Broad Shallow Shelf Research Articles

Boat effects on lake water clarity and the efficacy of a no-wake zone policy

Boating activity in shallow waters of lakes can lead to sediment resuspension and potential nutrient release, affecting water quality parameters. No-wake zones, to minimize boating-induced effects, are often defined based on distance from shore. This study assessed the effects of boating on shallow waters and the efficacy of an existing no-wake zone policy in a midsize lake. Two experiments were conducted: (1) Turbidity and pressure data were collected along a broad shallow shelf on the southern shore of Lake Tahoe in the summer of 2019 and (2) turbidity and pressure observations were conducted while boats traveled at speeds between 8 kmh−1 (5 mph) and 40 kmh−1 over a water depth of ≈ 2.5 m. Results showed that boat-induced sediment resuspension occurred at speeds exceeding 8 kmh−1, with turbidity increases of up to ∼7 NTU. Boat-induced filtered pressure signal was a maximum of 40% of the total daily wave activity, with peaks observed between 10:00 and 12:00. Nevertheless, during the summer and fall of 2019, winds were the dominant driver of sediment resuspension, contributing around 80% of the total wavefield. An evaluation of the current no-wake zone policy at Lake Tahoe suggested that an 8 kmh−1 (5 mph) speed limit is suitable for preventing wake effects. However, the policy falls short in preventing sediment resuspension in the northwestern and southern shores of Lake Tahoe. This calls for the consideration of multiple variables, such as lakebed sediment type and water depth, when defining no-wake zone policies to effectively mitigate boating-induced effects.

Holocene paleoenvironmental changes in mud area southwest off Cheju Island, East China Sea: Evidence from benthic foraminiferal assemblages and stable isotope records

A sediment core retrieved from the mud area southwest off Cheju Island (MASCI) was analysed for benthic foraminifera, grain size, and stable isotopes and constrained by AMS14C dating. Holocene paleoenvironmental changes in this broad shallow shelf were investigated, with special focus on the evolution of the MASCI and its related ocean eddy system. The results indicate that the East China Sea shelf has undergone four major environmental evolution stages during the Holocene. Prior to 9.8 cal. Kyr BP, the benthic foraminiferal assemblages were dominated by low salinity, shallow-water species [Ammonia beccarii (Linnaeus), Elphidium advenum (Cushman), and Cribrononion subincertum (Asano)], reflecting a near-coast-deposit environment. A noticeable transition of the benthic foraminifera, from an A. beccarii dominated to Ammonia compressiuscula (Brady) dominated assemblage, taking place at approximately 9.8 cal. Kyr BP together with a contemporaneous increase in δ18O and δ13C, provides evidence that a rapid rise in sea level occurred at this time in the East China Shelf. A gradual increase in δ18O between 9.8 and 6 cal. Kyr BP indicates a steady strengthening influence of open ocean water on the area under study. A sudden increase in the abundance of benthic foraminifera along with a decrease in the coarse fraction imply that an early eddy circulation system was established since almost 6 cal. Kyr BP in the MASCI. This eddy circulation system was quite weak but gradually strengthened between 6 and 3 cal. Kyr BP, possibly due to the intrusion of the Yellow Sea Warm Current (YSWC). A more stable mud deposition period, characterised by a high sedimentation rate of fine-grained sediment and lighter value of δ13C, starting at approximately 3 cal. Kyr BP, indicates that the MASCI was formed mainly during the late Holocene. We believe that the strengthened YSWC since 3 cal. Kyr BP might increase the trap efficiency of the eddy system, thereby accounting for the high sedimentation rate of mud deposits during this period.

