Showing papers in "Journal of Sedimentary Research in 1985"

The Effects of Sediment Transport on Grain-Size Distributions

Abstract: Changes in statistics (mean, sorting, and skewness) describing grain-size distributions have long been used to speculate on the direction of sediment transport. We present a simple model whereby the distributions of sediment in transport are related to their source by a sediment transfer function which defines the relative probability that a grain within each particular class interval will be eroded and transported. A variety of empirically derived transfer functions exhibit negatively skewed distributions (on a phi scale). Thus, when a sediment is being eroded, the probability of any grain going into transport increases with diminishing grain size throughout more than half of its size range. This causes the sediment in transport to be finer and more negatively skewed than its source, whereas the remaining sediment (a lag) must become relatively coarser and more positively skewed. Flume experiments show that the distributions of transfer functions change from having a highly negative skewness to being nearly symmetrical (although still negatively skewed) as the energy of the transporting process increases. We call the two extremes low-energy and high-energy transfer functions , respectively. In an expanded sediment-transport model, successive deposits in the direction of transport are related by a combination of two transfer functions. If energy is decreasing and the transfer functions have low-energy distributions, successive deposits will become finer and more negatively skewed. If, however, energy is decreasing, but the initial transfer function has a high-energy distribution, successive deposits will become coarser and more positively skewed. The variance of the distributions of lags, sediment in transport, and successive deposits in the down-current direction must eventually decrease (i.e., the sediments will become better sorted). We demonstrate that it is possible for variance first to increase, but suggest that, in reality, an increasing variance in the direction of transport will seldom be observed, particularly when grain-size distributions are described in phi units. This model describing changes in sediment distributions was tested in a variety of environments where the transport direction was known. The results indicate that the model has real-world validity and can provide a method to predict the directions of sediment transport

Silica diagenesis; II, General mechanisms

Abstract: Amorphous silica phases (opal-A) precipitate in nature due to the formation of dense colloids in supersaturated alkaline aqueous solutions with low relative concentrations of other ions. Opal-A dissolves and yields solutions of still relatively high silica content. In pore waters containing abundant cations, open framework polymers form which flocculate to yield opal-CT. Opal-CT becomes increasingly ordered, primarily due to preferential growth of cristobalite relative to tridymite and crystal size increase. Opal-CT dissolves to yield pore waters of low silica concentration, which allows slow growth of quartz crystals from monomeric solution. The quartz crystals then slowly increase in size and crystallinity. Carbonates appear to enhance opal-CT formation, possibly due to the activity of positively charged hydroxyl complexes. Hence, polymerization in relatively pure systems is involved in opal-A formation and polymerization in impure systems is involved in opal-CT formation, and slow growth from monomeric (low silica concentration) solutions is involved in precipitation of quartz in sedimentary realms.

Silica Diagenesis, I. Solubility Controls

Abstract: The diagenetic sequence of silica is examined in terms of several thermodynamic and kinetic concepts. The relationship between solubility and surface area or particle size is sufficient to explain simple opal-A opal-CT quartz transformations, suggesting that the role of temperature and other physical parameters may not be as critical as previously thought. Complexation and adsorption in an impure system, when added to the model, allow for possibilities of opal-A transformation directly to quartz, of clay and zeolite formation from an opaline silica source, and of variations in silica transformation rates due to presence of impurities in the system.

Kinetic Control of Morphology, Composition, and Mineralogy of Abiotic Sedimentary Carbonates

Abstract: Conventional thinking has long held that the abiotic precipitation of calcium carbonate occurs with a causal relationship between fluid My/Ca ratios and crystal morphology, crystal composition, and carbonate mineralogy, resulting in the formation of meteoric, equant, low-magnesium calcite and marine, acicular, high-magnesium calcite and aragonite. Problematically, calcites with varying amounts of incorporated magnesium occur either as equant or acicular crystals, and aragonite may coexist with calcite in either environment. Commonly, however, a systematic relation exists between crystal morphology, composition, mineralogy, and rates of reactant supply to growing crystal surfaces. For example, equant rather than acicular crystals of calcite form in modern, deep and/or cold marine, meteoric-phreatic, and deep-burial settings where the degree of carbonate saturation and/or rates of fluid flow are low. In areas of higher saturation and/or fluid flow, such as in warm, shallow-marine and meteoric-vadose environments, acicular calcite may predominate. This relation is also seen in systems in which aragonite and calcite form in intimate association. Aragonite precipitation is favored when rates of reactant supply are high; calcite forms when rates are low. Such relations suggest that crystal morphology, composition, and mineralogy are controlled by the kinetics of surface nucleation and the amount of reactants, principally carbonate ions, at growth sites. Precipitating phases are the ones which can best accommodate such excess reactants; ambient Mg/Ca ratios only indirectly control the nature of inorganically precipitated carbonate phases.

