Showing papers in "Journal of Sedimentary Research in 2001"

Microbially Induced Sedimentary Structures: A New Category within the Classification of Primary Sedimentary Structures

Abstract: Cyanobacterial films and mats syndepositonally influence erosion, deposition, and deformation of sediments. The biomass levels surface morphologies, and microbial mats stabilize depositional surfaces and shelter the sediment against erosion or degassing. Growing microbial mats dredge grains from their substrate upwards, whereas cyanobacterial filaments that are oriented perpendicular to the mat surface reach into the supernatant water and baffle, trap, and bind suspended particles. These and similar biotic-physical interactions are reflected in syndepositional formation of microbially induced sedimentary structures. We distinguish structures on bedding planes (leveled bedding surfaces, wrinkle structures, microbial mat chips, erosional remnants and pockets, multidirectional ripple marks, and mat curls) and internal bedding structures (sponge pore fabrics, gas domes, fenestrae structures, sinoidal laminae, oriented grains, benthic ooids, biolaminites, mat-layer-bound grain sizes). We propose to place this group of microbially mediated structures as a fifth category (bedding modified by microbial mats and biofilms) in Pettijohn and Potter's (1964) existing classification of primary sedimentary structures.

Quantitative Interpretation of Sedimentary Structures Formed by River Dunes

Abstract: New experimental data from a range of flumes and rivers have been used to test and develop a theoretical model for the thickness of sets of cross strata formed by the migration of subaqueous dunes. The distribution of dune height can be calculated directly from the distribution of cross-set thickness, and vice versa, independently of aggradation rate. Mean dune height is approximately 2.9 (± 0.7) mean cross-set thickness. The new models relating dune height to cross-set thickness have been tested successfully with data from flumes and from the Calamus and Mississippi Rivers. Furthermore, it is well known that dune height can be related approximately to formative flow depth. Although prediction of flow depth from cross-set thickness and dune height is imprecise, it provides a useful complement to other methods of estimating flow depth from sedimentary information.

Integrated Quantitative Stratigraphy of the Cenomanian-Turonian Bridge Creek Limestone Member Using Evolutive Harmonic Analysis and Stratigraphic Modeling

Abstract: Secular changes in bulk sedimentation have a significant impact on the preservation of climatic signals in rhythmically bedded strata. In this study a quantitative cyclostratigraphic methodology is developed to use these signal distortions as a means to reconstruct sedimentation. The method exploits a moving-window Fourier technique that we designate "evolutive harmonic analysis" (EHA). We calibrate this methodology to a specific set of geologic conditions (i.e., changes in bulk sedimentation) using a series of stratigraphic models and then apply the technique to the Cenomanian-Turonian Greenhorn Formation of the Western Interior basin. Application of EHA to strata that preserve a record of orbital forcing allows the reconstruction of a high-resolution sedimentation history in which bulk sedimentation rates and hiatuses are quantified. Using the data on bulk sedimentation rate, rates of accumulation for geochemical proxies representing detrital, biogenic, and authigenic contributions to the sediment can be calculated. By integrating these quantitative assessments of siliciclastic dilution, primary production, and depositional redox conditions within a high-resolution chronostratigraphic framework, we are able to deconvolve secular trends in hemipelagic sedimentation during the Late Cenomanian-Early Turonian in the Western Interior basin. This integrated quantitative stratigraphic approach allows a reevaluation of hypotheses for the accumulation of excess organic matter in the Western Interior basin during the interval representing Oceanic Anoxic Event II.

Styles of Failure in Late Holocene Highstand Prodelta Wedges on the Adriatic Shelf

Abstract: On the continental shelves of the northern Mediterranean basin, the late Holocene highstand systems tract (HST) prograded un- der the influence of major rivers, after the attainment of the present sea-level highstand (about 5.5 cal kyr BP). On the Adriatic shelf, the thickness distribution of the late Holocene HST reflects the location of major deltas on the western side of the basin and the geostrophic cir- culation, which prevents a more uniform sediment dispersal toward the center of the basin. Very high sediment accumulation rates (1 to 10 cm/year) resulted in the construction of a HST depocenter up to 35 m thick. This shore-parallel depocenter is affected by failure of limited displacement over as much as 40% of its extent. Gas impregnation is common in the topset region and occurs at very shallow levels (a few meters) below the sea floor. Five areas are characterized by a variety of sea-floor and subsurface crenulations. Although locally some of these crenulations have an intriguingly regular geometry, sediment failure is the most plausible mechanism for their formation. Sediment failure better explains the large variety of geometries that characterizes the coastal mud prism of the late Holocene HST. Furthermore, we observe that these crenulations occur only where the downlap surface at the base of the HST is disrupted and affected by geometries that are con- sistent with fluid escape processes. This relationship suggests that the basal surface acts as a weak layer for sediment failure. Failure occurred in variable water depths from the northern slope of the modern Po prodelta (10-20 m water depth) to the narrow shelf offshore Bari (40-110 m water depth). In all these areas the proximal part of the HST prodelta wedge is intensely gas-charged. The thickness and age of the sediment sections affected by failure are slightly differ- ent from place to place but appear everywhere younger than 5.5 cal kyr BP. Where failure affects the entire HST the detachment occurs on the downlap surface at its base. Failure geometries characterize the head region whereas compressional features, such as pressure ridges and mud diapirs, dominate in the toe region, ranging in depth between 70 and 110 m. Where failure is limited to the upper few meters of the HST, there is a clear lithologic change (decrease in carbonate fraction and grain size) across the basal surface. This lithological change reflects a switch in sediment supply from local Apennine rivers (below) to Po- derived mud; this change occurred at the onset of the Little Ice Age, documenting the indirect control of short-term climate change and hu- man impact on sediment architecture. The deformations affecting the late Holocene HST in the various areas show differences in internal geometry, but appear everywhere to be characterized by limited downward displacement and can be attri- buted to shear-dominated retrogressive failure. It is suggested that some degree of consolidation occurred immediately after mobilization, possibly induced by the escape of fluids. Nowhere has failure evolved into disintegration and flow, likely because the type of cyclic loading that triggered it was not prolonged over a long enough interval. Short-lived radionuclides in the uppermost stratigraphic layers, which postdate the failure in the area offshore Ortona, allowed us to quantify systematic changes in sediment accumulation rates as a func- tion of the underlying deformed sea floor. In areas of wavy sea floor, troughs show sediment accumulation rates of greater than 16 mm/yr, a figure that is fourfold the rate measured on the flanks of the troughs.

