Showing papers in "Journal of Sedimentary Research in 1991"

Habit of bacterially induced precipitates of calcium carbonate and the influence of medium viscosity on mineralogy

Abstract: Marine bacteria can and do induce the precipitation of calcium carbonate in the laboratory and in nature, which results in single crystals and aggregates of crystals (crystal bundles). The end-member forms of the crystal bundles observed are rods and spheres with numerous variations in between these forms, the most common of which is the dumbbell. Rounded dumbbells and single crystals resembling brushes appear to be unique to bacterially induced precipitates and, consequently, may serve to identify bacterially induced precipitates in the rock record. Viscosity of the growth medium is the single most important control over the mineral precipitated in the experiments conducted. The medium viscosity controls the ion diffusion rate and thus the rate of precipitation and, consequently, the mineral precipitated. In a liquid medium, circulation and the ion diffusion rate are both high, precipitation is rapid, and aragonite forms. In a gelatinous medium, there is little circulation, the ion diffusion rate is slow, the rate of precipitation is slow, and calcite is precipitated. The overall form of the resultant precipitate, rods or spheres, was the same regardless of the mineral composition.

Stratigraphic Sequences and Their Chronostratigraphic Correlation

Abstract: The Exxon sequence models of Posamentier and coworkers incorporate two concepts which I question. They suggest that rivers grade to a "bayline", but evidence from modern major rivers indicates that they do not. They also proposed that fluvial sedimentary accomodation space is generated by the seaward shift in equilibrium profiles during sea-level fall, but this is too simplistic a model, based on the use of schematic continental margin configurations which do not incorporate real slope and relief data. In most cases, a fall in base level leads to deltaic progradation and upstream fluvial incision. Most coastal plain fluvial wedges result from two processes: sea-level rise, with the drowning and filling of estuaries, and active source-area tectonism. Major tectonic events adjacent to foreland basins, such as terrane collisions, lead to the development of sequences of third-order type (duration approximately 1-15 m.y.). Changes in intraplate stress regimes resulting from changes in regional plate kinematics have a similar episodicity and can be shown to be responsible for some third-order cycles on extensional continental margins. Examination of the chronostratigraphic basis of the Mesozoic-Cenozoic Cycle Chart of Haq and coworkers reveals internal inconsistencies and an unrealistic downplaying of the built-in potential for error. This chart is not an approved global standard, and it should be used with extreme caution.

Relation of eustasy to stacking patterns of meter-scale carbonate cycles, Late Cambrian, U.S.A.

Abstract: An interbasinal study of Late Cambrian cyclic carbonate successions in the Appalachian and Cordilleran passive margins suggests that superimposed orders of eustasy controlled the development of large-scale depositional sequences and the component peritidal to subtidal meter-scale cycles that comprise them. The focus of this paper is on the small-scale cyclicity, its probable control by Milankovitch-forced sea-level oscillations, and how stacking patterns of meter-scale cycles can be used to define internal components of larger-scale sequences and estimate variations in relative sea level. Fining-upward peritidal cycles showing evidence of episodic emergence grade seaward into coarsening-upward subtidal cycles which lack evidence of emergence and form a continuum across the Cambrian carbonate platforms. Eustasy appears to exert the dominant control on the simultaneous development of peritidal and subtidal cycles on Late Cambrian carbonate platforms. Evidence for Milankovitch forcing of glacio-eustatic sea-level oscillations is provided by a 4:1 bundling of fifth-order meter-scale cycles ( 96 ky) within fourth-order cycles spanning tens of meters ( 440 ky) within the Big Horse Member of the Orr Formation in the House Range of Utah. The 4:1 bundling may manifest the short e centricity to long eccentricity ratio of the Milankovitch astronomical rhythms. Systematic changes in the stacking patterns of meter-scale cycles can be used in conjunction with Fischer plots to define long-term sea-level cycles. On Fischer plots of peritidal cyclic successions, long-term relative sea-level rises are characterized by thick, subtidal-dominated cycles with thin laminite caps. Long-term relative sea-level falls are defined by stacks of thin, laminite-dominated cycles that show brecciated cycle caps and quartz sands toward the relative sea level lowstand. On Fischer plots of dominantly subtidal cyclic successions, long-term sea-level rise is characterized by storm-dominated, open marine carbonate cycles or thick, deep ramp, shale-based cycles. Falling segments of the Fischer plot are characterized by thin, shallow subtidal cycles compos d of restricted lithofacies. Cycle stacking patterns (parasequence sets) provide the crucial link between the meter-scale cycles (parasequences) and the larger scale sequences and their component systems tracts. One- and two-dimensional models of peritidal and subtidal cycle development indicate that, whereas peritidal cycle thickness is primarily controlled by accommodation space, deeper subtidal cycle thickness is primarily controlled by sedimentation rate. Lithofacies within peritidal cycles reflect the sedimentologic response to fluctuations in sea level; lithofacies within subtidal cycles may be related to fluctuations in the zones of fairweather and storm-wave reworking that oscillated in harmony with sea-level fluctuations and may have acted as a subtidal limit to upward aggradation. The 2-D modelling illustrates how stacked peritidal to deep subtidal carbonate cycles with thicknesses, compositions and stacking patterns similar to the Late Cambrian of North America can be generated by ilankovitch-driven composite eustasy.

