Showing papers in "Journal of Sedimentary Research in 1990"

Classification of large-scale subaqueous bedforms; a new look at an old problem

Abstract: A Symposium entitled "Classification of Large-Scale Flow-Transverse Bedforms" was convened at the 1987 Mid-Year Meeting of SEPM in Austin, Texas with the purpose of examining the problems involved in classifying large subaqueous flow-transverse bedforms developed in fluvial, intertidal, and marine environments, and recommending changes in nomenclature. The consensus of the participants is that despite the wide spectrum of morphologies of large-scale flow-transverse bedforms (excluding antidunes), they all occupy a similar position in the lower-flow-regime sequence between ripples and upper plane bed. The wide variety of forms is a reflection of secondary effects such as channelization, fluctuating water levels, and unsteady and reversing flows. The bedforms appear not to fall into size classes with naturally occurring boundaries but rather form a continuum with spacing from just under 1 m to over 1,000 m. The symposium panel proposes, therefore, that they should have only one name, DUNE. Dune is preferred as it has historical precedence over other terms in use, such as megaripple and sand wave. The term "dune" should be modified by primary descriptors of shape (i.e., 2-D or 3-D) and size based on spacing (small (0.6-5 m), medium (5-10 m), large (10-100 m) or very large (> 100 m) and the adjective subaqueous when it is important to distinguish them from eolian dunes. The panel recommends a morphologically based classification that is descriptive, with an underlying genetic rationale. Second order descriptors such a sediment size and bedform superposition may be used to describe more thoroughly the variety of subaqueous dunes in nature.

Bed configuration in steady unidirectional water flows; Part 2, Synthesis of flume data

Abstract: Data from 39 flume studies that report equilibrium bed configuration as well as water temperature, flow depth, flow velocity, and sediment size are used to develop the best approximation to the relationships among bed phases (ripples, dunes, lower-regime plane bed, upper-regime plane bed, and antidunes) produced by flows of water over loose sediments. We use a three-axis graph with dimensionless measures of mean flow depth, mean flow velocity, and sediment size along the axes. The relationships are presented as a series of depth-velocity sections and velocity-size sections through the dimensionless diagram. Boundaries between stability fields of the bed phases were drawn as smooth surfaces that minimize misplacement of data points. A large subset of the data, for which reliable values of bed shear stress are reported, was used to represent the stability relationships of the bed phases in a graph of dimensionless boundary shear stress against dimensionless sediment size. The graph shows substantial overlapping of the fields for dunes, upper plane bed, and antidunes owing to the decrease in bed shear stress in the transition from dunes to plane bed with increasing flow velocity. The topology of bed-phase boundaries was guided by the relationships shown in the dimensionless depth-velocity-size diagram.

Production and Cycling of Calcium Carbonate in a Shelf-Edge Reef System (St. Croix, U.S. Virgin Islands): Applications to the Nature of Reef Systems in the Fossil Record

Abstract: Geologic reefs are the end-products of not only constructive processes that produce calcium carbonate, but also of destructive processes (e.g., bioerosion and wave action) that reduce solid substrate to sediment, and of physical processes that rework the reef fabric and transport sediment. A study was conducted at Cane Bay on the island of St. Croix, U.S. Virgin Islands to quantify the suite of processes that have influenced Holocene reef development. Data on carbonate production, reef accretion, bioerosion, and sediment transport were used to construct a detailed "carbonate budget" and to apply the findings of this study to the character of reefs observed in the fossil record. Total carbonate production on the reef ranged from 0 to 5.78 kg/m2 per year, with a reef-wide average of 1.21 kg/m2 per year (1.13 by corals; 0.02 by coralline algae; 0.06 by primarily molluscs, forams and echinoderms). Based on 7 cores, only 0.91 kg/m2 per year of this have been retained within the reef interior. The remaining 0.24 kg/m2 per year of sediment, along with the 0.06 kg/m2 per year contributed directly by molluscs, etc., are deposited within reef channels and are probably flushed from the reef by major storms. Of the material recovered in the cores, 58% was either loose sediment and rubble or open void space. Much of the recognizable coral material was demonstrably reworked and out of place. Accretion rates across the shelf generally increased with water depth, as a result of active slumping along the steepening reef face over the past 2,000-3,000 years. The importance of detrital material in the reef fabric and the major role played by secondary processes that constantly rework the substrate have resulted in a reef whose interior is more of a "garbage pile" than an in-place assemblage of corals cemented together into a rigid "framework." The physical similarities between the detrital fabric of the reef at Cane Bay and those found in many ancient deposits imply that the general processes operating along the Cane Bay shelf have persisted during much of the evolution of ancient reefs. It is proposed that our modern models overemphasize the importance of in-place framework as a criterion for an "ecologic reef."

Exploratory Flow-Duct Experiments on Combined-Flow Bed Configurations, and Some Implications for Interpreting Storm-Event Stratification

Abstract: Experiments were conducted to define equilibrium combined-flow bed configurations developed under a wide range of oscillatory (Uo) and unidirectional (Uu) velocity components, with a constant oscillation period of 8.5 sec and median grain size of 0.09 mm. In the range of flows studied (Uo, 0-0.80 m/sec; Uu, 0-0.25 m/sec) the bed phases are: no movement, small 2D ripples, small 3D ripples, large 3D ripples, and plane bed. Small 2D ripples are stable at low Uo and Uu; by increasing either velocity component the plan-view bed configuration becomes progressively less regular as small 3D ripples develop. Small 3D ripples are stable at low to moderate Uo and a wide r nge of Uu. Under purely oscillatory flow, as Uo increases beyond about 0.40 m/sec, large-scale, hummocky bed forms become stable. When a unidirectional component is applied, these bed forms quickly develop a downstream asymmetry and generally move in the direction of Uu. The dip angles of the lee flanks increase with increasing Uu, although they remain less than the angle of repose. At high oscillatory velocities (Uo, 0.70-0.80 m/sec), plane bed is the stable bed phase when Uu > 0.05 m/sec. Much hummocky cross-stratification shows a preferred dip direction of the coset laminae. Our experiments support the idea that such cross-stratification is produced by combined flow: the large 3D ripples produced at high oscillatory velocities and small to moderate unidirectional velocities would presumably generate what might be called anisotropic hummocky cross-stratification (if we had been able to aggrade the bed during the runs). This would be the case even in dominantly oscillatory flow; the experiments show that even a weak unidirectional current of several centimeters per second superimposed upon a strong oscillatory flow results in bed forms that move predominantly in a single direction and that leave sets of laminae that dip predominantly in that direction. In runs at large Uo, the purely oscillatory large-scale bed forms were quickly planed off as Uu was increased from zero, but a series of almost stationary gentle undulations on the bed surface persisted for indefinitely long times where the original topographic highs were situated, even at the highest unidirectional flow investigated, 0.25 m/sec. This suggests a possible origin for the gentle undulation observed in Cretaceous parallel-laminated shallow-marine sandstones described by Arnott (1987, 1988), with spacings up to a few meters and heights not much more than a centimeter.

