Showing papers in "Journal of Sedimentary Research in 1989"

Bedforms, primary structures and grain fabric in the presence of suspended sediment rain

Abstract: Flume experiments were conducted to determine how a pervasive rain of sand might affect the development of bedforms, internal structures, and grain fabric in a bed aggrading from this fallout. During otherwise standard flume runs, sediment similar to that in the bed (Mz = 2.13 ; = 0.63 ) rained from overhead bins extending the length of the 9-meter recirculating flume. Consequent rates of bed aggradation ranged to 4.2 cm min-1. The experiments are viewed as having particular relevance to deposition from turbidity currents. Under upper-plane-bed conditions, so long as low rates of sediment feed prevailed, parallel lamination developed from rapidly migrating, millimeter-scale bed-load sheets. At high rates of feed, lamination was suppressed, suggesting that rapid aggradation can account for the "massive" structure characteristic of Bouma A divisions of turbidites. When flow conditions were maintained within the dune stability field for clear-water runs, established dunes persisted at all sediment fallout rates examined (bed aggradation <= 4.0 cm min-1), apparently contradicting a suspicion that the underrepresentation of medium-scale cross-bedding in turbidites might be attributable to suppression of dunes by sediment rain. It was not feasible to test whether sediment rain might inhibit development of nascent dunes, however. Ripples proved stable across the range of fallout rates examined. Depositional fabrics, deduced from anisotropy of magnetic susceptibility, confirmed that under upper-plane-bed conditions (representative of Bouma divisions A and B) grain orientation is current-parallel and equally well developed for all sedimentation rates. Up-current imbrication, consistently steeper than what is typically found in upper-plane-bed stream deposits, increased in steepness in direct proportion to sediment fallout and may provide a useful basis for estimating aggradation rates during deposition of divisions A and B of ancient turbidites.

Field and Modelling Studies of Cambrian Carbonate Cycles, Virginia Appalachians: REPLY

Abstract: Field studies coupled with computer modelling of carbonate cycles in the Elbrook-Conococheague Formations (Middle to Upper Cambrian) in the Appalachians have helped define the mechanisms of formation. The sequence is up to 1.6 km thick and composed of cyclic, peritidal carbonates. These formed on an aggraded, rimmed shelf on a mature, passive continental margin. The cycles (1-7 m thick) have lower parts of subtidal/intertidal carbonate and caps of dolomitic laminite containing minor quartz arenite, shale and breccia. Sedimentation rates for these peritidal carbonate environments exceeded long-term subsidence of the platform (0.01 to 0.1 m/1,000 yr), thus, the shelf stayed near to sea level (i.e., was aggraded). Milankovitch (20,000 to 100,000 yr), low-amplitude sea-level fluctuations (few meters) superimposed on 3rd-order sea-level fluctuation of 1 to 3 m.y. duration and less than 20 m amplitude caused periodic submergence of the gently sloping platform (less than 0.02 m/km slopes), resulting in deposition of over 400 peritidal cycles in about 26 m.y. Cycles that formed during 3rd-order sea-level rise are thick and limestone-rich, whereas those formed during 3rd-order fall are thin, quartzose, and highly dolomitized. The modelling suggests that fewer and thinner cycles formed during the long-term falls compared to the rises, reflecting decreased accommodation. Cycle-frequency decreases into areas of lower subsidence rate. Open-marine cycles typical of the outer shelf and cycles formed during 3rd-order sea-level rise contain digitate bioherms, grainstone, and ribbon carbonates in lower parts. They reflect high subsidence rate and an open-marine setting. Inner shelf cycles and cycles that formed during 3rd-order sea-level fall commonly are thrombolitic, or have thin subtidal parts (grainstone, conglomerate or stromatolite) and reflec the low subsidence rates. Interior areas of Quaternary carbonate shelves are characterized by "incipiently" drowned facies and pinnacle reefs and are punctuated by soil/caliche horizons and karst surface that reflect 100 m glacio-eustatic sea-level fluctuations. In contrast, many ancient shelves, like the Cambro-Ordovician shelf of the Appalachians, were flat-topped and dominated by cyclic peritidal sequences lacking evidence of high amplitude sea-level fluctuations. Such aggraded shelves may represent the typical state of mature carbonate shelves that formed in response to low-amplitude Milankovitch sea-level fluctuations as opposed to the large-scale fluctuations that typified the Quaternary.

Whitings, a sedimentologic dilemma

Abstract: Whitings, drifting clouds of water, milky because of suspended carbonate, have been claimed to originate from either the action of bottom-feeding fish or direct precipitation of calcium carbonate. Five cruises during different seasons were made to the Great Bahama Bank to collect data pertinent to the controversy. Measurements of particulate concentrations average 10 mg/liter with a maximum of 20 mg/liter of carbonate sediment suspended in whiting water, compared with an average of 1.5 mg/liter for clear water outside the whitings. The particles are dominantly acicular aragonite, but Mg calcite composes as much as 20 percent of some whitings. Sedimentation rates, measured with fixed and drifting sediment traps, were as great as 34 g/m 2 /hr. Sediment suspended in whitings aggregated into silt- and sand-size fioccules and settled to the bottom of settling tanks within six hours, even on a rocking ship. Sediment in artificial whitings, created by stirring sediment from the bottom with a shrimp trawl, settled to the bottom in about the same time. Natural whitings, on the other hand, were never observed to dissipate. Because sedimentation from whitings occurs at rates sufficient to cause dissipation of the whitings within six hours, we conclude that the natural whitings are continually replenished by direct precipitation. The search for fish in whitings utilized sidescan sonar and fathometer imaging, shrimp trawls, rotenone, remote video, and direct scuba observation. These methods and 25 years of casual observations leading to this study indicate that fish are not involved in the formation of most Bahamian whitings. Several whitings were found over rocky or sandy bottoms where there was no mud available for fish to suspend. The distance of these whitings from areas of muddy bottom precluded their having been made elsewhere by fish. Stable carbon- and oxygen-isotopic analyses and Delta 14 C activity are interpreted to indicate that the suspended sediment in whitings contains some precipitated calcium carbonate and is not merely bottom sediment stirred into suspension. Estimates indicate that the amount of new carbonate produced in whitings on the Great Bahama Bank is substantially higher than that arising from algal production. Consequently, the amount of sediment transported to deep water may be much greater than previously thought.

