limestone and dolomite of the Belt and in Middle and Late Proterozoic rocks elsewhere in the world. Previous investigators have applied the term "molar-tooth" to several kinds of structures and attributed them to a variety of physical, biological, and chemical processes. We adhere to Bauerman's (1885) original citation of "molar-tooth" and limit our definition to blobs, vertical ribbons, and horizontal ribbons (O'Connor 1967, 1972; Eby 1977). Observations of molar-tooth samples suggest to us that ribbons formed as open voids through which gas and water flowed before being filled by fine, equant sparry calcite crystals. We tested the gas hypothesis by experimenting with mixtures of plaster of Paris, clay, water, sugar, and yeast in glass aquaria. Yeast metabolized the sugar, producing CO 2 gas that first formed bubbles in the mud slurry, nearly identical in shape to Belt "molar-tooth" blobs. The bubbles rose to the surface and pulled water behind them, stiffening the mud. We later sealed off the mud surface with plaster. The confined gas within the stiffened mud produced vertical and horizontal expansion cracks that were nearly identical in form to vertical and horizontal "molar-tooth" ribbon shapes. Finally, we precipitated fine equant crystals of calcium carbonate, identical to "molar-tooth" calcite, by mixing solutions of Na 2 CO 3 and CaCl 2 . The close replication of "molar-tooth" blobs and ribbons by biogenic gas convincingly explains the origin of the blob and ribbon forms. This explanation satisfies all the constraints of the data. We propose that some ribbons in the Chamberlain and Helena formations formed as gas cracks in water-saturated mud within a meter of the depositional surface, and were partially filled with fine calcite spar before compaction. Other cracks in the mud were first compacted and later filled by calcite. Still other "molar-tooth" structures probably formed deeper in the sediment column. Patterns of "molar-tooth" structure are commonly repeated within sedimentary cycles up to tens of meters thick, indicating that they may have formed during periods of lowstand between depositional cycles. Biogenic gas generated in Belt sediment probably included H 2 S, as evidenced by microcrystalline pyrite in "molar-tooth" calcite, CO 2 , and methane. Rapid calcite precipitation in the gas voids is probably necessary for "molar-tooth" preservation and may be a secular feature of the Middle and Late Proterozoic.

Does Clast Size Influence Fabric Strength

Abstract: Clast-fabric strength is commonly used as a quantitative tool to determine the depositional history of subglacial tills. This paper emphasizes the variance in fabric strength relative to clast size. Data on 46 clast-fabric analyses were sampled from two basal till localities in southeast Denmark and another eleven analyses from a basal till locality beyond a glacier in south Iceland. It is demonstrated that clasts longer than 2-3 cm show a stronger preference for parallel orientation than do shorter clasts. This implies that two operators sampling fabric data a few meters apart within the same till unit, but selecting clasts longer or shorter than 20 mm, may classify the till as two different types of basal till because of differences in clast size alone. Additionally, this study demonstrates considerable variations in fabric strength over short lateral distances within the same basal till bed. It is concluded that caution should be exercised in using eigenvalues to infer depositional processes and strain histories in subglacial environments; sedimentological criteria other than clast fabric must also be considered before drawing final conclusions on subglacial deposition.

Origin of fluvial grain-size trends in a foreland basin; the Pocono Formation on the central Appalachian Basin

Abstract: A widely recognized phenomenon of modern and ancient river systems is downstream decrease in grain size. Over the past two decades, theoretical formulations, observations of modern rivers, flume studies, and numerical models have significantly increased our understanding of sediment transport in mixed-grain-size river systems. These have established that downstream fining in modern and ancient rivers can be attributed to a combination of selective sorting, abrasion rate, and accumulation rate. However, no detailed multiple-grain-size sediment transport model has been used to address how much subsidence rate, sediment flux, water discharge, hydraulic geometry, and the mechanics of sediment transport influence grain-size distribution and facies belt development in ancient fluvial systems. We have combined the results of recent empirical and theoretical studies with a multiple-grain-size sediment transport model (MIDAS) to test the sensitivity of downstream fining trends to those controlling mechanisms. Our results demonstrate that subsidence and sediment feed rate are the most important mechanisms controlling downstream fining trends in a foreland basin if the evolution of hydraulic geometry is known. To illustrate the application of this result, we have replicated the textural trends of the Mississippian upper Pocono Formation and Burgoon Sandstone of Pennsylvania. This methodology can be applied to constrain plausible values of accumulation and subsidence rate for ancient alluvial deposits and enhances our ability to interpret paleohydraulic conditions from such facies in a foreland basin.

