Showing papers in "Journal of Sedimentary Research in 2004"
TL;DR: In this article, Petrographic, major, trace, and rare earth element compositions of sandstones from the upper Miocene Kudankulam Formation, Southern India, have been investigated to determine their provenance, tectonic setting, and weathering conditions.
Abstract: Petrographic, major, trace, and rare earth element compositions of sandstones from the upper Miocene Kudankulam Formation, Southern India, have been investigated to determine their provenance, tectonic setting, and weathering conditions. All sandstone samples are highly enriched in quartz (Q) but poor in feldspar (F) and lithic fragments (L). The major-element concentrations of these sandstones reveal the relative homogeneity of their source. Geochemically, the Kudankulam sandstones are classified as arkose, subarkose, litharenite, and sublitharenite. The CIA values (chemical index of alteration; mean value 44.5) for these sandstones and the A-CN-K diagram suggest their low-weathering nature. Similarly, their Fe2O3* + MgO (mean 2.7), Al2O3/SiO2 ( 0.09), K2O/Na2O ( 2.2) ratios and TiO2 contents ( 0.3) are consistent with a passive-margin setting. The Eu/Eu* ( 0.5), (La/Lu)cn ( 21), La/Sc ( 5.9), Th/Sc ( 1.9), La/Co ( 5.7), Th/Co ( 1.8), and Cr/Th ( 5.3) ratios support a felsic source for these sandstones. Chondrite-normalized REE patterns with LREE enrichment, flat HREE, and negative Eu anomaly also are attributed to felsic source-rock characteristics for Kudankulam sandstones. Total REE concentrations of these sandstones reflect the variations in their grain-size fractions. The source rocks are probably identified to be Proterozoic gneisses, charnockites, and granites of the Kerala Khondalite Belt, which must have been exposed at least since the late Miocene. Finally, the unusual Ni enrichment in the Kudankulam sandstones, unaccompanied by a similar enrichment in Cr, Co, and V, may be related to either the presence of pyrite in the sandstones or, more likely, the fractionation of garnet from the source rocks during transportation.
470 citations
TL;DR: In this article, a new equation for sediment fall velocity as a function of grain diameter for given values of fluid viscosity and fluid and solid density is presented, which is a fundamental parameter in the modeling and interpretation of fluviatile and coastal deposition.
Abstract: A new equation is presented for sediment fall velocity as a function of grain diameter for given values of fluid viscosity and fluid and solid density. Sediment fall velocity is a fundamental parameter in the modeling and interpretation of fluviatile and coastal deposition. The equation applies to the entire range of viscous to turbulent conditions, and its simple explicit form makes it easy to use in computer models and other applications in sedimentology, geomorphology, and engineering. The equation is derived from dimensional analysis and converges on Stokes' law for small grains and a constant drag coefficient for large grains. Its two physically interpretable parameters are easily adjusted for shape effects or for the use of sieve diameter rather than nominal grain diameter. It gives a close fit to published and new experimental data for both quartz sand and low-density materials, with no more error than previous equations of more complicated form.
436 citations
TL;DR: In this paper, a large number of experiments aimed at quantifying method and instrument uncertainty associated with laser diffraction analysis of fine-grained sediment is presented. But the results of these experiments are limited.
Abstract: In this paper, we present results from a large number of experiments aimed at quantifying method and instrument uncertainty associated with laser diffraction analysis. We analyzed the size distribution of fine-grained sediment ( 24 hours prior to analysis and using 60 seconds of ultrasonication during analysis. (2) Obscuration--a measure of the concentration of the suspension during analysis--produced the most reproducible results at about 20%. (3) Variations in refractive-index settings can significantly alter estimated grain-size distributions. (4) Assumed values for absorption (the degree to which sediment grains absorb the light) can have a profound effect on grain-size results. Absorption settings near 0 resulted in unexpected bimodal grain size distributions for sediments in the < 10 µm size fraction and significantly skewed the fine-grained tail of coarser samples, probably because of sub-optimal diffraction by particles with a diameter similar in size to the laser wavelength. Absorption settings closer to 1 produced very reproducible results and unimodal grain-size distributions over a wide range of refractive indexes. Our study has shown that laser diffraction can measure very fine-grained sediments (< 10 µm) quickly, with high precision ( 5% at 2 standard deviations), and without the need for extensive mineralogical determinations. These results make possible a new generation of studies in which high-resolution time-series data sets of sediment grain size can be used to infer subtle changes in paleohydrology.
355 citations
TL;DR: Autocorrelation between pixels in digital images of sediment can be used to measure average grain size of sediment on the bed, grain-size distribution of bed sediment, and vertical profiles in grain size in a cross-sectional image through a bed as discussed by the authors.
Abstract: Autocorrelation between pixels in digital images of sediment can be used to measure average grain size of sediment on the bed, grain-size distribution of bed sediment, and vertical profiles in grain size in a cross-sectional image through a bed. The technique is less sensitive than traditional laboratory analyses to tails of a grain-size distribution, but it offers substantial other advantages: it is 100 times as fast; it is ideal for sampling surficial sediment (the part that interacts with a flow); it can determine vertical profiles in grain size on a scale finer than can be sampled physically; and it can be used in the field to provide almost real-time grain-size analysis. The technique can be applied to digital images obtained using any source with sufficient resolution, including digital cameras, digital video, or underwater digital microscopes (for real-time grain-size mapping of the bed).
210 citations
TL;DR: In this paper, it is shown that calcite and vaterite formed in a gel medium in the presence of Myxococcus xanthus display a range of morphologies that depend on whether the bacteria are live or dead.
Abstract: It is thought that morphologies of bacterial carbonates can be used to identify microbial fossils and/or precipitates in sediments and rocks. This study shows that calcite and vaterite formed in a gel medium in the presence of Myxococcus xanthus display a range of morphologies that depend on whether the bacteria are live or dead. Metabolic activity of the bacteria induced: (1) aggregates of calcified bacteria formed at maximum supersaturation; (2) vaterite spheres (final growth stage of dumbbell fibrous-radiated aggregates); and (3) dipyramid- and disphenoid-like calcite crystals (combination of {011} and {0001} forms). Morphologies (2) and (3) developed at a lower supersaturation and are typically found in gel-like media. Dipyrimidal-like calcite crystals were also obtained abiotically in gel medium. Dead M. xanthus cells induced heterogeneous precipitation of calcite with rhombohedral morphologies at low supersaturation. A growth mechanism resulting from self-assembly of calcium carbonate nanocrystals may account for the observed morphologies, crystal microstructure, and crystallite size measurements. All of the above-mentioned morphologies of bacterial carbonate have been observed in other laboratory experiments and in continental and marine environments. However, all of them have also been produced abiotically, with the exception of calcified bacterial cells. This may make it more difficult to identify bacterial activity in the rock record. Nonetheless, bacterially induced alkalinization appears to be a prerequisite for the development of spherulitic and dipyramid- or disphenoid-like forms in natural mucilaginous biofilms and microbial mats. The morphologies reported here may facilitate the recognition of early and recent marine and continental microcrystalline bacterial carbonates and cements.