Cyclic Ramp-to-Basin Carbonate Deposits, Lower Mississippian, Wyoming and Montana: A Combined Field and Computer Modeling Study

Abstract: The Lower Mississippian Lodgepole/lower Madison Formations (20-225 m thick) developed along a broad (> 700 km), storm-dominated pericratonic ramp. Three types of fifth-order upward-shallowing cycles are recognized across the ramp-to-basin transition. Peritidal cycles consist of very shallow subtidal facies overlain by algal-laminated tidal flat deposits, which are rarely capped by paleosol/breccia layers. Shallow subtidal cycles consist of stacked ooid grainstone shoal deposits, or deeper subtidal facies overlain by ooid-skeletal grainstone caps. Deep subtidal cycles located along the outer ramp consist of basal sub-storm wave base limestone-argillite, overlain by storm-deposited limestone, which are capped by hummocky stratified to massive skeletal-ooid grainstone. Deep subtidal c cles pass downslope into ramp-slope facies composed to rhythmically interbedded limestone and argillite, with local deep-water mud mounds; no upward-shallowing cycles occur within the ramp-slope facies. Average cycle durations calculated along the outer ramp are between 30-110 ky. The fifth-order cycles are stacked to form three third- to fourth-order depositional sequences, which are recognized by regional transgressive-regressive facies trends and cycles stacking patterns. Ramp-margin wedges (RMW) developed during long-term sea-level fall and lowstand and consist of cyclic crinoidal bank and oolitic shoal facies, which pass downdip into deep subtidal cycles. Transgressive systems tracts (TST), which onlapped the ramp during long-term sea-level rise, include thick shallow and deep subtidal cycles; peritidal cycles are restricted to the inner ramp. Highstand systems tracts (HST) developed during long-term sea-level highstand and fall, and along the ramp are composed of early HST shallow subtidal cycles overlain by late HST peritidal cycles; shallow through deep subtidal cycles characterize the HST along the ramp-slope. Two-dimensional computer modeling of the cyclic sequences suggests that for the assumed water depths of facies, fifth-order sea-level oscillations of at least 20-25 m were required to generate deep subtidal cycles along the ramp-slope. Synthetic sequences run with fifth-order sea-level amplitudes End_Page 1194----------------------- and TST deposition, a feature not observed in the actual sequences. Other factors, in addition to fifth-order sea-level oscillations, likely played a role in generating synchronous peritidal and deep subtidal cycles during HST deposition. These factors may include short-term climatic changes, which influenced the depths to storm-wave reworking. The moderate-amplitude sea-level oscillations suggested by the cyclic sequences may reflect the initial effects of Carboniferous glaciation that was occurring in Gondwana.

Spectral Analysis of the Middle Triassic Latemar Limestone

Abstract: The Middle Triassic Latemar Limestone in the Dolomites region, northern Italy, consists of hundreds of vertically stacked, meter scale, platform carbonate cycles. Each cycle is composed of a shallow marine subtidal carbonate unit with a thin (centimeter scale) vadose diagenetic dolomite cap. These "Latemar couplets" are the products of repeated episodes of submergence/emergence with durations estimated at 20,000 yr. The couplets are arranged in upward thinning bundles of five, indicating the presence of a 100,000 year modulating component. We examined this high frequency cycling in the platform buildup using a variety of classical and advanced spectral analysis techniques. We found si nificant evidence for the presence of a Milankovitch forcing signal in this cycling. We also present evidence that the adjacent coeval basinal facies, the Livinallongo Formation, exhibits a similar bundling pattern in its cm-scale hemipelagic carbonate/shale alternations. We conclude that Milankovitch-forced, small amplitude sea-level oscillations were the most likely cause of these Middle Triassic carbonate rhythms.

Combined-Flow Model for Vertical Stratification Sequences in Shallow Marine Storm-Deposited Beds

Abstract: We develop a model to help explain the wide variety of vertical stratification sequences in shallow-marine sandstones This model, embodied in a set of vertical-sequence cartoons, predicts or accounts for vertical stratification sequences in beds of free sandstone deposited from a full range of decelerating combined flows, as well as from purely unidirectional and oscillatory flows According to the model, vertical stratification sequences in storm-deposited sandstone beds are the outcome of the time history of the bed configuration as deposition proceeds The idealized sequences are based on the initial velocities and subsequent deceleration histories of the unidirectional and oscillatory flow components Twenty qualitatively different sequences starting with plane bed are generated, and an additional 26 sequences result from omitting one or more of the lowermost intervals in succession The model thus accounts for 46 distinct kinds of sequences Five major simplifying assumptions are built into the model: bidirectional flows, linear deceleration of both unidirectional and oscillatory flow components, time-independent deposition rate, time-independent sediment size, and equilibrium bed configurations Owing to these assumptions, the model is likely to be applicable to only a small subset of vertical stratification sequences observed in shallow marine sandstones As more information on combined-flow bed configurations becomes available, these assumptions could be relaxed to provide a more refined and complete representation of vertical stratification sequences Since the model generates vertical sequences formed from any kind of decelerating flow, the Bouma sequence for deposits from purely unidirectional turbidity currents and also sequences resulting from decelerating purely oscillatory flows (similar in appearance to many proposed generalized tempestite sequences) are end-member cases in the model The model is also extended slightly to explain the geometry of the uppermost bedding surfaces of storm-generated sandstones by consideration of disequilibrium effects