Classification of Large-Scale Subaqueous Bedforms: A New Look at an Old Problem-SEPM Bedforms and Bedding Structures

Abstract: A Symposium entitled "Classification of Large-Scale Flow-Transverse Bedforms" was convened at the 1987 Mid-Year Meeting of SEPM in Austin, Texas with the purpose of examining the problems involved in classifying large subaqueous flow-transverse bedforms developed in fluvial, intertidal, and marine environments, and recommending changes in nomenclature. The consensus of the participants is that despite the wide spectrum of morphologies of large-scale flow-transverse bedforms (excluding antidunes), they all occupy a similar position in the lower-flow-regime sequence between ripples and upper plane bed. The wide variety of forms is a reflection of secondary effects such as channelization, fluctuating water levels, and unsteady and reversing flows. The bedforms appear not to fall into size classes with naturally occurring boundaries but rather form a continuum with spacing from just under 1 m to over 1,000 m. The symposium panel proposes, therefore, that they should have only one name, DUNE. Dune is preferred as it has historical precedence over other terms in use, such as megaripple and sand wave. The term "dune" should be modified by rimary descriptors of shape (i.e., 2-D or 3-D) and size based on spacing (small (0.6-5 m), medium (5-10 m), large (10-100 m) or very large (> 100 m) and the adjective subaqueous when it is important to distinguish them from eolian dunes. The panel recommends a morphologically based classification that is descriptive, with an underlying genetic rationale. Second order descriptors such a sediment size and bedform superposition may be used to describe more thoroughly the variety of subaqueous dunes in nature.

Dolomitization and Dolomite Neomorphism in the Back Reef Facies of the Bonneterre and Davis Formations (Cambrian), Southeastern Missouri

Abstract: The back reef facies of the Bonneterre and Davis Formations consists of dolomitized cryptalgalaminates and partially dolomitized peloid mudstones. These subfacies were deposited on tidal flats and in shallow lagoons that existed in the St. Francois Mountains of southeast Missouri during late Cambrian time. The back reef facies is bounded on the seaward side by dolomitized algal bioherms and grainstone banks which host the southeast Missouri Mississippi Valley-type (MVT) lead-zinc-copper ore deposits. Cryptalgalaminates are replaced by both fine crystalline, planar dolomite and coarse crystalline, nonplanar dolomite. Peloid mudstones are partially replaced, but mainly by coarse crystalline, nonplanar dolomite. Planar dolomite, replacing cryptalgalaminates, shows gradational transitions into coarse crystalline, nonplanar dolomite. The transitions are accompanied by progressive increase in average crystal size and an increased skewness toward the coarse size fraction. Planar dolomite is enriched in 18 O relative to stratigraphically equivalent marine limestones. Coarse crystalline, nonplanar dolomite, which replaces both cryptalgalaminates and peloid mudstones, has delta 13 C and delta 18 O values transitional between those of planar dolomite and late dolomite cements associated with MVT mineralization. Fluid inclusion analysis of nonplanar dolomite and associated dolomite cements indicates precipitation at elevated temperatures (60 to 114 degrees C) by saline, basinal fluids. These data indicate that the back reef facies was subjected to a complex diagenetic history involving both early and late diagenetic dolomitization and dolomite neomorphism. Fine crystalline, planar dolomite was formed during early diagenetic dolomitization of the cryptalgalaminate subfacies by a fluid approaching seawater composition. Coarse crystalline, nonplanar dolomite was formed as a result of neomorphism of pro-existing planar dolomite and dolomitization of peloid mudstones, after burial, accompanying MVT mineralization in southeast Missouri.

Experiments on Bed Configurations in Fine Sands Under Bidirectional Purely Oscillatory Flow, and the Origin of Hummocky Cross-Stratification

Abstract: We made experiments on bed configurations in bidirectional purely oscillatory flows at a wide range of oscillation periods T and maximum orbital velocities Um in two closed flow ducts, 0.1 m and 0.4 m wide. The motivation was that understanding of bed configurations in oscillatory flows with long periods and high velocities during storm-induced deposition in the shallow ocean is needed for interpreting the stratification in such deposits. The ripples produced were classified geometrically as two-dimensional (2D) or three-dimensional (3D) in plan-view geometry and dynamically as oscillatory-current ripples (dependent upon the patterns of flow and sediment transport specific to the existence of the oscillation itself) and reversing-current ripples (dependent on he existence of the current, in one direction or the other, during the oscillation). Runs in the small duct with effectively 0.19 mm and 0.30 mm sand showed the familiar small 2D oscillatory-current ripples at short T and low Um passing into much larger ripples, with spacing up to 2 m, at periods of 6-20 s and velocities up to 1 m/s, with no apparent limit on size with increasing T and Um. Even in such a narrow duct there was some evidence of 3D geometry of the large ripples. Runs in the large duct with effectively 0.11 mm sand at a single period of about 9.5 s showed a progression from small 2D oscillatory-current ripples through small 3D reversing-current ripples to large round-crested 3D oscillatory-current ripples with prominent superimposed 3D to 2D reversing-current ripples. The large 3D ripples, which increased in spacing to 1-3 m at the h ghest attainable Um of 1 m/s, shifted in position and changed in size with time, seemingly at random, causing substantial local and temporary erosion and deposition at a given point on the bed. There was no net aggradation during the runs, but a deposit was synthesized by taking a time series of sidewall bed profiles during a run in which large 3D ripples developed from an initially plane bed and changed irregularly in size and position as they grew toward equilibrium, and then plotting the profiles one above the previous with a constant upward increment. The resulting stratification is strikingly similar to much hummocky cross-stratification in ancient sandstones. This suggests that some hummocky cross-stratification is generated during sediment fallout from strong purely oscillatory flows at moderate to long oscillation periods as large 3D oscillatory-current bed forms develop from a planar bed during strong but waning flow.