157 citations
TL;DR: In this article, the origin and possible relation with calcified plant roots were studied in calcrete profiles within the Paleocene shallow-marine carbonate succession in southwestern Slovenia. And the morphologies intermediate between the typical Microcodium aggregates, composed of a single layer of individual, elongate pyramidal or prismatic crystals of calcite (measuring 100-500 µm in length and 20-70 μm in width) and calcified roots with multilayer arrangement of isodiametric cells were observed, and this support previous rhizogenic interpretations of Micro
Abstract: Microcodium is a problematic calcitic microfeature of many calcretes and calcareous paleosols in the Cretaceous and Tertiary continental and marine successions of the peri-Tethyan realm. The main controversy about the Microcodium structures is their origin and possible relation with calcified plant roots. Microcodium and rhizogenic (root-formed) microfabrics were studied in calcrete profiles within the Paleocene shallow-marine carbonate succession in southwestern Slovenia. The prominent laminar calcrete horizons contain abundant calcite aggregates, 150 μm to 1 cm in size, with perfectly preserved structural details of plant root tissues. Morphology and structure of these aggregates indicate that they formed through biologically controlled precipitation of calcium carbonate within the root cortical cells. The morphologies intermediate between the typical Microcodium aggregates, composed of a single layer of individual, elongate pyramidal or prismatic crystals of calcite (measuring 100-500 μm in length and 20-70 μm in width) and calcified roots with multilayer arrangement of isodiametric cells were observed, and this supports previous rhizogenic interpretations of Microcodium structures. Intermediate forms show that the typical Microcodium aggregates formed through morphological transformation of the root tissue by growth of the calcite within the cortical cells, which distorted the cell shape. Calcification of roots and the creation of Microcodium structures can be explained as an effective nutrient-acquiring mechanism used by certain types of terrestrial plants inhabiting nutrient-poor calcareous substrates. The widespread occurrence of Microcodium in almost unaltered shallow-marine limestones indicates that its formation took place during early stages of paleosol development, probably reflecting specific types of vascular plants of a pioneer community that were able to colonize carbonate substrates during the early phases of subaerial exposure.
152 citations
TL;DR: In this paper, the Mg/Ca ratio of echinoderms with preserved stereom was analyzed from 29 specimens, Cambrian to Eocene age, containing ossicles preserved either as Mg calcite, 3.3 to 8 mole% MgCO3, or as mixed calcite and dolomite, 5 to 12.5 mole% mg CO3.
Abstract: The alternation between abiotic calcite and aragonite precipitation in shallow seawater and the oscillation of seawater Mg/Ca ratio throughout the Phanerozoic are both popular hypotheses. Echinoderms with well-preserved stereom provide new empirical evidence that supports both these hypotheses. Analyses are reported from 29 specimens, Cambrian to Eocene age, containing ossicles preserved either as Mg calcite, 3.3 to 8 mole% MgCO3, or as mixed calcite and dolomite, 5 to 12.5 mole% MgCO3. Echinoderms with high mole% MgCO3 occur in the Early Cambrian and late Carboniferous to Triassic; low values come from the Silurian and Jurassic to Cretaceous. The average composition of echinoderms with preserved stereom does not fall below 3.5 mol% MgCO3 during the Phanerozoic and reaches its highest mean value of 16.0 mol% MgCO3 today. An empirical partition coefficient of 0.03182 for modern tropical echinoids is used to indicate the Mg/Ca ratio of ancient seawater from the Mg/Ca ratio of fossil stereom; some assumptions are involved and a likely error is calculated from the Mg2+ variation of modern echinoids. High mean seawater Mg/Ca ratios are calculated for early Cambrian (3.3) and the late Carboniferous to Triassic (2.3) but never reached today's value of 5.2; low Mg/Ca ratios (1.1) are indicated from Jurassic to Cretaceous echinoderms. The 29 echinoderm samples plot close to first-order Mg/Ca seawater oscillations derived from geochemical models and Mg/Ca ratios determined from fluid inclusions, but considerable discrepancies exist when shorter (106 Myr) time intervals are considered. Further data and improved understanding of Mg partitioning is required before accurate secular variation in the Mg/Ca of seawater can be determined from echinoderms. However, they are an underused resource in this context and provide an excellent seawater archive.
148 citations
TL;DR: Carbon isotope data from Upper Cambrian sections in three Laurentian basins in northern Utah, central Iowa, and western Newfoundland record a large positive d 13 C excursion (SPICE event) of up to 1 5
Abstract: Carbon isotope data from Upper Cambrian sections in three Laurentian basins in northern Utah, central Iowa, and western Newfoundland record a large positive d 13 C excursion (SPICE event) of up to 1 5‰. Peak d 13 C ratios are well dated by trilobite collections to the middle of the Steptoean Stage (Dunderbergia Zone) and occur during maximum regression associated with formation of the Sauk II- Sauk III subsequence boundary on the North American craton. Max- imum regression was marked by an influx of quartz sand into carbon- ate-platform settings in all three widely separated basins. In northern Utah, this quartz sand formed a thick sequence known as the Worm Creek Quartzite, which marks a conspicuous interruption of carbonate deposition during the Middle to Late Cambrian in the region. In west- ern Newfoundland, the thickness of the quartz sand unit is much re- duced but still marks a brief shutdown of the carbonate factory that is unique to the Cambrian shelf succession of the area. In the central Iowa area of the cratonic interior, an upward-shallowing carbonate succession culminates in cross-stratified trilobite grainstones at the peak of the SPICE in Dunderbergia Zone time, and the lowest point on the relative-sea-level curve is associated with the occurrence of coarse quartz sand derived from the encroaching shoreface. Although it is difficult to determine precisely the departure from baseline d 13 C that marks the beginning of the SPICE excursion in the stratigraphic successions analyzed, our results are consistent with a rise and subsequent fall in d 13 C tracking a major regressive-transgressive event recorded across northern Laurentia. The correlation of a major d 13 C excursion with regression is similar to that described for the Late Ordovician, for which the pattern has been attributed to either in- creased carbonate relative to terrigenous weathering rates as ice sheets covered up organic-matter-containing silicates at high latitudes or high productivity and organic-carbon burial driven by oceanic overturn. The lack of known Steptoean-age ice sheets that could have affected the ratio of carbonate to silicate weathering rates suggests that organic- carbon burial was the likely cause of the SPICE event. We suggest that increased weathering and erosion rates during relative sea-level fall (Sauk II-III) increased the burial fraction of organic carbon in an expanded region of fine-grained siliciclastic deposits in shelf and upper slope environments during the Steptoean.
135 citations
TL;DR: In this article, an occurrence of inclined heterolithic stratification (IHS) is described from a tidal point bar in a 40m-deep distributary of the macrotidal (tidal range 3.6-7.8 m), Han River delta, Korea.