Pedogenic and groundwater silcretes at Stuart Creek opal field, South Australia

Abstract: The opal mining areas of inland Australia have exposures in which a systematic association between near-surface silcrete and one or more silicified horizons at depth is displayed. In the Stuart Creek opal field, the deepest horizons are opalite and glassy quartzites in which all primary sedimentary structures are conserved. Higher in the section, lenses and nodules of quartzite occur in bleached, unconsolidated sands. Near the top of the section, a nodular and columnar silcrete displays numerous illuviation features. At the base of the section, the earliest phase of silicification is the pseudomorphic replacement of sedimentary clay minerals by opal. Subsequently, void linings of micro-laminated opal were formed, and fibrous silica was precipitated in residual cavities. In the middle part of the section, sedimentary clay minerals were replaced by microcrystalline quartz, while silicification of clay-free sands was achieved by overgrowth of detrital quartz grains. In both cases, residual voids were filled with chalcedony and euhedral quartz. In the upper part of the section, silicification produced microcrystalline quartz in the matrix of the host sediment and in titania-rich illuviation laminae at the base of voids and channels. The near-surface silcrete displays many features relating to infiltration and downward percolation of water. Variable rates of water percolation, as well as alternating periods of leaching and deposition, are inferred from macro- and micro-scale structures and fabrics. The presence of microquartz indicates that solutions contained comparatively low silica concentrations, but enough impurity elements to restrict the growth of large crystals. In the deepest horizons, the preservation of sedimentary structures, the occurrence of micro-laminated void cutans of silica, and the horizontal disposition of silicified pans or lenses suggest a relationship with former groundwater tables. Secondary silica is mainly opal, indicating that the precipitating solutions had high silica concentrations. In the middle part of the section, structures are similar to those at depth except that opal is lacking in the matrix and appears to have recrystallized to microquartz. Silicification may have commenced at a groundwater level, but it later proceeded in response to dissolution and recrystallization in the unsaturated zone above the water table. The different silicification processes occurred in the same landscape in response to different mechanisms. Near the surface, in the pedogenic silcrete, solutions appear to have dissolved silica during infiltration and concentrated it through evaporation during dry periods. At depth, there appears to have been a general acidification of the environment leading to destruction of sedimentary clay minerals and the consequent production of silica phases that had a comparatively high solubility. Opal was precipitated in these groundwater environments. During landscape dissection, the water table was lowered and former silicified horizons were stranded in the unsaturated zone. Here, percolating waters with relatively low concentrations of silica dissolved opal and precipitated microquartz. M anwhile, groundwater silicification proceeded at a deeper level at the new water table.

The influence of organic matter in organogenic dolomitization

Abstract: We hypothesize that the decomposition of organic debris derived from plants, planktonic animals, and algae by sulfate-reducing bacteria, and specifically by the enzymatic degradation of protein, is vital to organogenic dolomitization. The role of this degradation is to increase both alkalinity and pH of the pore waters, providing the necessary solution and surface chemistries for dolomitization to occur. Ammonia, derived by the enzymatic breakdown of proteins, is the only simple base available to supply the pH conditions necessary for dolomitization. Protein is abundant in organic matter derived from a marine source. Because of the large enthalpies of hydration of Mg (super +2) and Ca (super +2) ions, we propose that dolomitization proceeds with the transfer of neutral ion pairs, CaCO 3 and MgCO 3 , to and from the dolomite surface within a replacement zone. For this reason, dolomitization will occur only when the concentration of the CO 3 (super -2) ion is high and in large excess to the HCO 3 (super -) ion in solution and at the carbonate crystal surface. These conditions will be met when the pH and/or ionic strength is high and the carbonate alkalinity is also high.

The Bouma Division A and the Possible Duration of Turbidity Currents

Abstract: Experimental data (Arnott and Hand 1989) combined with theoretical arguments suggest that the transition in a turbidite from division A to division B occurs at a deposition rate of unsupported grains of about 6.7 10-4 m s-1 in the case of fine-medium quartz sand. A second rate may be calculated from properties measured at the transition from division B to division C (Allen 1971b). These rates together permit an estimate to be made of the order of magnitude of the duration of deposition from a turbidity current. The calculated order of a few tens of minutes for a Mio-Pliocene turbidite described and interpreted hydraulically by Komar (1985) is in broad agreement with his independent estimate based on the sequenc of sedimentary structures.

Modern Sediments and Facies Model for a Microtidal Coastal Plain Estuary, the James Estuary, Virginia

Abstract: Modern sediments of the James River estuary have been studied to characterize the lithofacies and to relate the facies pattern to estuary morphology and the energy regime. The estuary was formed by Holocene drowning of a river valley incised in Cenozoic coastal plain deposits. Morphologic analysis of sinuosity, width-depth ratios, and convergence characteristics reveals three compartments: 1) bay-mouth, 2) estuary funnel and 3) meander zone. Each compartment exhibits a characteristic lithofacies reflecting different proportions of wave, tidal and fluvial energy. These lithofacies form a longitudinal tripartite pattern, i.e., sand-mud--sand, with coarse-grained sediment at the energetic ends of the system. The seaward facies boundary is transitional as a result of mixing fluvial and ma ine sediment. In contrast, the landward boundary is abrupt as a result of a rapid seaward decrease in the river flood bedload which is partly attenuated by the tide. The tripartite facies develop in a transgressive wedge filling the path of the pre-Holocene fluvial drainage. A model is proposed for recognizing the sequence of ancient transgressive estuarine facies. The sequence fines upward except at the mouth and reflects the seaward evolution of environments from fluvial to fluvial estuarine, estuarine and marine estuarine.