The diurnal inequality of the tide as a parameter for recognizing tidal influences

Abstract: Tidal periodicities, in particular the neap/spring/neap cycle, can be recognized in the internal structure of large-scale cross-bedded sets. Another frequently occurring periodicity is the diurnal inequality of the tide. This is a regular variation of the water levels reached by successive high and/or low waters in semi-diurnal and mixed tidal systems. As a result, the strength of successive flood and ebb currents fluctuates. A particular depositional site in a tidal environment is frequently dominated by one of these two tidal currents. This can be reflected in the sediments in a bundle-wise building up of unidirectional cross-strata. During a dominant current stage, the bedform advances, and its displacement is reflected in the thickness of the forest deposits (bundles). The neap/sp ing sequence is reflected in variations of the bundle thickness over a period of 14 days. Variations in the strength of successive dominant currents due to the diurnal inequality of the tide are reflected in thick-thin variations in the thicknesses of successive bundles. The diurnal inequality of the tide occurs in semi-diurnal tidal regimes and in the semi-diurnal parts of mixed tidal systems. When the tidal currents within a semi-diurnal or mixed tidal regime drop to such low magnitudes that bedform migration ceases, for example around neap tide, the number of bundles within a neap/spring cycle may be (much) less than 28, and the depositional system can be incorrectly interpreted as a "diurnal tidal system." Then the reflection of the diurnal inequality of the tide in the bundle thickn sses may reveal that bedform migration was subject to semi-diurnal tides and that standstill periods in the bedform migration must have caused a reduction in the number of bundles per neap/spring cycle. Several examples from the recent literature are used to demonstrate this principle. In present-day diurnal systems the difference between low and high water is generally small and, moreover, the timespan between low and high water is twice as long as in semi-diurnal systems. The low tidal amplitude and the low frequency result in generally small tidal current velocities, which are rarely strong enough to produce large-scale megaripples with bundle cycles. Apart from the recognition of tidal influences in fossil sedimentary deposits, measurements of lunar cycles offer a tool for differentiating diurnal from semi-diurnal and mixed tidal systems in the geological past. Changes in the Earth-Moon rotational system may also be recognized.

Sedimentology, Paleoflow Dynamics and Flood History of JoKulhlaup Deposits: Paleohydrology of Holocene Sediment Sequences in Southern Iceland Sandur Deposits

Abstract: This paper examines the sedimentology of a series of deposits associated with catastrophic floods caused by subglacial volcanic eruptions in southern Iceland. A simple model is proposed for the interpretation of the sediments in terms of changes in flow characteristics of the fluid-sediment mix. A range of sediment types characteristic of these flood flows is identified, the predominant one being thick, massive, homogeneous, poorly structured, black pumice, fine gravels, with 65 p rcent silt + clay (Type E); and single lithology boulder deposits (Type F). These flood deposits alternate with, or are channeled into, more 'normal' sandur deposits, comprising heterogeneous, poorly sorted, clast-supported, imbricated, rounded cobble and pebble gravels (Type G). The dominant sediment sequences are interpreted as representing hyperconcentrated fluid-sediment mixtures, with dispersive stresses acting to create massive, poorly sorted, nongraded or inversely graded sediments, the latter characterized by large surface boulders. The vertical changes in sediment type reflect significant changes in flow conditions during transport and subsequent deposition, with an initial massive flood surge followed by flow deceleration, sediment 'freezing' and dewatering, producing a more fluid series of flows. The more fluid phases were associated with extensive erosion and channelization, cutting deep scour channels marked by residual streamlined hummocks, boulder lags, and fields of 'megaripples', leading downstream to a 'washed' sandur plain. The paper examines some of the problems associated with the application of paleoflow models for determining former flow dynamics. The models adopted suggest that most of the flows occurred as relatively mobile, low viscosity flows, with little internal resistance to shear. While sediment concentrations are estimated to have reached about 50 percent by weight, with maximum yield strengths between 2.9 103 and 4.2 105 dn cm -2, maximum viscosities ranged between only about 2 and 8 102 poises. A range of paleovelocity models applied to the flood sediment indicates flow rates of between abo t 4 and 13 ms-1, with peak discharges reaching a maximum of 3.3 105 m3 s -1. Dating of the deposits by 14C dating and tephrochronology indicates that at least 8 major sediment-producing jokulhlaups occurred over a 4,000-year period, between c 4500 yr BP and 1357 AD. Hence, although relatively rare, the effects of these floods still dominate the proglacial landscape, probably producing > 85 percent of the total thickness of proglacial sandur sediments.

Depositional Processes and Stratigraphy of Fluvially Dominated Lacustrine Deltas: Mississippi Delta Plain

Abstract: A geomorphic and stratigraphic investigation of lacustrine delta deposits in the Atchafalaya Basin, Louisiana, revealed that lacustrine delta formation is rapid and cyclic in nature. During the Holocene, thin, coarse-grained, regionally extensive, fluvially dominated lacustrine deltas filled interdistributary basins in the transition zone between the alluvial valley and the marine delta plain of the Mississippi River. One delta, Lake Fausse Pointe delta, prograded 6.5 km, partially filling the lake and covering more than 29 km2 in area in twelve years. Depositional processes in Lake Fausse Pointe ranged from suspension-settling of mud and organic matter during low sediment input to traction deposition of sand during floods. Hyperpycnal flow conditions, set up by the introdu tion of sediment-laden river water into the freshwater lake, induced underflows that scoured the lake bottom and deposited upward-coarsening lobes. The low relief of the Mississippi basin, a constant large volume of fine-grained sediment supply, and prevalent fluvial processes have formed fluvially dominated lacustrine deltas that differ sedimentologically and stratigraphically from "Gilbert-type" and brackish-water lacustrine deltas. Parallel-laminated prodelta mud, rippled to cross-laminated delta-front silty sand, and very finegrained to medium-grained distributary-mouth-bar sand are characteristic of rapidly deposited sediment, whereas rooted and burrowed sediments signify periods of minimal deposition. In a fluvially dominated setting, delta geometry and sediment distribution patterns are controlled by river-mouth processes, basin shape, and bathymetry. Thickest sand accumulations occur in linear, dip-elongate distributary-m uth-bar and natural-levee lobes that are separated laterally by mud-filled channels and interdistributary troughs. Lacustrine-delta deposition is driven by basin subsidence (regional downwarping and sediment compaction) and the concomitant development of distributary channels in the interdistributary basins. Numerous lacustrine-deltaic wedges, each bounded by rooted backswamp clay, are preserved multilaterally in Atchafalaya Basin sediments. Although lacustrine-deltaic deposits constitute much of the basin's sedimentary fill, each individual delta sequence records depositional events of only 100 years duration. These deltaic deposits represent basin-filling (aggradational) episodes that preceded the formation of marine delta complexes such as the Maringouin, Teche, La Fourche, and Atchafalaya deltas of the Mississippi River.