Surficial Sediments of the Great Australian Bight: Facies Dynamics and Oceanography on a Vast Cool-Water Carbonate Shelf

Abstract: The Great Australian Bight (GAB), the largest sector of the southern Australia continental margin, is a site of cool-water carbonate sedimentation throughout, ranging from locally warm-temperate inboard to cool-temperate outboard. Surficial sediments are a mixture of calcareous Pleistocene skeletal and lithic intraclasts (relict grains), and Holocene biofragments, with minor amounts of quartz inboard. The inner shelf is an area of abundant macrophytes and seagrasses, active carbonate sediment production and accumulation, and little relict sediment. The huge middle portion is a "shaved shelf" with active sediment winnowing and mostly relict sediment. The outer shelf and upper slope is a variably productive sediment factory characterized by prolific calcareous epibenthic growth on hard substrate subaqueous "islands" shedding particles into surrounding sands and muds. Patterns of Holocene sedimentation are linked to modern oceanographic parameters in this high-energy setting characterized by overall downwelling. Prolific rhodoliths occur on the NW inner shelf, where shallow summer waters are the warmest in the GAB. These warm, saline, nutrient-depleted waters then drift eastward across the shelf, suppressing heterozoan carbonate production on the central and eastern mid-shelf. This arrested production in the eastern GAB is countered locally by summer coastal upwelling along western Eyre Peninsula, with bryozoan-rich sediment extending well inboard onto the mid-shelf. The outer shelf and upper slope is an area of prolific bryozoan growth, likely linked to upwelling, except in the central GAB, a region of year-round downwelling, where the area is one of off-shelf fine sediment transport and carbonate mud deposition. These patterns, in the central GAB at least, are present in the underlying Holocene and Pleistocene, suggesting that the general modern oceanographic dynamics and resultant carbonate sedimentation have persisted throughout the Quaternary.

Avulsion Frequency, Avulsion Duration, and Interavulsion Period of Holocene Channel Belts in the Rhine-Meuse Delta, the Netherlands

Abstract: Avulsion parameters greatly influence alluvial architecture, because they determine channel density and interconnectedness. Reliable data on avulsion parameters is still scarce. From a detailed reconstruction of the Holocene avulsion history of the whole Rhine-Meuse delta we determined quantitative values for the avulsion parameters: avulsion frequency, avulsion duration, and interavulsion period. In the Rhine-Meuse delta the number of coeval channels is related to the avulsion frequency. Calibrated 14C intensity histograms are used to analyze 14C dates of beginning and ending river activity. The histograms show that instantaneous and gradual avulsions were almost equally important in the Rhine-Meuse delta. A high avulsion frequency occurred from 8000 to 7300 cal yr BP (a total of 17 avulsions, i.e., 2.43 avulsions/100 years). During this period the avulsion frequency was related to the high rate of sea-level rise, which induced fluvial sedimentation in the present near-coastal area. After 7300 cal yr BP avulsion frequency decreased as a result of a continuously decreasing rate of sea-level rise. From 7300 to 3200 cal yr BP avulsion frequency was low; 35 avulsions took place within 4100 years (0.85 avulsions/100 years). Approximately 5000 cal yr BP the coastal barriers became closed and large-scale peat formation occurred. This resulted in fixation of the river channels and low cross-valley gradients, reducing the chances for avulsion. Between 3200 and 1400 cal yr BP avulsion frequency was high again (a total of 34 avulsions, i.e., 1.89 avulsions/100 years) as a result of increased discharge and/or within-channel sedimentation, which enhanced chances for avulsion. The period of activity of channel belts shows no significant trend on the time scale of the Holocene. It is highly variable and averages 1280 ± 820 cal yr. The avulsion duration fluctuates between less than 200 and 1250 cal years and averages 335 cal years. The avulsion duration shows no significant trend through time and remained constant until at least 1900 cal yr BP. Avulsion duration seems not to be influenced by aggradation rate. Average interavulsion period is shorter than the period of activity of channel belts, and is approximately 945 cal years. In the Rhine-Meuse delta, on the time scale of the Holocene, interavulsion period and avulsion duration are on average constant. Therefore, the number of coeval channels is directly related to avulsion frequency. Available evidence from other rivers and deltas around the world, however, suggests that the relationship between these avulsion parameters is not everywhere the same.

An Indicator of Water-Column Anoxia: Resolution of Biofacies Variations in the Kimmeridge Clay (Upper Jurassic, U.K.)

Abstract: An Indicator of Anoxicity (IA), defined as the ratio (Pyrite Fe + Oxide Fe)/Total Fe, can be used to recognize sediments deposited under an anoxic water column. Modern marine sediments deposited under oxygenated bottom waters mainly have IA values 0.5. IA values in the central Kimmeridge Clay (Upper Jurassic, UK) for fissile, organic-rich shales indicate that deposition occurred principally from anoxic bottom waters with infrequent oxygenation events.

Origin of Inner-Shelf Mud Deposit in the Southeastern Yellow Sea: Huksan Mud Belt

Abstract: A variety of previous sedimentological results, together with newly obtained hydrodynamic data, are summarized and reinterpreted with respect to the most likely origin of the Huksan Mud Belt (HMB) in the southeastern Yellow Sea. The HMB consists of two mud units that have similar grain texture and clay mineralogy but can be differentiated on seismic profiles because the lower unit is generally harder than the watery upper unit. The hydrographic measurements together with satellite imagery clearly indicate that the suspended plume covering the HMB originates from the west coast of Korea. The suspended plume becomes enhanced by winter monsoon winds beginning in late fall. Most of the studies of clay mineralogy, geochemistry, and isotherm distribution also suggest that the HMB has been derived from the west coast of Korea, particularly from the Keum River. An approximate evaluation of the depositional time for the HMB, based on its dimensions and the discharge rate of the Keum River, suggests a time duration of the order of 14,000 years. Hydrodynamic measurements across two transects over the northernmost HMB reveal that suspended matter travels southwest toward the main body of the HMB in a well-constrained corridor ultimately connected with the Keum River. Time-series (12.5 h) hydrodynamic measurements at a site over the HMB suggest that the winter-season suspended plume carries concentrations of suspended matter one to two orders of magnitude higher than those of the summer. Silt grains of 5-6 , well sorted through numerous resuspension events, settle out onto the HMB, forming distinctive silt laminae alternating with clay-rich laminae.