Abstract: An occurrence of inclined heterolithic stratification (IHS) is described from a tidal point bar in a 40-m-deep distributary of the macrotidal (tidal range 3.6-7.8 m), Han River delta, Korea. The channel bank demonstrates a convex-upward profile with intermittent presence of wave-formed scarps and terraces near the low-water level. The vertical succession of IHS is approximately 25 m thick and dips into the channel with angles reaching 14°. The IHS overlies 15 m of trough cross-bedded sand deposited in the channel thalweg and lower point bar. Even though the channel as a whole is ebb dominated, the preserved cross bedding is predominantly flood directed because the mutually evasive nature of the ebb and flood currents causes the point-bar surface to be flood dominated. This pattern may be a common feature of tidal point bars. The IHS itself consists of interstratified fine sand, sandy silt, and silt with an upward-fining textural trend. Tidal rhythmites are well developed in the middle and upper intertidal zone, and may also be present in the subtidal zone, but are poorly developed near the low-water level because of wave action. Seasonal discharge variations of the Han River are not obvious in the deposits, because the large size, distal location, and energetic tidal environment of the studied channel reduces the impact of river-stage fluctuations. Despite the moderate salinity levels, bioturbation is rare, except in the upper intertidal zone, because of the rapid sedimentation and energetic conditions.
127 citations
TL;DR: In this paper, a combination of calcitic belemnite samples from the Basque-Cantabrian basin of northern Spain have been analyzed to improve our current understanding of the chemistry of seawater as an index of paleoceanographic changes during the Early Jurassic, a period punctuated by times of oceanic anoxia and global mass extinctions.
Abstract: Well preserved calcitic belemnite samples from the Basque-Cantabrian basin of northern Spain have been analyzed to improve our current understanding of the chemistry of seawater as an index of paleoceanographic changes during the Early Jurassic, a period punctuated by times of oceanic anoxia and global mass extinctions. Because the oxygen-isotope composition of calcite depends not only on temperature but also on salinity, we present here a combination of Sr/Ca, Mg/Ca, and δ18O analyses that helped us to evaluate both effects separately. Comparison between the temporal records and cross plots of these geochemical parameters shows a good correlation of Mg/Ca with δ18O and a weak correlation of Sr/Ca with δ18O.
Comparison of our belemnite δ18O record with coeval O-isotope profiles from other paleogeographic domains suggests the existence, despite differences in the absolute isotopic values, of a major negative shift during the serpentinus Zone that is regionally reproducible. This event is linked to the Early Toarcian ocean anoxic event (OAE), and suggests the existence of global changes in paleoclimatic and paleoceanographic conditions that are reflected largely in the geochemical composition of seawater. Compared to our results, however, other Early Toarcian δ18O data reported from northern European basins are generally depleted ∼1.3-1.6‰ In contrast, Mg/Ca ratios display similar values. These differences in δ18O data suggest a salinity component for the northern European data and provide evidence for a north-to-south salinity gradient within the north European interior (Boreal) seaway during the Early Jurassic. This study suggests that the secular variation of Mg/Ca is a useful proxy for discriminating between temperature and salinity effects on the oxygen isotope composition of fossil shells, and highlights the prospect of using the Mg/Ca ratios of belemnite calcite to calculate paleotemperatures independently of salinity. The findings of our study may help to calibrate δ18O curves in other basins by removing the salinity component of their δ18O records, which may help to calculate water density gradients between the Jurassic seas of northern and southern Europe.
108 citations
TL;DR: The mixing-zone diagenetic model predicts alteration in only part of the mixing zone, and because the alteration recorded in these cores shows continuous alteration through the entire mixing zone as mentioned in this paper, they consider the mixing of waters itself to be largely irrelevant.
Abstract: Three cores from Great Bahama Bank and southern Florida contain examples of mixing-zone diagenesis where typical shallow-water, aragonite-rich sediments were altered to low-Mg calcite. The 40–50 m thick mixing zones are identified on the basis of downcore covariant trends from depleted meteoric (δ18O = -3.2 ± 0.7‰; δ13C = -1.6 ± 1.8‰) to enriched marine isotopic values (δ18O = +1.0 ± 0.3‰; δ13C = +1.9 ± 0.7‰). The age of the mixing zones varies from late Pliocene to Pleistocene, so these cores provide three independent examples of mixing-zone diagenesis. Mixing-zone diagenesis produced a limestone composed of low-Mg calcite with petrographic fabrics including moldic porosity, blocky to dogtooth calcite cementation, and recrystallization of lime mud to micrite and microspar. The mixing-zone diagenetic model predicts alteration in only part of the mixing zone. Because the alteration recorded in these cores shows continuous alteration through the entire mixing zone, we consider the mixing of waters itself to be largely irrelevant. Rather, diagenesis was probably driven by the difference in solubility between aragonite and calcite with some additional dissolution of aragonite driven by the microbial oxidation of buried organic matter. The mixing zone has been proposed as a likely location for dolomitization. With the exception of some clearly sea-floor dolomite in one core (Unda), the mixing zones in these cores lack dolomite despite the sediment clearly having been altered in the mixing zone. The lack of dolomite in these mixing zones, combined with the extensive literature questioning the chemical reasoning for mixing-zone dolomite, supports rejection of the mixing-zone dolomite model.
TL;DR: This paper examined 39 samples of siliceous sinter from the low-temperature (35°C) palisade microfacies at Orakei Korako and Te Kopia geothermal fields, New Zealand, in three different postdepositional environments (least-disturbed, heat-affected, weathered).
Abstract: The opal-A to opal-CT silica phase transformation in New Zealand's siliceous hot-spring deposits (sinter) is accompanied by recurring textural changes at the micron scale. Recognition of these changes is essential to extract paleohydrological, paleoenvironmental, and paleobiological signatures from ancient hydrothermal systems, and to recognize diagenetic overprints upon primary depositional signals. We examined 39 samples of siliceous sinter from the low-temperature (< 35° C) palisade microfacies at Orakei Korako and Te Kopia geothermal fields, Taupo Volcanic Zone, New Zealand, in three different postdepositional environments (least-disturbed, heat-affected, weathered). Friable to indurated samples were collected from modern, presently silicifying microbial mats in the outflow of nearly neutral alkali chloride springs, to sinter 3,500 years old. The progressive mineralogical change from noncrystalline opal-A to paracrystalline opal-CT was traced by X-ray powder diffraction and scanning electron microscopy. One diagenetic sequence with two morphological pathways, spherical and smooth (polymeric and monomeric deposition, respectively), was identified at the micron scale. Both pathways ultimately lead to formation of typical, opal-CT bladed lepispheres. Along the spherical pathway, initial morphological restructuring includes formation of circular holes (< 0.1 to 1.0 µm in diameter) in opal-A spheres (< 3.0 µm in diameter). These changes correlate with a shift in the maximum intensity of the opal-A X-ray scattering broadband, from 4.0 A to 4.09 A, the characteristic position for opal-CT. An early transitional diagenetic stage is indicated in both sphere and smooth silica pathways by development of hexagonal platelets and a composite opal-A/-CT scattering broadband that is sharp-peaked in comparison, and centered at 4.09 A. A still sharper-peaked, composite opal-CT/-A X-ray diffraction band follows, which has developed an incipient tridymite shoulder; it is found in samples that display both hexagonal platelets and incipient "fuzzy" opal-CT lepispheres. At this late transitional stage, patchy replacement textures are visible at the macroscale. Only with the appearance of numerous, well-developed, opal-CT bladed lepispheres is a typical, sharp-peaked (4.09 A), opal-CT X-ray trace evident, with a well-defined tridymite shoulder. This diagenetic process can produce either porous or vitreous fabrics, depending on the degree to which infill of silica has reduced porosity and increased density of the deposit. This study provides a textural-mineralogical context for understanding silicification of microbial communities in geothermal settings and their subsequent diagenetic modifications. These modifications continue long after geothermal activity has ceased, with little or no burial, and occur in horizons or localized patches of a given deposit. Moreover, postdepositional conditions, such as heat overprinting or weathering, influence the diagenesis of siliceous sinter by increasing its maturation rate.
