Showing papers in "Journal of Sedimentary Research in 1984"

The Effect of Grain Size on Detrital Modes: A Test of the Gazzi-Dickinson Point-Counting Method

Abstract: Differing methods of determining detrital modes of sand/sandstone have been developed by different "schools" due to different goals and different geologic settings. The Gazzi-Dickinson method of point counting was developed to maximize source-rock data, while minimizing the time, effort, and expense of gathering such data. Use of the method minimizes variation of composition with grain size, thus eliminating the need for sieving and multiple counts of different size fractions. Unsorted samples of any sand size may be used, thus allowing direct comparison between modern sands and poorly sorted ancient sandstones. The application of actualistic petrologic models relating composition to tectonic setting thus is facilitated. The unique aspect of the Gazzi-Dickinson method of point counting is the assignment of sand-sized crystals and grains within larger fragments to the category of the crystal or grain, rather than to the category of the larger fragment. In addition, every attempt is made to reconstruct original detrital compositions in spite of subsequent alterations. Six unconsolidated Holocene sand samples derived from a variety of source rocks in north-central New Mexico were collected, sieved, impregnated, sectioned, stained and point-counted, using both traditional and Gazzi-Dickinson methods. Results of these counts provide a comparative test of traditional and Gazzi-Dickinson methods. There are two reasons for variation of modal composition with grain size: 1) the breakage of fragments into constituent grains, and 2) actual mineralogic variation with grain size. The Gazzi-Dickinson method successfully eliminates the first source of compositional grain-size dependency. No point-counting method eliminates the second source. Use of the Gazzi-Dickinson method on unsorted samples produces results that are consistent with those from different size fractions of the same samples. Lithic-fragment compositions (for example, LmLvLs, QpLvmLsm) are especially consistent and provide the most useful parameters for relating composition to source rock and, ultimately, to tectonic setting.

Travertines: Depositional Morphology and the Bacterially Constructed Constituents

Abstract: Investigation of travertine accumulations throughout central Italy and the west-central U.S. has shown that the carbonate is precipitated in response to both inorganic and organic processes. Individual deposits range up to 85 m thick and hundreds of square kilometers in areal extent; all of the carbonate is low-magnesian calcite. Water chemistry, temperature, and morphology of the accumulation greatly influence the constituents comprising these deposits. Harsh environmental situations favor inorganic deposits, while increasingly more moderate conditions result in the formation of a greater abundance of organically precipitated material. Morphological variations of travertine deposits recognized include 1) waterfall or cascade, 2) lake-fill, 3) sloping mound, fan, or cone, 4) terraced ound, and 5) fissure ridge. Gross morphology, internal stratification, and constituents comprising these deposits vary systematically depending upon the type of accumulation and chemistry of the waters. Bacterially precipitated calcite forms a large percentage of the carbonate in many travertine accumulations, exceeding 90% of the framework grains comprising some of the lake-fill deposits. Bacteria are among the first taxa to inhabit and reproduce in harsh spring environments and produce a variety of fascinating constituents. The bacteria are primarily rods, generally 0.2 µm in diameter and less than 1.0 µm in length. The rods readily decay resulting in calcite crystals loaded with micropores. The basic building block of bacterially constructed travertine is a clump of bacteria averaging 20 µm in diameter enclosed in a single crystal of calcite. Aggregates of these crystals produce a variety of deposits including 1) crudely laminated carbonate mud, 2) finely laminat d layers of mud, 3) intraclasts, 4) foam rock, and 5) shrubs. The shrubs are most striking, commonly forming layers 1-3 cm thick but also producing bacterial pisoids. At some locales, bacterial stromatolites composed of layers of shrubs alternating with finely laminated layers of bacterial mud comprise essentially the entire deposit. The shrub layers are the result of flourishing summer growth of bacteria and, furthermore, show remarkable daily laminae 0.1-0.5 mm thick. The importance of bacteria in the formation of travertine and their universally recognized abundance in modern sediments provides an impetus for a reexamination of the role of bacteria in the origin of other types of ancient deposits.