Turbidite Variability and Architecture of Sand-Prone, Deep-Water Slopes: Eocene Clinoforms in the Central Basin, Spitsbergen

Abstract: The architecture and turbidite variability within six wedge-shaped (downslope-thinning), sand-prone slope accumulations are documented from Eocene shelf margins on Spitsbergen. The Central Basin formed as a small foreland or piggy-back basin, and the studied turbidites accumulated mainly on the slope portion of sand-prone clinoforms that developed during depocenter migration and infilling of the basin. The shelf-margin clinoforms have amplitudes (minimum water depths) from 100 to 350 meters, and their shelf, slope, and basin-floor segments are well imaged, and can be walked out along many of the mountainsides. Only a small percentage of the clinoforms are sand prone, and these developed when sea level occasionally fell to or below the shelf edge. Of the sand-prone clinoforms, some had their sand budget partitioned mainly out onto the basin floor (basin-floor fans), but most trapped the sand on the slope only. The latter are now visible as downslope-thinning wedges, some 2.5-3.5 km in downdip extent. The turbidites within this type of clinoform have been examined and classified. The lower-slope to base-of-slope segment of the studied clinoform complexes are dominated by lobes consisting of broad, shallow channels and sheet-like turbidites, becoming heterolithic and muddy out on the basin floor. Beds on the lower slope vary from thick (up to 4.5 meters), ungraded or laminated sandstones, to thinner ungraded sandstones with coarse cappings. The middle-slope segment of clinoform complexes is dominated by narrow channels (chutes) that feed downslope to progradational chute-mouth lobes. Chutes contain ungraded and laminated sandstone beds up to 3 m thick, whereas the chute-mouth lobes show alternations of thinner, ungraded to laminated or rippled sandstones. These lobes become more heterolithic and muddy downslope. The shelf-edge to upper-slope segment of clinoform complexes is dominated by upward-coarsening and -thickening sheetsands of steep-fronted shelf-edge deltas. The sandsheets of the delta front can be traced updip into mouth-bar and distributary-channel sandstones. It is argued that shelf-margin accretion, represented by the sand-prone slope wedges, was achieved mainly by sand-laden currents that flooded from the shelf edge as hyperpycnal flows. This hypothesis is supported by: (1) the direct connection between channel and mouth bar systems at the shelf edge, and the turbidites of the slope lobes, (2) the systematic progradational character of the slope lobes, (3) the absence of large-scale slump scars, gullies, or canyons on the slope, and (4) the dominance of a type of turbidite that implies deposition from sustained flow. Detailed examination of the architecture of one of the slope wedges shows that there are unconformities developed within the stratigraphy below the shelf edge and that these erosional terraces beheaded the deltas perched on the uppermost slope. The erosion surfaces indicate fall of sea level to this position. Despite the magnitude of this fall (up to 80 meters), the lack of canyons on the slope prevented the construction of basin-floor fans. Such falls of sea level, on non-canyoned slopes, simply promote sand-prone accretion of the shelf margin.

Facies-Related Diagenesis and Multiphase Siderite Cementation and Dissolution in the Reservoir Sandstones of the Khatatba Formation, Egypt's Western Desert

Abstract: In the fluvio-deltaic, quartz-arenitic sandstones of the Jurassic Khatatba Formation in the Salam field (Egypt9s Western Desert), diagenesis and sedimentary facies control reservoir quality. Fluvial channel sandstones have the highest porosities (10-15%) and permeabilities (100-600 md), in part because of siderite cementation, which (1) inhibited compaction and quartz cement and (2) was later dissolved, creating intergranular secondary porosity (1/4 of total porosity). Fluvial crevasse-splay and marine sandstones have the lowest reservoir quality because of an abundance of depositional kaolin matrix and pervasive, shallow-burial Fe-dolomite cement, respectively. Siderite precipitation was multiphase and separated by distinct dissolution events. The earliest siderite precipitated near surface, within suboxic tropical coastal swamps containing predominantly meteoric waters. Some influence of marine waters is indicated by local enrichments in Mg and Ca. The next major siderite generation shows a trend to decreasing Mn and Ca contents, and is of shallow-burial origin. The last major siderite phase is Mg rich and interpreted as deeper-burial in origin. Some dissolution occurred during shallow burial related to climatically controlled meteoric water fluxing under unconformities. The most important dissolution, however, occurred during deep burial, resulting in (1) a major corrosion surface predating the last Mg-rich zone, (2) selective dissolution of some earlier zones, and (3) secondary porosity. This burial dissolution is interpreted to have been caused by cooling of compactional waters expelled from the basin along major faults. Other diagenetic phases observed include early-diagenetic pyrite, kaolin, quartz, bitumen, and late-diagenetic barite and illite. Kaolinite precipitated at shallow depths ( 130°C) during and/or after rapid Late Cretaceous burial. Quartz developed in two phases, separated by oil migration. This paper illustrates that, in fluvio-deltaic quartz arenites deposited under the influence of humid tropical climate, reservoir quality can be largely controlled by the contrasting pathways of carbonate diagenesis followed by the different sedimentary facies. This paper also documents a case in which siderite dissolution generated significant secondary porosity in reservoir sandstones, and where both siderite cementation and dissolution took place in multiple phases during different diagenetic stages, including early, shallow-burial, and deep-burial diagenesis. The present study also shows that, in multiphase siderite cements, the earlier growth zones can be selectively dissolved and replaced by later siderite zones (i.e., recrystallized) during burial diagenesis. These findings contrast with the general thought that siderite cements are not susceptible to generation of significant secondary porosity by dissolution and that earlier-formed siderites are essentially stable during diagenesis.

Origin and Growth Mechanism of Authigenic Chlorite in Sandstones of the Lower Vicksburg Formation, South Texas

Abstract: Authigenic chlorite, occurring as grain coatings, pore fillings, and rosettes, is common in deltaic sandstones of the lower Vicksburg Formation in South Texas. Chlorite rosettes are isolated and occur late in the diagenetic history, whereas grain-coating and pore-filling chlorite occur early, predating the development of secondary porosity and the precipitation of quartz overgrowths and Fe-poor calcite. Grain-coating and pore-filling chlorite, which are interstratified with approximately 15 percent 7 A layers, are Fe-rich with a mean Fe/(Fe+Mg) ratio of 0.68. Stable-isotope data (18O averages 13.5o/oo (SMOW)) indicate that the grain-coating and pore-filling chlorite began precipitating between 20°C to 40°C within the upper 1.8 km (6000 ft) of burial. Sandstones of the lower Vicksburg Formation were deposited in a relatively shallow deltaic environment as part of a large fluvial system draining the volcanics of Sierra Madre Occidental, Mexico and Trans Pecos, Texas. Weathering of these volcanics brought large amounts of particulate Fe to the river mouth. An increase in salinity in the distributary channel complex caused Fe flocculation and deposition. During periods of high river discharge, or when coastal currents acted to concentrate particulate Fe, large amounts of particulate Fe were deposited in the delta system. This resulted in thick layers of Fe-rich clay (odinite?) around grains. During periods of low river discharge, or when coastal currents caused wide distribution of particulate Fe, less particulate Fe was deposited. It is proposed that the grain-coating and pore-filling chlorite began forming through the transformation of an Fe-rich clay precursor in the formation-water hydropressure zone following Ostwald processes. The presence of this synsedimentary Fe-rich clay, and its recrystallization to Fe-chlorite, strongly influenced fluid flow and played a large role in developing the diagenetic heterogeneity seen in lower Vicksburg sandstones today.