MN and FE-Rich Black Travertine Shrubs: Bacterially (And Nanobacterially) Induced Precipitates

Abstract: Black shrubs (i.e., arborescent forms) are present within hot-water travertine deposits from Morocco. The shrubs are commonly a few centimeters high and laterally grouped along individual laminae. Other than in mineral composition, the black shrubs closely resemble the calcite shrubs from other hot-water travertine deposits (e.g., Bagni di Tivoli, Italy, and Mammoth Hot Springs, Yellowstone National Park). They are composed of a variety of minerals that are Mn-rich and also have a significant Fe-oxide component. Similar Mn- and Fe-rich shrub-like forms have been described from desert varnish, deep-sea Mn nodules, and marine carbonates (Frutexites). SEM analyses show that bacterial bodies are densely packed within the Mn-rich black travertine shrubs whereas no bacterial bodies are evident within the enclosing aragonite and calcite laminae. The bacterial bodies range in shape and size from rods (commonly 0.8 m long by 0.1 m in diameter), to spherical forms (commonly 0.3 m in diameter) that form chains up to 1 m long, to nanobacterial spheres (150 to 200 nm in diameter) that make up densely packed chains commonly 15 m in length. It is well documented within the microbiological literature that, not only do bacteria have the ability to induce the precipitation of Mn and Fe, they can concentrate Mn in mineral precipitates from waters with very low to essentially undetectable concentrations of Mn. The black Mn-rich shrubs within the Moroccan travertines, other hot-water travertines, and related features such as desert varnish, deep-sea Mn nodules, and Frutexites are postulated to also be the result of bacterially induced precipitation.

Alluvial Fans and Fluvial Distributary Systems, Oligo-Miocene, Northern Spain: Contrasting Processes and Products

Abstract: Sediments of alluvial fans and distributive fluvial sediments were deposited by contrasting processes during the Oligo-Miocene in the north-central Ebro Basin, northern Spain. The coeval depositional systems can be separated on the basis of lithofacies criteria. A number of localized deposits, each covering only a few square kilometers and lying adjacent to the south Pyrenean thrust front, are made up of beds of poorly sorted conglomerate, sandstone, and mudstone that show a radial decrease in bed thickness and grain size. These deposits are interpreted as the products of sheet-flood events on alluvial fans that formed at the basin margin. Covering much larger areas of the basin (several thousand square kilometers) are conglomerate, sandstone, and mudstone as lensoid and irregular sheet bodies incised into thin-bedded sheets of sandstone and mudstone. These are interpreted as the deposits of fluvial systems that are considered to be distributary in form because the channel size and grain size of channel-fill deposits decrease down paleoflow and because there is a radial pattern of paleocurrents. At the distal margins, the fluvial facies interdigitate with ephemeral lacustrine and alluvial-plain deposits. The deposits of the alluvial fans and the fluvial distributary systems are petrographically distinct, reflecting separate source areas. Important components of the alluvial-fan deposits are limestone clasts derived from the adjacent thrust front. The deposits of the fluvial systems are mainly sand and subordinate gravel derived from rocks of the axial zone of the Pyrenean mountain belt. The deposits of these systems interdigitate, and there are no transitional forms between the two within this stratigraphic package.