TL;DR: In this article, different sample preparation techniques accentuated different components of the heterogeneous biofilm, thus resulting in vastly different textures and distributions, and the authors concluded that the cultured bacteria produced a biofilm that had a different surface texture and was more susceptible to sample preparation artifacts than the in situ biofilm.
Abstract: When biofilms (aggregations of bacteria and extracellular polymer secretions) in samples from the Carter Sandstone of Alabama were prepared for scanning electron microscopy (SEM) using different dehydration techniques, the organic material had visibly different textures and distributions. In order to assess whether the variation was attributable to sample preparation or to inherent biofilm heterogeneity, each of five techniques were tested 3 to 10 times on small (1 cm) pieces of the Carter Sandstone containing either a strain of bacteria cultured from and reintroduced into the rock, or an in situ biofilm grown by injection of nutrients through core samples. The techniques tested were (1) air drying alone; (2) fixation in 10% glutaraldehyde with air drying; (3) ethanol dehydration with hexamethyldisilazane (HMDS) drying [2.5% glutaraldehyde, ethanol dehydration, and HMDS]; (4) ethanol dehydration with critical-point drying; and (5) ethanol and acetone dehydration with critical-point drying. Unpreserved control samples were either imaged wet in an environmental scanning electron microscope (ESEM) or vacuum-dried for SEM. Observations were based on SEM microscopy of over 60 samples and study of over 150 photomicrographs. In both experiments, the original morphology of individual bacteria was best preserved by ethanol dehydration with HMDS drying, ethanol dehydration with critical-point drying, or ethanol-acetone dehydration with critical-point drying. Critical-point drying preserved bacteria but stripped away mucilaginous material, revealing filamentous structures within the biofilm. These filaments, along with masses of microspheres (nannobacteria?) and the smooth mucilaginous outer layer, also occur in wet samples studied by ESEM, and are, therefore, not dehydration artifacts. However, different sample preparation techniques accentuated different components of the heterogeneous biofilm, thus resulting in vastly different textures. The cultured bacteria produced a biofilm that had a different surface texture and was more susceptible to sample preparation artifacts than the in situ biofilm. Use of more than one sample preparation technique is recommended in order to avoid bias.
TL;DR: In this article, a three-tier hierarchy of depositional cyclicity of alluvial strata of latest Cretaceous and earliest Tertiary age are continuously exposed along Dawson Creek, in Big Bend National Park, west Texas, U.S.A.
Abstract: Paleosol-bearing alluvial strata of latest Cretaceous and earliest Tertiary age are continuously exposed along Dawson Creek, in Big Bend National Park, west Texas, U.S.A., and exhibit a three-tier hierarchy of depositional cyclicity. Meter-scale, fluvial aggradational cycles (FACs) occur as fining-upward successions that are gradation- ally overlain by paleosols or are sharply overlain by the coarser- grained base of the succeeding FAC without an intervening paleosol. FACs stack into decameter-scale, fluvial aggradational cycle sets (FAC sets) that also fine upward, and from base to top contain either a grad- ual upsection increase in soil maturity and soil drainage or a somewhat symmetrical pattern of increasing and decreasing paleosol maturity. Longer-period trends of FAC thickness, lithologic proportions, paleosol maturity, and paleosol drainage indicate that two complete, and two partial, hectometer-scale fluvial sequences occur within the study in- terval. From base to top, each sequence is characterized by an asym- metric increase and decrease in FAC thickness, a decrease in the pro- portion of sand-prone fluvial facies, an increase in paleosol maturity, and better paleosol drainage. Whereas FACs and FAC sets are interpreted to record cyclic epi- sodes of channel avulsion and stability, and longer-term avulsive chan- nel drift within the alluvial valley, respectively, fluvial sequences may coincide with third-order sea-level changes within the North American Western Interior Seaway. As such, the Cretaceous-Tertiary (K-T) transition within the Tornillo Basin may provide an example of me- gascale stratigraphic cyclicity that is controlled by eustatic sea level within a fully fluvial succession. Thickening and thinning successions of FACs record a third-order period of accelerating (transgressive- equivalent) and decelerating (highstand-equivalent) base-level rise, and subsequent base-level fall (falling stage- to lowstand-equivalent). Se- quence boundaries are placed at the sharp inflection between thinning and thickening FACs. Sand-prone facies and immature, more poorly- drained paleosols are associated with the transgressive-equivalent por- tion of each sequence, and mudrock-dominated overbank facies and their associated mature, well-drained paleosols are associated with the highstand- and falling stage-equivalent.
TL;DR: In this article, the authors show that along-strike variations in the rate of transgression were caused by the variable inner-shelf gradients, which increase towards the west, and the orientation of the Sabine incised valley and associated terraced fluvial deposits, which trend northeast-southwest (parallel to shore).
Abstract: The low gradient east Texas coast and inner-continental shelf, from Sabine Lake at the Texas-Louisiana border to the western end of Galveston Island, experienced extreme along-strike variations in rates of Holocene coastal retreat. Around 7.7 ka a barrier shoreline was located approximately 55 km offshore. Toward the western end of Galveston Island, the shoreline retreated 55 km, occupying a position on the lagoon side of the Island by about 5.3 ka. Toward the Texas-Louisiana border, the shoreline retreated more gradually, occupying a position seaward of Sabine Bank by 5.3 ka. Between 4.7 ka and 2.8 ka the shoreline at Sabine Bank retreated 30 km, while Galveston Island prograded seaward. Bolivar Peninsula began to accrete around 1.5 ka. Heald and Sabine banks, located on the inner continental shelf above terraced fluvial deposits of the Trinity-Sabine incised valley, are the only preserved remnants of these former shoreline positions. Fluctuating rates of sea-level rise were not the forcing mechanism behind episodes of rapid shoreline retreat because these events were localized. Rather, along-strike variations in the rate of transgression were caused by the variable inner-shelf gradients, which increase towards the west, and the orientation of the Sabine incised valley and associated terraced fluvial deposits, which trend northeast-southwest (parallel to shore). As shorelines retreated over fluvial deposits, these served as local sand sources that enabled barrier islands to persist Figure 1. Map of the east Texas inner shelf showing locations of Sabine, Heald, and Shepard banks. Paleotopographic map of the Pleistocene surface (thin contours; Siringan 1993) delineates the Trinity-Sabine incised valley, which formed during the last lowstand of sea level (Thomas and Anderson 1994). Bathymetric contours are shown as thick lines with bold numbers. Dots are core locations. End_Page 405------------------------ offshore, out of equilibrium with sea level. Once these sand sources became depleted, and/or sea level reached some critical threshold, barrier shorelines became stranded offshore as banks, and new shorelines formed landward. The geologic setting of coastal areas, specifically antecedent topography, plays a primary role in controlling coastal evolution. To accurately forecast long-term (centennial to millennial) coastal evolution, it is essential that impacts associated with variations in the underlying geology of coastal areas be incorporated into coastal forecasting models.