Detrital Zircon Geochronology of Taconian and Acadian Foreland Sedimentary Rocks in New England

Abstract: U-Pb ages of detrital zircons in lower Paleozoic sedimentary rocks from New England permit evaluation of the relationships between tectonic activity and provenance of foreland sediments deposited in response to the Taconian and Acadian orogenies. The Lower Cambrian Poughquag Quartzite was deposited on the Laurentian passive margin. Poughquag detrital zircons (n = 40) are mostly concordant to slightly discordant with 207Pb*/206Pb* ages of 1.67-0.87 Ga and are consistent with a provenance from the nearby Grenville Structural Province. Two Poughquag zircon grains record 206Pb*/238U ages of about 0.64 Ga and 0.55 Ga and could have been derived from rift-related magmatic rocks. The Middle Ordovician Austin Glen Member of the Normanskill Formation and Lower Silurian Shawangunk Formation represent clastic-wedge sedimentation in the Laurentian foreland deposited in response to the Taconian orogeny (ca. 470-430 Ma). Zircons from these units (n = 41 for Austin Glen, n = 29 for Shawangunk) are also predominantly of Grenville age (207Pb*/206Pb* ages of 1.46-0.94 Ga). One zircon from the Austin Glen records an age of 3.29 Ga, consistent with derivation from North American Archean terranes. No zircon from either unit (or reported in the literature) can be interpreted as recording an Ordovician (Taconian) age. Devonian Catskill "redbeds" represent synorogenic to postorogenic foreland sedimentation associated with the Acadian orogeny (ca. 400-360 Ma). Zircons from one sample (n = 45) are all near concordant and have a bimodal age distribution. Approximately 60% of grains have 207Pb*/206Pb* ages of 1.23-1.00 Ga and the other 40% have 206Pb*/238U ages of 0.47-0.42 Ga. Very different proportions of Precambrian and Paleozoic ages are seen in the Catskill sample if zircons are selected on the basis of size or optical clarity, and this observation highlights the possibility of sampling bias if detrital zircons are not randomly selected for U-Pb analysis. The 0.47-0.42 Ga population is consistent with derivation from rocks crystallized during the Taconian orogeny. No analyzed zircon in this sample records an unambiguous Devonian (Acadian) age. Accordingly, during early Paleozoic evolution of the Laurentian foreland basins in New England, detrital zircons do not appear to record contemporaneous orogenic activity but instead are restricted to recycling of the preexisting continental margin. It is possible that substantial topography (e.g., accretionary prism) impeded sediment transport from the active orogen to the foreland basin.

Sedimentary Facies and Mineralogy of the Late Pleistocene Umukuri Silica Sinter, Taupo Volcanic Zone, New Zealand

Abstract: The Umukuri silica sinter is a large, late Pleistocene hot-spring deposit exposed along the eastern upthrown block of the Umukuri Fault, one kilometer southwest of the active Orakei Korako geothermal area, Taupo Volcanic Zone, New Zealand. Uplift along the fault has frozen sinter maturation to produce a characteristic silica-phase stratigraphy revealed by X-ray diffraction. Paracrystalline opal-CT dominates upper layers; opal-C occurs throughout middle and lower horizons; and recrystallized fabrics of microcrystalline quartz constitute lowermost exposed layers. Original and secondary matrix fabrics in the sinter include: fine-grained, porous, friable; dense, vitreous; and massive-mottled, diffusely layered. Original fabrics combine with silicified plant matter, peloids, pisoids, sinter fragments, detrital grains, diatoms, ostracodes, and filamentous to tufted microbial remains, to form nine depositional microfacies. Thinly laminated, plant-rich, and palisade types dominate. Sinter breccia and wavy-laminated varieties also are common, whereas curved laminae with lenticular voids ( = bubble mats), clotted, peloidal, and pisolitic microfacies are minor. Umukuri microfacies represent silicification in mid to low temperature waters on sinter apron terrace and slope areas, and in distal, marshy settings. Closely spaced, lateral and vertical intercalation of various microfacies in outcrop implies changing local flow and temperature conditions. No facies typical of high-temperature, proximal vent areas have been identified. Comparison with modern thermal spring analogs suggests that original Umukuri sinter fabrics recorded varying degrees of polymerization vs. monomerization of juvenile opal-A. Mineralogical and textural modification of matrix fabrics reflects a microscale, incremental continuum, following granular or fibrous habits along solution-precipitation pathways. Late-stage quartz rims and infills pores throughout the sinter.

Shoreline Autoretreat Substantiated in Flume Experiments

Abstract: Under conditions of constant sediment supply (S) to the basin and a steady rate of rise (A) in relative sea level, the autoretreat theory predicts that shorelines can retreat landwards at a relatively early stage of delta growth and attain an "autobreak" state, after which the existing subaqueous slope begins to be starved of sediment and thereby lose its clear delta-front configuration. A numerical model based on this theory further suggests that shoreline migration depends on the inclination of basin's basement slope () and subaqueous depositional slope () and, to a lesser degree, on the inclination of subaerial depositional slope (). Two-dimensional flume experiments in which a miniature delta is subject to steady forcing (A = const > 0, S = const > 0), were conducted to test the expectations of the autoretreat theory. The experiments consisted of two different series of runs: one was of " modulation" associated with a narrow range of the ratio of water discharge to sediment discharge (q/S), the other was of "q/S modulation" with kept nearly constant. The q/S ratio was mostly responsible for changes in but also affected to some extent. All experimental runs substantiated the landward turnaround feature of shoreline accretion (i.e., autoretreat) and the geometrical break of the delta-front shape during its retreat phase (i.e., autobreak). The obtained shoreline trajectories hold their geometrical patterns even after they were nondimensionalized in terms of the S/A ratio. A key hypothesis confirmed in the experiments is that A and S can never balance each other. Both of the experimental series brought some systematic change of shoreline trajectories, but the variations of shoreline trajectory due to modulation were much more striking than those related to q/S. Even these contrasting results are consistent with the numerical predictions. The present experimental results support the applicability of the autoretreat concept to natural environments and help clarify the processes governing the regression and transgression of a shoreline on a constructional margin.