Emplacement of clay into sand by infiltration

Abstract: Laboratory experiments and examination of core samples from Holocene sand bodies demonstrate that vadose infiltration of muddy water through sand is an effective mechanism by which clay can be emplaced into sand. Infiltrated clays form coatings on, and bridges between, sand-sized framework grains. Clay particles are mostly oriented parallel to grain surfaces in coatings and perpendicular to grain surfaces in bridges. The amount of clay emplaced as coatings and bridges is determined by complexly interacting variables that include sand grain size, clay particle size, clay particle shape, concentration of suspended sediment, and clay composition. Of these, relatively coarse sand sizes and high concentrations of suspended sediment appear to promote the emplacement of the most infiltrated clay. In Holocene sands, the formation of grain coatings and bridges is facies dependent. Infiltrated clays are most effectively emplaced in environments characterized by high suspended sediment concentrations, fluctuating water levels, and minimal sediment reworking (e.g., point bar and delta plain). In contrast, virtually no infiltrated clay is emplaced in environments characterized by low suspended sediment concentration and continuous reworking (e.g., beach and tidal delta).

Diagenesis of saline pan halite; comparison of petrographic features of modern, Quaternary and Permian halites

Abstract: Petrographic studies of modern saline pan halites (Saline Valley, CA; Salina Omotepec, Baja California, Mexico) and Quaternary shallow-buried (0-200 m) halites (Saline Valley, CA; Bristol Dry Lake, CA; Searles Lake, CA; Qarhan saline pan, Qaidam Basin, China; Lake Uyuni, Bolivia) show that the diagenetic modification of halite begins contemporaneously with deposition, is most intense within the upper few meters of burial, and is essentially complete within the first 45 m of burial. Halite crusts from modern saline pans that have undergone repeated episodes of flooding, evaporative concentration, and desiccation contain abundant syndepositional diagenetic features. These "mature" modern halites are dominated by dissolution textures and fabrics (formed during flood stages) and cementati n textures (formed during desiccation stages). Interlayered mud beds contain varying amounts of displacive halite crystals. At shallow burial depths, halites retain many textural features of "mature" modern saline pan halite. Halites below the first few meters are no longer susceptible to dissolution from floodwaters but continue to be cemented by clear halite. Within the first 10 m of burial, cementation reduces the porosity of halite crusts to less than 10%. The remaining pore spaces are completely filled by burial depths of approximately 45 m. Displacive growth of halite in muds continues at shallow-burial depths and is probably limited to the first few tens of meters of burial depth. The mechanisms for cementation and displacive growth of halite at shallow-burial depths probably include (1) evaporative concentration of groundwater brines and (2) cooling of surface brines when they sink below the sedime t surface. Undeformed halites from the Permian Salado and Rustler Formations of New Mexico contain delicate syndepositional textures and abundant clear halite cements that are comparable to those observed in modern saline pan halites and shallow-buried halites. The Permian halites are interpreted to have undergone a depositional and early diagenetic history similar to the modern and Quaternary analogs. Complete cementation of saline pan halites at shallow burial depths has important implications for the origin of saline formation waters in sedimentary basins. Parent evaporite brines may not be stored in the pores of halite rocks and later expelled during burial compaction if the rocks are cemented early, and tightly crystallized halite rocks may also impede the downward migration of dense syndepositional brines.

Pervasive Subsurface Dolomitization of the Nisku Formation in Central Alberta

Abstract: The Nisku Formation in the subsurface of central Alberta, Canada, contains pervasive dolomites that occur mainly as a replacement of reefal limestones. The degree of pervasive dolomitization increases downdip along a structural homocline from about 25 to 100 vol-% and associated replacive anhydrites increase from about 5 to 20 vol-%. Pervasive dolomites occur as two types. One type consists of almost non-porous and impermeable mosaics of cloudy crystals that have diameters of 5-300 microns. The other type has significant intercrystalline porosity and permeability and consists of cloudy and clear crystals ranging in size from 100-500 microns. Cathodoluminescence zonation is rare, but modulated structures observed with TEM are common. Both types of dolomite have strong XRD ordering refl ctions and are nearly stoichiometric. Sr-concentrations (25-125 ppm) increase and Mn-concentrations (20-400 ppm) decrease upwards in several cores. The boron contents of both dolomite types (20-50 ppm) are enriched relative to other diagenetic phases (< 18 ppm). The Sr-isotope ratios vary from 0.7089-0.7107 and are enriched relative to Devonian and younger seawater. Oxygen and carbon isotope ratios cluster around -5.8 permil PDB and +2.0 permil PDB, respectively, and are similar to those of remnant calcites. From the combination of stratigraphic, petrographic, petrophysical, and geochemical data, it is inferred that both types of matrix dolomite formed contemporaneously, from the same fluids, in a flow regime with predominantly upward fluid movement, at temperatures between about 40-50°C, and at burial depths of about 300-1,000 m. Most anhydrites formed originally as gypsum during the advanced stages and after matrix dolomitization, and later recrystallized to the anhydrites now present. Based on all accumulated data and on mass balance calculations, at least two types of subsurface flow could have caused pervasive dolomitization and gypsification/anhydritization: 1) expulsion of burial compaction water, i.e., chemically modified seawater, with or without additional contributions of M -bearing formation waters from deeper parts of the basin (e.g., Middle Devonian brines); and 2) thermal convection of formation fluids originally underlying and/or overlying the Nisku.