Estuarine Deposits of the Albian Paddy Member (Peace River Formation) and Lowermost Shaftesbury Formation, Alberta, Canada

Abstract: Outcrop exposures of the Peace River Formation in northwestern Alberta contain evidence of significant relative sea-level fluctuations that occurred during the middle to late Albian. These include Paddy Member channels that incised into the Cadotte Member shoreline deposits and the subsequent estuarine-frill. The upper Cadotte Member contact is irregular and siderite-cemented; it has been scoured into and infilled by sandstone of the Paddy Member. The lower sandstone of the Paddy Member is crossbedded, with abundant comminuted, carbonaceous debris. Channels cut into the Cadotte Member are up to 5 m deep and are locally overlain by in situ coal and roots. The channels are infilled with alternating couplets of moderately to intensely bioturbated sand and mud which form inclined h terolithic stratification: mud plugs are also present. Internally these sands are ripple and parallel cross-laminated. This part of the Paddy Member is interpreted as estuarine fill that resulted from a relative sea-level rise. The channel fill is overlain by intensely bioturbated, locally rooted, finely interbedded sandstone, siltstone and shale, with minor reworked bentonites, interpreted as tidal flat deposits. Tidal flat sediments are overlain by 2 to 3 m of scoured, parallel-laminated sandstone and capped by 1.5 m of planar-tabular crossbedded sandstone interpreted as shoreface and transgressive, estuary-mouth deposits, respectively. Evidence for tides throughout the Paddy Member includes inclined heterolithic stratification, mud couplets, reactivation surfaces, reversing paleoflows and compound crossbeds. The incised channels and related fill are correlative with a sequence of paleosols 300 km southwest in the Boulder Creek Formation. A brackish influence is indicated by the presence of a few species of peridinioid dinoflagellates occurring in abundance at certain levels and the trace fossil assemblage. The Paddy Member is overlain by marine mudstone of the Shaftesbury Formation. A 20 to 30 cm thick, wave-rippled layer of fish teeth, fish bones and pebbles, near the base of the Shaftesbury Formation, can be traced along the Peace River for at least 75 km. The layer of fish remains is interpreted as transgressive lag.

Geometry, Lateral Variation, and Preservation of Downlapping Regressive Shelf Deposits: Eastern Tyrrhenian Sea Margin, Italy

Abstract: The shelf of the eastern Tyrrhenian Sea margin changes substantially in width, shelf-break depth, and sea-floor steepness over relatively short distances, largely owing to pronounced lateral changes in subsurface geologic structure. Remnants of late Pleistocene prograded coastal deposits are locally preserved on the middle and outer parts of the shelf. We recognize two major controls on the distribution, lateral extent, thickness, and preservation potential of these prograded deposits. First, prograded (downlapping) deposits formed only where the physiographic shelf break was deeper than the lowstand shoreline, thus providing accommodation space for the lowstand deposits. Second, the proximity and relative size of sediment sources and the local coastal dispersal system apparently infl enced the geometry of the deposit. Midshelf and shelf-margin bodies composed of seaward-steepening downlapping reflectors were deposited as thin to thick continuous prograding sheets over an irregular eroded shelf surface and onto the shelf edge during the latest fall and lowstand of sea level. Decreased sedimentation at the end of this lowstand resulted in net erosion. During sea-level rise, shoreface erosion created a major marine erosional (ravinement) surface landward of the 120-m isobath, and much of the downlapping deposits were removed. Preservation of these downlapping deposits is largely a function of their thickness. Thick continuous deposits are common on the shelf edge, whereas on the midshelf only thin remnants are preserved locally where depressions or morphologic steps were present in the shelf surface.

Braided Fluvial to Marine Transition: The Basal Lower Cambrian Wood Canyon Formation, Southern Marble Mountains, Mojave Desert, California

Abstract: The Lower Cambrian Wood Canyon Formation, recognized over a 39,000 km2 belt through the southern Great Basin and Mojave Desert, was examined in the southern Marble Mountains, eastern Mojave Desert, southeastern California. There it crops out as a well-exposed 90-130 m thick, feldspathic sandstone nonconformably overlying Proterozoic basement. Previous investigators have suggested a tide-dominated, nearshore, shallow marine origin for the entire unit in the eastern Mojave, based on broad-scale stratigraphic trends and localized fossil occurrences in the upper member of the formation. In contrast to a regional approach, the present study uses a detailed lithofacies analysis in a smaller study area to suggest that a fluvial to marine transition occurs within these strata. The middle and upper members of the Wood Canyon Formation are recognized in the southern Marble Mountains and are divisible into six distinct lithofacies that possess gradational contacts where exposed: cross-stratified sandstone (Facies A); mediumpebble conglomerate (Facies B); planar-stratified sandstone (Facies C); sandstone and thin mudstone (Facies D); mudstone with intercalated sandstone (Facies E); and horizontally laminated siltstone (Facies F). Facies A-D, contained within the middle member, comprise the bulk of Wood Canyon sediments and record the depositional characteristics of a transition zone from a distal alluvial braid plain to a tidally influenced, fluvial-dominated braid-delta complex. Cross stratification in the braid-plain sediments displays a strongly unimodal pal ocurrent pattern, suggestive of downstream-migrating sinuous-crested megaripples. Braid-delta sediments, however, show a more dispersed paleocurrent pattern and contain a sparse trace fossil assemblage. A change from fluvial-dominated to marine-dominated processes is recorded in upper member rocks (Facies E and F). Facies E contains many of the traits characteristic of a low- to mid-tidal flat sequence, such as coarsely interlayered bedding, flaser and lenticular bedding, microripple marks, and locally abundant Rusophycus and Planolites trace fossils. Facies F, which dominantly consists of horizontally laminated siltstone, is interpreted as the product of an estuarine lagoon that developed behind and within a discontinuous barrier of stranded braid bars.