TL;DR: In this article, the authors show that the case for deposition of the Miocene evaporites of the sub-Mediterranean under shallow-water conditions is based on interpretations that are either equivocal or incorrect.
Abstract: The idea that the Mediterranean Sea dried out completely during the late Miocene (Messinian, 5-6 Ma) is now widely accepted. This idea, first published in 1972 (Hsu 1972a; Hsu 1972b) grew out of the interpretation by Deep Sea Drilling Project (DSDP) scientists that the evaporites recovered from beneath the floor of the Mediterranean by drilling on DSDP Legs 13 (1970) and 42A (1975) were of shallow-water origin. Since that time, there have been a number of advances in understanding of evaporite depositional processes, and with this in mind, we have reexamined the evidence provided by the DSDP evaporite cores. We show that the case for deposition of the Miocene evaporites of the sub-Mediterranean under shallow-water conditions is based on interpretations that are either equivocal or incorrect. Several features of these evaporites used by the DSDP workers as evidence of shallow-water conditions are more compatible with deposition under deep-water (below wave base) conditions, and others can only be considered as of uncertain origin. In addition, coring has sampled only the upper few tens of meters of the evaporites, a factor that seriously limits any interpretation of the origin of the deposit as a whole. In view of these findings, we suggest that the question of the origin of the Miocene evaporites beneath the floor of the Mediterranean Sea be reexamined by the geologic and oceanographic communities.
TL;DR: In this article, the lowermost 300 m thick Fan A-F turbidite system was studied and the facies associations include channel fills and correlative lateral overbank deposits of thin sheet-like and rippled sandstones.
Abstract: Six individual turbidite systems, informally called Fans A-F, were deposited in the Laingsburg area of the southwestern Karoo Permian foreland basin. This study concentrates on the lowermost, 300 m thick "Fan A" system. Facies associations include channel fills and correlative lateral overbank deposits consisting of thin sheet-like and rippled sandstones. Massive- and thin-bedded frontal sheet sandstones form down-dip extensions to channel systems. Identification and correlation of mudstone-dominated intervals from field mapping and oblique aerial photostratigraphy delineates a high-resolution internal stratigraphy of the fan system which, coupled with 4000 m of logged section, allows detailed geometrical and architectural analysis. Seven individual depositional units have been mapped within Fan A, and these are interpreted as the deep-basin sedimentological expressions of high-frequency lowstand systems tracts, separated by high-frequency transgressive and highstand condensed intervals. Stacking patterns of the seven lowstand fan units that make up Fan A record early progradation (units 1 to 3), a backstep (unit 4), followed by further progradation (units 5 and 6). Retrogradation during unit 7 records abandonment of the whole Fan A deposystem. Coupled with facies analysis, paleocurrents reveal unusual paleotransport patterns that are interpreted as a consequence of structural deformation of the basin floor. Paleotransport indicators reveal that sediment pathways are strongly parallel to the structural grain, while some point to sediment pathways that crosscut the dominant structural grain.
TL;DR: Celestine (SrSO4) is a minor diagenetic mineral phase in marine carbonate sediments, where it is thought to form during the transformation of Sr-bearing aragonite or biogenic calcite to low-Sr calcite or dolomite as mentioned in this paper.
Abstract: Celestine (SrSO4) is a common minor diagenetic mineral phase in marine carbonate sediments, where it is thought to form during the transformation of Sr-bearing aragonite or biogenic calcite to low-Sr calcite or dolomite. The largest known sedimentary celestine occurrences, however, contain several million metric tons of SrSO4 and cannot be as readily explained. These large deposits occur as epigenetic replacements and cavity-fill in coastal marine carbonate and evaporite sequences. Host rocks range in age from Silurian to Pliocene. There is variable evidence of associated hydrothermal activity, and the Sr typically has a radiogenic signature. One of the keys to explaining the origin of these massive celestine deposits is determining why celestine and not barite (BaSO4), which is orders of magnitude less soluble, is the replacement phase. Although precipitation of massive celestine beds during the early stages of evaporation of sea water has been invoked by some, this hypothesis is not supported by either experimental or field observations, which indicate most sea-water Sr is removed as dispersed sulfate phases during the halite stage of evaporation. The association of large celestine deposits with coastal marine carbonate-evaporite sequences more likely reflects in part the significant difference in the geochemical behavior of Sr and Ba during burial diagenesis. The concentration of Sr in subsurface sedimentary fluids is buffered by silicate-carbonate mineral assemblages, and the concentration of Sr increases significantly with increasing salinity. The concentration of Ba, in contrast, is controlled by equilibrium with barite and is inversely related to the concentration of dissolved sulfate. Basinal fluids having the highest Sr/Ba ratios, and thus those most likely to form celestine rather than barite, have high salinities and moderately high levels of sulfate. The precursors to such waters can be produced in coastal marine settings by the evaporation of sea water. As these brines reflux into underlying or laterally adjacent sediments, they can leach substantial amounts of Sr as they attempt to achieve chemical equilibrium with their host sediments. Barium concentrations are kept low by the presence of dissolved sulfate. If these diagenetically altered fluids are then discharged into beds containing calcium sulfate minerals and/or sulfate-rich waters, the precipitation of celestine would result. Porosity produced by uplift and karstification of associated carbonates would help focus fluid flow.
TL;DR: In this paper, the authors show that the compositional heterogeneity in the opal-A, as highlighted by BSEI, must reflect variation in the water content of opal A.