Geostrophic circulation or shallow marine turbidity currents? The dilemma of paleoflow patterns in strom-influenced prograding shoreline systems

Abstract: In ancient storm-influenced prograding shoreline sequences, sole marks (mainly tool marks) from hummocky crossstratified storm deposits are commonly oriented normal to paleoshoreline and the trend of paleobathymetric contours in the basin. Asymmetrical tool marks typically indicate flows directed offshore. Several workers have attributed their formation to storm-generated, shallow marine turbidity currents. This interpretation conflicts with observations from modern shelves, where storm-driven circulation generally is geostrophically balanced, and time-averaged bottom currents approximately parallel bathymetric contours and the local shoreline. The resolution of these apparently conflicting observations may lie in the realization that tool marks (and many other small paleoflow indicators) form almost instantly as the result of instantaneous flow conditions very near the bed. Beneath storm-generated flows in the shallow ocean, instantaneous and time-averaged characteristics of the bottom boundary layer generally exhibit little similarity. Storm-generated tool marks are formed by the movement of large tools within the thin (less than 1 m) inner boundary layer resulting from the superimposition of waves and currents. The orientation of the peak instantaneous shear stress moving large tools under such combined flows mainly reflects wave-orbital motions, which typically are normal to shore. The magnitude of stress is greatly increased in the offshore direction (and decreased in the onshore direction) by superimposition of a steady current with an offshore component of flow, but the direction of stress is only slightly affected. In ancient storm-influenced sequences, therefore, shore-normal tool marks generally were not formed by turbidity currents; rather, their orientation is best attributed to shoaling waves approaching the coast at a very high angle. Asymmetrical tool marks are directed offshore due to enhanced shear stress on the offshore stroke of waves superimposed on a geostrophic current with an offshore flow component. Tool marks do not reflect the time-averaged bottom-flow direction; in fact, they provide almost no information concerning steady bottom currents. In contrast, high-angle cross-beds (formed in coarser sediment by the migration of dunes and sandwaves), although relatively rare in offshore storm deposits, generally reveal approximately shore-parallel flows in ancient systems. Cross-beds closely reflect the time-averaged flow direction in the outer boundary layer of a geostrophic current, for three reasons: 1) the net transport direction for sand moving as bed load beneath a combined flow lies between the directions of peak instantaneous shear stress and time-averaged shear stress; 2) large ripples disrupt the thin inner boundary layer; and 3) long time intervals (relative to wave-induced velocity oscillations) are required to form large ripples.

Albitization of Detrital Plagioclase in Triassic Reservoir Sandstones from the Snorre Field, Norwegian North Sea

Abstract: Detrital plagioclase (An12-An28) in Triassic reservoir sandstones of the Lunde Formation in the Snorre Field, offshore Norway has been partially to completely albitized during burial diagenesis (depths of about 2,500-3,000 m; 75-100°C present temperature). Calcite (CaCO3 > 99 mole %) and kaolinite were formed as by-products of plagioclase albitization. Sodium-rich detrital plagioclase (An2-An11) are usually unalbitized. Detrital K-feldspar is fresh or, rarely, partly albitized. Differences in albitization behavior of detrital plagioclase and K-feldspar are shown to be related to the chemistry of formation waters, reactivity of kaolinite, and temperature. The diagenetically formed albite pseudomorphs are very pure (Ab > 9 ), well-ordered low-albite, dark-luminescent, vacuolized, mostly untwinned, and comprised of numerous euhedral albite crystals. Some of the microporosity within the albite pseudomorphs is caused by volume reduction during albitization of the detrital plagioclase. The amount of albite formed in the pseudomorphs is related to the molar proportion of albite component in the detrital unstable Ca-rich plagioclase. The microporosity formed during albitization is proportional to the anorthite component, which is believed to have dissolved and resulted primarily in the formation of calcite and kaolinite. The albitization of detrital plagioclase in the studied sandstones can thus be explained without assuming the consumption of significant amounts of Na+ from pore solutions.

Infiltrated Clays in Fluvial Jurassic Sandstones of Reconcavo Basin, Northeastern Brazil

Abstract: Fluvial sandstones and conglomerates of the Sergi Formation, a Jurassic unit of Reconcavo Basin, northeastern Brazil, contain significant amounts of interstitial detrital clays, which are interpreted as the product of early mechanical infiltration developed in an arid/semi-arid environment. Within such an environment, the lowered water table allowed muddy waters of episodic runoff to infiltrate through the coarse alluvium, concentrating the clays in the upper phreatic zone near the sources of influent seepage. Petrographic textures considered to be typical of mechanically infiltrated (MI) clays are herein presented, using examples from the Sergi Formation. These textures include 1) ridges and bridges, 2) geopetal fabrics, 3) loose aggregates, 4) anisopachous coatings of tangentially-accreted lamellae (cutans), 5) massive aggregates, and 6) shrinkage patterns developed during diagenesis. In addition, their detrital aspect as revealed by SEM observations and the presence of impurities in the interior of the clay aggregates were also used as evidence for the interpreted origin of the clays. In most hydrocarbon reservoirs in the Sergi Formation, the clays described in this paper are among the main controls on porosity and permeability distribution. Commonly, zones with maximum clay concentration are the main internal permeability barriers within the Sergi Formation reservoirs.

Is there evidence for geostrophic currents preserved in the sedimentary record of inner to middle-shelf deposits

Abstract: Unidirectional, oscillatory and combined-flow paleocurrent data from many ancient wave-dominated coastlines indicate that sediment transport was directed offshore at angles of 70 to 90° from the inferred local paleoshoreline. Solemarks and parting lineations from turbidite-like storm beds and solemarks from hummocky beds indicate that flows transporting sediment offshore were orthogonal to the local shoreline. The shore-normal orientations of solemarks, parting lineation and asymmetric ripples on hummocky cross-stratification and their orthogonal relationship with respect to wave-ripple crest orientation indicate that the dominant direction of wave approach at the sediment-water interface was also orthogonal to the shoreline. The formation of hummocky cross-stratification by an al ng-shore flow component superimposed on oscillatory wave motion is not supported by paleocurrent data. Rather, much of the evidence from the ancient record suggests that hummocky cross-stratification forms under high-energy, oscillatory-flow with a weak, shore-normal, combined-flow component. The orientation of wave-ripple crests from the lower shoreface to offshore-transitional zone of wave-dominated coasts can be used to approximate the local paleoshoreline trend. The paleocurrent data from the ancient examples suggest a strong predominance of offshore-directed sediment transport, as opposed to the nearly shore-parallel orientation of currents observed in modern geostrophic flows.