High Production and Highstand Shedding from Deeply Submerged Carbonate Banks, Northern Nicaragua Rise

Abstract: Because the shallow isolated carbonate banks on the northern Nicaragua Rise, the Nicaragua/Honduras and southern Jamaica carbonate shelves, and many other modern carbonate banks worldwide, are covered by an average of 20 to 30 m of water and by a thin blanket of coarse carbonate sediments, other carbonate sedimentologists have considered these banks good examples of "incipiently drowned", or even "drowned" carbonate banks. However, based on recent research on the northern Nicaragua Rise, we can demonstrate that these banks currently are healthy producers of large volumes of periplatform sediments (fine aragonite and magnesian calcite), which are exported almost entirely to the deep surrounding slopes. These sediments, which were deposited during the past 9000-10,000 years, form peripl tform wedges on the middle and upper slopes, which are seen clearly on 3.5 kHz seismic profiles. Radiocarbon ages of the wedge surface sediment range between 230 and 610 YBP and provide clear evidence for the contemporaneous production of sediments on the shallow bank and shelf and their instantaneous export to the upper slopes. For the last 5000 years, sedimentation rates ranged from 2000 mm/ky off Pedro Bank to 1300 mm/ky off Jamaica. These rates are somewhat lower than, but of the same order of, magnitude as sedimentation rates on the western (leeward) slopes of Great Bahama Bank. Deposition of the metastable carbonate sediments discussed in this paper occurred as soon as sea level rose to flood the bank and shelf which flank Walton Basin, following the last glacial lowstand. They com rise a wedge-shaped package of transgressive and highstand sediments on the upper and middle slopes of Pedro Bank and the southern Jamaica shelf.

Enso and Sunspot Cycles in Varved Eocene Oil Shales from Image Analysis

Abstract: On the premise that sequential changes in varve thickness offer a proxy for climatic variations, we investigated varve thickness in three core segments from the distal lacustrine oil shales (Tipton and Laney members) of the Green River Formation, by means of an image analysis program. Of two strong bimodal periodicities one, at 4.8-5.6 years, is interpreted as an El Nino type (ENSO) phenomenon of atmospheric dynamics, while the other, at 10.4-14.7 years, is interpreted as the sunspot cycle, originally recognized in this formation by Bradley (1929, 1931). Weaker periodicities may exist at ca. 8 and 33 years--the latter also recognized by Bradley. Taken in conjunction with the work of Bradley (1929, 1931) and of Crowley et al. (1986), this suggests that some but not all of the oil shale of the Green River Formation is truly varved and can be used to infer climatic time-series. Automated instrumental scans have greatly extended the possibilities of varve studies as approaches to climatic variations in time, but careful selection of facies in their geological framework is necessary.

Cyclicity in the Green River Formation (Lacustrine Eocene) of Wyoming

Abstract: Basinal facies of the Green River Formation have two main modes, lacustrine and playa. The lacustrine mode (Tipton and Laney members) accumulated mainly varved oil shale. Here annual cycles are recorded as varves. Variations in verve thickness demonstrate El Nino (ENSO)-type and sunspot cycles (Ripepe et al., this volume). Milankovitch-scale cycles are not obvious in lithic variations, but gamma ray logs record 1) precessional variations with a mean period (varve-timed) of 19.5 ka, and 2) a bundling of these in the ca. 100 ka eccentricity cycle. In the playa mode (Wilkins Peak Member), the lithic succession oil shale-trona-dolomitic marlstone records the precessional drying up of a lake and is again bundled in sets of 5, by the 100 ka eccentricity rhythm. The Tipton Member persisted f r 450 ka, the Wilkins Peak Member for ca. 1 Ma.

Stable Isotope Evidence for the Origin of the Urkut Manganese Ore Deposit, Hungary

Abstract: The stratiform sedimentary manganese ore deposit at Urkut, Hungary contains primary ore that is exclusively manganese carbonate. It shares many geological, sedimentological, mineralogical, and geochemical characteristics with other organic-rich, sediment-hosted, marine manganese deposits. Samples from two stratigraphic sequences through the marlstone-hosted Mn carbonate ore at Urkut were studied. Stable isotope compositions of both Mn-rich and Mn-poor carbonates were analyzed and are discussed, together with the geologic background of the deposit, rock and mineral chemistry, and x-ray diffraction mineralogy. The 13C values of carbonates (-1.24 to -30.78) show a negative linear correlation with Mn contents and a negative exponential correlation with total organic carbon contents. These relations suggest that the Urkut deposit was produced by early diagenetic precipitation of manganese carbonate. Data are consistent with models for bacterial metabolic pathways. Thus, mineralization was a consequence of bacterially mediated diagenetic reactions that involved Mn reduction via one or two coupled reactions: the oxidation of organic matter with Mn oxyhydroxide reduction, or the oxidation of FeS produced as a byproduct of seawater sulfate reduction with Mn oxyhydroxide reduction. xygen-deficient water in the depositional basin acted as a carrier and reservoir for Mn+2 before its deposition.

Pebble shape (and size

Abstract: The mathematical technique of R-mode factor analysis was applied to the long, intermediate and short axes of one synthetic random data set and three sets of natural pebbles to extract the three fundamental factors related to these parameters One factor corresponds to size, which is best described by the mean of the axes Two further factors correspond to shape: sphericity and disc-rodness These factors are revealed whether synthetic data or natural pebble samples are analyzed Extending the factor analysis to include shape indices revealed which indices are equivalent, which are most useful, and which should be discarded Sphericity is best described with the Corey shape index, S/(IL), and disc-rodness is best described with the disc-rod index, (L - I)/(L - S) Accordingly, the most effective shape diagram is a triangular plot of these two shape indices; the three end-members of shape are spheres, discs and rods Shape is best investigated using this shape diagram; a 2-D technique such as contouring point density on this diagram should be used to determine the mean and standard deviation of shape "While others ask themselves 'What is there to do?', I ask myself 'What is there that I can avoid doing?'" Masanobu Fukuoka, One Straw Revolution