Provenance of lower Cambrian quartz arenite on southwestern Baltica: Weathering versus recycling

ABSTRACTLower Cambrian quartz arenite deposits have a world-wide occurrence and are also present on Baltica. However, the processes influencing the deposits from source to sink have not been accordingly investigated. The provenance of these deposits is crucial for the understanding of the extent of chemical weathering in the cratonic drainage area and reworking at the broad shallow shelves of Baltica during early Cambrian time. Provenance analysis and study of weathering effects was done for lower Cambrian sandstone from southern Scandinavia, including southern Norway, southern Sweden, and Bornholm (Denmark). For the quartz-arenite sandstone of the Ringsaker Member and the Hardeberga Formation, predominantly moderately weathered felsic–intermediate plutonic and meta-igneous source terranes are suggested from negative Eu anomalies, high LaN/YbN, and low to moderate Ti/Nb ratios, as well as trace amounts of plutonic lithoclasts. Similarly, a felsic–intermediate igneous and metamorphic signature is indicated in the heavy-mineral assemblage and a dominance of dark cathodoluminescence of quartz in a special study of samples from Bornholm, thus suggesting a mixed provenance of local granitoid–orthogneissic Mesoproterozoic basement and distant sources from the Transscandinavian Igneous Belt for the Hardeberga Formation on Bornholm. High percentages of mostly rounded ultra-stable heavy-mineral grains, quartz, and extremely high SiO2/Al2O3 ratios indicate a compositionally and texturally mature sand that was subjected to extensive to moderate weathering following removal of detritus from the source area. The main controls on the alteration of framework composition and the heavy-mineral assemblage are interpreted as surface weathering, based on moderate Chemical Index of Alteration values and meteoric flushing, and reworking by waves, based on an estimated low amount of feldspar before burial. Thus, the maturation of lower Cambrian quartz arenite on southwestern Baltica can be ascribed to a combination of processes including weathering in the source-rock area, extensive reworking, and early diagenesis. This study also highlights the importance of understanding alteration processes affecting the deposits from source to sink, and to be careful to describe quartz-rich rocks as simply recycled deposits.

The Ability of Barotropic Models to Simulate Historical Mean Sea Level Changes from Coastal Tide Gauge Data

The nature of mean sea level variation over the global coastal ocean is considered based on 219 historical tide gauge records and three barotropic ocean circulation models forced by reanalysis surface air pressure and wind stress. The consistency of the models and their ability to reproduce the data are considered on nonseasonal timescales (seasonal cycles and linear trends removed) from bimonthly to multidecadal over 1900–2010. Models consistently simulate stronger sea level variability at higher latitude, higher frequency, between winters, and over broad shallow shelves and semi-enclosed marginal seas; standard deviations in modeled monthly sea level grow from 1–2 cm on average at low latitude (0°–30°) to 5–10 cm at high latitude (60°–90°), with larger values simulated over some shelf areas (e.g., North Sea). Models are more consistent over narrow shelf regions adjacent to deep basins and less consistent along the broad shallow continental shelf. On monthly timescales, discrepancies between models arise mostly from differences in model configuration (e.g., fine vs. coarse horizontal resolution), whereas model configuration and surface forcing (i.e., choice of atmospheric reanalysis) contribute comparably to model differences on annual timescales. Model solutions become more uncertain at earlier times (e.g., prior to 1950). The models show more skill explaining variance in tide gauge data at higher latitude, higher frequency, between winters, and over broad shallow shelves and within semi-enclosed marginal seas; at middle and high latitudes (poleward of 45°), model sea level solutions on average explain 30–50% of the monthly variance and 35–70% of the variance from one winter to the next in the tide gauge data records. Statistically significant relationships between the model solutions and observational data persist on long decadal periods. The relative skill of individual models is sensitive to region and timescale, such that no one model considered here consistently performs better than the others in all cases. Results suggest that barotropic models are useful for reducing noise in tide gauge records for studies of sea level rise and motivate additional model comparison studies in the context of sea level extremes.

Chaetetes morphology, environment, and taxonomy

Chaetetes, a hypercalcified sponge, formed expansive biostromes in Atokan strata of the Hueco Mountains, West Texas, where they grew in very shallow water on a broad shallow carbonate shelf. The Chaetetes there have tabular, columnar, compound-columnar, and branching shapes, and grew to heights of greater than 1 m. In most cases, they grew from a point source with an axial growth style. The laminar growth style occurs rarely, overlying growth interruption surfaces and where initial growth was on a hard surface such as solitary rugose corals. From the analysis of these fossils in the context of their inferred paleoenvironment, and integrating observations and available data from other localities, we conclude that the morphology of Chaetetes—their shape and growth style—was determined primarily by sedimentation parameters concurrent with their growth. These parameters were substrate character—texture and firmness—and both overall rate of sediment accumulation and variations in the rate relative to the rate of Chaetetes growth. Six species of Chaetetes were described previously in North America. Four of these are too incompletely known to be of taxonomic value. The other two, Chaetetes milleporaceus and Chaetetes favosus, form a continuum of morphotypes that is interpreted here to represent a single species, C. milleporaceus by priority.