Epigenetic Dolomitization and the Origin of Xenotopic Dolomite Texture

Abstract: Xenotopic texture, which is commonly observed in pre-Cenozoic rocks, is defined here as a mosaic of anhedral crystals with irregular or curved intercrystalline boundaries and, usually, undulatory extinction. Xenotopic dolomite texture is similar in appearance to neomorphic limestone textures. Idiotopic dolomite texture (euhedral to subhedral crystals with straight, intercrystalline boundaries) contrasts with xenotopic texture and is commonly observed in both Cenozoic and more ancient dolomites. Texture may be controlled by the temperature at which crystals grow. Crystal growth theory predicts that at low temperature a smooth crystal surface is energetically favored, and atoms are added to crystal faces layer by layer with dislocations acting as nucleation sites. This results in faceted crystals and euhedral to subhedral crystal mosaics. Above a "critical roughening temperature" (CRT), a rough surface is energetically favored, surface nucleation does not require dislocations, and atoms are randomly added to the crystal surface, resulting in nonfaceted growth and an anhedral crystal mosaic. It is hypothesized that a "critical roughening temperature" exists for dolomite above 50°C. Xenotopic dolomites are produced by dolomitization of limestone and/or neomorphic recrystallization of dolomite at elevated temperature (above the CRT) after burial. Idiotopic dolomites are produced below the CRT by near-surface processes and are occasionally produced above CRT if growth-inhibiting effects of the solution or impurities stabilize crystal faces. Calcite has a CRT below 25°C and, therefore, develops anhedral crystal mosaics (characteristic of neomorphic texture) both at near-surface and at elevated temperatures. Synthetic xenotopic dolomite was produced in the laboratory at 250°C and 300°C by dolomitization of aragonite and calcite skeletal fragments and by recrystallization (neomorphism) of nonstoichiometric Cenozoic dolomites. Xenotopic dolomite resulted from the metamorphic recrystallization of the idiotopic Hueco dolomite (Permian, Texas) near the Marble Canyon intrusion, at temperatures between 350°C and 600°C. Hydrothermal dolomitization of periclase-calcite marble near the intrusion also resulted in a xenotopic texture. Xenotopic dolomite in the Galena Group (Ordovician, Wisconsin) was produced by neomorphism of a preexisting dolomite during the emplacement of lead-zinc sulfides at temperatures between 80°C and 227°C. In the Trenton Limestone (Ordovician, Michig n), xenotopic dolomite replaced limestone during the migration of hot (> 50°C) fluids along fracture systems. Xenotopic dolomite was not observed in Cenozoic dolomites that were subjected to temperatures near surface values only.

Subaerial Debris-Flow Deposition in the Upper Paleozoic Cutler Formation, Western Colorado

Abstract: Outcrops of the lower part of the Cutler Formation near Gateway, Colorado, contain proximal alluvial-fan deposits of two principal lithofacies. The diamictite lithofacies (D) consists of a variety of sharply bounded, massive, muddy beds which are interpreted as deposits of subaerial debris flows. The gravelly sandstone lithofacies (S) consists of amalgamated beds of laminated and cross-bedded conglomeratic sandstones which are interpreted as water-deposited sediments. Clasts in S are rarely larger than 0.5 m in maximum diameter, but clasts in D are commonly much larger (up to 8 m). Both normal and inverse grading are visible in D, but grading is generally not well developed in either lithofacies. Imbrication of clasts, although more pronounced and more consistent in S, is fairly commo in both lithofacies. Four subfacies of diamictite are distinguishable on the basis of clast packing and grading: massive matrix-supported (Dram), graded matrix-supported (Dmg), inversely graded clast-supported (Dci), and massive clast-supported (Dcm). Given the hydrodynamic and rheologic properties of debris flows as reported by previous workers, four debris-flow regimes are postulated on the basis of varying water content and clast interaction. Subfacies Dram and Drag typify deposits of flows having viscous clast interactions; subfacies Dci represents deposits of flows having inertial or colliding clast interactions. Subfacies Dram comprises deposits of flows with demonstrably high yield strengths. Subfacies Dcm represents deposits of flows in which yield strength was insufficient to support most clasts. This classification specifically recognizes turbulence, selective deposition, and bedload transport of clasts as essential characteristics of certain dilute debris flows. It also accommodates as debris-flow deposits some beds which have features usually associated with aqueous deposition, including clast frameworks, channelization, imbrication, and normal grading.

Reactive Surface Area of Skeletal Carbonates During Dissolution: Effect of Grain Size

Abstract: Major advances have recently been made in quantifying the chemical parameters which control carbonate mineral reaction kinetics in natural waters. One of the primary factors influencing rates of reaction is the amount of surface area per unit mass available for reaction. For relatively smooth, nonporous carbonates, the relationship between reactive and total surface area measured by gas adsorption has proven simple. However, the dominant carbonates in the marine environment are biogenic and exhibit complex microstructures. We have performed experiments to determine dinghy what proportion of the total surface area of these biogenic carbonates is actually available for reaction during dissolution. Three biogenic carbonates having widely differing microstructures (Halimeda, coral, and echinoid) were chosen for study and comparison with rhombic calcite. The influence of grain size on the relation between total surface area, determined from gas adsorption, and reactive surface area, determined relative to rhombic calcite during dissolution in constant composition solutions, was of primary concern. Unlike rhombic calcites, total surface area for texturally complex biogenic grains varied slightly or not at all with increasing grain size. However, both rhombic calcite and biogenic carbonate dissolution rates were inversely related to grain radius. This implies that much of the total surface area of biogenic carbonates is unavailable for reaction with the aqueous solution. Dissolution rates were a function of both grain size and microstructure. We have defined an empirical roughness factor which quantifies the difference in reactive surface area between rhombic calcite and a given biogenic grain of equivalent size. The roughness factor increased with grain microstructural complexity but was independent of grain size over the grain-size range (51-513 microns) studied. The maximum roughness factor value observed was 7 for Halimeda relative to a value of 1 for rhombic calcite. Our findings indicate that both microstructure and grain size can play important roles in controlling reactivity of biogenic carbonates during diagenesis. The relative significance of grain size versus microstructure for carbonate dissolution rates in natural systems is discus ed.