Grain Fabric in Hummocky Cross-Stratified Storm Beds: Genetic Implications

Abstract: A detailed analysis of grain fabric in samples from hummocky cross-stratified (HCS) storm beds suggests that these shallow marine sandstones were deposited from waning combined (oscillatory/unidirectional) flows. Particle a-axis orientations in the deposits, in plan view parallel to visible lamination, vary widely but have a mean orientation approximately normal to unidirectional paleoflow indicators (sole marks) and subparallel to regional shoreline. This alignment of a-axes is attributed to deposition under combined flows with a multidirectional oscillatory component produced by wind-induced surface gravity waves with a modal crest orientation approximately parallel to the shoreline. Imbrication of particles varies in both the horizontally-laminated and HCS intervals of the HCS stor beds, due to waxing and waning of boundary shear stress caused by the oscillatory component of the flow. In the basal, horizontally-laminated interval of an HCS storm bed, grain imbrication angles vary about a mean of approximately 13° from the horizontal, dipping into the paleoflow inferred from sole marks. This preferred imbrication direction is attributed to boundary shear stress influenced by the unidirectional component of the combined flow. However, quasi-cyclic variation in imbrication angle over a broad range and periodic reversal of imbrication direction suggest that the oscillatory component was also significant during deposition of the horizontally-laminated interval. In HCS intervals, imbrication angles vary about a mean that is parallel to visible laminae, suggesting th t oscillatory flow had become predominant. The sequence of structures and changing patterns of fabric through these HCS storm beds is consistent with deposition under waning combined flows. Erosional scour, forming the sharp base and unidirectional sole marks, occurred in response to combined flows with an effective unidirectional component. The unidirectional component continued to influence the direction of boundary shear stress during deposition of the horizontally-laminated sands but waned and became negligible relative to a strong oscillatory component during formation of HCS.

Orbital Forcing and Sedimentary Sequences

Abstract: "The variation of eccentricity is connected with the most celebrated attempts to determine a limited portion of geologic time. In the elaboration of the theory of the ice age that bears his name, Croll correlated two important epochs of glaciation with epochs of eccentricity computed to have occurred 100,000 and 210,000 years ago. As the analysis of the glacial history progresses, these correlations will eventually be established or disproved, and should they be established it is possible that similar connections may be made between events far more remote". (Gilbert 1900, p. 15-16). "But the precessional motion pulses steadily on through the ages, like the swing of a frictionless pendulum. Its throb may or may not be caught by the geological process which obtains in a particular province and in a particular era, but whenever the conditions are favorable and the connection is made, the record should reflect the persistence and the regularity of the inciting rhythm". (Gilbert 1900, p. 18).

Episodic dolomitization of late Cenozoic carbonates in the Bahamas; evidence from strontium isotopes

Abstract: The minimum stratigraphic age and the timing of dolomitization have been constrained in sediments of late Cenozoic age in the Bahamas (Little Bahama Bank--LBB, Andros, New Providence, San Salvador, Great Inagua and Mayaguana) using the Sr-isotope composition (87Sr/86Sr) of dolomites. Most dolomite sequences investigated are composites of several dolomitization episodes. Five phases of dolomitization have been recognized, the most extensive of which are early Late Miocene and Late Pliocene in age. Two minor phases occurred later during the latest Pliocene/early Pleistocene and late Pleistocene. The fifth phase (late Early Miocene) is only recognized in San Salvador. Commonly, dolomitization episodes are stacked, indicating an increasing age with depth, Only in San Salvador are younger dolomites interspersed with older ones. The Late Pliocene episode on LBB is the best constrained, lasting probably less than 0.6 Ma and causing extensive dolomitization. The shade of the dolomite body, together with the age of diagenesis, provides support for a dolomite origin from seawater in a hydrologic environment characterized by a mixing zone induced seawater circulation. Variable depth of burial of Middle Miocene dolomitized strata indicates non-uniform subsidence behavior of Bahamian carbonate platforms. Strontium-isotope ratios of other late Cenozoic dolomites (Hope Gate Formation, Jamaica; Seroi Domi Formation, Netherlands Antilles; Rangiroa Atoll; Niue Atoll--Aharon et al. 1987) also indicate early Late Miocene and Late Pliocene dolomite ages. Simultaneous dolomitization in the Bahamas, the Caribbean and the Pacific appears to occur during times when the marine 87Sr/86Sr ratio is either constant or decreasing. This suggests that dolomitization of carbonate platforms and the evolution of Sr-isotope ratios in seawater are linked, possibly via eustatic sea-level fluctuations.