Iceberg Drop, Dump, and Grounding Structures from Pleistocene Glacio-Lacustrine Sediments, Scotland

Abstract: Dropstone, dump, and grounding structures, resulting from the melt-out of debris contained in floating and grounded icebergs, are described from a Pleistocene ice-marginal, glacio-lacustrine sequence in Scotland. Dropstones display bending penetration, rucking, and complete rupture of stratum occurring beneath them, and onlap above them, with the degree of deformation varying as a function of the size, shape, and axial disposition of the clast and of the sediment type into which they fell. Dump structures are conical mounds of gravel or diamict formed by the break-up and overturning of dirt-laden icebergs and the consequent release of large quantities of debris to the lake floor. Grounding structures are caused by the grounding of icebergs, the down-warping of underlying lake-floor se iment, and the subsequent in situ melt-out of contained debris to form isolated troughs of diamict.

Paleochannel Patterns Inferred From Alluvial Deposits: A Critical Evaluation Perspective: PERSPECTIVE

Abstract: Interpretation of paleochannel patterns from ancient alluvium involves reconstruction of sinuosity and degree of braiding. It is emphasized here that the geometry, flow, and sedimentary processes of the different channel patterns form a continuum, such that similarities between their deposits are more significant than differences. While many commonly cited sedimentary criteria for distinguishing channel patterns are shown to be invalid, the following are considered useful: 1) proportion of channel fills relative to lateral-accretion deposits, which increases with degree of braiding; 2) mean grain size of channel fills relative to lateral-accretion deposits, which decreases with increasing sinuosity; 3) paleocurrent variance, which may indicate sinuosity; and 4) bankfull discharge, slo e, and width/depth of paleochannels throughout the channel belt, as long as they can be quantitatively reconstructed from channel-bar and channel-fill deposits. Utilization of these criteria requires examination of completely preserved sections of channel belts and overbank deposits in large outcrops. Presently existing facies models do not convey the diversity of different channel types and facies; those that are not three-dimensional and lack scales and paleocurrent information are of limited value.

Relative Sea Levels and Coastal Sedimentation in Southeast Australia in the Holocene

Abstract: In recent years, Australian studies of relative sea-level change have shown reasonable consistency on the termination date of the postglacial marine transgression at c. 6,500 14C years BP. The following "stillstand" period has been characterized in some regions by a slight falling trend from a high stand, or in other areas by a level not easily distinguished from the present. The latter situation occurs on the embayed coast of New South Wales. Here studies of depositional environments and sequences within bed-rock embayments have linked the Holocene sea-level history to patterns of sedimentation on the inner shelf, in bay barriers and in estuaries. Deposition of quartz-rich sand in shoreface, flood-tidal delta and backbarrier environments occurred during the marine transgre sion, and the subsequent "stillstand." Shell fragments from these deposits have been radiocarbon dated. Shoreline displacement continues to the present day, partly in response to changes in availability of sand in the shoreface zone, and partly in response to variations in regional storminess patterns. "[A]nd the great shroud of the sea rolled on as it rolled five thousand years ago." HERMAN MELVILLE,Moby Dick

Resedimented conglomerates in the bottomsets of Gilbert-type gravel deltas

Abstract: A marine, mass-flow-dominated, fan-delta sequence (Espiritu Santo Formation, Pliocene, Vera basin, southeastern Spain) and its resedimented conglomerate beds are described and interpreted. The studied sequence from the base upward contains 1) shelf mudstones (Cuevas Formation); 2) an up to 40-m-thick sequence of sand and conglomerate sheets with primary dip of up to 10° (lower delta slope, LDS); 3) up to 40 m high, planar to tangential delta foresets with primary dip of up to 25° (upper delta slope, UDS). Most of the LDS conglomerates have a matrix of lime mud (now recrystallized), silt, sand, granules, and pebbles containing scattered, weakly imbricated cobbles. There is little change in texture in the downcurrent direction, but the grain size decreases slightly. Conglomerate beds thicker than 2 m are mostly due to amalgamation, as inferred from remnants of mud-layer horizons occurring within these beds. Conglomerates are dominantly nongraded, although at the base, a few centimeters of thin, inversely graded zone may be present. Some beds show in their distal segments coarse-tail, normal grading covered by diffusely stratified conglomerate. Good exposures indicate that gravel deposition started just beyond the slope break (zone of hydraulic jump). The grain fabric suggests laminar-flow behavior of a plastic-viscous gravel mass (debris flow with dominantly frictional strength). The flat, eroded top of the gravel beds, and the presence of cobble-sized, imbricated clasts there suggest reworking by a strong current, probably at the time of their deposition. It is therefore suggested that file LDS conglomerates originate from the transformation of a delta-front slide either into a combination of high-density turbulent flow and debris flow (lower part), or into an initially fully turbulent flow. In the latter case, the debris flow would have originated from gravity transformation of the turbulent flow (Fisher 1983). In both ca es, the faster-moving turbidity current enhances the mobility of the debris flow. The LDS conglomerate beds are intercalated with dominantly structureless (pebbly) mudstone beds which are locally laminated (suspension sedimentation). The structureless mudstones probably originate from a combination of bioturbation, dewatering, resedimentation, and deformation of overriding, gravelly sediment flows. The UDS conglomerates contain clast- and matrix-supported beds which are dominantly nongraded. The matrix consists of some (recrystallized) lime mud, silt, sand, and small pebbles. These beds are inferred to have been deposited by debris flows similar to those of the LDS, but with relatively higher strength values, as inferred from the steep (25°) depositional slope. The tangential geometry of many UDS foreset beds has been explained by flow transformation (dilution) and acceleration (decrease of flow viscosity), enhancing its mobility. The regular occurrence of LDS conglomerate beds within bottomset fines is a modification of the classical Gilbert-type delta. Therefore, the term bottomset-modified Gilbert-type delta is applied to illustrate the presence of major delta-front failures.