The Castlegate Sandstone of the Book Cliffs, Utah: Sequence Stratigraphy, Paleogeography, and Tectonic Controls

Abstract: Earlier stratigraphic work had predicted that at the type section of the Castlegate Formation, the Castle Gate, near Price, Utah, the unit consists of two sequences separated by a sequence boundary representing approximately one million years of unrecorded time. Although the type section is well exposed, it consists of a monotonous succession of braided fluvial sandstones and no obvious boundary can be identified using facies criteria--this is a good example of a "cryptic sequence boundary." Petrographic data indicate, however, a significant change in detrital composition 20 m above the base of the section, at a through-going erosion surface that is therefore interpreted as the sequence boundary. Revised sequence correlations, together with other petrographic data and regional paleocurrent patterns, provide the basis for a model of the paleogeographic evolution of the area. Rocks assigned to the Castlegate Sandstone comprise two or possibly three sequences formed at times of slow regional subsidence. Erosional sequence boundaries and tilts in paleoslope between each sequence record thrust loading and unloading of the basin and the growing influence of intrabasinal upwarps, movement of which was beginning to be affected by Laramide movements toward the end of Castlegate sedimentation.

Ikaite Tufa Towers in Ikka Fjord, Southwest Greenland: Their Formation by Mixing of Seawater and Alkaline Spring Water

Abstract: Ikka Fjord in southwest Greenland is the type locality for ikaite, a metastable hexahydrate of calcium carbonate. Here, ikaite forms submarine tufa columns rising up to 18 m above the fjord bottom. The columns are spectacular examples of abiogenic carbonate precipitation in a cold seawater environment and so far represent the only known formation of ikaite in seawater. We have analyzed the 2H, 13C, 14C, 18O and 87Sr contents and major-element and minor-element compositions of ikaite and of the different water types in and around Ikka Fjord in order to understand the formation of the Ikka columns. Water from inside the columns is fresh but alkaline, rich in ions of sodium, bicarbonate, and carbonate, and has a stable-isotope composition identical to precipitation falling on the nearby Gronnedal-Ika igneous complex, and we conclude that the tufa columns form over submarine springs issuing from the bottom of Ikka Fjord. Dissolution of unidentified sodium carbonate minerals in carbonatites in the igneous complex probably accounts for the peculiar chemistry of the springs. Ikaite supersaturation and precipitation is controlled by mixing of spring water and seawater at the cold (< 6°C) fjord bottom, and precipitation is accelerated by the formation of hydrated CaCO3o (aq) ion pairs, while phosphate ions in the spring water act as inhibitor for precipitation of anhydrous carbonates. We propose that the spring water seeps in Ikka Fjord literally create their own conduits in the form of the vertical, chimney-like columns with ikaite forming continuously at the tops of the columns. The only limits to upward growth are winter ice and the fresh-water layer capping the fjord during summer.

Modern Sedimentation on the Shoreface and Inner Continental Shelf at Wrightsville Beach, North Carolina, U.S.A.

Abstract: The geologic framework and surficial morphology of the shoreface and inner continental shelf off the Wrightsville Beach, North Carolina, barrier island were mapped using high-resolution sidescan-sonar, bathymetric, and seismic-reflection surveying techniques, a suite of over 200 diver vibracores, and extensive seafloor observations by divers. The inner shelf is a sediment-starved, active surface of marine erosion; modern sediments, where present, form a patchy veneer over Tertiary and Quaternary units. The lithology of the underlying units exerts a primary control on the distribution, texture, and composition of surficial sediments, as well as inner-shelf bathymetry. The shoreface is dominated by a linear, cross-shore morphology of rippled scour depressions (RSDs) extending from just seaward of the surf zone onto the inner shelf. On the upper shoreface, the RSDs are incised up to 1 m below surrounding areas of fine sand, and have an asymmetric cross section that is steeper-sided to the north. On the inner shelf, the RSDs have a similar but more subdued cross-sectional profile. The depressions are floored primarily by shell hash and quartz gravel. Vibracore data show a thick (up to 1.5 m) sequence of RSD sediments that unconformably overlies ancient coastal lithosomes. In this sediment-starved inner shelf setting, rippled scour depressions probably form initially on preexisting coarse-sediment substrates such as modern lag deposits of paleofluvial channel lithosomes or ancient tidal inlet thalwegs. Interannual observations of seafloor morphologic change and the longer-term record contained in vibracores suggest that the present seafloor morphology is either relatively stable or represents a recurring, preferential morphologic state to which the seafloor returns after storm-induced perturbations. The apparent stability is interpreted to be the result of interactions at several scales that contribute to a repeating, self-reinforcing pattern of forcing and sedimentary response which ultimately causes the RSDs to be maintained as sediment-starved bedforms responding to both along-shore and across-shore flows. Sediment accumulation from over 30 years of extensive beach nourishment at Wrightsville Beach appears to have exceeded the local shoreface accommodation space, resulting in the "leaking" of beach and shoreface sediment to the inner shelf. A macroscopically identifiable beach nourishment sediment on the shoreface and inner shelf was used to identify the decadal-scale pattern of sediment dispersal. The nourishment sediment is present in a seaward-thinning wedge that extends from the beach over a kilometer onto the inner shelf to waters depths of 14 m. This wedge is best developed offshore of the shoreline segment that has received the greatest volume of beach nourishment.

Relationship between Spring and Geyser Activity and the Deposition and Morphology of High Temperature (> 73°C) Siliceous Sinter, Yellowstone National Park, Wyoming, U.S.A.