Transgressive systems tracts and onlap shellbeds from mid-Pleistocene sequences, Wanganui Basin, New Zealand

Abstract: Sequences in the Castlecliff section (mid-Pleistocene, Wanganui Basin, New Zealand) are composed of transgressive and highstand systems tracts, and were deposited in response to fifth-order (c. 100 ky) and sixth-order (c. 40 ky) glacio-eustatic fluctuation on the paleo-Taranaki Shelf. Castlecliff transgressive systems tracts (TST), the topic of this paper, are ideally represented by fining-upward facies successions up to 22 m thick. Above a basal sequence-bounding ravinement surface, these successions usually comprise a subtidal cross-bedded shell-rich gravel (onlap shellbed) which is overlain by a fining-upward succession of inner-shelf siltstones and silty fine sandstones. TST successions are capped by mid-cycle condensed shellbeds (discussed in detail elsewhere) which represent terrigenous sediment starvation at the seaward feather-edge of TST deposition. Castlecliff TST facies architecture differs substantially from other examples of transgressive stratigraphy. First, Castlecliff TST are relatively thick in comparison to overall cycle thickness. Second, sequence-bounding ravinement surfaces at the base of Castlecliff TST everywhere rest directly on shelf siltstone constituting the truncated highstand systems tract of the underlying sequence; this indicates erosion of (1) coeval paralic TST landward of the ravinement surface, (2) the lowstand coastal-plain comprising the sequence boundary sensu stricto as well as any incised fluvial channels, and (3) the presumably coarsening-upward upper part of the underlying sequence. Such differences can be reconciled by consideration of local factors relating to the active tectonic setting of Wanganui Basin. Deposition occurred in a storm-influenced shelf embayment adjacent to a rising hinterland. In this setting, high-energy transgressive shorefaces scoured 10-20 m. At the same time, the embayed coastline provided a trap for the abundant sediment supply.

Siderite concretions from nonmarine shales (Westphalian A) of the Pennines, England; controls on their growth and composition

Abstract: Back-scattered electron microscopy has been used to examine the microstructure of nonmarine-shale-hosted siderite concretions. The concretions are composed of 50-100 micrometer, zoned crystallites, which exhibit no noticeable center-to-edge variation within any individual concretion. This indicates that siderite crystallites nucleated at virtually the same time across the entire concretion and that the concretions did not grow by radial addition of siderite layers around a central nucleus. Further siderite precipitation took place by crystal growth onto the nuclei. The total proportion of siderite in any part of the concretion bears no simple relationship to the porosity of the enclosing shale at the time of precipitation, and growth by passive precipitation in pore space is unlikely. Integration of microprobe data with bulk mineral-chemical and stable-isotope data suggests that the siderite crystallites are composed of an Fe-Mn-rich end member with a delta 13 C value of approximately +10 per mil and a Mg-Ca-rich end member with a delta 13 C value of approximately 0 per mil to -5 per mil. The mineral-chemical and stable-isotope compositions of these concretions resulted from microbially mediated processes operating close (<10 m) to the sediment-water interface, during methanogenesis. Methanogenesis can generate low-delta 13 C as well as high-delta 13 C carbonate cements, hence deep-burial diagenetic reactions, such as decarboxylation of organic matter, need not be invoked to generate solutes for siderite precipitation.

Comparing sea-ice sediment load with Beaufort Sea shelf deposits; is entrainment selective?

Abstract: Modern dispersal of sea-ice-rafted debris (IRD) is important for the Arctic Ocean sediment budget from sources to sinks. Sediment entrainment occurs mainly through the action of small ice crystals (frazil) attaching to sedimentary particles in shallow water, a mechanism that could be selective. The principal source for entrainment of IRD by suspension freezing into the Beaufort Gyre, western Arctic Ocean, is the adjacent shallow ( 250 m) lithology, and carbon and carbonate content of IRD in the Beaufort Gyre were compared to sediments from the probable source surface, in order to determine whether preferential entrainment occurs with any of these sediment parameters. IRD is generally much finer grained than the source surface, showing that silt- and clay-size particles are preferentially entrained by frazil ice, although anchor ice can locally incorporate very high percentages of sand and coarser clasts. The coarsest IRD is also the most poorly sorted. The clay mineralogy of the < 2 m IRD fraction is very similar to that of the source surface, indicating no selective entrainment within the clay fraction. The lithology of sand in IRD also matches that of the source surface, although the number of coarse grains is too small (< 100) in most samples for a statistically meaningful count. The average organic-carbon content of IRD is three times higher than that of the source surface, but we attribute this to summer algal growth on ice floes rather than to selective entrainment. A relatively low carbonate content in IRD may be because much of the carbonate in the source is of silt size while about 50% of the IRD measured is clay size. The low carbonate content may also reflect solution under acidic summer conditions on sea ice. Selective export of silt- and clay-size particles by ice rafting from the shallow shelf with time should lead to the formation of a slightly coarser lag, even though some of the dirty ice drops its sediment load in the entrainment area. Further mineralogical and lithological analysis on IRD promises to become a useful tool for the study of sediment dispersal paths by drift ice in the Arctic today and in the past, and also for the study of sources of anthropogenic pollutants found in sea ice.