Petrographic image analysis; I, Analysis of reservoir pore complexes

Abstract: There exists a need to relate the petrology of reservoirs (pore geometry, surface areas of mineral phases and pores) to geophysical and petrophysical data. The end result is improved assessment of reservoir quality as well as better interpretation of well logs and seismic data. Petrographic Image Analysis (PIA) was developed from the beginning to interface with petrophysical/geophysical data. PIA relies upon computer-based image analysis using pattern recognition/classification programs, and so information can be obtained very rapidly--the rate simply tied to sophistication of the computer in use. PIA consists of a critical mix of hardware and software which perform four separate functions: 1) image acquisition; 2) image digitization; 3) image segmentation; and 4) image analysis. A sp cial effort has been made to characterize the geometry of the pore complex. Separate spectra related to pore size and pore roughness are generated from each image. In addition, surface area per unit volume of pore can be estimated. Pore spectra can be decomposed and classified using pattern recognition/ classification algorithms or used directly to estimate physical parameters.

Large-scale Facies Sequences In Alluvial Overbank Environments

Abstract: Gradational coarsening-upwards or fining-upwards sequences (order of meters thick) in overbank environments can be produced by progressive crevasse-splay/levee progradation or abandonment while the channel belt remains unchanged in position. In contrast, meter-scale facies sequences related to periodic channel-bell avulsion may show relatively abrupt changes in facies and paleocurrent direction. Simulation models suggest that the number of such avulsion-related facies changes in any overbank sequence bounded above and below by channel-belt deposits is on the order of 1 to 10; this is confirmed using an ancient example. In this example the average thickness of each avulsion-bounded overbank facies sequence was 3 m: assuming an average recurrence interval for avulsions of about 1,000 ye rs, the mean overbank deposition rate was about 3 mm per year.

Rippled Scour Depressions on the Inner Continental Shelf Off Central California

Abstract: Side-scan sonar records taken during the recent Coastal Ocean Dynamics Experiment (CODE) show elongate, shore-normal rippled depressions of low relief on the inner continental shelf off central California between Bodega Bay and Point Arena. These features extend up to 2 km from the coast into water depths of up to 65 m. The proposed mechanism for their generation is storm-generated bottom currents associated with coastal downwelling during the late fall and winter which scour the surficial fine-sand sediment and expose the coarser-sand substrate in the depressions. The zones of most intense erosion and the irregular spacing of the features may be controlled by submerged rock ledges and other prominent coastal features. The large straight-crested ripples within the depressions (heights to 40 cm; wavelengths to 1.7 m) are probably formed by large-amplitude, long-period surface waves generated by winter storms.

Interactions between the Ebb-Tidal Delta and Landward Shoreline: Price Inlet, South Carolina

Abstract: Patterns of erosion and deposition along the shoreline of Price Inlet, South Carolina, are strongly influenced by ebb-tidal delta processes. The ebb delta goes through cycles of growth and decay (15-20% change in volume) which last from 4 to 7 years. In the growth phase, large intertidal bar complexes are formed on both sides of the main ebb channel by the coalescence of landward-migrating swash bars. During this stage, waves refracting around the southern bar complex produce a reversal in the longshore-transport direction on the downdrift shoreline. This results in little sediment escaping the inlet and delta system. The sand-trapping mechanism ceases and the delta reduces in volume when the bar complexes migrate onshore and attach to the beach. The inlet shoreline also goes through cycles of 7- to 42-years duration in which its offset configuration changes. The shift in shoreline alignment is controlled by the orientation of the main ebb channel and the resulting ebb-tidal delta morphology. When the inlet has a downdrift, offset configuration, the ebb-tidal delta is skewed along the updrift shoreline. In this position bar complexes preferentially accrete along the updrift beach causing a progradation of the shoreline. At the same time, the downdrift shoreline erodes due to little sediment bypassing the inlet. This pattern of sedimentation eventually produces an updrift, offset inlet. The erosional-depositional processes are reversed when the ebb flow in the channel thalweg is deflected off the prograding updrift, inlet shore ine. This causes a clockwise rotation of the main ebb channel and a shift in the ebb delta to a position along the downdrift shoreline. In this configuration the updrift shoreline erodes and the downdrift shoreline accretes. This study documents the close relationship that exists at Price Inlet, South Carolina, between the ebb-tidal delta and landward shoreline. Evidence is presented to show that this relationship may hold for other mixed-energy tidal inlets as well.

Epiphyton and Renalcis; diagenetic microfossils from calcification of coccoid blue-green algae

Abstract: Epiphyton and Renalcis, common components of lower and middle Paleozoic reefs, are end-members of a spectrum of diverse, micritic microfossils that often occur alongside or intergrown with each other. Five salient morphotypes are distinguished: dendritic (typified by Epiphyton), septate, clotted, chambered, and saccate (the last three usually considered Renalcis). Especially noteworthy among common intergrowths, intermediate forms, and cooccurrences are Epiphyton branches attached to Renalcis chambers. Such observations argue that these microfossils may not be deliberately precipitated skeletons of genetically distinct organisms. It is proposed that they represent precipitation of calcite, probably high-Mg, within colonies of coccoid blue-gree algae in the environment of growth, soon after death of the algae. Entire "thalli" accreted apically by repeated growth and calcification of these colonies. The various forms, genera, and species resulted from size variation of colonies, frequency of calcification, and whether whole colonies or only their outer sheaths were replaced. Epiphyton and Renalcis are therefore "diagenetic taxa."