Paleochannel patterns inferred from alluvial deposits; a critical evaluation

Abstract: Interpretation of paleochannel patterns from ancient alluvium involves reconstruction of sinuosity and degree of braiding. It is emphasized here that the geometry, flow, and sedimentary processes of the different channel patterns form a continuum, such that similarities between their deposits are more significant than differences. While many commonly cited sedimentary criteria for distinguishing channel patterns are shown to be invalid, the following are considered useful: 1) proportion of channel fills relative to lateral-accretion deposits, which increases with degree of braiding; 2) mean grain size of channel fills relative to lateral-accretion deposits, which decreases with increasing sinuosity; 3) paleocurrent variance, which may indicate sinuosity; and 4) bankfull discharge, slope, and width/depth of paleochannels throughout the channel belt, as long as they can be quantitatively reconstructed from channel-bar and channel-fill deposits. Utilization of these criteria requires examination of completely preserved sections of channel belts and overbank deposits in large outcrops. Presently existing facies models do not convey the diversity of different channel types and facies; those that are not three-dimensional and lack scales and paleocurrent information are of limited value.

Dolomitization in a Mixing Zone of Near-Seawater Composition, Late Pleistocene, Northeastern Yucatan Peninsula

Abstract: Patches of dolomite occur in cores of reefal limestone from the shallow subsurface on the northeastern coast of the Yucatan Peninsula. This limestone accumulated during an interglacial high stand of sea level about 200,000 years ago. Dolomitization was preceded by freshwater diagenesis, including precipitation of sparry calcite cement, stabilization of Mg-calcitic skeletal fragments, and partial dissolution of aragonitic components. This suggests a predolomitization lowering of sea level with the consequent freshening of pore water. The subsequent precipitation of dolomite indicates a return to high sea level with the consequent increase in Mg/Ca ratio of pore water. Dolomitization took place during a brief high stand of sea level, either shortly after deposition about 200,000 yr BP, r, more likely, about 125,000 yr BP. Dolomite occurs both as microcrystalline replacement dolomite and as cement. The cement is part of the following diagenetic sequence: 1) limpid euhedral-subhedral calcian dolomite crystals, 2) zoned dolomite crystals with zones formed by variations of the calcium/magnesium ratio in dolomite, 3) layers of alternating calcian dolomite and magnesian calcite or calcite, and 4) calcite. This sequence represents the progressive freshening of ground water during the initial stage of a fall in sea level. Average cation composition of the limpid dolomite cement is Ca57Mg43 (electron microprobe analysis). Zoned cement crystals are composed of Ca57-59Mg43-41 layers and Ca62Mg38 layers. Most of the higher-calcium dolomite layers are dissolved, forming hollow-zone crystals. In cement with alternating dolomite and calcite zones, the calcite is Ca99-97Mg1-3 (low-Mg calcite) and Ca96-93Mg4-7 (Mg calcite). The dolomite and Mg calcite zones are partially to totally leached. 18O compositions of Yucatecan dolomite and of modern ground water suggest dolomite precipitation from mixed water ranging from about 75% seawater, 25% freshwater to nearly all seawater. (Isotope analyses are for the most stable calcian dolomites; more soluble, calcium-rich dolomite presumably is analyzed with calcite and thought to be isotopically lighter than the less soluble dolomite.) In the cement sequence, the most stable dolomite is followed by more soluble dolomite as ground water becomes less saline. Isotope analyses, together with position of dolomite in the cement sequence, suggest the most stable calcian dolomite (including limpid dolomite) precipitated from mixed water with large proportions of seawater, and the less stable, more calcian dolomite precipitated from fresher mixed water.

Depositional Facies of a Mud Shoreface in Suriname, South America--A Mud Analogue to Sandy, Shallow-Marine Deposits