Abstract: Six classes of alkaline hot springs and geysers are here distinguished in Yellowstone National Park based on activity and eruptive style: (1) non-surging, non-boiling springs, (2) gently surging, non-boiling springs, (3) non-surging to gently surging, boiling springs; (4) strongly surging springs, (5) fountain geysers, and (6) cone geysers. Each displays morphologically distinctive siliceous sinter deposits in the high-temperature (> 73°C) parts of the vent and outflow systems. Non-surging and gently surging, non-boiling springs that do not overflow their rims develop thin digitate to lilypad-like sinter rims that grow horizontally over the pool surfaces. Similar springs with rim overflow have rims that accrete vertically. Boiling springs often have nearly circular, steep-sided pools and rims dominated by splash and rim-overflow and composed mainly of spicular and columnar sinter. Fountain geysers, characterized by broad fountain-like eruption bursts, have a central vent surrounded by a zone of spicular sinter beyond which is a broad, flat-bottomed moat, but lack steep sinter cones. Cone geysers have narrow vents, jet-like discharges that cool rapidly and fall close to the vent, and steep-sided spicule-dominated sinter cones. Sinter deposition takes place within three main zones: (1) fully subaqueous sides and bottoms of vent pools, outflow basins, and outflow channels; (2) periodically wetted splash zones surrounding hot springs and geysers; and (3) largely subaerial parts of the rim and outflow areas wetted mainly by overflow and surging and that dry quickly. Fully subaqueous surfaces are covered by fine earthy particulate siliceous sediment (PSS) composed largely of amorphous silica spherules and spherule aggregates that appear to have precipitated around heterogeneous nuclei, including microbes. Splash zones of hot springs and geysers are characterized by spicular sinter. Largely subaerial zones, wetted mainly by rim overflow and surging with little or no splash, show complex varieties of terraced, columnar, and knobby sinter. The highest silica deposition rates occur where surfaces are wetted frequently and evaporate to dryness, such as the tops of spicules, columns, and knobs. Associated lower areas and crevices that remain wet or moist show slow accumulation of PSS. Most continuously wet surfaces in high-temperature (> 73°C) hot spring and geyser vent and outflow systems are probably covered by microbial biofilms. Although microbes may locally influence rates of silica deposition and sinter microstructure, the results of this study suggest that the depositional sites, morphology, and macrostructuring of high-temperature sinter are controlled largely by the processes of surface wetting, evaporation, and drying.

On the Origin and Significance of Pyrite Spheres in Devonian Black Shales of North America

Abstract: Devonian black shales deposited on the North American craton contain abundant Tasmanites cysts that are typically preserved as flattened circular discs on bedding planes. Work by the present authors shows that cysts can be preserved as pyrite-infill casts that are expressed as sand-size whole and geopetal half-spheres of pyrite. At the bases of thin black shale layers these occur in situ at many stratigraphic levels in the distal prodelta facies of the Catskill Delta complex of New York, as well as in laterally equivalent black shales in Tennessee and Kentucky. Reworked pyrite casts, usually dominated by whole spheres, form lenticular lag accumulations and hydraulic placers, together with plant debris and phosphatic particles (bone debris, conodonts). An earlier model for the formation of pyrite spheres in gas bubbles is rejected in favor of formation within uncompressed Tasmanites cysts. Direct observation of cyst cuticle in association with pyrite spheres suggests that localized bacterial sulfate reduction in Tasmanites interior voids led to formation of localized pyrite deposition, in a manner similar to that described from certain ammonoid chamber settings. Cyst fill commenced with formation of framboidal pyrite, followed by later diagenetic pyrite cementation between framboids. These fills show geopetal features and appear to have formed within the redox zone below the sediment-water interface. Although described here from the Upper Devonian, comparable pyrite textures are also known from Proterozoic, Cambrian, Ordovician, and Silurian sediments. They probably occur throughout the sedimentary record, and in mudstone successions they may prove to be an important source of sand-size grains in areas far removed from the basin margins. As such they may be important for detection of erosive events and strong bottom currents, and provide valuable information about the depositional history of mudstone successions.

Equatorial Aridity in Western Pangea: Lower Permian Loessite and Dolomitic Paleosols in Northeastern New Mexico, U.S.A.

Abstract: Lower Permian strata have been extensively cored in the subsurface of the Bravo Dome field, northeastern New Mexico. Analysis of core indicates that these strata consist of conglomeratic and sandy fluvial deposits and volumetrically significant eolian silt (loessite). Fluvial facies dominate the lower half of the study interval and include matrix-supported, massive conglomeratic debris-flow units and laminated arkosic sandstone, whereas loessite dominates the upper half of the study section and consists of massive, well-sorted quartzose siltstone that locally reaches thicknesses as much as 120 m in the greater study region. Paleosols are present throughout the study interval and consist of protosols and dolosols, commonly exhibiting vertic features. Dolomite that is interpreted to be of pedogenic origin is an unusual but volumetrically significant component in these paleosols. Paleogeographic reconstructions and paleomagnetic data indicate that these strata accumulated at equatorial (3-8°) latitudes, but depositional and pedogenic evidence both suggest seasonally wet to markedly arid conditions from early Wolfcampian to early Leonardian time. The loessite covers a substantial area (> 6000 km2), making this the largest pre-Cenozoic loess accumulation yet documented. This is significant, because loess generally suggests arid to semiarid conditions. Intercalated paleosols in the loessite section record repeated cessation of silt influx coupled with landscape stability, which we relate to high-frequency oscillation between dry and slightly wetter conditions, possibly attributable to glacial-interglacial climatic conditions that prevailed at low latitudes. At a lower frequency, the evolution from a predominance of fluvial to primarily eolian strata, in tandem with changes in pedogenic character, reflect a long-term aridification for the study interval. These data corroborate independent inferences of monsoon-induced equatorial aridity in western Pangea and help constrain the timing of the zonal-to-monsoonal transition to earliest Permian time.

Physical Mechanisms of River Waterfall Tufa (Travertine) Formation

Abstract: Waterfall tufa is widely distributed around the world, especially in tropical and subtropical karst areas. In these areas river water is generally supersaturated with respect to calcite, and the precipitation occurs mainly at waterfall and cascade sites. Development of waterfall tufa has been described as simply being the result of water turbulence. We believe, however, that three physical effects can lead to tufa deposition at waterfall sites: aeration, jet-flow, and low-pressure effects. The three physical effects are induced by two basic changes in the water: an accelerated flow velocity, and enlargement of the air-water interface area. These two changes increase the rate of CO2 outgassing and the SIc, so that a high degree of supersaturation is achieved, which then induces calcite precipitation. These "waterfall effects" have been simulated in laboratory and field experiments, and each of them can accelerate, or trigger, calcite precipitation. Field measurements of river water chemistry also show that tufa deposition occurred only at waterfall sites. In these experiments and observations, waterfall effects play the most important role in triggering and accelerating CO2 outgassing rates. Field and laboratory observations indicate that plants and evaporation also play important roles in tufa formation. Growth of algae and mosses on tufa surfaces can provide substrates for calcite nucleation and can trap detrital calcite, accelerating tufa deposition. However, the prerequisite for such deposition at waterfall sites is a high degree of supersaturation in river water, which is mainly caused by waterfall effects. Evaporation can lead to supersaturation in sprays and thin water films at a waterfall site and cause the precipitation of dissolved CaCO3, but the amount of such deposition is relatively small.