Sea ice sediment entrainment and rafting in the Arctic

Abstract: Large areas of first-year sea ice containing disseminated loads of silt- and clay-sized materials are common off the northern coast of Alaska. Sand and coaser material is also found in this ice in the form of distinct masses of sediment-laden ice. Sediment is entrained into the sea-ice cover during fall storms when frazil, or small discs of ice in suspension, is actively forming in the supercooled water column. At this time, sea ice is unconsolidated and very mobile, making long-range ice rafting possible. Sediment-rich ice may be advected off the shelf and, upon melting, deposit material in the deep Arctic basin. The amount of sediment incorporated into the ice cover varies considerably from year to year, depending on the location and severity of storms accompanying freezeup. In 1978 the seasonal ice cover in an area off northern Alaska carried 16 times as much sediment as the total annual suspended sediment input from adjacent coastal rivers. Sea-ice sediment transport is important in the overall sediment budget in northern polar regions, particularly the transport of fine-grained material, but entrainment mechanisms are poorly understood.

Petrogenesis of Cenozoic, Temperate Water Calcarenites, South Australia: A Model for Meteoric/Shallow Burial Diagenesis of Shallow Water Calcite Sediments

Abstract: The Gambler Limestone is one of several extensive, shallow-water shelf carbonates of Eocene to Miocene age exposed along the southern margin of Australia. It is a muddy to grainy bryozoan calcarenite, with accessory benthonic foraminifers and echinoids. The sediments, originally composed almost entirely of calcite or Mg-calcite, have been in vadose and phreatic environments for over 10 Ma, yet are virtually unlithified. The only cements of any consequence are epitaxial on echinoids. Numerous karst features, dolines, caves, speleothems and surface karren attest to prolonged residence in the meteoric zone. The Gambler Limestone is presently one of the best fresh water aquuifers in Australia. Most flow occurs through intergranular pores in sediments with over 30% porosity. Cementation is by minor intergranular pressure solution which has developed under overburden of less than 100 m. The overlying Naracoorte Limestone (Miocene), a calcarenite of warmer water aspect, contains numerous aragonite molds and is well-cemented. We propose that such cool water limestones are a better model for the meteoric diagenesis of calcite sediments of all ages than are aragonite-rich tropical sediments. It is probable that many similar early and middle Paleozoic calcite limestones may have been in the meteoric zone for prolonged periods yet contain little or no petrographic or geochemical record of such exposure.

Petrography and composition of authigenic feldspars, Oligocene Frio Formation, South Texas

Abstract: Detrital feldspar assemblages (1 to 4 km burial depth) in Oligocene sandstones from South Texas are extensively altered by dissolution. In some samples, cementation in secondary pores, principally by albite but also by authigenic K-feldspar and other minerals, occurs subsequent to the dissolution and can lead to wholesale replacement textures. Solid-state diffusion of cations has not been important in the replacement process. Information from conventional light microscopy, SEM examination, and back-scattered electron imaging suggests that relative rates of dissolution versus replacement varied widely among different detrital grains through time, even on the scale of a thin section. Dissolution of detrital feldspars and precipitation of relatively pure albite and K-feldspar take place ver a large depth interval and thus can be markedly separated in time. Vacuolized albite occurring as irregular veins within detrital grains is associated with little visible secondary porosity and forms when precipitation is rapid relative to dissolution of the detrital grain. More rapid dissolution results in formation of larger-scale intragranular secondary pores, commonly filled by subsequent precipitation of clear albite within the former grain volume. Electron microprobe analyses show that the composition of vacuolized albite and authigenic K-feldspar deviate somewhat from ideal compositions presumed typical for authigenic feldspars. Regardless of their cause, these subtle variations in composition have important ramifications for basin-scale mass balance calculations requiring accurate estimates of the volume of authigenic feldspar.

Decelerating Holocene sea-level rise and its influence on Southwest Florida coastal evolution; a transgressive/regressive stratigraphy

Abstract: The Ten Thousand Islands (TTI) are a myriad of low-relief mangrove islands that lie along the low energy, subtropical southwest Florida coast. The region has been subjected to a relative rise in Holocene sea level, which has continuously decelerated to its present rate. Data derived from surface and subsurface sampling indicate that the Holocene sediment package of the TTI area consists of two sediment sequences. The lower sequence is transgressive and was generated as coastal salt marsh and/or terrestrial environments were submerged and replaced by a shallow coastal marine setting. The upper sediment sequence consists primarily of 1) biogenic shallowing upwards sequences or 2) thickened mangrove peat layers, reflecting island emergence and shoreline stabilization, respectively. Island emergence compartmentalized the area, further reducing wave and current energy and promoting the infilling of the protected bays through deposition of organic-rich shelly mudstone and wackestone. Based on coastal stratigraphy and 14C dates, the formation of his transgressive/regressive sediment sequence is directly related to changing rates of Holocene sea-level rise, reported to have occurred between 3,500 and 3,200 YBP. Continued regressive sedimentation could eventually generate a 5-10 m thick transgressive/regressive sediment couplet, with an aerial extent of over 300 km2, in roughly 10,000 years. This externally forced sediment couplet compares remarkably well to individual small scale transgressive/regressive cycles which repeat tens to hundreds of times throughout the geologic record (see James 1984). The results of this study thus support the allocyclic mechanism as a viable working hypothesis for the generation of these small scale rock cycles.

Cathodoluminescence in diagenetic calcites; the roles of Fe and Mn as deduced from electron probe and spectrophotometric measurements

Abstract: Quantitative spectrophotometric measurements of cathodoluminescence (CL) intensities of calcite cements from the Mississippian Lake Valley Formation are compared with Mn and Fe electron microprobe analyses of the same calcites. CL intensities of these calcites show excellent positive correlation with Mn concentrations at approximately constant Fe/Mn ratio and within narrow ranges of Fe contents (r = 0.76 to 0.97). Our data suggest that Fe/Mn ratios may control the maximum CL intensity of calcite. Below this maximum, at constant Fe/Mn ratios, CL intensity can vary significantly and is controlled mainly by absolute concentrations of Mn and Fe. CL intensities are therefore a function both of absolute concentrations of Mn and Fe and of Fe/Mn ratios. The data further suggest that CL intensities in calcites with low Fe concentrations (< 1,000 ppm) are more sensitive to Mn activation than to Fe quenching. CL spectra on the calcites show no evidence of rare earth element activators. Visual estimates of CL intensities of these calcite cements compare moderately well with intensities measured with the spectrophotometer. Spectrophotometrically measured CL intensities of calcites in any single visually estimated category (bright, moderate, dull, non-luminescent) overlap with those from adjacent categories. Thus spectrophotometric measurements are distinctly superior to the visual estimates. Our data indicate that calcites with more than about 100 ppm Mn will luminesce even in the presence of moderate amounts of Fe. In the absence of detectable Fe, calcites with about 150 to 650 ppm Mn have moderate CL, and those with Mn contents greater than about 700 ppm have bright CL. In the presence of detectable Fe, the amount of Mn required to impart moderate or bright CL increases with Fe content. Furthermore, Fe does not extinguish Mn-activated CL up to about 10,000 ppm Fe, the maximum of our data. These results are in general agreement with the electron microprobe study of Grover and Read (1983), in partial agreement with the data of Frank et al. (1982), and in poor agreement with the data of Fairchild (1983). Considering the poor precision of the electron probe at low element concentrations (< 100 ppm), our data are reasonably consistent with the published ion microprobe and atomic absorption data (Mason 1987; ten Have and Heijnen 1985; Richter and Zinkernagel 1981) which imply that Mn concentrations in the 30 to 50 ppm range are sufficient to activate cathodoluminescence in calcites.