Diagenetic albitization of detrital K-feldspars in Jurassic, Lower Cretaceous and Tertiary clastic reservoir rocks from offshore Norway; II, Formation water chemistry and kinetic considerations

Abstract: Formation waters from clastic reservoirs offshore Norway are saturated or supersaturated with respect to albite deeper than 2.5 km, and support the view of Saigal et al. (1988) that albitization of K-feldspar is presently active in these sandstones. Thermodynamic and mass constraints show that the albitization process is more sensitive to potassium removal than sodium supply. The temperature of albitization commencement coincides more or less with the estimated onset temperature of illitization of smectite (70-80°C). Interbedded shales seem to be the major potassium sink and source of sodium. Kinetic considerations indicate that at temperatures below 145°C the dissolution of K-feldspar is always faster than albite growth. Combined with replacement constraints, the kinetic data also explain the observed coarsening of growing albite crystals with increasing burial temperature.

Chemical Weathering of Fluvial Sediments During Alluvial Storage: The Macuapanim Island Point Bar, Solimoes River, Brazil

Abstract: Modification of sediment composition by chemical weathering depends both on the overall intensity of weathering and on its duration. At Macuapanim Island, Solimboes River (Amazon main channel), Brazil, a number of alluvial deposits have been exposed to intense tropical weathering for differing lengths of time. In the very fine sand fraction, the detrital component shows a steady and marked increase in compositional maturity with increasing sediment age: immature sands of arkosic to litharenitic composition show a progressive loss of lithic fragments, feldspars, and accessory minerals, with a concomitant enrichment in quartz. This process produces first-cycle sands whose detrital component is of quartz arenite composition. Among the salts, quartz is enriched over time relative to more nstable phases, particularly plagioclase. This enrichment is probably largely caused by the destruction of plagioclase through chemical weathering, but silt-size quartz may also be "produced" through the breakdown of sedimentary and metasedimentary lithic fragments in the sand fraction. In the clay fraction, an increase in the relative abundance of smectite accompanies the breakdown of volcanic lithic fragments in the sand fraction, and probably reflects the degradation of volcanic glass. The clay fractions of the oldest samples examined contain less smectite, somewhat more kaolinite, and markedly more vermiculite as compared to younger samples, but an extensively-leached, cation-poor clay fraction analogous to the quartz-dominated silts and sands is never attained. The compositional mat rity of the clay fraction is everywhere lower than that of associated silts and sands, perhaps because of a continual influx into the clay fraction of immature cation-rich clays derived from the physical breakdown of lithic fragments in coarser size fractions. This effect is enhanced by the translocation of fines, whereby younger, less-weathered fine material is juxtaposed with older, more mature coarse material in the lower levels of the alluvial stratigraphy. Reworking of floodplains during channel migration results in the reincorporation of older material stored on the floodplains into the sediment load. If sediments have been intensely weathered during storage on the floodplain, then the result will be a net dilution and replacement of younger, relatively immature sands transported by the river by older, more mature material from the floodplain. The relative proportion of the older sands in the bedload will increase downstream, resulting in a progressive downstream increase in the compositional maturity of the bedload of the river.

A Fluid Inclusion Study of Quartz-Cemented Sandstones from Offshore Mid-Norway--Possible Evidence for Continued Quartz Cementation During Oil Emplacement

Abstract: Jurassic sandstones from the oil zone in a well from the Haltenbanken area offshore mid-Norway contain both aqueous and hydrocarbon fluid inclusions within quartz overgrowths. Homogenization temperatures for the inclusions range from 92°C up to the present formation temperature of approximately 118°C. Since the inclusions mostly are located at the boundaries between quartz clasts and quartz overgrowths, the homogenization temperatures indicate that quartz cementation started at temperatures slightly below 92°C. Poikilotopic ferroan calcite cement usually predates formation of quartz overgrowths, and most calcite cement must be precipitated at temperatures below 90°C, although in one calcite cemented zone ferroan calcite encloses quartz overgrowths with inclusions trapped at temperatures of up to 105°C. None of the examined hydrocarbon inclusions homogenized below 102°C, and this suggests that oil started entering the reservoir at temperatures of around 100°C. The burial history of the examined sandstones indicates that temperatures of 100°C were not reached before the late Pliocene and that filling of the reservoir has taken place during the last few million years. Homogenization temperatures approximately equal to present formation temperature suggest that quartz cementation may still be in progress, and that emplacement of oil did not necessarily stop precipitation of quartz cement. Possible continued quartz cementation after oil filled the reservoir could be explained by dissolution of quartz at stylolites and grain contacts as transport of dissolved quartz from these sources might possibly take place by diffusion through irreducible water.

Fabric-retentive, non-luminescent brachiopods as indicators of original delta 13 C and delta 18 O composition; a test

Abstract: Fabric-retentive, non-luminescent Helderbergian brachiopods from open marine deposits, analogous to brachiopods used in other studies to determine paleo-oceanic stable isotopic composition, exhibit depleted 13C values immediately below the erosional contact and more enriched values of 13C at greater stratigraphic distances below this surface. In addition, 13C values of these brachiopods display a marked shift across the erosional surface, and values above it are essentially the same as those 30 m or more beneath the surface. Complementing these trends, whole rock analyses, metastable, high- agnesian calcite constituents (echinoderms), as well as early equant calcite cement, also exhibit an increased depletion trend in 13C values as the Helderberg-Oriskany contact is approached from below. Fabric-retentive luminescent, as well as obviously recrystallized brachiopods, display stable isotopic values similar to those of fabric-retentive, non-luminescent brachiopods. Thus, "best-preserved" fabric-retentive, non-luminescent brachiopods exhibit values similar to obviously recrystallized brachiopods, diagenetically altered metastable constituents (echinoderms), stable early meteoric cements, and whole rock samples. Helderberg strata of New York State were affected by early meteoric diagenesis during a Siegenian exposure event prior to deposition of the overlying marine Oriskany Sandstone. Meteoric diagenesis during this exposure gave rise to a characteristic carbon isotopic depletion trend approaching the Helderberg/Oriskany contact. All of the skeletal constituents underwent isotopic exchange during early diagenetic exposure to meteoric waters. Consequently, the practice of using fabric-retentive, non-luminescent brachiopods as indicators of their original chemistry and, by extension, paleo-oceanic chemistry is questioned.