Abstract: Immense volumes of argillaceous muds are being deposited along the world's longest continuous mud coastline which faces the open equatorial Atlantic. Not only are these muds deposited in an environment normally associated with sand deposits, but, just as in sandstones, sedimentary structures within these muds record the dynamic processes that deposited them. This study investigates a 100-km-long portion of the Suriname coast utilizing cores, bathymetric surveys, and field observations from nearshore and innershelf environments. Using terminology borrowed from sand coastlines, the innershelf/nearshore of Suriname is divided into transition zone, shoreface, foreshore, and backshore. The transition zone consists of a mix of offshore sands and shoreface muds. Shoreface and foreshore are subdivided into bank and interbank zones. These zones are transitory because mud banks, which make up the bank zones, migrate westwardly. Mud banks are up to 5 m high, 50 to 60 km long, 10 to 20 km wide, and oriented at oblique angles to the coastline. These banks generally consist of fluid muds which are so fluid that they interact with surface waves causing them to be altered and damped. Banks are attached to the shoreline and commonly produce progradation of the coastline. Substrate within the interbank zone is more consolidat d than within the mud banks. Because of lack of wave dampening, wave energies are relatively high within the interbank zone, producing erosion of the shoreface and shoreline. Sand beaches are generally formed within the erosional interbank shoreline. The vertical sedimentary sequence within the shoreface muds is a record of bank migration and other sedimentary processes present. Wave and current energies are recorded by scour-and-fill features, discontinuity features, micro cross-laminations, lenticular laminations and parallel to subparallel laminations. Laminations appear to be created by boundary-layer shearing within fluid muds which are above wave base and made mobile by wave-generated, shore-parallel currents. Large-scale evidence for migration of the mud banks is westward-dipping acoustical reflectors within the banks. These reflectors, which are over a kilometer apart, mark the boundaries of annual migration events. Biogenic structures are generally absent within the fluid bank muds but are present within the more consolid ted interbank zone and associated with discontinuity features which are buried interbank surfaces. Migration and vertical stacking of mud banks are recorded by vertical sequences of massive and laminated muds, which were deposited by mud banks, and discontinuity features, which were formed by interbank zones. The coastal deposits of Suriname are a mud analogue to nearshore and innershelf sand deposits. Transition zones of both mud and sand systems reflect a transition from offshore to shoreface processes and sediments. Both mud and sand shoreface/foreshore systems reflect sediment input and wave, current, and tidal processes that act on them. Mud banks of Suriname resemble linear sand ridges on the continental shelf of the eastern U.S.A., in shape, oblique orientation to the coastline, and orientation with respect to dominant direction of transport processes. The sequence of massive and laminated muds with discontinuity features resembles laminated to burrowed sequences found in ancient and Recent nearshore and shallow-marine sands.

Derivation of the Original Isotopic Composition of Permian Marine Cements

Abstract: Diagenesis of former aragonite and magnesian calcite marine cements of the Permian Reef Complex has resulted in a fine-scale intermixture of two distinct secondary phases, a luminescent and a nonluminescent calcite. Multiple carbon and oxygen isotopic analyses of these marine cements from any single sample result in a linear covariant trend, reflecting random proportions of the two calcite phases contained in individual analyses. The convergence of four trends derived from samples of former aragonite marine cement collected at four sites along a single Upper Capitan paleoslope fixes the isotopic composition of the nonluminescent calcite at a value of -2.518O, +5.313C. Likewise, covariant trends from former aragonite cements in the Lower Capitan and the youngest Upper Capitan yield nonluminescent calcite compositions of -2.818O, +5.213C and -0.718O, +5.813C, respective y. The single covariant trend generated from former magnesian calcite cements does not converge with equivalent former aragonite covariant trends, indicating that the precursor mineralogy influences the isotopic composition of the nonluminescent calcite. Considered as a whole, the invariance of the nonluminescent calcite signature, the timing of luminescent calcite emplacement, and the fine scale of the fabric retention suggest that the precipitation of the nonluminescent calcite occurred while isolated from effective isotopic exchange with external diagenetic fluids. A direct consequence of this low water/rock ratio, "closed" system is that the isotopic signature of the nonluminescent calcite also represents the original isotopic composition of the marine cement. This is further support d by the preservation of primary differences between the original isotopic compositions of aragonite and magnesian calcite cements. The constancy of the marine isotopic signature throughout the reef massive and foreslope facies requires that the Delaware basin water mass was relatively well mixed. Comparison of marine compositions throughout the range of the Capitan samples indicates that, although the oxygen composition of the marine cements was apparently constant through much of the Guadalupian, a substantial enrichment in 18O occurred at the very end of the Guadalupian. This relatively abrupt change, recording a concomitant change in the oxygen composition of the Delaware Basin water mass, was probably the result of increased restriction of the Delaware Basin, and marked the first step in the sequence of events which led to the deposition of the deep-basin evaporites of the Castile formation.

Enhanced Carbonate Petrography Using Fluorescence Microscopy

Abstract: Fluorescence microscopy can recognize depositional facies and delineate diagenetic fabrics and porosity relationships in certain highly dolomitized or recrystallized limestones previously uninterpretable using standard petrographic techniques. This tool has other valuable applications in the study of carbonate rocks. These other applications include differentiating carbonate cements from neomorphic spar, enhancing depositional and diagenetic fabrics, rapidly evaluating mineralogical stabilization and recrystallization in Holocene and Pleistocene deposits and improving delineation of porosity and pore geometries in sedimentary rocks. Fluorescence microscopy is a rapid, easily used and nondestructive technique that can be applied to unpolished thin sections or rock slabs. By itself, or in conjunction with other petrographic and geochemical tools, it has the potential to expand our understanding of carbonate diagenetic processes and products, in particular dolomitization and neomorphism, and carbonate porosity evolution. We strongly encourage routine usage of fluorescence microscopy in future carbonate petrographic studies.