Exceptions to the Relationship Between Plate Tectonics and Sandstone Composition

Abstract: Routine use of QFL, QpLsmLvm, and LmLsLv plate-tectonic provenance diagrams has led to the recognition of error populations. For these sandstones, the interpretation of tectonic setting based on data exclusive of petrography does not coincide with the interpretation of tectonic setting based on their location on compositional diagrams. The anomalous sandstones can be separated into at least four categories: i) sandstones deposited during the transition between tectonic regimes may be derived, in part, from relict source rocks; ii) the relative enrichment of sandstones in detrital quartz by weathering and/or depositional reworking may lead to an inaccurate interpretation of tectonic setting from compositional data; iii) sandstones deposited in tectonic settings as yet unrepresented on he provenance diagrams may plot between the provenance fields or overlap existing fields; iv) the role of detrital carbonate rock fragments directly affects the location of data points on provenance diagrams, and is an unresolved problem. The first two categories represent permanent problems in the use of sandstone composition as an indicator of plate-tectonic setting, whereas the last two may only be temporary problems that can be solved by more data and a consensus on determining detrital modes. Recognition of anomalous sandstones in the rock record will reduce the probability of erroneous interpretations of tectonic setting and enhance the use of sandstone petrology as an indicator of plate-tectonic setting.

Sedimentology and Ichnology of the Cretaceous-Tertiary Boundary in Denmark: Implications for the Causes of the Terminal Cretaceous Extinction

Abstract: Trace fossils and other sedimentologic evidence of bioturbation in latest Cretaceous and earliest Tertiary marine deposits in Denmark provide important clues to understanding the ecology of the sea bottom immediately before, during, and after the terminal Cretaceous boundary event. Trace-fossil associations at all the onshore Danish boundary sites indicate a general upward shoaling of the late Maastrichtian chalk sea, which was accompanied by an abrupt shift from a carbonate ooze sea floor to a marly, clay-rich, unstable substrate at the boundary. Although the substrate condition may have become more stable in the early Danian, water depths never did return to their former deep level. There is evidence of localized anoxia during the deposition of some boundary sediments (in the Fish Clay) in shallow parts of the Danish Maastrichtian sea, but no clear evidence of anaerobic conditions occurring at the era boundary was observed in boundary strata deposited in deeper water near the center of the basin. In fact, earliest Danian marl there is totally bioturbated. A pulse of calcite dissolution in shallow water coincided precisely with the era boundary, and this event played a major role in the formation of the Fish Clay in eastern Denmark, which is a condensed series of smectitic clay-rich layers from which much calcite has dissolved. The effects of this dissolution episode, however, were not so extreme in the marl that was deposited at the same time in slightly deeper water in northwestern Denmark. Evidence from Cretaceous-Tertiary boundary sequences in Denmark and elsewhere suggests that no single catastrophe can account for the major biotic extinctions that occurred at the end of the Cretaceous Period. The primary causal factors of the terminal Cretaceous extinction event appear to be drastic global sea-level regression occurring simultaneously with extensive volcanism on land and a strong pulse of calcite dissolution in ocean surface waters.

Influence of Grain Size and Temperature on Intergranular Pressure Solution, Quartz Cementation, and Porosity in a Quartzose Sandstone

Abstract: Cathodoluminescence petrography and scanning electron microscopy (SEM) reveal that grain size and temperature significantly influenced intergranular pressure solution, quartz cementation, and porosity evolution in the quartzose Hartshorne Sandstone (Middle Pennsylvanian) of the Arkoma basin. Among samples from each locality, a negative, linear relationship exists between mean grain size and volume of quartz dissolved by intergranular pressure solution. In contrast, a generally positive relationship exists between mean grain size and volume of quartz cement, although considerable variability exists in this relationship. A positive relationship also exists between mean grain size and porosity. Intergranular pressure solution directly reduced minus-cement porosity by causing tighter pack ng of detrital grains; it thereby controlled volumes of both cement and primary porosity. For this reason, little primary porosity is preserved in pressolved, finer-grained sandstones even though they contain small absolute volumes of quartz cement. Appreciably more primary porosity may be retained by unpressolved, coarser-grained sandstones even though they contain larger absolute volumes of quartz cement. These relationships also vary with thermal maturity, which increases eastward across the basin. This thermal maturity trend is the result of elevated temperatures associated with Mesozoic rifting and intrusive events in the Mississippi embayment. Among samples of equal grain size, more intergranular pressure solution, less quartz cement, and less porosity are evident in areas of higher thermal maturity. As is the case with grain size, the influence that temperature exerted on intergranular pressure solution is, in turn, reflected in volumes of quartz cement and porosity. Thus, primary porosity tends to be preserved preferentially in areas of lower thermal maturity even though the absolute volume of quartz cement is generally higher. In areas of higher thermal maturity, more quartz has been dissolved by intergranular pressure solution than is present as cement whereas the opposite is true in areas of lower thermal maturity. This suggests that intergranular pressure solution was an important agent of mass transfer during diagenesis of these quartzose sandstones.