Dynamics of bedforms in the lower Mississippi River

Abstract: The size and roughness characteristics of dunes in the Mississippi River are not predicted well by experimental and theoretical relations, even though intensive flow measurements were made in the study area. Although dunes increase in scale with increasing discharge of water and sediment, the development of multiple dune sizes and nonuniform flow obscure the relationship of dune geometry to synoptic hydraulic variables. Some nonuniformity is caused by the development of large bed undulations from kinematic waves that can deform into compound dunes, but most of it is related to flow convergence and divergence in pools and riffles, varying flow geometry with increasing stage, and reach-controlled relations between flow and energy loss. Even though changes of bedform size are not found to lag the flow changes because sand transport is large, a considerable volume of sediment is required to initiate and propagate the largest compound dunes. This means that they might be profiled in different stages of growth and have a varying effect on the flow during their evolution spatially and temporally.

Controls on Cyclostratigraphy of Lower Cretaceous Carbonates and Evaporites, Cupido and Coahuila Platforms, Northeastern Mexico

Abstract: The Lower Cretaceous Cupido (Barremian-Aptian) and Coahuila (Albian) carbonate platforms of northeastern Mexico exhibit thick successions of meter-scale cycles deposited in three unique paleoenvironmental settings. (1) The Cupido shelf lagoon is composed of peritidal carbonate cycles deposited in the protected lee of a reef-rimmed to barrier-shoal margin. (2) The restricted Coahuila ramp interior consists of cyclic alternations of subtidal evaporites and peritidal carbonates. (3) The deep-water parts of both the Cupido and Coahuila platforms are composed of foraminiferal wackestones and lime mudstones interspersed with firmgrounds and hardgrounds in a "cyclic" arrangement. Vertical successions of meter-scale evaporitic cycles and peritidal cycles exhibit systematic stacking patterns that build into intermediate-scale high-frequency sequences (tens to hundreds of meters thick), and large-scale composite sequences (hundreds of meters thick) that can be correlated across the Cupido and Coahuila platforms. These large-scale stacking patterns are interpreted to reflect longterm accommodation events and, when combined with the scale-independent architecture of all genetic units, permit the inference that all three meter-scale cycle types on the Cupido and Coahuila platforms are also governed by relative sea-level change. The composition, thickness, and number of meter-scale cycles within individual high-frequency sequences can be highly variable across the Cupido and Coahuila platforms, however, even though the overall upward-shallowing patterns are evident. The lateral complexity of cycle architecture and distribution is interpreted to be a natural response to fluctuations in regional climate interacting with autogenic processes such as variations in carbonate production and dispersal, intensity and frequency of tropical storms and monsoons, thermohaline circulation patterns, and ambient ocean chemistry and temperature. These interacting processes created laterally variable physiographic and oceanographic conditions across the Cupido and Coahuila platforms, complicating the sedimentary record generated by the composite sea-level signal. Well-documented evidence from Barremian-Cenomanian pelagic cycles throughout the Tethyan seaway strongly indicates that Milankovitch-driven climatic changes operated during the Early Cretaceous. Contemporaneous shallow-marine cyclicity in several locations suggests that these climatic changes may have had globally widespread effects. In an effort to link the shallow-water and deep-water realms, we propose a model whereby Milankovitch-driven global climatic changes generated low-amplitude, high-frequency eustatic fluctuations through some combination of thermal expansion and contraction of ocean water, waxing and waning of small ice caps and alpine glaciers, and changes in the storage capacity of aquifers and lakes to produce meter-scale cycles across Lower Cretaceous shallow-marine platforms.