Closed-System Burial Diagenesis in Reservoir Sandstones: Examples from the Garn Formation at Haltenbanken Area, Offshore Mid-Norway

Abstract: The Middle Jurassic Garn Formation of the Haltenbanken area has been studied using mineralogical and geochemical data from 21 wells, ranging in burial depths from 2.0 to 4.1 km relative to seafloor (RSF). K-feldspar and plagioclase contents show variations on a regional scale both laterally and as a function of burial depth. The content of pore-filling authigenic illite increases sharply, and the content of K-feldspar and kaolinite decreases in Garn sandstones presently at depths greater than 3.6-3.7 km RSF (120-130°C). The depletion in K-feldspar below 3.7 km RSF is not accompanied by lower potassium values in the bulk chemical composition (wt % K2O). This suggests that the potassium released during K-feldspar dissolution is retained in the sandstones and is precipitated as illite. The variations in bulk contents of potassium and sodium are therefore considered to be related principally to primary variations in sandstone mineralogy. The shallower sandstones ( 1/3) have a K-feldspar:kaolinite ratio greater than one. The deeply buried (> 3.7 km RSF) sandstones with similar potassium contents contain excess K-feldspar and most of the kaolinite is illitized. However, deeply buried sandstones containing an average of 0.38 wt % K2O (K/Al molar ratio < 1/4) contain a significant amount of kaolinite but negligible K-feldspar. This suggests that the K-feldspar:kaolinite ratio before the onset of illitization was less than one, and hence that the kaolinite-illite reaction has been restricted by an insufficient supply of potassium (absence of K-feldspar). This illustrates how illitization of kaolinite depends upon K-feldspar as a local source of potassium. Prediction of illitization in sandstones, therefore, must be based on integration of models for provenance, facies, and early diagenesis in addition to burial and thermal history. The formation of pore-filling authigenic illite in these sandstones is an important influence on the total reservoir quality.

Powder X-Ray Diffraction Analysis of Homogeneous and Heterogeneous Sedimentary Dolostones

Abstract: The %Ca (molar Ca/(Ca + Mg) 100) in sedimentary dolomite, which ranges from 48 %Ca to 62.5 %Ca, can be determined by atomic absorption spectrometry, electron microprobe analysis, or powder X-ray diffraction analysis. The latter procedure, which relies on the fact that the position of the d104 reflection depends linearly on the Ca content, offers an easy and relatively inexpensive method of determining %Ca in dolomite. To date, such analyses have assumed that the dolostone is homogeneous with respect to the %Ca content of its constituent dolomite crystals. Herein, the XRD technique is extended to compositionally heterogeneous dolostones. A considerable amount of important information can be derived from X-ray diffractograms of dolostones if they are analyzed with peak-fitting techniques (PF-XRD). Specifically, this technique allows (1) determination of the %Ca with an accuracy of ± 0.5 %Ca, (2) identification of heterogeneous dolostone samples formed of more than one population of dolomite crystals as defined by their %Ca content, and (3) derivation of the proportion of each population of dolomite crystals with an accuracy of ± 10%. The PF-XRD technique is a powerful analytical tool that provides valuable information on the major-element composition of low-Fe dolomite. Compared to other techniques, the PF-XRD technique is accurate, rapid, relatively inexpensive, and easy to use.

Geochemistry of the Floodplain Sediments of the Kaveri River, Southern India

Abstract: The floodplain sediments of the Kaveri River, southern India, derived from Archean gneissic and charnockitic source regions, show interbedding of silty (4-4.7 ) and sandy units (1.4-3.7 ). The geochemistry of silty beds is remarkably uniform at a given location and over a lateral distance of nearly 250 km; the sandy beds have more variable chemical compositions, yet are comparable to those of silty beds except for the diluting effect of quartz. Silty sediments retain the geochemical signature of prominently exposed source rocks for almost all elements and provide evidence of tectonic instability in the source region. The floodplain sediments contain all size grades (sand, silt, and clay), which may have resulted in minimizing the biases imposed on suspended and bedload sediments due to sorting. The low Chemical Index of Alteration (CIA), the dominance of unweathered primary minerals, and the minor proportion of smectitic clay all suggest that the region has been subjected to little chemical weathering. This is possible if the region has undergone recent uplift, exposing fresh Archean rock to surface denudation. The formation of fertile farmland along the Kaveri River course and its delta is related to these recent geological processes.

In Situ Stable Isotopic Evidence for Protracted and Complex Carbonate Cementation in a Petroleum Reservoir, North Coles Levee, San Joaquin Basin, California, U.S.A

Abstract: Knowledge of the evolution of carbonate cementation in hydrocarbon reservoirs is key to understanding the history of fluid flow during petroleum accumulation. The Stevens sands is a sequence of marine shales and deep-sea fan sands that was deposited within the Miocene Monterey Formation in the south-central part of the San Joaquin basin, California, during the upper Miocene (10-6 Ma). Rapid, high-precision in situ oxygen and carbon isotopic analyses of carbonate phases using the ion microprobe operated in multi-collection mode, in conjunction with electron microprobe analyses, indicate that carbonate cement zones within the Stevens sands at North Coles Levee (NCL) have had a complex and protracted fluid history. Three main generations of carbonate cement were identified. The relative timing of carbonate cement precipitation within the Stevens sands at NCL was estimated using the thermal and burial history of the San Joaquin basin, in situ oxygen isotope data, and cementation temperatures derived from equilibrium oxygen isotope fractionation factors for calcite-water and dolomite-water. Precipitation of these cement zones began soon after sediment deposition ( 7 Ma) and is ongoing. Early dolomite was precipitated at a temperature of 10°C, near the sediment-water interface, and soon after sediment deposition. Calcite cements, which are the most abundant variety, precipitated semicontinuously between 4 Ma and 5 Ma, at temperatures between 50°C and 65°C, and depths of 800 m to 1300 m. Fe-dolomite, which is paragenetically late, appears to have precipitated at temperatures near 100°C in response to pore-pressure reduction, which accompanied exploitation of the gas cap within the last 35 years. Carbon in these cements was likely derived from several sources including marine, maturing hydrocarbons, and a zone of methanogenesis.

Middle Holocene Sea-Level Rise and Highstand at +2 M, Central Texas Coast

Abstract: New data suggest a revised picture of middle Holocene sea-level change for the Texas Gulf of Mexico coast, and suggest reevaluation of coastal evolution. First, brackish marsh facies with calibrated radiocarbon ages of 7.7 to 7.8 ka have been recovered from depths of −8.5 to −9 m in a core from the ancestral Colorado River delta, and are interpreted to represent a sea-level pinning point. Second, a series of ridges along the Copano Bay margin farther south consist of shelly mud and fine sand with subtidal foram assemblages, occur at elevations of 1.95 m above the modern intertidal zone, and have produced calibrated radiocarbon ages on foram tests of ca. 6.8 to 4.8 ka. These ridges are interpreted to represent relict shallow subtidal to intertidal spits that provide minimum sea-level positions for the middle Holocene, and are now emergent because of later sea-level fall. In aggregate, these data show rates of sea-level rise during this time period that are very comparable to, or even lower than, published eustatic rates, and suggest a middle Holocene sea-level highstand for this non-uplifting, non-rebounding, and very slowly subsiding part of the North American coastline.