The Effects of Callianassa Bioturbation on the Preservation of Carbonate Grains in Davies Reef Lagoon, Great Barrier Reef, Australia

Abstract: Coring, air-life excavation, and tracer-sediment experiments in the lagoon-floor sediments of Davies Reef revealed that callianassid shrimp had produced, during their feeding, a 5-60-cm-thick surface layer of moderately to poorly sorted gravel-free sediment, above very poorly sorted gravelrich sediment. The subsurface gravel is epilithic in origin close to reef framework but consists predominantly of infaunal molluscs through most of the lagoon. These infaunal skeletons tend to be buried rapidly in a well-preserved state escaping the microboring that attacks all grains on the surface. Two geologically significant points emerge; first, the surface layer will not get preserved in its present form and any analyses of surface samples can give a false impression of the accumulating sedimen pile and the prevailing hydraulic regime, and second, the combination of continuous fine-sediment recycling and surface microboring can lead to a bias in the fossil-reefal limestone record, with epilithic organisms seemingly subordinate to infaunal molluscs.

Reactivation Surfaces in Subtidal Channel Deposits, Oosterschelde, Southwest Netherlands

Abstract: Recognition of unsteadiness in flow velocities is an important factor in the identification of tidal deposits. One consequence of such unsteadiness is the production of reactivation surfaces -erosion surfaces developed within a cross-stratified set. Reactivation surfaces can be produced in a unidirectional flow system, by the migration of a faster-moving megaripple over a slower one ("overtaking"). The form of the surface is dependent upon the relative sizes of the two megaripples concerned and the flow conditions under which they migrate. Other types of reactivation surfaces are produced by the action of a subordinate current, which erodes the lee side of the dominant-current megaripple. The morphology of the reactivation structures depends on the size of the megaripples concerned (the examples considered here had heights in the range 0.2-2.5 m) and, most importantly, on the relative strengths of the dominant and subordinate currents. Where the subordinate current is relatively weak, only the tops of the dominant-current foresets are eroded. With increasing subordinate-current strength, the whole foreset height is subjected to erosion, and a large angular discordance is produced between the large-scale foresets and the reactivation surface. Such strong erosion drastically alters the megaripple relief--the lee-side slop is much reduced. Recovery of the lee side during the subsequent dominant-current phase is gradual, as can be seen from the development of small-scale structures immediately overlying the reactivation surfaces. The regular variation in the strength of the subordinate current during the neap/spring cycle is often reflected in the morphology of the reactivation surfaces. In such examples, the reactivation surfaces formed during spring-tide periods have a large angular discordance with the foresets, indicating that there was considerable erosion by the subordinate current. Reactivation surfaces formed during periods between spring and neap, in contrast, are developed only in the upper parts of the foresets, suggesting that subordinate-current erosion was relatively modest, while the deposit of the neap tides themselves show no evidence of any erosion by the subordinate current.

Black-pebble occurrence and genesis in holocene carbonate sediments (Florida Keys, Bahamas, and Tunisia)

Abstract: Black carbonate lithoclasts, along with blackened peloids, ooids, skeletal fragments, and entire limestone beds are found in the shallow subtidal, intertidal, and supratidal zones of modern and ancient carbonate environments. They are often associated with pedogenic features such as root-traces or caliche. The blackening is due to impregnation of the sediment by dissolved, colloidal, or finely particulate organic substances in an anoxic and alkaline environment or microenvironment. The organic matter is derived from decayed algae and/or decayed or burnt higher terrestrial plants. Iron sulfides contribute to the blackening, especially in samples containing algal matter. A complex interplay of adsorption of organic matter on carbonate-crystal surfaces, neomorphism, and microcrystalline cementation in the vadose or freshwater phreatic zones is thought to fix the organic matter and make the black coloration relatively resistant to oxidation. Black pebbles form through reworking of the preferentially cemented and blackened sediment by coastal erosion. They are mostly relics because the less-consolidated host sediment is washed away. Black pebbles may thus be valuable indicators of ancient coastal and terrestrial environments.