Numerical Assessment of Dissolution Versus Replacement in the Subsurface Destruction of Detrital Feldspars, Oligocene Frio Formation, South Texas

Abstract: Dissolution is the main process responsible for modifying detrital feldspar assemblages in the Oligocene Frio Formation of South Texas, accounting for up to 80% of feldspar modification in completely altered assemblages. This finding is supported through two independent calculations, one based on an estimate of initial feldspar content of the sandstones, the other on estimated initial composition of the feldspars themselves. Volumetric loss of detrital feldspar on the observed scale over such a short time span has major implications for provenance interpretations and for understanding the basin-wide mass balance of several major diagenetic components. The wide variation of feldspar content and composition at any given depth (temperature) attests to the premier significance of fluid co position and flux in driving the reactions that bring about dissolution of feldspars in these rocks.

Provenance Characteristics of Detrital Opaque Fe-Ti Oxide Minerals

Abstract: We have investigated petrogenetic characteristics of detrital opaque Fe-Ti oxide minerals (DOPQ) in sand samples collected from Holocene streams draining exclusively igneous and metamorphic source rocks in the Rocky Mountain region under a semi-arid climate, and from the Appalachian Mountains under a humid climate. Our sampling strategy allows us to compare and contrast the properties of detrital mineral grains not only from igneous and metamorphic source rocks, but also those from dissected-arc and recycled-orogen provenances. Reflected-light petrography including extensive modal analysis and electron probe microanalysis of polished grain mounts were the two principal methods of data gathering. Between 25% and 50% of all DOPQ show oxidation exsolution and/or other lamellar intergrowth textures, i.e., they are polymineralic grains. Lamellar intergrowth texture is present in about twice as many grains derived from igneous source rocks as from metamorphic source rocks. Lamellae thinner than 2 µm are predominant in grains derived from dissected-arc provenance, whereas lamellae thicker than 10 µm are predominant in grains derived from recycled-orogen provenance. Exsolution lamellae along {111} planes of magnetite (three directions) and presumably along {0001} planes of ilmenite (one direction) characterize dissected-are and recycled-orogen provenance, respectively. Detrital ilmenites derived from the igneous source rocks studied show a wide range of TiO2 co tent with a mode around 47%, whereas those from the metamorphic source rocks show a tight cluster around 52% TiO2 content. DOPQ derived from metamorphic source rocks contain < 0.5% MgO, whereas those from igneous rocks may contain up to 1.2% MgO. Both MgO and MnO are low, 0.3% and 2.5%, respectively, in DOPQ derived from dissected-arc provenance, but may be relatively high, up to 1.2% and 8.0%, respectively, in grains derived from recycled-orogen provenance. Whereas no single character of DOPQ may be diagnostic of provenance, discriminant function analysis of the complete set of data show that collectively they are diagnostic in 95% to 100% of cases. We recommend the use of 1) presence of exsolution texture, 2) minimum lamellae widths and the number of crystallographic orientation of the lamellae, and 3) chemical composition including minor elements of DOPQ as important properties for provenance identification. This should supplement and refine existing criteria for provenance determination.

Multistage Dolomitization in Rainbow Buildups, Middle Devonian Keg River Formation, Alberta, Canada

Abstract: More than 80 partially to completely dolomitized buildups of Middle Devonian Keg River Formation occur in the Rainbow sub-basin of northwestern Alberta. In six buildups (A, B, E, F, G, and Tehze), four petrographic types of dolomites are identified: gray, fine crystalline dolomite; floating dolomite rhombs/patches; matrix dolomite; and saddle dolomite. The petrographic and geochemical data indicate that these dolomites probably formed in three principle stages of dolomitization during progressive burial. Gray, fine crystalline dolomite (less than 1% of total dolomite by volume) is interpreted as penecontemporaneously dolomitized lime muds by either normal marine waters or evaporitic brines during early exposure. The following evidence suggests that this dolomite probably formed early: 1) it usually occurs in fractures and breccias presumably related to the early exposure of the buildups; 2) it is absent in rugs and molds that formed in the subsurface environment; and 3) locally, clasts of gray, fine crystalline dolomite are embedded in marine limestones. Rhomb/patch dolomite is the major type of dolomite in dolomitic limestones and is widespread throughout the limestone buildups. Matrix dolomite is fabric destructive and is most abundant in the dolostone buildups. Both these dolomites probably formed during shallow to intermediate burial because they 1) postdate stylolites, 2) are widespread throughout the buildups and crosscut various facies, and 3) have light oxygen isotope compositions and low Sr but high Fe and Mn concentrations. Dolomitizing fluids for dolomite rhombs/patches and matrix dolomites were possibly derived from 1) mechanical compaction, 2) dehydration of the adjacent Muskeg gypsum at relatively shallow burial depths, and 3) intermediate-depth basinal fluids conducted updip along porous and permeable conduits. During D vonian and carly Carboniferous times, compaction fluids were probably responsible for the early dolomite rhombs and patches. However, the main phase of matrix dolomitization took place later, probably during Late Mississippian to Jurassic time when the east side of the basin was intermittently tilted and uplifted, causing updip flow of intermediate-depth basinal brines. Saddle dolomite occurs as late-stage vug and fracture fillings. It is most depleted in 18O but most enriched in Mn and Fe, suggesting precipitation during deeper burial from warm basinal fluids. These fluids were expelled during Upper Cretaceous sedimentary and tectonic loading of the western side of the basin and then moved up the east side of the basin along existing platform, fault, and fracture conduits. The source of the Mg is uncertain. Some or most of the Mg may have resulted from chemical compaction of the earlier matrix dolomite.