Alluvial Paleosols and Pedofacies Sequences in the Permian Lower Beaufort of the Southwestern Karoo Basin, South Africa

Abstract: Three types of paleosol profiles are described as representative of the range of soil types that developed on the ancient alluvial plains of the southwestern Karoo Basin during the Late Permian. Continuous 3-dimensional exposures of the Lower Beaufort (Adelaide Subgroup) strata allow detailed sedimentological, taphonomic, and paleopedological observation of thick floodplain sequences that contain numerous "stacked" paleosol profiles. Various floodplain strata contain characteristic paleosol profiles that are regulated by the periodicity and competence of overbank flooding, both of which decrease with increasing distance from the contributing channel. Paleosols developed in levee or channel-bank deposits are immature calcic Entisols displaying evidence of early carbonate translocation but without well-developed horizonation. Proximal floodbasin paleosols, at the foot of the meanderbelt slope, are mature calcic Vertisols with more fully developed profiles containing zones of both carbonate and clay illuviation as well as some groundwater calcretes. Slickensided stress argillans in the B-horizons are indicative of multiple cycles of wetting and drying. Distal floodbasin paleosols are dominated by hydromorphic and evaporitic processes as a result of their axial position in the floodbasin. Sheet-like networks of mudcrack casts composed of calcretized aeolian sand and silt suggest that dust was a major source of calcium carbonate to the e soils. The paleosols are interpreted as having formed under warm to hot, semi-arid climatic conditions with a strongly seasonal rainfall of 500 to 800 mm per annum and a mean annual moisture deficiency. The geomorphic setting was similar to the Holocene Indo-Gangetic alluvial plain. Vertebrate burrow casts in the proximal floodbasin facies indicate a minimum depth to watertable of 150 cm. The distribution of rooted horizons reflects dense vegetation flanking major drainage channels and sparse cover in the distal floodbasin. A simple stratigraphic sequence of once laterally continuous paleosols is termed a pedofacies. Stacked sequences of pedofacies within approximately 20-50 m thick, and 100-250 m thick stratigraphic successions reflect second- and first-order sedimentary cycles respectively. Both scales of cyclicity are attributed to autocyclically controlled avulsions of differentially aggrading channels in the first instance and of complete drainage networks in the second.

Shelf-wide erosion, deposition, and suspended sediment tranport during Cyclone Winifred, central Great Barrier Reef, Australia

Abstract: Observation of shelf sediments collected immediately before and after Cyclone Winifred crossed the central Great Barrier Reef shelf (1 February 1986) confirmed that the storm produced a normally graded, mixed terrigenous-carbonate storm layer extending 30 km offshore in water up to 43 m deep. Distinct post-Winifred changes in the cross-shelf distribution of organic carbon and carbonate in the mud-fraction of the sediment suggest that suspended sediment transport was extensive and that the storm layer had multiple sediment sources. On a shelf-wide scale, the storm layer is composed almost entirely of reworked shelf sediment. Erosion depths were greater on the mid-shelf (20-40 m water depth) than on the inner shelf( 6.9 cm and 5.1 cm, respectively. Particles finer than medium sand were eroded and transported out of the mid-shelf. The inner-shelf portion of the storm layer formed by the combination of three sediment sources, including 1) seaward transport of terrigenous sediment in buoyant freshwater flood plumes, 2) resuspension and settling of inner-shelf sediment, and 3) resuspension and shoreward transport of mid-shelf sediment. Mass-balance calculations predict that at least 10-30% of the inner-shelf storm layer is composed of mid-shelf mud. Combined wave and wind-forced currents probably resuspended mid-shelf material and drove the suspended fraction alongshelf and shoreward a minimum distance of 15 km. The results suggest that tropical cyclones are capable of sporadic but efficient cross-shelf transport of suspended sediment. On shallow cyclone-prone shelves, suspended sediment may easily be exchanged between adjacent sedimentary facies. In ancient shelf sequences, the transport history of the mud will be complex, and stratigraphically equivalent facies may have similar mud types but completely different sands.

Sedimentology and Mineralogy of Dolomitic Coorong Lakes, South Australia

Abstract: Coorong dolomites are true "primary dolomites"; that is, Coorong dolomite is precipitating where there was nothing before and is not replacing an earlier carbonate mineral. Dolomites in the various lakes of the Coorong coastal plain in South Australia occur in three mineralogical associations; Dolomite ± Mg-calcite (widespread) Dolomite ± Magnesite (common in northwest) Dolomite ± aragonite ± hydromagnesite (rare and only in northwest). Most Holocene dolomite occurrences in the near coastal lakes of the Coorong region are associated with Mg-calcite. Only in the more arid settings found at the northwestern end of the Coorong region, near Salt Creek, is dolomite commonly found in association with magnesite and occasionally with hydromagnesite and aragonite. Dolomite in the near surface sediments of lakes near Salt Creek such as Pellet Lake and Milne Lake is often associated with magnesite in the lake center and Mg-calcite about the lake margin. The dolomites associated with magnesite tend to be more magnesian-rich than dolomites associated with Mg-calcite, a direct reflection of the chemistry of the mother waters. Waters which precipitate magnesite are typically more concentrated and more magnesium-rich than waters pre ipitating Mg-calcite, hence co-precipitated dolomites will also be more magnesium-rich. Textures found in vertical sequences from lakes in the Coorong coastal plain are largely independent of mineralogy; similar vertical transitions occur whether the lake is filling with carbonate, magnesian carbonates, or gypsum. Dolomite has been the main mineral of concern in previous geological studies of this area, yet it makes up no more than 10% of the carbonate minerals forming surficial deposits across the coastal plain. The hydrological model of the Coorong lakes should not be used to explain widespread dolomitization in ancient supratidal and shelf carbonates. Coorong counterparts in the rock record are a facies mosaic of lacustrine carbonate sitting in a much more extensive paleoaquifer.