Eolian origin of upper Paleozoic sandstones, southeastern Utah

Abstract: Throughout its outcrop in southeastern Utah, the Permian Cedar Mesa Sandstone contains large-scale crossbedding and is devoid of marine fossils except for sand-size skeletal fragments. Although this rock unit has been interpreted by several recent workers as shallow-marine, stratification identical to that documented in modern eolian dunes is ubiquitous in the Cedar Mesa. Thin, inversegraded laminations within crossbeds represent the unique product of climbing wind ripples. This type of stratification is the best evidence for an eolian origin of the Cedar Mesa; such an interpretation is strongly supported by other evidence. Vertebrate trackways and simple, small, cylindrical burrows constitute a distinctively nonmarine ichnofauna. Rhizoliths ("root casts") are abundant at many stratig aphic levels. In the underlying Rico and Hermosa formations, sandstones with physical and biogenic sedimentary structures identical to those in the Cedar Mesa are interbedded with crossbedded limestones with abundant megafossils and marine traces. Contacts between sandstones and limestones are sharp; gradational relationships would be expected if both sandstones and limestones accumulated subaqueously. Siliciclastic pebbles are present in some of these carbonate rocks, suggesting that the well-sorted nature of the sandstones cannot be attributed to the unavailability of coarse clasts. Palcocurrent data from the sandstones and limestones show significantly different vector means and variance. Southward pinchout of marine carbonates indicates that winds were onshore. Winnowing of silicicla tics from coarser carbonate detritus created a strong dichotomy in bulk mineralogy between marine and eolian sediments. Reinterpretation of the sandstones necessitates reappraisal of ideas concerning the mineralogical maturity of these sediments. The relative importance of eolian and shallow-marine transport processes to the destruction of labile siliciclastics is unknown.

Role of Hydraulic Sorting in the Origin of Fluvial Placers

Abstract: A necessary condition for placer development is selective sorting at the grain scale by size and density. The effects of differential entrainment, suspension, and transport on an initial distribution composed of medium-size quartz and 10% fine-size magnetite were modeled by solving the Einstein bedload function for specific grain friction velocities (U (super *) ') in the range 3-63 cm.sec (super -1) and bed roughnesses of 0.55, 2, 5, and 10 mm. For any value of U (super *) ' and for both mineral densities, the transport rate for all sizes in the initial distribution decreases with increasing roughness, the decrease being greatest in the finer sizes. The concentration of magnetite transported in the flow increases with increasing U (super *) ' for each roughness and decreases with increasing roughness for each U (super *) '. The settling velocity ratio of magnetite to quartz in transport decreases with increasing U (super *) ' for any value of roughness. For any value of U (super *) ', the ratio first decreases then increases with increasing roughness, ranging from 1.36 at low U (super *) ' and roughness to 0.76 at high U (super *) ' and intermediate roughness. These results are due to variations in the reactive angles of grains and the extent to which grains hide in the lower, inner zone of the boundary layer. Concentrations of heavy minerals at the bed, bar, and system scales are explained using these results.

Sources of Periplatform Carbonates: Northwest Providence Channel, Bahamas

Abstract: Sources and quantities of periplatform carbonate sediments have been determined in Northwest Providence Channel, Bahamas, by an end-member method employing geochemical, textural, petrographic, and SEM data. The Holocene sediment over most of this open seaway, 200 to 2,000 m deep, between Great Bahama Bank and Little Bahama Bank, is a drape of largely fine grained lime mud 50 cm thick. Shallow platform sources contribute 75-90 percent; the remainder is planktonic foraminifera, pteropods, and coccoliths. Mud (< 62 µm) from each bank and the planktonic source are each geochemically distinct and can be quantified. The two shallow platform sources dominate sedimentation in the deep basin. Trend-surface maps of mud derived from each bank reveal no windward-leeward asymmetry in cont ast to reported sand-transport patterns. This indicates that the fines are swept off both platforms by storm and tidal exchange. Concentric distribution of fines relative to source indicates no offset by currents, suggesting that fecal pelleting must rapidly remove fine sediment from the water column. Canyon valleys and intercanyon highs have equal thicknesses of Holocene sediment, which indicates that pelagic processes of deposition presently dominate over gravity-flow processes. Confirmation of this depositional process is that the deposition rate increased from 2 cm/1,000 years to 10 cm/1,000 years when postglacial sea level flooded the bank tops. Thus 80 percent of the present sedimentation rate results from bank-top contribution. Furthermore, the calculated mass of mud traceable to Little Bahama Bank agrees excellently with the independent estimate of overproduction there. In cores, bank sediments resulting from the most recent sea-level highstand sharply overlie planktonic sediments, writing a sea-level history in alternating stable and unstable mineral suites.

Petrology and Geochemistry of Rhizoliths from Plio-Pleistocene Fluvial and Marginal Lacustrine Deposits, East Lake Turkana, Kenya

Abstract: Recent studies of rhizoliths from the Koobi Fora Formation (Plio-Pleistocene) of East Lake Turkana, Kenya, indicate that their shapes vary with the depositional environment of their host sediment. Vertical rhizoliths are associated with channel-bar and overbank deposits of fluvial origin. These rhizoliths represent the tap roots of phreatophytes that lived in well-drained, upland settings. Horizontal rhizoliths predominate in facies ascribed to beaches, lagoons, and floodplains associated with ancient Lake Turkana. They are produced by water-emergent aquatic macrophytes and coastal grasses that grow in saturated or poorly drained soil environments. The rhizoliths consist of sparry and micritic calcite cements whose petrography and geochemistry reflect differences in diagenesis between the two types of root systems. The vertical roots contain little organic debris and have poorly developed meniscus and microstalactitic cements. These attributes reflect precipitation of the calcite in well-oxygenated, well-drained vadose conditions. In contrast, the cements of the horizontal root systems lack textures indicative of vadose conditions, contain abundant clay and plant debris, and display Mn concentrations as high as 4.5 cation percent. These features confirm that the horizontal rhizoliths developed in chemically reduced, water-saturated conditions. Bacterial decay of the plant debris maintained reduced Eh conditions while also supply ng abundant Mn2+ to the pore solutions. Groundwater that had moved through volcanic strata may have also contributed Mn2+. The Mn may have been incorporated in the calcite cements through a combined process of chemisorption and ionic substitution of Mn2+ as well as by inclusion of manganese oxides.