A Comparison of Clast Fabric and Shape in Late Precambrian and Modern Glacigenic Sediments

Abstract: Northeastern Svalbard contains a well-preserved glacigenic sequence of Vendian (Late Precambrian) age in which terrestrial, lacustrine, intertidal, and marine glacial facies are represented. Diamictite facies are of two main types, massive and weakly to strongly bedded. Diamictite field appearance and relationships with other facies, however, provide insufficient information to allow a clear interpretation of the precise mode of glacial deposition. By combining this information with data concerning the fabric of clasts in modern glacigenic sediments (where the mode of glacial deposition is better known), a wider understanding of the processes of glacial deposition in Vendian time is obtained. Several diamictite samples were interpreted to be of lodgement tillite facies, and material o water-laid and glacigenic sediment flow origin was also recorded. Clast shape analyses indicate that most glacial debris was transported, at least for a time, at the glacier bed. Little angular supraglacial debris was present, implying that the ice mass was of ice-cap or ice-sheet dimensions, with few subaerial rock exposures. Clast shape in the Late Precambrian deposits was not useful in distinguishing between the variety of lithofacies derived from basally transported glacial material. Too few systematic studies of clast striation in modern glacigenic sediments are yet available for this criterion to be helpful in distinguishing between tillite facies. Lithofacies analysis, together with clast fabric and shape studies, provide useful and complementary lines of evidence concerning the ode of deposition of ancient diamictite sequences. An understanding of the depositional processes and patterns of modern glacial sedimentation provides a fundamental framework for such studies.

Rate of dissolution of carbonate sediments by microboring organisms, Davies Reef, Australia

Abstract: Skeletal carbonate sediments on the lagoon floor of Davies Reef (Great Barrier Reef, Australia) are subject to intense attack by microboring organisms, principally algae. The rate at which this microboring process dissolves carbonate was measured experimentally. Fresh, unbored, molluscan sand samples placed in the shallow lagoon (5 m water depth) experienced a weight loss of about 0.9 g/30 g sample (or 3%) in one year due to dissolution of carbonate by microborers (revealed by direct measurement of weight change and point-count analysis). This figure may be converted to about 350 g CaCO3 dissolved/m2 lagoon floor/year, which is equivalent to between 18 and 30% of the sediment influx rate to this lagoon, averaged over the past 9,000 years. Consideration of the exp rimental design suggests that the experiment underestimated the true rate of microboring in lagoon floor sediments. We conclude that the extent of microboring in carbonate sands may provide information on the rate of sediment deposition, and that dissolution of carbonate sediments by microborers is a significant factor in whole-reef CaCO3 budgets.

Biofabrics as Dynamic Indicators in Nummulite Accumulations

Abstract: Biofabric interpretation of skeletal accumulations consisting of "biologically standardized" nummulite tests (larger foraminifera) is based upon (1) the ratio of small-bodied A-forms (megalospheric) and large-bodied B-forms (microspheric), and (2) on their packing and imbrication features. In the Eocene Mokattam Formation of Egypt, Nummulites gizehensis occurs in limestones with four basic biofabrics. In wackestones, the nummulites "float" throughout the matrix, and A-forms strongly dominate over B-forms, approximating the ratio reported for undisturbed assemblages. Packstones, however, reflect various degrees of sedimentary reworking and winnowing of the original substratum by physical processes. For packstones with edge-wise imbrication and A-forms strongly dominating over B-forms, in situ winnowing of fine-grained matrix by waves can be inferred. In contrast, packstones composed of imbricated B-forms suggest hydraulic sorting and selective removal of A-forms by currents. Packstones composed almost entirely of A-forms represent transport-sorted allochthonous deposits. Biofabric analysis thus allows us to understand better the dynamics of skeletal accumulations such as nummulite banks, whose commonly complex and extensive buildups can form hydrocarbon reservoirs.

Mudstones and Thin-bedded Turbidites Associated with the Upper Cretaceous Wheeler Gorge Conglomerates, California: A Possible Channel-Levee Complex

Abstract: Three conglomeratic thinning- and fining-upward sequences in the Upper Cretaceous at Wheeler Gorge, California have previously been assigned to submarine-channel environments. There is continuing controversy as to whether the channels represent inner to midfan environments, or whether they are incised into basin-plain facies. To shed further light on the problem, the mudstones and turbidites below and above the conglomerates are described here in detail. Below the conglomerates, the sequence is dominated by mudstones interbedded with very fine grained turbidite sandstones in beds up to 5 cm thick. There is only one slumped horizon, about 240 m below the conglomerates. By contrast, the mudstones above the conglomerates contain more and thicker turbidites (beds up to 47 cm), which occur in at least three thinning- and fining-upward sequences. Bouma sequences (A, AC, BC) are common, whereas below the conglomerates, the thin turbidites show only graded bedding (very fine sandstone to mudstone) or ripple cross lamination. Slumping is much more common above the conglomerates; at least eight distinct slumped horizons are scattered through about 33 m. The slumped turbidites are interpreted as channel-margin or levee facies, and the thinning- and fining-upward sequences of relatively thin, unchannelized turbidites suggest a levee environment with gradual lateral migration of the channel away from the depositional site, yielding the thinning and fining upward. Below the conglomerate complex, the thinness of the turbidites (average 1 cm, maximum 5 cm), the absence of thinning- or thickening-upward sequences, and the dominantly muddy nature of the rocks, suggest a depositional environment on a basin floor far from any channel influence. The overall sequence of units records the incision of a channel complex (the conglomerates) into quiet basin-floor mudstones. Following deposition of all of the conglomerates, there was lateral shifting of the channel, with concurrent deposition of a channel-levee complex. The channel-levee complex is a variant of the more general lobate submarine-fan model. In the fan model, the fan is fed by a single channel: the channel-levee complex replaces that feeder channel by a group of channels. Similarly, the few branching channels on the lobate midfan may be replaced by extensive channel-levee complexes with little development of smooth lobes.