Laminar Micrite Crusts and Associated Foreslope Processes, Red Sea

Abstract: Forereef slopes in the Red Sea of Sudan exhibit a uniform biozonation that is independent from the topography of the slopes. Below - 120 m, ledges protrude horizontally from sleep cliffs of barrier reefs and atolls as well as from patches of in situ lithified slope sediment on inclined fringing reef slopes. Free surfaces and cavities within these ledges are partly covered by laminar micrite crusts of 7-20 mm thickness. The ledges are formed by an organic framework of living azooxanthellate corals, bryozoans, serpulids and fossil red algae. They are affected by repeated episodes of boring, infilling, and cementation which obliterate much of the original fabric. Concomitant cementation and boring result in asymmetric cement linings which often show geopetal fabrics. The laminar m crite crusts, however, show no significant traces of bioerosion, which are a typical feature of fossil deep-water stromatolites. Two types of lamination have been observed: 1) vertically stacked, irregular, anastomosing laminate with ragged outlines and little lateral persistence which are often accentuated by iron staining, and 2) 0.5-mm-thick laminae of light-grey micrite, rich in fine bioclasts, showing smooth, thin coatings of dark homogenous micrite, which are weakly fluorescent in ultraviolet light. No algal films are preserved, but micritic tubes 10-30 µm in diameter, often bifurcated, form a dense network on the upper surface of the crusts by overgrowing and binding particles. Associated aragonitic rosettes 30-40 µm in diameter may represent calcified coccoid algae or bacteria. If true, these organisms suggest a formation of the latter type of crust by biogenic processes not related to the photic zone. Fenestral porosity and Frutexites textures are absent. Radiocarbon dates from corals within the ledges provide ages of 10,500 YBP. From the time-related decrease in oxygen isotope ratios (versus PDB) of boring micrite (18O = +3.72), micrite crusts (18O = +1.69), and unlithified mud (18O = +0.47) within the youngest voids of the hard substrates, we conclude that crust formation must have taken place in shallower water when sea level was lower during the Early Holocene rise of sea level.

Late-Stage Dolomitization of the Lower Ordovician Ellenburger Group, West Texas

Abstract: Petrography of the Lower Ordovician Ellenburger Group, both in deeply-buried subsurface cores and in outcrops which have never been deeply buried, documents live generations of dolomite, three generations of microquartz chert, and one generation of megaquartz. Regional periods of karstification serve to subdivide the dolomite into "early-stage", which predates pre-Middle Ordovician karstification, and "late-stage", which postdates pre-Middle Ordovician karstification and predates pre-Permian karstification. Approximately 10% of the dolomite in the Ellenburger Group is "late-stage". The earliest generation of late-stage dolomite, Dolomite-L1, is interpreted as a precursor to regional Dolomite-L2. L1 has been replaced by L2 and has similar trace element, O, C, and Sr isotopic signatures, and similar cathodoluminescence and backscattered electron images. It is possible to differentiate L1 from L2 only where cross-cutting relationships with chert are observed. Replacement Dolomite-L2 is associated with the grainstone, subarkose, and mixed carbonate-siliciclastic facies, and with karst breccias. The distribution of L2 is related to porosity and permeability which focused the flow of reactive fluids within the Ellenburger. Fluid inclusion data from megaquartz, interpreted to be cogenetic with Dolomite-L2, yield a mean temperature of homogenization of 85 ± 6°C. On the basis of temperature/18Owater plots, temperatures of dolomitization ranged from approximately 60 to 110°C. Given estimates of maximum burial of the Ellenburger Group, these temperatures cannot be due to burial alone and are interpreted to be the result of migration of hot fluids into the a ea. A contour map of 18O from replacement Dolomite-L2 suggests a regional trend consistent with derivation of fluids from the Ouachita Orogenic Belt. The timing and direction of fluid migration associated with the Ouachita Orogeny are consistent with the timing and distribution of late-stage dolomite. Post-dating Dolomite-L2 are two generations of dolomite cement (C1 and C2) that are most abundant in karst breccias and are also associated with fractures, subarkoses and grainstones. 87Sr/86Sr data from L2, C1, and C2 suggest rock-buffering relative to Sr within Dolomite-L2 (and a retention of a Lower Ordovician seawater signature), while cements C1 and C2 became increasingly radiogenic. It is hypothesized that reactive fluids were Pennsylvanian pore fluids derived from basinal siliciclastics. The precipitating fluid evolved relative to 87Sr/86Sr from an initial Pennsylvanian seawater signature to radiogenic values; this evolution is due to increasing temperature and a concomitant evolution in pore-water geochemistry in the dominantly silic clastic Pennsylvanian section. A possible source of Mg for late-stage dolomite is interpreted to be from the dissolution of early-stage dolomite by reactive basinal fluids.

Subaqueous shrinkage cracks in the Devonian of Scotland reinterpreted

Abstract: Abundant lenticular, straight to curving cracks on bedding planes are commonly interpreted as subaqueously formed shrinkage cracks ("syneresis cracks"), the two most influential examples being those in the Green River Formation (Picard 1966) and the Devonian Orcadian Basin of Scotland (Donovan and Foster 1972). Cracked surfaces in the Orcadian Basin deposits show all gradations between lenticular cracks and well developed polygons of subaerial origin. Some lenticular cracks are inferred to have formed during subaerial desiccation. Many of the simplest lenticular cracks also have the shapes of gypsum crystals and originated as pseudomorphs. The gypsum crystals were often important in providing nuclei for later desiccation cracks. More complex crack patterns of incipient to fully formed desiccation polygons have arisen from desiccation of surfaces containing gypsum crystals or pseudomorphs resulting from gypsum crystal dissolution. Cracks of all types are preserved by sand infills introduced by wind transport across dried-up lake floors. Reassessment of the literature reporting subaqueous cracks in rocks shows that many of these cracks are likely to have originated in a similar way, partly as gypsum crystals, and partly by limited subaerial desiccation. The remaining examples have either already been reinterpreted in other ways, or are insufficiently documented to rule out a subaerial origin. There is no clear evidence for subaqueously formed cracks being preserved in rocks.