Offshore Transport and Sand-Body Formation: Evidence from a Steep, High-energy Shoreface, Southeastern Australia

Abstract: Elongate, shore-parallel sand bodies lie on the lower shoreface of the embayed and cliffed coast of central New South Wales, Australia. These sand bodies are 10 to 30 m thick, extend almost continuously for 40 km parallel to the coast, and display a pronounced convexity on the sea floor in profile. Sediment sampling and detailed seismic reflection and side-scan sonar profiling provide evidence that the upper part of these large lobate sand bodies consists chiefly of sand transported downslope from the upper shoreface and surf zone. The sand bodies consist entirely of marine sand probably derived locally from retreating embayed beaches and in part from sandstone cliffs. Channels and sediment lobes oriented normal to the shoreline and internal structure indicate growth by seaward progradation, and surface morphology indicates coalescing of lobes and damming of bed-load sediment against bed-rock ridges in water depths of 70 to 80 m. The precise mechanism, or mechanisms, of seaward transport on the shoreface is unresolved. We surmise that during storm events, disturbance by large waves and seaward transport by downwelling bottom currents result in sand movement onto and across the sand-body surface at depths of 40 to 80 m. Concurrent modification of the sand bodies by shore-parallel processes is suggested by textural trends, bed forms, and the overall alongshore continuity of the sand bodies.

A Carbonate Submarine-Fan Sequence from the Jurassic of Portugal

Abstract: A prograding, carbonate submarine-fan sequence is described from the Toarcian-Aalenian of western Portugal. The sequence is predominantly composed of shallow-water carbonate grains but can be interpreted using the current siliclastic submarine-fan models. Basin-plain to outer-fan, braided-channel midfan, and large channel-fill sequences can be recognized. The source of the carbonate grains was a platform developed on an uplifted Hercynian horst to the west of present mainland Portugal. The fan sequence overlies restricted basin deposits with organic-rich mudstones and shales. This association of restricted basin facies and a thick (over 300 m) carbonate grainstone could be an attractive hydrocarbon play. However, such limestone sequences are easily misinterpreted as in situ shelf carb nates.

Relation of paleovegetation to geometry and cyclicity of some fluvial carbonaceous deposits

Abstract: The fluvial lower Eocene Willwood Formation (Bighorn Basin, Wyoming) is composed largely of red, oxidized, overbank sediment that has been extensively altered by soil formation. Within this matrix are two different types of dominantly fine grained, drab- to dark-colored deposits that contain abundant plant remains. The first are areally restricted, lenticular bodies that truncate underlying mudstone layers. These are interpreted as having formed in abandoned sections of channels. Deposits of the second type are tabular, as much as 10 km in lateral extent, and rest conformably on other floodplain sediment. These units show a cyclic arrangement of sedimentary subunits. Sedimentologic and paleobotanic evidence suggests that these tabular deposits accumulated under a sequence of vegetatio al types including, in succession, a floodplain marsh, a swamp forest, and a drier-ground floodplain forest. These interpretations stress the interaction of vegetational succession, soil characteristics, and fluvial morphology in creating the chemical conditions that led to the preservation of organic matter. The stratigraphic distribution of the two types of deposits within the Willwood Formation suggests that in the Bighorn Basin, tectonism exerted a greater influence on the geometry of carbonaceous deposits than did climate.

Supratidal evaporitic dolomite at Ojo de Liebre Lagoon; mineralogical and isotopic arguments for primary crystallization

Abstract: This pair deals with a new site of dolomite formation in supratidal evaporite flats, at the southernmost edge of the Ojo de Liebre evaporitic complex (Baja California, Mexico). The sedimentary context is mostly siliciclastic, except for calcareous outcrops around the flats. Both chemical and isotopic data on the interstitial solutions demonstrate a mixing between Mg2+-rich marine-derived brines and calcium bicarbonate-bearing continental waters, which are subjected to concentration by capillary evaporation. The lack of any in situ solid CaCO3 in the sedimentary matrix argues for direct crystallization of dolomite from aqueous solutions. This is documented by the oxygen isotope compositions of the dolomite which are consistent with deposition under equilibrium conditions of ambient temperature and 18O of water of the interstitial solutions. Furthermore, the low 13C values of the dolomites and the absence of living plants or organisms in the basin indicate that a fraction of the total dissolved inorganic carbon reservoir is biogenic and is mostly provided by continental groundwaters. Therefore, crystallization of dolomite occurs within the sands infiltrated by solutions whose state of saturation relative to dolomite is mainly governed by the chemistry of continental groundwaters mixing with the inflowing marine brines.