Holocene Sediment Budgets for the Basins of the California Continental Borderland

Abstract: Two Holocene sediment budgets have been compiled for the California Continental Borderland. The first detailed budget includes the six northern inshore and central basins that capture almost 40% of the total sediment input to the entire borderland. In the second budget, the data for the six basins has been added to estimates of accumulation for the remaining borderland area to produce a total regional budget of lower precision than the smaller, six-basin region. Natural river influx to the borderland approximates 13 1010 tons for the Holocene epoch, and biological input has been estimated at about 2 1010 tons. Eolian and ocean current influx have each been estimated at about 1 1010 tons. In sum, the total deposited volume for 104 yrs is about 14 1010 tons, and the estimated total influx is of the order of 17 1010 tons, which suggests that a net of 1-3 101010 tons for the Holocene high sea-level period. The six high-accumulation-rate basins are located adjacent to the largest river sources, the major zones of coastal upwelling, and the main northern eddies of the California Current. These high-accumulation sites are typically also areas where mass movement and fan development are localized. The sediment-accumulation rate is much larger than subsidence rate, and flat-floored basins are typical. In contrast, the southern and outer, low-accumulation basins are areas of draped sediment, with accumulation rate less than subsidence rate, and are the sites of penecontemporaneous deformation of the accumulating, fine hemipelagic sediments. Few fans or major mass movements are seen. Significant sedimentation is mainly confined by topography to less than 20% of total margin area. Thus, the moderate to low total sediment influx is amplified by the restricted receiving area to produce locally thick deposits of limited areal extent. This pattern is characteristic of Neogene southern California basins and illustrates that stratigraphic thickness is not a valid indicator of sediment influx.

Textures in Layered Silicates: Progressive Changes Through Diagenesis and Low-Temperature Metamorphism

Abstract: Phyllosilicates in samples from Gulf Coast cores and the Martinsburg Formation near Lehigh Gap, Pennsylvania (which together represent a continuous sequence from mudstone to slate) are shown by TEM studies to possess imperfections on a massive scale and to exhibit a progressive increase in perfection through diagenesis and low-grade metamorphism In shallow portions (1,750 and 2,450 m) of the Gulf Coast core, smectites have irregular and curved layers with high dislocation densities and many small-angle, grainlike boundaries In both the deeper portion (5,500 m) of the Gulf Coast sediments and in the Martinsburg mudstone samples, gram perfection increases in illites, but a complex mosaic structure with small-angle grain boundaries and edge dislocations still prevails In the Martinsbu g slate, phyllosilicates exhibit layers which are continuous and straight and have few imperfections Original clays which form at low temperatures have highly imperfect and metastable structures Changes with increasing diagenesis are akin to annealing processes, with increasing perfection of grains being an approach to a more stable, strain-free structure Temperature, pore water, time, and other variables affecting these changes act only to promote reaction rates to the defect-free, stable structures

The Depositional Record of Sandy, Versicolored Tidal Flats (Mellum Island, Southern North Sea)

Abstract: Cyanobacteria fix and bind sediments and generate versicolored, laminated patterns in quartz sand flats. They are developed in the lower supratidal zone (between mean high water and mean high water springs). The cyanobacteria mats protect the sediments from erosion and desiccation and provide food for marine and terrestrial invertebrates. Burrowing and grazing polychaetes, amphipods, and gastropods are present as well as a diverse meiofauna. The bioturbation structures are described. At some places the population density of the macrofauna averages 20,000 individuals per m2. Typical are smooth surfaces and erosional pockets with rippled sand. The deposits consist of laminated sand in which organic-rich mat generations are intercalated. The sediment is medium to fine-grained sand. The development of versicolored systems by interaction of migrating cyanobacteria and episodic low-rate sedimentation was studied in the laboratory. The depositional record reveals characteristics from both, the terrestrial and the marine surrounding. There are bioturbation structures from marine and terrestrial burrowing animals, hard skeletal parts from shelled marine organisms and plant roots of halophytes. Fenestrae produced by burrowing beetles are observed. The described mats characterize the type of quartz sandy potential stromatolites.