Abstract: Sinter and geyserite, formed largely of amorphous opaline silica (opal-A), precipitate at many hot-spring and geyser systems like those found on the North Island of New Zealand. Opal-A is a hydrated silicon dioxide (SiO2.n H2O) that may contain silanols, trapped water, and/or absorbed water. Little is known about the factors that control the distribution of water (H2O + OH) in sinter and geyserite. Backscattered electron images (BSEI) of geyserite and sinter commonly reveal sectors and/or alternating laminae that are characterized by differences in their average atomic weights. Electron microprobe (EMP) and scanning electron microscope (SEM) analyses, however, show that differences in the average atomic weights cannot be attributed to the presence of other elements (e.g., Au, Ag) or minerals (e.g., sulfur, kaolinite, jarosite). The compositional heterogeneity in the opal-A, as highlighted by BSEI, must reflect variation in the water content of the opal-A. Integration of BSEI and EMP analyses for Si and O shows that opal-A contains "excess" O that must be bound into the H2O and OH. The excess O can be used a proxy for the total water (H2O + OH) content of the opal-A that forms geyserite and sinter. Geyserite and sinter from the Whakarewarewa geothermal area is formed of "dry" opal (average 5-6 wt % H2O + OH) and "wet" opal (average 12-13 wt % H2O + OH). Interpretation, based largely on the distribution of these different types of opal in the geyserite and sinter, suggests that the "wet" opal forms as a result of rapid precipitation whereas the "dry" opal forms as a result of slower precipitation. Such information must be integrated into any model that attempts to explain the evolution of geyserite and sinter deposits.
TL;DR: In this article, a review of current practice in coastal science and engineering with regard to quantitative determination of longshore drift is presented, concluding that coastal scientists and engineers have been trapped in an expected universe of long-shore-transport sand volumes without critical assessment of assumptions made in pioneering studies.
Abstract: On the basis of a review of current practice in coastal science and engineering with regard to quantitative determination of longshore drift, we conclude that coastal scientists and engineers have been trapped in an expected universe of longshore-transport sand volumes without critical assessment of assumptions made in pioneering studies. As a result, workers in this field have come to accept a range of sand volumes and a range of techniques to measure or predict these volumes, stabilized by opinion of the leading experts. The shortcomings of previous studies and subsequent practice, however, indicate that these transport volumes and techniques may well be wrong. Certainly at present we have no dependable, verifiable, and consistent field measure of net or gross instantaneous or annual longshore transport volumes against which predictions can be compared. Both field measurements and mathematical model results, especially as used for applied purposes and expressed in annual terms, are suspect for a number of reasons. These include questionable assumptions that lag far behind our current understanding of shoreface processes, "fudge factor" coefficients for equations, the lack of storm transport measurement, and a cascade of uncertainties that moderate current practice. Once determined, annual longshore-transport sand volumes tend to be long-lived and in all cases are unverifiable by field measurement. More fundamentally, the question is raised whether an earth surface system as complex and variable as longshore transport on beaches can ever be quantitatively modeled or measured successfully. Qualitative mathematical modeling remains a valid and useful approach to understanding the nature of sedimentary processes.
TL;DR: In this paper, the authors presented a well-exposed example of the geological record of the birth, life, and death of a mud volcano, showing that the chimneys were the pathway for the expulsion toward the sea floor of gas-and sediment-charged fluids likely originated from destabilization of methane gas hydrates.
Abstract: Chaotic deposits are frequently reported in the geological literature and are commonly interpreted as olistostromes or tectonic melanges. A chaotic complex in the Cenozoic succession of Monferrato (NW Italy) consists of interbedded mud breccia and burrowed silty clays that are pierced by sheared mud breccias and embed carbonate-cemented blocks. These may be represented by microcrystalline limestones or strongly cemented matrix-supported breccias locally containing remains of chemosymbiotic organisms (lucinid bivalves). Moreover, cylindrical concretions, up to 15 cm in diameter and 1 m long, occur in the chaotic complex and crosscut bedding planes at high angles. The cement of all these lithified portions is mainly dolomite characterized by low 13C values (from -10.3 to -23‰ PDB) and 18O values up to +7‰ PDB. The 13C values testify to precipitation of carbonates induced by microbial oxidation of methane, whereas the markedly positive 18O signature, ubiquitous in the cylindrical concretions, is the evidence for the presence and destabilization of gas hydrates. The studied section provides a well-exposed example of the geological record of the birth, life, and death of a mud volcano. Unsheared, soft mud breccias represent mud flows along the flanks of the volcano, whereas sheared mud breccias are the result of the injection of unconsolidated overpressured fine-grained sediments, both taking place during "eruptive" phases. They were followed by more quiet stages of hemipelagic sedimentation, burrowing, and CH4 seeping. The cylindrical concretions represent the first described ancient example of the chimneys observed in present-day mud-volcano settings. They are the remnants of a cold-seep plumbing network that crosscut the mud volcano edifice. The chimneys were the pathway for the expulsion toward the sea floor of gas- and sediment-charged fluids likely originated from destabilization of methane gas hydrates. The association of mud breccias and methane-derived carbonates may not be due to mass gravity flows but can be primary and, therefore, is a diagnostic criterion for recognizing chaotic deposits due to mud volcano activity in the geological record.
TL;DR: In this paper, the preservation of pedogenic mud aggregates in alluvial sediments, basin infill dynamics, and paleosol development in ancient dryland river systems is discussed.
Abstract: Mudrocks with well preserved pedogenic mud aggregates in the alluvial Upper Triassic Lunde Formation, northern North Sea, provide information about preservation potential in alluvial sediments, basin infill dynamics, and paleosol development in ancient dryland river systems. Sand-size to millimeter-size pedogenic mud aggregates composed of densely packed clay and silt dominate the floodplain facies. These aggregates are present both in situ in paleovertisol profiles and as reworked aggregates which form thick floodplain units. In situ aggregates are characterized by homogeneous composition, closely associated with pedogenic slickensides and crystallaria, and display evidence of translocation of clays around them. The reworked mud aggregates are heterogeneous and are mixed with reworked carbonate fragments, rip-up clasts, and large, fresh unweathered micas. These pedogenic mud aggregates have survived compaction down to at least 2500-3000 m of burial without any significant overpressure. High rates of sedimentation resulted in only poor to moderate pedogenic overprinting, which could have destroyed the aggregate texture. The preservation of the aggregates was also related to early carbonate cementation and the formation of robust aggregates suitable to resist disintegration during both reworking and burial. The presence of such mud aggregates emphasizes the importance of mud transported as aggregates and not as suspended load in ancient dryland river deposits. Superimposed pedogenic structures on the reworked aggregates indicate a highly dynamic system with pedogenesis, erosion, reworking and redeposition. In floodplain mudrocks, where the texture of reworked aggregates is lost, the mudrock beds attain a massive and structureless morphology. Such units can be confused with more developed paleosol horizons, and in the present study this process is termed pseudo-pedodestratification. Neglecting or overlooking the presence of the mud aggregates also results in the misinterpretation of alluvial mudrocks and paleosol development, and criteria are presented for recognizing in situ and reworked aggregates.
TL;DR: In this paper, the authors investigated the relationship between the flow dynamics of turbidity currents entering a minibasin and the stratal architecture of their deposits and developed a dimensionless ponding number to compare the geometry of the deposits with the dynamics of the flows that filled the basins.