Authigenesis of Impact Spheroids in the K/T Boundary Clay from Italy: New Constraints for High-Resolution Stratigraphy of Terminal Cretaceous Events

Abstract: Detailed chemical and mineralogical analyses of authigenically altered impact spheroids contained in the Cretaceous/Tertiary (K/T) boundary clay of the northeastern Apennines provide information about the chemical conditions on the sea floor of a deep pelagic basin immediately after the impact of a large extraterrestrial object against the Earth's surface. The temporal alteration of global meterologic conditions resulting from this catastrophic event caused the death of most planktonic and nektonic biomass of the oceans. Dead organisms settled on the seafloor creating low-pH, low-Eh conditions at the water-sediment interface. Reducing and slightly acidic waters percolated in the top sediment, causing reduction and leaching of iron and dissolution of the top surface of the calcareous o ze. The suppression of biogenic calcium carbonate production in the upper water column, and the reducing conditions in the seafloor, lasted for a period not longer than 50 Ka. During this time, only detrital silt and clay were deposited in the basin, along with the fallout material derived from the impact cloud. The fallout included spherules of sub-millimeter size which condensed from vaporized and/or molten terrestrial target rock admixed with the vaporized impactor. The heterogeneous chemical composition of the spherules, and the wide range of shapes of relict crystals preserved in them, suggest that they were originally made of a suite of high-temperature quenched minerals ranging in composition from silica-rich/iron-poor ones (plagioclase), to iron-rich/silica-poor ones (spinel-beat ng clinopyroxene and olivine). The slightly reducing conditions of the seafloor favored the authigenic growth of K-feldspar in Si-rich spherules, and of glaucony in the Fe-rich ones. In some parts of the paleobasin, characterized by stronger reducing conditions, K-feldspar authigenesis did not occur, and spheroids were preferentially altered to pyrite and glaucony. The resumption of carbonate deposition resulting from the post-K/T plankton bloom coincided with the restoration of normal oxidizing conditions in most parts of the paleobasin. Such conditions caused the arrest of glauconitic growth and the alteration of the most Fe-rich spherules to goethite. Bioturbation activity also resumed in the top sediment, causing slight vertical mixing in the K/T boundary clay layer. In addition to the mineralogical and chemical description and the interpretation of the authigenic phases contained in the Italian K/T boundary clay, this paper includes a high-resolution stratigraphic analysis of the boundary clay in the Petriccio section which is the best preserved among the several K/T sections studied in the northeastern Apennines. The most relevant discovery from this study is that the impact spherules are concentrated in the central part of the clay layer along with the peak of an iridium anomaly, whereas grains of quartz with multisets of parallel lamellae, which are considered a product of impact shock-metamorphism, are mostly concentrated in the bottom part of the layer. This clear decoupling may represent several thousands of years of delay in deposition of ridium and spheroids with respect to shocked quartz and suggests two impact events closely spaced in time. However, further high-resolution analyses in other sections in the northeastern Apennines and elsewhere around the world is advised for more definitive confirmation for a terminal Cretaceous multiple-impact hypothesis.

Bar Sedimentology of the Squamish River, British Columbia: Definition and Application of Morphostratigraphic Units

Abstract: In a 20-km river reach of the Squamish River, British Columbia, in which channel planform changes progressively downstream from braided to meandering, there is a corresponding down-valley change in bar type from mid-channel compound bars to bank-attached compound bars to point bars. On each of 10 bars studied, within-bar facies and particle size trends relate directly to the spatial distribution of local-scale depositional environments on bar surfaces. Four fluvial morphostratigraphic units, defined in terms of their surface morphology and sedimentary characteristics, are differentiated: bar platform, chute channel, ridge, and remnant floodplain. The morphology, scale, and sedimentologic character of each morphostratigraphic unit are described. Compound bars are composed of extensive bar platform units dissected by chute channels, with remnants of other morphostratigraphic units. Within-bar facies and particle size trends are highly irregular, whether viewed down-bar, across-bar or vertically. Sediment sequences on point bars exhibit both around-the-bend and lateral trends. Bar platform units are transitional laterally to alternating ridge and chute features away from the main channel, with floodplain sequences beyond. Analysis of bar sedimentology using fluvial morphostratigraphic unit analysis provides a more reliable basis for palaeoenvironmental reconstruction than do existing procedures based on facies associations at the channel bedform scale.

Macrotidal mudflats of the southwestern Korean coast; a model for interpretation of intertidal deposits

Abstract: Extensive muddy tidal flats along the western coast of Korea are distinctly different from other intertidal environments used as standard models for intertidal sedimentation (e.g., the North Sea). The muddy Korean tidal flats lack extensive, dendritic drainage systems, seaward barriers, landward salt marshes, and most sedimentary structures considered diagnostic of intertidal environments (i.e., flaser, wavy, and ripple-laminated bedding). Fifty-one cores were collected from intertidal and subtidal areas of Nam Yang Bay to investigate the distribution of texture, composition, structure, and accumulation rate of these muddy sediments. Sediments coarsen seaward (from about 8.8 phi to 4.6 phi) and downcore, indicating that the tidal flats are actively accreting, with finer-grained, high-tidal- and mid-tidal-flat sediments prograding over the coarser-grained, low-tidal-flat sediments. Sedimentary structures show a trend of increasing preservation of physical stratification (mm-scale parallel to slightly wavy-bedded laminae) toward the mid-tidal flat. Pb-210 accumulation rates are highest on the mid-tidal flat (5-9 mm/yr) and decrease both seaward and landward to 1-2 mm/yr. Upper portions of intertidal cores are laminated and lower portions are mottled; the opposite is true of subtidal cores. Different processes dominate these two environments on both short and long timescales. The most significant difference between the muddy Korean tidal flats and the sandier North Sea setting is the lack of intricate drainage networks. Drainage channels on the Korean tidal flats seem to be relatively stable. Thus, channel-fill sequences do not compose a major part of muddy intertidal deposits. Muddy intertidal deposits would be more difficult to identify in the rock record compared to sandy intertidal deposits because of the lack of channel-fill sequences (which compose the bulk of sandy intertidal deposits) and because the gradients in sedimentological properties are gradual and subtle.