Diagenesis of Quaternary Bahamian beachrock; petrographic and isotopic evidence

Abstract: Carbonate beachrock is a common feature on many beaches of San Salvador Island, Bahamas. These well-indurated slabs of cemented beach sand are jointed normal and parallel to the shoreline and slope gently seaward. Constituent sediment grains are identical to those of adjacent, unlithified beach sands. Beachrock exposed along the northern, western, and southern coastline of the island is commonly characterized by aragonite-needle cement, micritized grains, and micritic envelopes, which are features of early marine-phreatic diagenesis. These beachrocks have whole-rock 13C values of + 4.75 (PDB) and 18O values of - 0.75 (PDB), which are identical to carbon and oxygen isotopic compositions of isolated aragonite-needle cement and unlithified beach sand. Inclusion of Recent artifacts in the rock indicates that these deposits are Holocene. Beachrock from one locality on the western side of the island is covered with a calcrete crust, indicating prolonged subaerial exposure and vadose weathering associated with the formation of organic-rich soils. Petrographic features include low-Mg calcite cement, evidence of neomorphism of aragonitic cement and micritic rims to low-Mg calcite, random needle-fiber calcite, calcified filaments, and micritic networks. Whole-rock 13C values of these beachrocks range from + 3.66 to - 6.85 and 18O from -2.16 to - 3.68. The wide range of 13C values in these samples is probably a function of the relative proportions of meteoric-vadose to meteoric and/or marine-phreatic cements in individual samples. Isolated meteoric-phreatic cements are only slightly enriched in the light isotopes relative to marine cements, whereas meteoric vadose cements are significantly lighter. The diagenetic features suggest the beachrock passed through marine-phreatic to meteoric-phreatic and meteoric-vadose diagenetic environments. This diagenetic sequence commonly forms in response to sea-level lowering. Evidence of deposition in a shoreline environment and prolonged residence in meteoric-phreatic and meteoric-vadose environments suggests that these beachrocks may be late Pleistocene.

Field Experiments on Vertical Mixing of Sand in the Surf Zone

Abstract: The depth to which sand in the surf zone is vertically mixed by waves was determined from large numbers of cores taken in situ in eight tracer experiments performed on high-energy beaches. The most common tracer concentration profile showed a maximum at the bed surface and a monotonic decrease with depth; significant numbers of irregular profiles were also found and classified. Several methods of estimating the mixing depth were compared. In the method finally selected, the mixing depth was defined as that depth which includes 80% of the tracer in a core. Reliability of the results was reinforced by a consistency check made possible by the use of different colored tracers. Local mixing depths exhibited considerable variation. However, the average value for the surf zone was found to increase only slightly over time intervals of 2 to 5 hr. The most common on-offshore distribution of the mixing depth was bimodal, with maxima near the breaker line and near the shoreline. The average mixing depth alongshore was almost constant for distances of up to 200 m. The experiment-average mixing depth, Z, was well correlated with the significant breaking-wave height, Hb, and the relation Z = 0.027 Hb was obtained.

Substrate Destruction and Sediment Production by the Boring Sponge Cliona Caribbaea on Grand Cayman Island

Abstract: The boring sponge Cliona caribbaea is ubiquitous on the shallow terraces around Grand Cayman Island, British West Indies, occurring as brown sheets covering and growing over both live corals and hard substrate Throughout the zone of the radial grooves, on the southwest corner of Grand Cayman, C caribbaea covered 5% of the total bottom area between -1 and -10 m Values up to 10% occurred commonly Lateral extension rates were 4 cm/yr The sponge removes an average of 20% (up to 45%) of the substrate, which has an average specific gravity of 17 Conservative estimation of typical rates of substrate removal yields 8 kg/m2/yr, a value of about double the typical rates of reef calcification The silt produced by the sponge is rapidly transported downslope

Glauconitic sediments on the continental shelf off Vancouver Island, British Columbia, Canada

Abstract: The tectonically active Vancouver Island continental margin is characterized by a very narrow (< 80 km) continental shelf and an extremely steep continental slope. The shelf is marked by high wave and current energies on the inner, nondepositional shelf and very slow rates of sediment accumulation on the outer shelf. Within the muddy sands of the outer shelf and slope, glauconitic grains are frequently an important constituent of the sand fraction, occurring in four principal forms: (a) locally abundant, pale green infillings of benthic foraminifera; (b) very abundant, pale to moderately dark green, dull grains displaying varying degrees of fracturing of the external surfaces; (c) scarce, moderately dark green, altered and expanded mica grains; and (d) scarce, very dark green, small (< 200 µm) grains with polished, highly fractured surfaces. Mineralogically, the grains mainly reflect their origins as detrital sediments. They consist most commonly of quartz, plagioclase minerals, chlorite, smectite, and illite. Also, a mixed-layer mineral, smectite-illite, occurs in significant amounts in glauconitic grams but is absent in unaltered mud fragments and in the clay fraction of sediments from the outer shelf. The appearance of this mineral reflects the first stages of glauconite authigenesis. Bulk analyses of glauconitic grains yielded K2O values of less than 2%, reflecting their very early stage of evolution, but 19-28% Fe2O3, several times the level in the surrounding matrix sediments. Microprobe analyses and scanning electron microscopy show that glauconitic smectite forms in the pores and along fractures in the host grains, commonly as a boxwork arrangement or rosette clusters of blades. Fe2O3 and K2O values at these sites can reach 32.5% and 7.5%, respectively. Several lines of evidence, including radiocarbon age determinations on glauconitized foraminifera tests, indicate a Recent age for the Vancouver Island shelf glauconitic grains.