Abstract: Small intraslope basins (~100 km^2), or "minibasins," such as those found on the continental slope of the Gulf of Mexico, have been filled predominantly by turbidity currents. Each minibasin is the result of local subsidence and is partially or completely isolated from neighboring basins by ridges formed from compensational uplift. We undertook a series of experiments to investigate the relationship between the flow dynamics of turbidity currents entering a minibasin and the stratal architecture of their deposits. The experiments were performed using continuous-feed turbidity currents and surge-feed turbidity currents. A dimensionless ponding number is developed to compare the geometry of the deposits with the dynamics of the flows that filled the basins. The experimental surging turbidity currents created a deposit that was notably more ponded than the deposits of continuous turbidity currents.
TL;DR: This paper explored the relationship between substrate type (classified on the basis of both habitats and facies) and the water depth at which they occur on a modern well-studied shallow rimmed shelf margin.
Abstract: Many stratigraphic studies of ancient shallow-water carbonate platforms focus interpretation on the primacy of changes in accommodation, inferred by assuming a direct and deterministic link between depositional facies and water depth. The purpose of this study is to test this assumption explicitly and quantitatively by exploring relations between substrate type (classified on the basis of both habitats and facies) and the water depth at which they occur on a modern well-studied shallow rimmed shelf margin. GIS analysis of digital benthic habitat and bathymetry maps of the shallow ( 8 m) South Florida shelf illustrate that most substrate types occur across a range of water depths. Similarly, at greater water depths, habitats and facies are more constrained (more dominance of one class, more deterministic), with generally decreasing dominance (more diversity) at shallower water depths. At a scale of shelf-to-basin transects, facies and habitats clearly are related to water depth. Yet, on this narrower ( 10 km) shallow shelf margin, at the scale of this study, water depth and habitats and facies are not uniquely related or linked. This general lack of correspondence between bottom type and water depth may be a manifestation of landscape disequilibrium, a state in which habitats and facies do not fully reflect ambient environmental conditions, perhaps recording the influence of rapid change in sea level. Alternatively, it may reflect the impact of variables other than water depth. Although more general consequences of these results for other depositional settings remains to be evaluated, they do emphasize the fact that variables other than bathymetry may significantly influence the ecological and sedimentologic attributes of depositional surfaces. They do allow for the quantification of some of the potential uncertainties inherent in the interpretation of analogous ancient platforms, and can serve to focus future observations on the causes and consequences of vertical and lateral heterogeneity in ancient stratigraphic sequences.
TL;DR: In this article, a new borehole, 210 mbsf (meters below sea floor) deep, was used to reveal a shallowing-upwards succession, the younger part of which is punctuated by a series of erosion surfaces.
Abstract: A new borehole, 210 mbsf (meters below sea floor) deep, drilled in Ribbon Reef 5 on the Great Barrier Reef off Cooktown, NE Australia, reveals a shallowing-upwards succession, the younger part of which is punctuated by a series of erosion surfaces. Nine depositional units have been defined by lithological changes and are numbered sequentially from the base of the hole upwards. Aminostratigraphy, magnetostratigraphy, radiocarbon dating, uranium series dating, and modeling together with strontium ratios have been applied in an attempt to establish a chronology of accumulation. Carbonate deposition began about 770 ka ago in a relatively deep-water slope environment and is represented by a series of debris flows. Lithoclasts within these rocks, indicate that older limestones already existed in the area. Subsequent accretion involved the downslope accumulation of grainstones and wackestones, sometimes cross-laminated, characterized by intervals with abundant rhodoliths and scattered, probably reworked, corals. Four units at the base of the hole reflect deposition that probably began during isotope stage 16 and continued through stage 15 from about 770 to about 564 ka. Unit 5 probably extended to stage 11 (about 400 ka), and unit 6 to stage 9 ( 330 ka). Typical reefal associations of corals and calcareous algae were established in this area only above depths of about 100 m in the borehole, units 5-4. The succession is apparently unbroken to an erosion surface at 36 mbsf indicating subaerial emergence. The lack of evidence of emergence below this surface reflects progressive accretion or progradation or both. Two younger erosion surfaces define further periods of lowered sea level. Unit 7 is attributed to deposition during isotope stage 7, but erosion during stage 8 resulted in the preservation of only 8 m of unit 7 limestones. Unit 8 is correlated with stage 5 (125 ka), and unit 9 is interpreted as Holocene (post 7,700 ka). The limited thicknesses of units 7, 8, and 9 are considered to reflect erosion. The progressive shallowing brought the depositional surface within the zone exposed during lowstands, and there is no sedimentological evidence that aggradation was restricted by a lack of accommodation.
TL;DR: Stable-isotope compositions were determined for 166 microsamples of aragonite from the bivalve Venericardia hatcheplata collected from the early Eocene Hatchetigbee Formation in southwestern Alabama as mentioned in this paper.
Abstract: Stable-isotope compositions were determined for 166 microsamples of aragonite from the bivalve Venericardia hatcheplata collected from the early Eocene Hatchetigbee Formation in southwestern Alabama. These were milled from two transects, one across the left valve hinge plate and one along the growth axis. Amplitude and spacing of about nine years of seasonal variation is very regular, with peaks and troughs occurring at equal intervals and reaching consistent compositions except for some attenuation in both parameters during the last three years of life. Assuming an ice-free world with an ocean 18O value of about -1.0‰, and using the aragonite-water fractionation equation of Grossman and Ku (1986), isotope-derived paleotemperatures are somewhat warmer ( 31 °C) than temperatures presently recorded anywhere on the Earth's surface, and exhibit significantly greater seasonality ( 12 °C) than regions with the warmest mean temperatures. It is therefore likely that these very negative and extremely seasonal 18O values reflect the influence of seasonal discharge of 18O-depleted river water into the Eocene Gulf of Mexico. Comparison of these data with (1) mean annual temperature versus seasonal temperature variation from modern climates, (2) isotopic compositions reported from coeval molluscs, and (3) isotopic compositions of global meteoric precipitation and United States rivers suggest that mean annual temperature along this portion of the Eocene Gulf coast was in fact about 26 °C, and varied seasonally from a winter low of about 25 °C to a summer high of 27 °C. Differences between these temperatures and those derived from direct conversion of oxygen isotope values reflect the influence of seasonal influx of fresh (18O value of -4.7‰) water. During deposition of this portion of the Hatchetigbee Formation, ambient water 18O values varied between winter values of about -1.0‰ to less than -3.5‰ during the summer. Freshwater discharge served to seasonally depress salinity to 10 to 16 ppt along this part of the Gulf coast. The early Eocene was perhaps the warmest time interval of the past 65 million years, primarily manifest as lower equator-to-pole temperature gradients. In this context, an Eocene mean annual temperature of 26 °C is not much higher than modern surface water temperatures of about 24 °C at 29° 30 N, immediately off the modern Alabama coast. Seasonal temperature variation of 2 to 3 °C about a mean of 26 °C, however, is significantly lower than that off coastal Alabama (19 to 30 °C), and occurs only along modern equatorial coasts at latitudes less than 19°. This implies that early Eocene tropical conditions extended well beyond modern latitudinal limits.
TL;DR: In this article, the process of sediment-wave initiation is inferred from cores and high-resolution seismic-reflection profiles, and it is shown that most lower-velocity and density overflows occur on both flanks of the wave.