Detrital Magnetite as a Provenance Indicator

Abstract: Detrital magnetite grains carry unique petrographic and chemical fingerprints that can be used in provenance research. Petrographic analysis of 2,941 detrital magnetite grains in Holocene sands from felsic plutonic and volcanic, intermediate volcanic, mafic plutonic and volcanic and metamorphosed mafic/ultramafic parent rocks has found that 58% are polymineralic. Polymineralic detrital magnetite grains are most common in sands derived from felsic volcanic, mafic plutonic, and metamorphosed mafic/ ultramafic parent rocks. Sands from felsic volcanic parent rocks are characterized by grains with composite- or trellis-type magnetite-ilmenite intergrowths (c/(c + t) > 0.5), whereas sands from marie plutonic and metamorphosed mafic/ultramafic parent rocks are characterized by grains with exsolved pleonaste or ulvospinel. Monomineralic (homogeneous) grains are characteristic of sands derived from felsic plutonic and intermediate volcanic sources. Sands from mafic volcanic sources are characterized by a 1:1 relationship between polymineralic (grains with trellis- or composite-type magnetite-ilmenite intergrowths, (c/(c + t) < 0.5) and monomineralic grains. A ternary plot, with homogeneous grains, grains with trellis- or composite-type magnetite-ilmenite intergrowths, and grains with exsolved pleonaste or ulvOspinel as corners, successfully grouped grains from these parent rocks into fields with minimum overlap. Electron probe microanalysis of 433 homogeneous grains identified the presence of a chemical fingerprint. Step-wise discriminant function analysis found that TiO2, MgO, V2O3, and Al2O3 could best discriminate between detrital magnetite grains from felsic plutonic and volcanic, intermediate volcanic, and mafic plutonic parent rocks. Within this control set, detrital magnetite grains were correctly classified 98% of the time. However, grains from mafic volcanic and metamorphosed mafic/ultramafic sources were not chemically distinct, emphasizing the importance of integrating petrographic and chemical analyses in provenance research.

Geochemical Imprint of Meteoric Diagenesis in Holocene Ooid Sands, Schooner Cays, Bahamas: Correlation of Calcite Cement Geochemistry with Extant Groundwaters

Abstract: The geochemistry of meteoric calcite cements and their parent waters on three small Holocene ooid-sand islands in the Schooner Cays, Bahamas, is explicated in terms of observed hydrologic and hydrochemical processes. Dissolved Mg is derived soley from admixed seawater or aerosols. The Mg contents of the cements (0.4 to 3.0 mole %) are thus a salinity indicator and suggest cementation in waters composed of 16 mmoles/liter per yr). Variations in Mg and Sr contents across cement mosaics reflect long-term temporal fluctuations in the amou t of aragonite-to-calcite alteration and incursions of brackish waters. The pore-water oxygen reservoir is water-buffered and derived from meteoric rainfall and seawater mixing, but is seasonally enriched in 18O by evaporation. The 18O values of the cements (-3.4 to -5.0 PDB) reflect the relative proportions of cements formed during and immediately after the rainy season versus those formed during the dry season. The pore-water carbon is derived from organic respiration, atmospheric carbon, and dissolved aragonite. Organic respiration dominates the 18C of the vadose cements (-7.8 to + 1.1 PDB) and the 13C of phreatic groundwaters (-2.2 to -14.0% PDB). In contrast, 13C-enriched phreatic cements (-1.2 to +3.7% PDB) reflect carbon from dissolved aragonite, and are not in carbon isotopic equilibrium with the present groundwaters. This case example reveals the complex geochemical signal that can result from even a very simple diagenetic history, such as a single phase of meteoric diagenesis acting on a purely aragonitic sediment.

Physiographic controls on the composition of sediments derived from volcanic and sedimentary terrains on Barro Colorado Island, Panama

Abstract: The phenomena that control sand composition are complex, dynamic, and interlinked. Although source-rock lithology is of critical importance in determining the allowable range of sand compositions, chemical weathering may overprint the source-rock signal substantially. In the humid tropical environment of Barro Colorado Island, Panama, source rocks of similar lithological character produce sediments of differing compositions in different physiographic environments. Moreover, sediments derived from source rocks of dissimilar mineralogical composition but produced on similar physiographic terrains may be nearly indistinguishable. Physiography influences the composition of sediments released from the source areas by controlling the duration of chemical weathering. On relatively flat terrains, rates at which loose solids are generated by weathering may exceed rates at which slope processes can remove this material. Soils are exposed to the weathering environment for long periods of time and are strongly leached. Consequently, weathering products are cation poor and contain relatively immobile elements. Incompletely weathered detritus is not abundant and is greatly enriched in chemically resistant phases. On steeper slopes, potential solid erosion rates exceed weathering rates. Residence times of minerals within soils are short, resulting in weathering products that are cation rich and contain an abundance of uns able phases. On Barro Colorado Island, fluvial sediments are dominated by authigenic particles composed of secondary weathering products. Although the stability of many such particles in larger transportational and depositional systems and in the diagenetic environment has yet to be assessed, they may be more important constituents of fluvial sediments in tropical environments than has been previously recognized.

Crustose coralline algae from the Flower Garden Banks, northwestern Gulf of Mexico; controls on distribution and growth morphology