Carbonate Debris Flows, Cow Head Group, Western Newfoundland

Abstract: The Cow Head Group is an allochthonous Cambro-Ordovician sequence of deep-water limestone conglomerates, mudstones to grainstones, minor quartz sandstones, siltstones and shales. Platy-clast conglomerates and boulder-rich megaconglomerates as much as 100 m thick exhibit features similar to those described for subaerial debris flows; that is, flat, sharp bases with only minor evidence for erosion, poorly developed clast fabric with tilting of platy clasts at right angles to the flow direction, boulders floating within flows or projecting above the tops of flows, irregular flow tops indicating primary relief above the sea floor, and tapered flow margins (snouts). Snout geometry and the extent of boulder projection both allow calculation of debris static strength. These results, combined with measured flow thickness, permit calculation of minimum and maximum paleoslope gradients. Original debris static strengths were in the range of 103 to 105 dynes/cm2, similar to the observed range in subaerial flows. Calculations yield bottom gradients of l-l 8°, but independent field data suggest that the paleoslope dip was on the order of only a few degrees. This constraint suggests that debris strength was substantially reduced during flow, probably by excess pore-fluid pressures, thus permitting flow on very gentle slopes. The paleoslope orientations used in this study are substantially different from those previously published by Hubert et at. (1977).

Distribution of Calcrete in Holocene Coastal Sands in Relationship to Climate, Southwestern Australia

Abstract: Seven study areas located over 320 km of coastal plain in southwestern Australia show the distribution of groundwater and vadose calcrete within Holocene coastal sands (age <7,100 yr BP,) over a regional climate gradient. The calcrete exhibits a range of structures that include rhizoconcretionary, mottled, massive, and laminar types. Groundwater calcrete, composed of mottled, massive, and laminar types, occurs as a thin sheet in the zone of capillary rise just above the water table. Vadose calcrete is predominantly rhizoconcretionary. At all sites where it is well developed the groundwater and vadose calcretes are closely associated with vegetation. Calcrete is well developed in southern areas that are humid. Further north, as the rainfall decreases and evaporation increases, calcrete is less common until it is absent or rare at the semiarid northern limit of the study region. Its limit of occurrence is correlated with a region whose rainfall is ca 800 mm/annum, evaporation is ca 1,900 mm/annum, and mean summer temperature is 23 degrees C. This study shows, therefore, that a trend toward a drier climate may not be conducive to the development of calcrete, although such a view prevails in the literature.

Evidence of Tide, Storm, and Wave Interaction on a Precambrian Siliciclastic Shelf: The 1,700 M.Y. Ortega Group, New Mexico

Abstract: The 1,700 m.y. Ortega Group in northern New Mexico is a sequence of quartz arenites and subordinate mudstones in excess of 1,000 m. Sedimentation took place in diverse shallow-shelf environments under the influence of tidal, wave, and storm processes. Time-velocity asymmetry of tidal flow on the inner shelf produced large-scale trough cross beds and tabular-planar cross beds with multiple reactivation surfaces. In contrast, tabular cosets structured internally by small-scale trough cross beds and herringbone cross beds resulted from symmetrical tidal flow on the inner shelf. Fair-weather waves reworked the upper surface of tidal sand bodies on the inner shelf during quiet periods within the tidal rhythm. During storms, sand was entrained into the water column on the proximal inner she f leaving winnowed pebble lags at the top of tidal sand bodies. Sand transported by storm-induced gradient currents was deposited on the outer shelf as discrete lobes. Progradation of sand lobes produced 1-11-m-thick genetic packages which display an upward thickening of sandstone beds and are capped by cosets of tabular, planar-laminated sandstones. Storm sands were reworked by wave processes on the proximal outer shelf, whereas storm-deposited sands on the distal outer shelf remained unmodified. No evidence of tidal processes exists on the outer shelf. Distribution of facies and paleocurrent patterns indicates that the shelf sloped to the south and experienced an overall transgression which culminated in the drowning of the outer shelf with onlap of black, basinal muds. Absence of shallow-water turbidites below storm wave base reflects low gradients on the shelf. Vertical transition from tide- to storm-dominated sedimentation in the Ortega Group does not represent a change in depositional style with time on the shelf. Rather, all facies coexisted across the shelf, with tidal processes prevailing on the shallow inner shelf and storm processes on the outer shelf. The German Bight in the North Sea and parts of the Bering Sea off Alaska are Holocene counterparts for the depositional framework envisaged for the Ortega Group; in both, pro imal, tide-dominated environments pass distally into deeper-water, storm-dominated environments.

Chemical control of carbonate phases; implications from Upper Pennsylvanian calcite-aragonite ooids of southeastern Kansas

Abstract: Ooids in grainstones from the Spring Hill Member of the Pennsylvanian Plattsburg Limestone in southeastern Kansas were precipitated as both aragonite and calcite. Either phase predominated in different outcrops, in different units within an individual outcrop, in different grains within an individual sample, and in different portions of an individual ooid cortex. Selective diagenetic dissolution of aragonite cortices gave rise to a variety of fabrics reported from many other politic limestones. These include oomoldic porosity, sparry cortices, strings of spastolithic (deformed) and elephantine (trunk-to-tail) connected ooids, and displaced nuclei. These and other reported occurrences of fabrics which record the former presence of aragonite are not evenly distributed through Phanerozoic time. Rather, they predominate during times of general continental emergence. While this distribution supports the postulations of Mackenzie and Pigott (1981) and Sandberg (1983) that the dominant mineralogy of ancient marine limestones may have corresponded closely to positions of global sea level, fabrics of the Plattsburg Limestone demonstrate that evaluating changes in ooid compositions with geologic time is not a simple "either aragonite or calcite" proposition.