Diagenesis of Devonian Reefal Carbonates in the Oscar Range, Canning Basin, Western Australia

Abstract: The Oscar Range, part of the extensive Devonian reef complex of the northern Canning Basin, is a large atoll (80 10 km) in which Frasnian and Famennian (Late Devonian) reefs grew around an emergent island of Precambrian metasedimentary rocks. Many sediments in the Oscar Range had high depositional porosities, but this porosity was largely occluded by diagenetic alteration. Diagenesis included marine cementation, dolomitization, pressure solution, burial cementation, and karstification. Field mapping, petrography, cathodoluminescence, and carbon and oxygen isotope analyses have been used to document six major diagenetic episodes that affected Oscar Range sediments. Earliest cementation occurred in the marine environment, where reef-margin and associated subfacies developed abundant non-luminescing radiaxial fibrous and microcrystalline calcite cements. Based on convergent covariant isotopic trends, marine cements had a characteristic Late Devonian oxygen signature of -4.5 (±0.5) (PDB) and a carbon signature of +2.0 (±0.5) (PDB). In well-cemented parts of the reef complex, synsedimentary neptunian fissures formed and were filled with cement and sediment. Cements in these fissures have isotopic ratios that are essentially identical to the values determined for marine cements, thereby suggesting that the fissures acted as conduits f r seawater flow and cementation early in the history of the reef complexes. Marine diagenesis was followed by marine-burial and possible meteoric-phreatic cementation, and dolomitization. Marine cements are overgrown by non-luminescing scalenohedral cements that grade into banded bright- and non-luminescing blocky calcite cements that are common on the platform but are missing in many reef-margin and marginal-slope deposits. Banded luminescence may represent fluctuating reducing and oxidizing conditions in pore waters across the intermittently emergent platform. The reef complexes, however, were generally drowned reefs with little chance for prolonged meteoric exposure. This sedimentologic evidence suggests that banded-luminescing cements may have formed in the marine-burial environment. The next cement generation consists of non-luminescing, non-ferroan bloc y calcites that occur throughout the reef complex. Stratigraphic relationships combined with isotopic results (18O = -6.0 to -9.0; 13C = +2.0) suggest that this spar is a burial cement resulting from Late Devonian through Early Carboniferous burial. The reef complex was exhumed and eroded in the Late Carboniferous. Dedolomite, dolo-moldic porosity, pisolitic calcrete crusts, iron oxide stained blocky cements, and deep (> 200 m) sinkholes formed at the unconformity surface. Non-ferroan, irregularly banded, moderate- to bright-luminescing blocky calcite precipitated throughout the reef complex. Unconformity-related cements, calcrete crusts, and dedolomites have distinctive isotopic signatures that show little variation in oxygen, -7.0 to -9.0, but have a broad range of carbon values, +1.5 to -8.0. Highly negative carbon values are probably related to the interaction of meteoric fluids with isotopically light soil gas at the er sion surface. In post-Carboniferous time, most of the remaining porosity was filled with non-ferroan, bright-luminescing blocky calcite that formed by pressure solution/reprecipitation during burial. Isotopic analyses (18O = -8.0 to -12.0; 13C = +1.5) support a burial origin for this generation of cements. These cements fill primary porosity, tension gashes along stylolites, and hairline fractures along which hematite is leached. A final generation of ferroan, homogeneous, dull- to non-luminescing blocky calcite cement formed in a late-burial event that, according to isotopic evidence (18O = -10.5 to -15.0; 13C = +2.0 to -9.0), was probably related to movement of basinal brines through the reef complexes.

Observed Parameters for Turbidity-Current Flow in Channels, Reserve Fan, Lake Superior

Abstract: Fine-grained tailings discharged from a taconite-ore processing operation near the shore of Lake Superior produced turbidity currents that transported the sediment from a small delta into deep water at Silver Bay, Minnesota. Deposition over nearly 20 years produced a sublacustrine fan with two prominent channels. During 1972 and 1973, a current meter anchored 5 m above the lake floor adjacent to one of the channels recorded episodic turbidity-current flow events lasting as long as two weeks. To understand flow parameters for turbidity currents better, a short-term experiment within a channel on Reserve Fan in 1975 measured those variables not previously directly observed for channelized turbidity currents: flow thickness, flow density, and concurrent velocity. The observed flow thickn ss, approximately 16 m, is nearly four times the channel depth. Calculations using the average flow speeds (8 to 12 cm/sec) and the dilute concentration of the flow as measured during the experiment yield a value for the drag coefficient that is in remarkable agreement with estimated values commonly used for deriving speeds of turbidity currents using dimensions of submarine channels and properties of the sediments.

Significance of Loessite in the Maroon Formation (Middle Pennsylvanian to Lower Permian), Eagle Basin, Northwest Colorado

Abstract: Quaternary loess deposits are widespread on the earth's surface, yet pre-Quaternary loess deposits have rarely been reported. The Maroon Formation (Middle Pennsylvanian to Lower Permian) of the Eagle Basin, northwest Colorado, includes a siltstone-dominated facies interpreted as loessite (lithified loess) along its downwind basin margin. The section of inferred loessite in the Maroon Formation is locally at least 490 m thick and consists in large part of structureless and nearly structureless beds of homogeneous sandy siltstone. Bed contacts are generally planar to undulatory and are either horizontal or are characterized by gentle relief. Loessite beds are separated by common claystone drapes and weakly developed paleosols, and by rare pond deposits, channel deposits, and eolian-ripp e-laminated deposits. The loess interpretation is based on 1) the homogeneity and dominance of the sandy silt grain-size; 2) the relative lack of primary sedimentary structures; 3) the gentle character of most bedding contacts and the common mantling of irregular depositional topography; 4) the inferred paleogeographic setting; and 5) the absence of suitable alternative interpretations. The loessite grades laterally into mixed fluvial-eolian deposits of the Maroon Formation in the main part of Eagle Basin, which served as the loessite sediment source. Deposition of the Maroon Formation was probably strongly affected by cyclic climatic changes synchronous with fluctuations in late Paleozoic continental ice sheets. The paleogeography and paleoclimatology of the Maroon Formation depositional system are not unique, suggesting that there are probably many other ancient loessites that have gone unrecognized.