Evolution of Continental Slope Gullies on the Northern California Margin

Abstract: A series of subparallel, downslope-trending gullies on the northern California continental slope is revealed on high-resolution seismic reflection profiles imaging the uppermost 50 m of sediment. The gullies are typically 0 m wide and have 1 to 3 m of relief. They extend for 10 to 15 km down the slope and merge into larger channels that feed the Trinity Canyon. In the lower half of the 50 m stratigraphic section, the gullies increase in both relief and number up section, to maxima at a surface 5 to 10 m below the last glacial maximum lowstand surface. Gully relief increased as interfluves aggraded more rapidly than thalwegs. Erosion is not evident in the gully bottoms, therefore gully growth was probably due to reduced sediment deposition within the gullies relative to that on interfluves. As the gullies increased in relief, their heads extended upslope toward the shelfbreak. At all times, a minimum of 10 km of non-gullied upper slope and shelf stretched between the heads of the gullies and the paleo-shoreline; the gullies did not connect with a subaerial drainage network at any time. Gully growth occurred when the gully heads were in relatively shallow water ( 200 m paleo-water depth) and were closest to potential sediment sources. We suggest that prior to the last glacial maximum, the Mad River, then within 10 km of the gully heads, supplied sediment to the upper slope, which fed downslope-eroding sediment flows. These flows removed sediment from nearly parallel gullies at a rate slightly slower than sediment accumulation from the Eel River, 40 km to the south. The process or processes responsible for gully growth and maintenance prior to the last glacial maximum effectively ceased following the lowstand, when sea level rose and gully heads lay in deeper water ( 300 m water depth), farther from potential sediment sources. During sea-level highstand, the Mad River is separated from the gully heads by a shelf 30 km wide and no longer feeds sediment flows down the gullies, which fill with sediment from the distal Eel River. Approximately one-half of the subsurface gullies have no expression on the seafloor, because they have completely filled with sediment following the last glacial maximum lowstand of sea level.

Anatomy of a Modern Coral Reef Flat: A Recorder of Storms and Uplift in the Late Holocene

Abstract: Cores from Kabira Reef, Ishigaki Island, southwest Japan, reveal the internal structure and temporal changes in sedimentary process of a complete coral reef flat. The reef crest caught up with sea level at about 4000 yr BP. Since then, it expanded oceanward and the reef pavement has grown landward. The backreef structure is composed of bioclasts derived from the reef framework. Of all the bioclasts, corals and coralline algae are most abundant. Coral fragments coarser than -3.0 o have dominated the backreef sediments since about 2000 yr BP. Benthic foraminiferal tests first occurred at about 4000 yr BP, and their abundance increased significantly starting around 2000 yr BP. Shallowing of the reef crest is also indicated by the presence of the shallow-water benthic Foraminifera Baculogypsina sphaerulata tests after 2000 yr BP. The date 2000 yr BP is coincident with tectonic uplift at Kabira Reef (Kawana 1989). We consider this uplift to have caused a relative sea-level fall that aided the deposition of coral fragments transported from the reef pavement by storms. The relative sea-level fall also caused subaerial exposure of the reef crest during low tides and the transition of reef-building organisms from corals to shallow-water species of benthic Foraminifera, resulting in a change noticeable in the constituents of backreef sediments. The results of this study suggest that the coral reef flat, and especially the backreef, can be a faithful recorder of relative sea-level changes.

Conditioning a Process-Based Model of Sedimentary Architecture to Well Data

Abstract: Prediction of sedimentary architecture for modeling of fluid flow in hydrocarbon reservoirs and aquifers is accomplished mainly using stochastic, structure-imitating models, because these can be conditioned to data from wells, seismic profiles, and outcrop analogs. Conditioning implies that modeled architecture fits all available observations. However, the sedimentary architecture simulated by such models is commonly unrealistic. Process-based (forward) models potentially provide more realistic prediction and understanding of sedimentary architecture, but these models are not widely used because conditioning to well, seismic, or outcrop data is considered to be very difficult. We show here that conditioning of process-based models to well data is possible in principle, using a 3D alluvial-architecture model as an example. This model considers the formation of alluvial deposits as a predominantly deterministic process, with a single channel belt moving by avulsion over an aggrading floodplain. However, the initial floodplain topography is simulated by a random field, thus yielding different model output for each run. Monte Carlo simulation was used to produce model realizations that fit five hypothetical vertical wells within predetermined tolerance bands. Such simulation allows calculation of the probability of occurrence of channel-belt deposits for each 3D cell in the 3D block of sediments generated by the model, as well as the probability distributions of volumes of channel-belt deposits and connectedness ratios. Adding more conditioning wells increases the precision of model predictions. Application of this approach in practice will require a major effort, particularly in overcoming the anticipated large amounts of computing time.

Glaucony in Ocean-Margin Sequence Stratigraphy (Oligocene-Pliocene, Offshore New Jersey, U.S.A.; ODP LEG 174A)

Abstract: Glaucony occurs in abundance in clinoform-top and clinoform-toe positions within Atlantic-margin depositional sequences (offshore New Jersey, U.S.A.; Oligocene to Pliocene). Thin section and backscatter scanning electron microscope (BSEM) analyses indicate that grains of glaucony commonly formed in situ within burrows in deep-water (600-1000 m), clinoform-toe settings; fragmentation of glaucony grains in the matrix is most likely to have occurred through animal disturbance (e.g., ingestion and displacement by burrowers). Deep-water glaucony occurrences in the distal clinoform-toe positions show a pattern of association with quartz-sand abundance: in the most distal settings quartz sand is a minor component and its importance increases with proximity to the clinoform fronts. We hypothesize that these glauconitic sands, which commonly have erosional bases, formed by sediment starvation during relative sea-level rise and highstand, when the sandy clinoform fronts (deposited during sea-level lowstand) were abandoned. During particular times of regional sediment starvation over Oligocene to Middle Miocene time, biologically mediated erosion and transport were thus dominant processes in distal clinoform-toe settings, and in situ glaucony grains were mixed with quartz sand grains derived by degradation of the clinoform front. From Middle Miocene time onwards, development of submarine canyons may have restricted redistribution of quartz sand to discrete conduits. Sedimentary fabrics exhibited by shallow-water (