Abstract: Detailed study of fine-grained migrating sediment waves and processes of sediment deposition along the modern, active, meandering channel of the Zaire turbidite system was made on recent cruises. The data used in this study include EM12 bathymetry and backscatter imagery, 3.5 kHz profiles, high-resolution seismic-reflection profiles and Kullenberg piston cores. Migrating sediment waves are built under several different flow energies and hydrodynamic conditions (subcritical or supercritical condition) according to their location along the modern Zaire channel. The process of sediment-wave initiation is inferred from cores and high-resolution seismic-reflection profiles. Overflow density and sediment supply delivered to levees appear as critical parameters for wave initiation. In other cases, a preexisting wavy topography is observed that acts as a template favoring sediment-wave initiation. Core-to-core correlation between the two flanks of a single sediment wave reveals no major differences in the lithofacies, suggesting no change of the dynamics of overflows during sediment deposition on both flanks of the waves. Comparison of number, thickness, and type of individual beds in cores collected on both flanks of the wave suggests that most lower-velocity and density overflows depositing sediments on the upstream flank bypass on the downstream flank. This process is probably related to a strong decrease then increase of the bed shear stress on the upstream and downstream flank of the wave, respectively. Interpretation of lithofacies suggests that construction of migrating sediment waves is closely linked to repeated successive spillovers from the head and body of a single channelized turbidity current. Construction of a single wave is rapid, less than 5000 years.
TL;DR: In this paper, detrital zircons from 13 sediment samples from the Rhone and Rhine drainages were dated by the fission-track method to study downstream changes in detritual fissiontrack grain-age distributions in large river systems draining the European Alps.
Abstract: Zircons from 13 sediment samples from the Rhone and Rhine drainages were dated by the fission-track method to study downstream changes in detrital fissiontrack grain-age distributions in large river systems draining the European Alps. The orogen-parallel Rhone River shows a zircon fission-track grain-age distribution similar to that in its Alpine source areas. This signal is well preserved because there are Alpine sources along its full length and input from non-Alpine sources is small. In contrast, only the headwaters of the north-flowing Rhine River are located in the Alps. As a consequence, Alpine-derived zircons, which are distinguished by young fission-track ages, become progressively diluted downstream by older ages from zircon sources external to the Alps. Nevertheless, the Alpine component is persistent and can easily be detected in sediments more than 1000 km downstream at the Rhine delta. These results demonstrate that fission-track dating of detrital zircons can provide useful information about orogenic processes, even where sediments have been transported hundreds of kilometers from the orogenic source, crossing ephemeral lakes and subsiding basins. Deposits along the lower reaches of the river appear to have a short-residence time (< 1 Myr), and thus many of these deposits serve to smooth out variations in the supply of sediment from fast-eroding heterogeneous sources in the Alpine headwaters of these drainages. For the Rhone and the Rhine rivers, we show that fast erosion in the Alps accounts for most of the sediment load. This finding supports a widespread observation that the sediment in most large continental drainages is usually derived from a small part of the drainage, where uplift, relief, and erosion rates are greatest.
TL;DR: The comparison between amino acid and monosaccharide composition of purified cyanobacterially produced organic matter and that of intramineral organic matrices associated with carbonate precipitates emphasizes the importance of dicarboxylic acids released by the decay of cyanobacterial sheaths, in CaCO3 formation.
Abstract: Hemispheroidal domes (microbialites) produced by natural populations of filamentous cyanobacteria belonging to four distinct Phormidium species, and one probable new species of Schizothrix were collected alive from 0-25 m depth habitates in the lagoon of Tikehau atoll (Tuamotu, French Polynesia). This study aims to establish the biochemical control on in-situ carbonate precipitation processes ("organomineralization" processes) occuring merely in the alveolate network of non-coalescent microfibrils that characterizes the degraded parts of the microbialite domes. The comparison between amino acid and monosaccharide composition of purified cyanobacterially produced organic matter and that of intramineral (soluble and insoluble) organic matrices associated with carbonate precipitates emphasizes the importance of dicarboxylic (aspartic and glutamic) acids, released by the decay of cyanobacterial sheaths, in CaCO3 formation and demonstrates that the in situ precipitation of ultra-fine micrites is a highly selective process regarding the available external organic matter. This diagenetic process is thought to result from incipient hydrolysis of cyanobacterial S-layer proteins attached to extracellular polysaccharide fibrils composing the sheath. Taxonomic affinity of cyanobacterial populations responsible for microbialite construction is one of the major factors allowing biochemical discrimination of in-situ precipitated carbonates, indicating that specific mucilages or their degradational products are guiding forces for the calcification processes. Another possible source for the formation of carbonate-associated organic matrices is derived from metabolites (e.g. mucus) released in water by lagoonal dwelling benthic organisms.
TL;DR: In this article, the authors examined the size frequency relations of sediment bodies across surfaces of sediment accumulation, and tabulated numbers and areal extents of lithotopes across two rather dissimilar settings, across the Persian Gulf and around the Caribbean island of Antigua.
Abstract: Among broadly different types of sedimentary successions, those that accumulate in nearshore settings are perhaps unique in that a very wide range of rock types recurs with high frequency over relatively short stratigraphic intervals. This vertical lithofacies heterogeneity suggests that deposition of any sedimentary unit at any site was short-lived, that many different sediment types existed within a complex mosaic over the depositional surface, and that this facies montage repeatedly moved across surfaces of accumulation in response to eustatic, sedimentologic, and/or tectonic forcing. Data on numbers and thicknesses of lithofacies units from several dozen such sequences define the size frequency distribution expected if horizons of lithologic change occur randomly within sedimentary successions. Walther's law suggests that the amount of vertical lithologic heterogeneity manifest in a sedimentary sequence should reflect the degree of lateral heterogeneity in sediment composition that existed across depositional surfaces. If so, this repetition should also be manifest in sizes and areal distributions of sediment patches constituting the depositional mosaic. In order to examine size frequency relations of sediment bodies across surfaces of sediment accumulation, we tabulated numbers and areal extents of lithotopes across two rather dissimilar settings, across the Persian Gulf and around the Caribbean island of Antigua. Although compositions and lateral extents of different sediment types exhibit broad lateral variation across interior to margin transects in either area, lithotope abundances and areal extents define frequency distributions that are in excellent agreement with those anticipated for a facies mosaic of elements whose mean linear widths constitute an exponential frequency distribution. These are the size frequency distributions one would expect if mosaic element boundaries occur randomly across regions of sediment accumulation, and suggest that distributions of lithotope areas from Holocene surfaces are directly analogous to lithofacies thickness frequencies from ancient successions. In addition to the fact that both of the size frequency distributions attest to the importance of stochastic processes during sediment deposition, they also afford numerical descriptors of abundances and areal extents of Holocene lithosomes that can then be used as controlling parameters in computational models of sediment accumulation. Importantly, results derived from such simulations are based not on inference of water depth varying in response to change in subsidence, sea level, and rate of sediment accumulation but on statistical attributes intrinsic to modern and ancient marginal marine successions.