Extensive submarine lithification in a cave in the Belize barrier reef platform

Abstract: Extensive deposits of submarine cements discovered in a submerged Pleistocene cave in the outer barrier-reef platform off Belize, Central America, occur in an unusual setting--on and within countless calcareous projections formed by serpulid worms on the ceiling of the cave. The cements are predominantly dense submicrocrystalline and microcrystalline to porous chalky or submicrosucrosic magnesium calcite (approximately 15 mole % MgCO3), and they contain minor amounts of acicular aragonite, which generally forms a lining inside the aragonitic serpulid tubes. Radiocarbon dates, three-year artificial substrate experiments, and evidence of the ceiling's original relief indicate a low rate of accumulation for the serpulid-cement projections. The characteristic 20-60 µm peloidal texture associated with magnesium calcite submarine cements is well developed here and there within the projections. These peloids are physiochemical precipitates, having formed during the development of the submarine cement deposits, which accumulated preferentially on substrates that have a slow rate of accumulation and that experience little or no disturbance of their sediment cover. Artificial substrates placed on the ceiling of the cave for three years revealed that the initial stage of precipitation of magnesium calcite from seawater is in the form of a submicrocrystalline magnesium calcite "dust."

Bioclastic carbonate sedimentation on a high-latitude, tide-dominated shelf; Northeast Orkney Islands, Scotland

Abstract: Shell-sands and gravels cover much of the shallow Orkneys shelf at 59°N, accumulating locally into 30-m-high banks at rates up to 540 g/m2/yr (67 cm/l,000 yr). Overall, the Orkney shelf sedimentation rate is approximately 10 cm/l,000 yr, compared with 3 cm/1,000 yr for the entire Scottish continental shelf. Major sandbanks are located off headlands that produce circulation loops in the tidal flow. Regional sandwave orientations reveal a clockwise transport of sediment around the islands, probably resulting from storm-wave reinforcement of the tidal asymmetry combined with the net inflow of Atlantic water into the North Sea. Carbonate production is high from the Modiolus epifauna. Modiolus shell gravels, commonly containing Glycymeris, pass laterally into comminuted shell-sands. Within the euphotic zone (down to 40 m) dead shells are weakened by echinoid biting, algal boring, and limpet grazing. Boring by fungi and clionid sponges and grazing by chitons are common but not depth-restricted. Sediments contain 89-95% carbonate on the level bottom offshore, but 94-99% in sandwaves. Mean values for the main skeletal components are--bivalves 46%, barnacles 18%, bryozoans 11%, and serpulids 7%. Calcareous algal gravels occur in sheltered areas less than 20 m deep. Bryozoa typify lower-energy offshore environments, while more durable barnacle and serpulid debris is concentrated in sandwave fields. The sediments are dominantly calcitic and have a high preservation potential.

Burial Dedolomite in the Mississippian Madison Limestone, Wyoming and Utah Thrust Belt

Abstract: Calcitization of dolomite in sedimentary carbonate sequences is often interpreted as a near-surface process reflecting either an erosional unconformity within a sequence or late, postburial weathering. In the Madison Group of the Western Overthrust Belt there is evidence of widespread dedolomitization. Petrographic, isotopic, and trace element analyses of Madison dedolomite indicate that multiple phases of calcitization have occurred in several different diagenetic environments. Isotopic trends in replacement calcite suggest progressive burial interrupted by tectonic thrusting, fracturing, and uplift. Burial dedolomite in the Madison Limestone is characterized by depleted oxygen (-8.0 18O PDB) and carbon (-8.6 to 17.5 13C PDB) isotopic compositions relative to other replacive calcite phases. Introduction of depleted carbon into the system via hydrocarbon alteration is suggested by the depleted carbon isotopic composition of burial replacive calcite. Trace element data also indicate diverse origins for the various dedolomite phases. The bu ial-related dedolomite has low iron and manganese (Fe = 200 ppm; Mn = 30 ppm) and moderate strontium (Sr = 350 ppm) while other replacive components have higher iron and manganese and trace levels of strontium.

Sedimentology of a Late Pleistocene Submarine-Moraine Complex, County Down, Northern Ireland

Abstract: The Killard Point moraine formed when Late Pleistocene ice grounded on the isostatically depressed coastal lowlands of County Down, Northern Ireland, at a time of relatively high sea level. The moraine consists of three major lithologic associations which prograded for about 1 km from the ice-grounding zone into a glaciomarine environment. At the base of the succession a diamicton association is characterized by massive and stratified diamictons that are mainly debris flows. It contains a minor ice-rafted debris component. The overlying sand association was deposited from sediment-gravity flows of low to intermediate viscosity. The gravel association was deposited by high-density sediment-gravity flows. Massive mud beds or drapes occur throughout the sequence and are related to low-density turbidity or contour currents. Sequences of this type are more common in the geological record than the literature suggests and are often explained in terms of terrestrial-based deglacial models. This calls for a sedimentological reappraisal of many "glacigenic" deposits which accumulated in areas subject to deep isostatic depression during the Late Pleistocene.