Geometry and Depositional Sequences of the Mississippi Canyon, Gulf of Mexico

Abstract: The Mississippi Canyon is one of several large canyon systems which have developed seaward of large rivers around the shelf margin of the Northern Gulf of Mexico. High-resolution geophysical and borehole data are combined to re-evaluate canyon geometry and sedimentology. Incision of the canyon appears to have taken place prior to 30,000 years BP. Canyon widening by slope failure and mass-movement processes resulted in arcuate re-entrants containing slumped debris separated by residual knolls. Much of the sediment associated with canyon formation and widening was transported down-slope, by-passing the upper canyon. Thick, laminated pro-deltaic sediments, greater than 30,000 years old, comprise the basal canyon fill. Rapid deposition along the canyon axis began about 19,000 years BP, ap arently associated with down-canyon mudflows from a nearby lowstand delta. As canyon filling progressed, mass-movement processes declined, resulting in interbedded pro-deltaic and debris flow sediments. The sediment fill was deposited in a complex cut and fill sequence suggesting a much older canyon than previously thought. Deltaic sedimentation retreated northward away from the canyon at about 7,500 years BP, and since that time a Holocene pelagic drape has been deposited.

Potassic Diagenesis of Cambrian Sandstones and Precambrian Granitic Basement in UPH-3 Deep Hole, Upper Mississippi Valley, U.S.A.

Abstract: The Mt. Simon Sandstone (Cambrian) in core from Hole UPH-3 in northwestern Illinois contains three distinct diagenetic zones. A zone with K-feldspar overgrowths characterizes the upper 51 m of Mt. Simon Sandstone. An intermediate, underlying sericite zone is about 126 m thick, and a lowermost feldspar zone is about 87 m thick. Authigenic K-feldspar in the Mt. Simon Sandstone occurs in several modes: overgrowths, rhombs, penetration-twinned crystals, chert-like aggregates surrounded by coarser K-feldspar, mosaics of anhedral to euhedral crystals, and as replacements of microcline. Sericite (I/S) occurs as pore-filling aggregates of minute flakes. Quartz overgrowths are common throughout these three diagenetic zones. K-feldspar was the first to form, closely followed by quartz, then by ericite, and lastly, possibly by a second addition of quartz overgrowth. The upper 6.1 m of Precambrian granite gneiss basement exhibits similar potassic alteration to the overlying Mt. Simon Sandstone. The uppermost 2.5 m of the basement rock is a probable weathering profile in which pedogenic clays were replaced by sericite (I/S) and chlorite. Sericite in the basement rock was likely formed during the same episode as in the Mt. Simon Sandstone, based on 18O values. Authigenic K-feldspar occurs as pore-fillings, overgrowths, replacements of microcline, and intergrown with phyllosilicate. Below the profile it partially replaces plagioclase. Similar potassic alteration of the Mt. Simon Sandstone of UPH-3 occurs in cores of both UPH-1 and 2, indicating a lateral extent of about 7 km. A K/Ar date of 394.6 ± 6 Ma was obtained from authigenic K-feldspar of the basal Mt. Simon Sandstone. This Early Devonian age is within the range of ages from authigenic K-feldspar in Ordovician tuffs of the Upper Mississippi Valley, suggesting that the K-feldspar is the product of a single, regional diagenetic event. K/Ar dates of 271.3 ± 4.6 Ma, 254.4 ± 4.6 Ma, and 214.1 ± 4.6 Ma were obtained from authigenic sericite of the Mt. Simon Sandstone. The Devonian age for the K-feldspar and Permian ages for the sericite are nearly coeval with times of uplift of the Findlay and Pascola Arches respectively. These diagenet c episodes are attributed to regional migration of basinal brines caused by groundwater recharge on these uplifted arches. Potassium for both the K-feldspar and sericite possibly came from the Michigan and Illinois basins, respectively. Devonian and Permian dates for potassic diagenesis possibly coincide with two episodes of Pb-leaching of basement rocks at 400 Ma and 260 Ma. Thus the potassic fluids may have leached Pb and Zn from Precambrian basement, particulary paleosols, and then formed Upper Mississippi Valley Pb-Zn deposits.

delta 18 and delta 13 C variations in Late Devonian marine cements from the Golden Spike and Nevis reefs, Alberta, Canada

Abstract: For geologists and geochemists concerned about the chemical and thermal evolution of oceans, deciphering primary marine 13C and 18O values from ancient marine carbonates can provide useful data for the physical and chemical modelling of ancient oceans. Although abiotic aragonite and magnesian calcite marine cements have commonly been overlooked because of their susceptibility to diagenetic alteration, petrographic and chemical analysis of radiaxial fibrous calcite from two Upper Devonian (Frasnian) pinnacle reefs (Golden Spike and Nevis) of central Alberta indicates that these magnesium-rich cements preserve original marine 18O and 13C values. In the pursuit of primary marine isotopic compositions it is necessary to characterize the isotopic compositions of diagenetic cement phases. Subaerial exposure and meteoric diagenesis of the interior of the Golden Spike reef is recorded by vadose cements, caliche, and clear, non-ferroan, meteoric calcite spar with distinctive black and yellow cathodoluminescence (CL) zones. 18O-13C crossplots of caliche and meteoric phreatic cement data form an "inverted J-curve" typical of rock-water reactions in meteoric systems. A late phase of calcite spar with variable inclusion density and Fe2+ content and diffusely-zoned red-orange CL is associated with late fractures and stylolites, and it precipitated during subsequent burial of the reef. Unaltered centers of radiaxial fibrous cement crystals are nonluminescent, inclusion-free, and enriched in magnesium, while diagenetically altered crystal terminations, inter-crystalline boundaries, and crystal substrates are brightly luminescent, inclusion-rich (microdolomite and fluid inclusions), low-Mg calcite. Altered cloudy marine cements have variable 18O and invariant 13C values which define a trend that diverges from isotopically heavier unaltered marine cements. The isotopic compositions of inclusionrich marine cement are coincident with those of meteoric phreatic spars. Although burial spars are present in all samples, petrographic and chemical data suggest that portions of the marine cement crystals were altered by meteoric fluids. The 18O values of unaltered radiaxial fibrous calcite are similar (invariant) throughout both reefs, although marine cements from older (deep) parts of both reefs are enriched in 13C relative to younger (shallow) parts. This records temporal changes in the 13C value of marine bicarbonate within the Alberta Basin. Variability of 18O and 13C values of unaltered marine cements is very small (± 0.5 for a given suite of samples), making these cements ideal for h gh time resolution studies of the isotopic evolution of ancient oceans. The presence of well preserved magnesium-rich calcite from Late Devonian reefal sequences suggests that marine cements throughout the Phanerozoic should be examined more carefully, as they may preserve primary isotopic signatures. Independent confirmation of primary isotopic compositions is found in similar values for marine cements from Middle Frasnian reefs of the Canning Basin (Australia) and the Dinant Synelinorium (Belgium). The 18O values for the well preserved marine cements of this study are considerably lower than modern marine carbonate values (by 3 to 4). Detailed chemical and petrographic analysis indicates that diagenetic alteration is not the cause of these low 18O values. In addition, such a difference cannot be fully explained by warmer Late Devonian oceans. These data suggest that Late Devonian seawater had a 18O value lower than modern oceans. This in turn, suggests that the balance of low temperature silicate weathering and high temperature seawater-basalt exchange reactions must have been different from today's.