Abstract: The East and West Flower Garden Banks in the northwestern Gulf of Mexico represent the northernmost tropical coral reefs and Caribbean coralline algal flora on the North American continental shelf. Coralline algae are the dominant framework builders and sediment contributors below 50 m on the banks. On the basis of growth form and taxonomic composition, coralline algae on the Flower Garden Banks can be divided into four overlapping depth zones: upper coral reefs (20-30 m), Madracis-algal ridges (30-40 m), algal nodule zone (45-80 m), and deep algal reefs (50-90 m). Corallines on the upper coral reefs are dominated by Hydrolithon, Lithophyllum, and Porolithon. Their growth form as thick, laterally restricted crusts and knobs results from intensive grazing by parrotfish and urchins. Grazing is diminished on the Madracis-algal ridges below the upper reefs, and corallines grow here as laterally extensive plates, binding coral rubble. Dominant corallines on the ridges are Lithophyllum, Paragoniolithon, and Tenarea. Algal nodules (rhodoliths) o the deeper bank slopes are the most widespread algal growth form on the Flower Garden Banks. On the basis of shape, internal structure, and composition, rhodoliths are divided into an upper zone (where they have a spherical shape with concentric laminae) and a lower zone (discoidal shape with irregular laminae). Diminishing current velocity with increasing depth controls nodule morphology. The red alga, Peyssonnelia, and the foraminifer, Gypsina, are two non-coralline components that contribute substantially to nodule growth. Reduced current velocity in the lower nodule zone enables corallines and Peyssonnelia to develop laterally extensive pavements and 1-2 m high algal reefs. Taxonomic composition, high framework porosity, and irregular internal growth morphology f the deep algal reefs distinguish these structures from shallow algal ridges described elsewhere. The 90 m isobath at the Flower Garden Banks approximates the upward limit of a turbid nepheloid layer composed of continually resuspended sediment. Reefal structures within this water layer are dead and are now covered by fine sediment.

Coastal Mixing Zone Dolomite, Forward Modeling, and Massive Dolomitization of Platform-Margin Carbonates

Abstract: Dolomite of mixed-water origin occurs in three late Pleistocene raised reef terraces in southeastern Barbados, West Indies. Dolomite concentrations range from trace quantities to locally complete, with a general increase in dolomite percentage with increasing terrace age. Most of the dolomite is replacive, both mimetic and fabric selective, with limpid pore-lining dolomite cements common in the oldest terrace. Stratigraphic relationships suggest that mixing zone dolomitization occurred locally during three temporally separate sea level events. The rapidity of Quaternary glacio-eustasy constrains dolomitization to have occurred within about 5,000 yr in each terrace. Rapid, recurring dolomitization resulted from development of similar hydrologic and hydrochemical conditions during each ea level event. Barbados data are used as conceptual input for a computer-based forward model which simulates the development of thick, platform-margin sections of mixing zone dolomite through the interaction of sea level fluctuations, sedimentation, rapid recurring dolomitization, and subsidence. Sensitivity tests of the model indicate that the most important parameters for producing thick sections of platform-margin dolomite are: (1) dolomitization rate, (2) sea level still stand duration, and (3) rate of sea level change. Dolomite thus formed may provide nuclei for later burial dolomitization or may provide an intrabasinal magnesium source for stepwise stabilization of platform interior dolomites.

Tidal Deposits Associated with Low-Sulfur Coals, Brazil Fm. (Lower Pennsylvanian), Indiana

Abstract: The interval between the Lower and Upper Block Coal Members of the Brazil Formation (Lower Pennsylvanian, Indiana) records a transgression of brackish water tidal deposits over a coastal domed(?)-peat field, followed by a regression, and then re-establishment of the peat-forming environment. Within the study area, the interval averages 8 m in thickness and consists of a basal sequence of laminated mudstone that coarsens upwards to thinly interlayered sandstone and mudstone (lenticular, wavy, and flaser bedding). This is capped by an upward-fining sequence into a rooted mudstone. Sedimentation rates can be estimated by derailed thickness measurements of vertically stacked mudstone laminae and sandstone layers that thicken and thin rhythmically. This rhythmic thickness variation can be elated to neap-spring tidal-current cyclicity and indicates that sedimentation varied from 1 cm/yr to 1 m/yr. Locally, as much as 8 m of accommodation space for the sediments appears to have been generated and mostly filled in a matter of decades to a few centuries. Such rapid rates of filling indicate the possibility that the accommodation space was generated by compaction of the transgressed peat and mud-rich sediments.

Organic-Rich, Radioactive Marine Shale: A Case Study of A Shallow-Water Condensed Section, Cretaceous Shaftesbury Formation, Alberta, Canada

Abstract: Organic-rich radioactive shales are a common regional feature resting on Cretaceous transgressive surfaces in western Canada. The basal shale in the Shaftesbury Formation (Late Albian) from the Peace River area of northern Alberta is characterized by high gamma-ray, high resistivity, and low neutron wireline log signatures. Three facies, in ascending order, are present within the basal Shaftesbury Formation: 1) a brackish-water estuarine shale; 2) a restricted, marginal-marine shale which is radioactive; and 3) an open-marine, normal salinity shale. The radioactive shale contains an abundance of large, lenticular algal cysts (cf. Lancettopsis lanceolata Madler 1963) which are rare in overlying and underlying shale. The algal cysts and high organic content may be the locus of th radioactivity. The total organic carbon content ( 6%) and sulphur content ( 3.4%) of the radioactive shale also are higher than the shale above and below, with a different mineralogy as well. The radioactive portion of the basal Shaftesbury shale has the characteristics of a condensed section; it is directly above a ravinement surface and transgressive-lag deposit which, in turn, locally overlie estuarine sediments deposited within an incised valley. Other characteristics include evidence of low oxygen values, low concentrations of benthonic foraminifera, and evidence of a slow sedimentation rate. Palynological, micropaleontological, and geochemical results indicate that the radioactive s ale was deposited in restricted, marginal marine conditions and that overlying shale shows a progressive deepening to nearshore, open-marine conditions. This radioactive shale does not represent the deepest water sediments of the transgression but is a condensed section deposited in relatively shallow water.

Geostrophic Circulation or Shallow Marine Turbidity Currents? The Dilemma of Paleoflow Patterns in Storm-Influenced Prograding Shoreline Systems