Tidal current ridges in the southwestern Yellow Sea

Abstract: Large tidal current ridges (tidal sand banks) in the southwestern Yellow Sea are located offshore of northern Jiangsu Province, China. These ridges radiate to a depth of 20-30 m. The dimensions of the ridge system are about 200 km long and 90 km wide. Their morphology indicates the strong influence of tidal currents. The ridges are composed of well-sorted fine sands (size range is 2-4 ). The grain size distributions of the sands show that the sands are transported and deposited by tidal currents. The ridge system formed on the abandoned Yellow River and ancient Yangtze River deltas has an abundant sand supply from the former deltas. Of particular importance is the existence of two tidal wave systems in the Yellow Sea: the rotary ti al wave system in the southern Yellow Sea and the Pacific progressive tidal wave system propagated into the southern ridge area of the southwestern Yellow Sea. Under the interaction of the two tidal wave systems, the huge tidal current system in the southwestern Yellow Sea has formed.

Sediments and Sedimentary Sequences on a Modern Macrotidal Flat, Inchon, Korea

Abstract: Enormous, unbarred tidal flats fringe the west coast of Korea. Near Inchon, where spring tides range between 8 and 9 m, the intertidal expanse is more than 4 km wide. This low-energy regime results in three broadly intergradationai modern subfacies: l) an intensely bioturbated inner flat of slightly sandy mud; 2) a wavy-bedded middle flat of clayey sandy silt; and 3) a ripple-laminated outer flat of bioturbated sandy silt to silty sand. The mid-flat region is less distinctive sedimentologically than the mixed-flat subfacies of North Sea tidal flats, and flaser and lenticular bedding are rare. Well-developed intertidal drainage networks and landward salt marshes are absent. Vibracores reveal two additional sequences underneath the modern sequence, their contacts defined by scour horizons and shell concentrations. The basal sequence is characterized by irregularly oxidized, intensely-to-totally bioturbated argillaceous sediment with scattered wavy beds and abundant in-situ plant roots. These deposits suggest a transition from a shallow subtidal or low intertidal environment to a salt marsh developed in a protected intracoastal setting. The overlying intermediate sequence also consists principally of bioturbated fine sediment with scattered wavy beds, but plant roots are absent. Stratigraphic distributions of mollusk shells and other features ally this sequence with landward parts of the modern sequence.

Storm-dominated Sedimentation on the Inner Shelf of the Canadian Beaufort Sea

Abstract: The inner shelf of the Canadian Beaufort Sea is characterized by a succession of near-surface fine-grained facies changes. Seaward of shoreface sands and silts, these facies are: (1) thin-bedded silt and clay couplets (

The Effects of Neogene Thrusting on Deposition in the Bermejo Basin, Argentina

Abstract: In the Bermejo basin of western Argentina, semi-arid, sandy, low-gradient sedimentation has accompanied deformation of the Frontal Cordillera and Precordillera since the mid-Miocene. Utilizing the precise geochronology established for strata at Sierra de Huaco, we document an immediate basinward migration of facies coincident with thrusting in the central Precordillera (8.7 Ma). An earlier shift in environment (10.3 Ma) is related to a change in drainage patterns produced by more distant deformation in the western Precordillera. Evidence from Sierra de Huaco suggests that depositional environments are largely independent of the net subsidence rate. Sedimentologic features observed at Sierra de Huaco bear directly on the nature and scale of the processes that operate in sandy, ephemeral settings: 1) lateral accretion was the dominant process within an ephemeral braidplain (the Jarillal Formation); 2) fining-upward cycles in sandflat deposits were a consequence of the dynamics of unconfined flows coupled with lobe switching on alluvial fans (the Huachipampa and Quebrada del Cura Formations); and 3) unconfined flow deposition in the Bermejo basin occurred (and still occurs) over distances on the order of 50 km from point sources at alluvial fan heads.

Experiments on the thickness of beds deposited by turbidity currents

Abstract: Turbidites were produced experimentally by releasing suspensions of spherical beads from a lock into a 6 m long horizontal channel. Flow properties were varied by changing initial flow geometry, and sediment size, sorting, and concentration in the initial suspension. Initial flow geometry was varied using two lengths of lock (0.29 and 0.59 m) and four depths (0.15, 0.2, 0.3 and 0.4 m). To vary the properties of the initial suspension, three nearly uniform sizes (2.0, 2.7, and 3.6 phi) of glass beads and one (2.5 phi) of plastic beads (specific gravity 1.52) were used, and all experiments were run at concentrations of 20% and 40% by volume. The effect of poor sorting was investigated by mixing the coarse and fine glass beads to produce a poorly sorted sediment with a median grain size qual to that of the intermediate-size glass beads. Bed thickness was almost uniform near the source except for beds deposited from poorly sorted sediment suspensions, which tended to be wedge-shaped. High concentration produced more uniform bed thickness, and (for equal volume of sediment) shorter, thicker beds. Increases in lock height and length increased bed thickness almost equally. If all other variables were held constant, bed thickness was directly proportional to grain size. The dimensionless ratio of the square of bed thickness divided by the volume of suspension (per unit width) was found to be directly proportional to the dimensionless ratio of settling velocity divided by the speed of the head of the current. The 'constant' in the regression equation appears to depend mainly on the sediment concentration. The experimentally determined equation gives plausible values for turbidity current velocities when extrapolated to scales over a million times larger than those of the experiments.