Abstract: In ancient storm-influenced prograding shoreline sequences, sole marks (mainly tool marks) from hummocky crossstratified storm deposits are commonly oriented normal to paleoshoreline and the trend of paleobathymetric contours in the basin. Asymmetrical tool marks typically indicate flows directed offshore. Several workers have attributed their formation to storm-generated, shallow marine turbidity currents. This interpretation conflicts with observations from modern shelves, where storm-driven circulation generally is geostrophically balanced, and time-averaged bottom currents approximately parallel bathymetric contours and the local shoreline. The resolution of these apparently conflicting observations may lie in the realization that tool marks (and many other small paleoflow indicators) form almost instantly as the result of instantaneous flow conditions very near the bed. Beneath storm-generated flows in the shallow ocean, instantaneous and time-averaged characteristics of the bottom boundary layer generally exhibit little similarity. Storm-generated tool marks are formed by the movement of large tools within the thin (less than 1 m) inner boundary layer resulting from the superimposition of waves and currents. The orientation of the peak instantaneous shear stress moving large tools under such combined flows mainly reflects wave-orbital motions, which typically are normal to shore. The magnitude of stress is great y increased in the offshore direction (and decreased in the onshore direction) by superimposition of a steady current with an offshore component of flow, but the direction of stress is only slightly affected. In ancient storm-influenced sequences, therefore, shore-normal tool marks generally were not formed by turbidity currents; rather, their orientation is best attributed to shoaling waves approaching the coast at a very high angle. Asymmetrical tool marks are directed offshore due to enhanced shear stress on the offshore stroke of waves superimposed on a geostrophic current with an offshore flow component. Tool marks do not reflect the time-averaged bottom-flow direction; in fact, they provide almost no information concerning steady bottom currents. In contrast, high-angle cross-beds (formed in coarser sediment by the migration of dunes and sandwaves), although relatively rare in offshore storm deposits, generally reveal approximately shore-parallel flows in ancient systems. Cross-beds closely reflect the time-averaged flow direction in the outer boundary layer of a geostrophic current, for three reasons: 1) the net transport direction for sand moving as bed load beneath a combined flow lies between the directions of peak instantaneous shear stress and time-averaged shear stress; 2) large ripples disrupt the thin inner boundary layer; and 3) long time intervals (relative to wave-induced velocity oscillations) are required to form large ripples.

Tidal Channel, Levee, and Crevasse-Splay Deposits from a Cambrian Tidal Channel System: A New Mechanism to Produce Shallowing-Upward Sequences

Abstract: Two cliff exposures, 1 km and 0.7 km long and 10 m high, of the Middle-Upper Cambrian Waterfowl Formation were mapped in detail on the Costigan Thrust in southwestern Alberta. The formation here comprises 5 rock types: intraformational conglomerate; cross-stratified grainstone; wavy-bedded grainstone-mudstone; massive dolomite mudstone; and prism-cracked, cryptmicrobial-laminated mudstone. These rock types form a grainstone body in the upper 7 m of the outcrops and 4 fining-upwards sequences in the lower 3 m. The grainstone body is divided into 5 storeys, each of which has a basal erosion surface overlain by an intraformational conglomerate with interbedded thrombolites. Storeys contain lateral accretion bedding that is composed of decimeter-thick bedsets which are inclined up to 12 d grees relative to the basal erosion surfaces. The fining-upwards sequences comprise basal cross-stratified grainstone overlain by wavy-bedded grainstone-mudstone that is capped by prism-cracked, cryptmicrobial-laminated mudstones. Three fining-upwards sequences are laterally continuous. However, one fining-upward sequence pinches out along the outcrop face. We interpret the grainstone body as the deposits of laterally migrating, sinuous tidal channels. It is not clear whether the stacking of channel deposits into storeys represents superimposed channel-bar and channel-fill deposits from a single channel belt or superimposed channel-belts. The fining-upwards sequences may be shallowing-upwards sequences in the sense of James (1984) and represent aggrading-prograding tidal flat deposits. A ternatively, these sequences may be crevasse-splay and levee deposits analogous to fluvial overbank flood sequences. In this case, these sequences may not be related to eustatic sea level changes, although they resemble classic shallowing-upwards sequences and "punctuated aggradational cycles."

Origin, Structure, and Evolution of a Reattachment Bar, Colorado River, Grand Canyon, Arizona

Abstract: In a channel expansion, flow can separate from the bank, creating a zone of relatively weak recirculating current. Bars that accumulate in this weak flow near the point where flow reattaches to the bank are called reattachment bars. As a reattachment bar evolves, the recirculation zone may fill with sediment and restrict flow from the main channel. The increasingly restricted flow over the bar causes ripples to replace dunes and causes the sediment size to fine; the resulting vertical sequence resembles that of point bars. Seasonal and daily flow fluctuations in the Grand Canyon complicate this idealized sequence. Changes in discharge alter the geometry of recirculation zones, flow within the recirculation zones, the location of depositional and erosional sites, the kind of bedform an migration direction of bedforms on the bar, and the transported sediment size. Dunes and ripples within a recirculation zone migrate in a rotary pattern in response to the recirculating flow. Ripples near the reattachment point often resemble oscillation ripples in morphology and dynamics. The reversing flow that creates these ripples is caused by fluctuations in location of the reattachment point. These fluctuations cause flow near the reattachment point to reverse in an upstream-downstream direction, thereby producing symmetrical, reversing ripples with crests that trend normal to the bank. Low rates of ripple migration in the reversing flow, accompanied by rapid deposition, cause these ripples to climb at a high angle. At increasing distances from the reattachment point, the reversing flow is less balanced, and the ripples climb at lower angles as they migrat upstream and downstream. Although these observations were made in a bedrock canyon, the same processes operate in alluvial and tidal channels and are important in adjusting the shape of channels on point bars and concave benches and behind bedforms that become emergent at low stage. Reattachment bars can be recognized by the rotary flow patterns and by symmetrical, reversing, vertically climbing ripples.

Rates of sediment accumulation in a tidal freshwater marsh

Abstract: Rates of sediment accumulation were determined for a Delaware River tidal freshwater marsh utilizing radiometric, palynological and sediment flux estimating techniques plus historical reviews. All techniques showed good agreement, indicating that tidal freshwater marshes are capable of preserving evidence of processes and events that have shaped the estuary through time. Prior to 1940, the marsh was a slowly accreting swamp accumulating material at the rate of 0.04 cm yr-1 prior to colonization of the region in the late 1600s and a somewhat more accelerated rate of 0.12 cm yr-1 prior to the introduction of regular tides in 1940. Between 1940 and 1988 average rates of accumulation ranged between 1.04 and 1.38 cm yr-1, being highest near the tidal channe . During the period 1954-65, rates averaged 1.67 cm yr-1 due to increased storm activity. Since 1966, storm activity has decreased and sediment accumulation rates have averaged 0.97 cm yr-1, reflecting these changes. The current average rate of accumulation is four times the rate of sea-level rise for this region of the estuary. It is hypothesized that sediment accumulation will continue to exceed sea level rise until the marsh surface approximates mean high water.