Roles of sorption and tube-dwelling benthos in the cycling of phosphorus in Bering Sea sediments

Adsorption of dissolved phosphate onto iron-hydroxides has been shown to be one of the primary regulators of phosphorus cycling in sediments. Bioturbation and bioirrigation by benthic infauna modify this cycling by accelerating the transport of dissolved and particulate phosphorus and by changing rates of reactions that occur in the sediment, such as the adsorption of phosphate by amorphous iron hydroxides. Hydrographic processes vary regionally in the Bering Sea and nutrient exchange between the sediments of the broad shallow shelf and overlying water may influence water column productivity. These characteristics make the Bering Sea a good study site for examining the processes that influence sedimentary cycling of phosphorus. To examine these processes, we collected samples in four domains (southern middle shelf, southern outer shelf, southern off shelf (consisting of the continental slope and Bering Sea basin) and northern Bering shelf) based on hydrographic regime. At each station we directly measured phosphate flux and sediment oxygen consumption using whole–core incubations. We also measured infaunal burrow abundances, amorphous iron-hydroxide concentrations and phosphate sorption. We found that three out of the four domains had a high affinity for trapping phosphate in the sediment, as indicated by their adsorption coefficients (6.59–81.81). However, the measured phosphate fluxes could not be explained by the adsorption capacity of the sediment alone. The results indicated that on the middle shelf, the phosphate flux positively co-varied with infaunal burrow abundances. The high number of organisms in this domain (10–32 burrows per 50cm2 core) enhances the flux of phosphate to the overlying water. Controls on the phosphate flux on the middle shelf cannot be properly understood unless benthic infaunal abundance is taken into account.

Transport timescales for identifying seasonal variation in Bass Strait, south-eastern Australia

A three-dimensional σ-coordinate ocean model with realistic forcing is used to derive and compare flushing time, residence time and water age distributions in Bass Strait, a broad shallow shelf sea in south-eastern Australia which exhibits seasonal variation in water mass properties and circulation. Results illustrate flushing out of most old water occurs each winter–spring period and the circulation in summer increases age of remnant older water in the Strait. A correlation between water age and salinity is found in winter which is the result of advective “erosion” of the interior water mass. The presence of an area in the south-eastern part of the interior with low flushing is suggested to result from a number of physical factors including the presence of quasi-stationary eddies which entrain age, leading to relatively older water residing there.

A note on the occurrence of the encrusting foraminifera Homotrema rubrum in reef sediments from two distinctive hydrodynamic settings

Sediment samples from two different reef environments were analyzed for their foraminiferal content: the Sioba/Rio do Fogo reefs, located in a broad shallow shelf dominated by strong currents (Rio Grande do Norte State), and the Praia do Forte reefs, located in a narrow shelf under the influence of wind-induced waves (Bahia State). The recorded foraminiferal fauna, from forty-six samples, is represented by 113 species, being the encrusting species Homotrema rubrum present in nine samples from the Sioba/Rio do Fogo reefs and in 11 samples from Praia do Forte reefs. Homotrema fragments recovered from Sioba/Rio do Fogo are mostly polished with rounded edges (51.59%) and some have a whitish color, whereas the specimens from Praia do Forte reefs are predominantly composed of reddish fragments (82.41%) with sharpened points (63.88%). The high wave energy on the reef environment of Praia do Forte is responsible for the fragmentation of living Homotrema tests (red color), which are deposited near to their source area. The effects of the strong current system operating in the broad continental shelf of Sioba/Rio do Fogo is the major cause of sediment reworking, producing polished grains accumulated in its reef surroundings.

Hydrographic changes in the Canada Basin of the Arctic Ocean, 1979–1996

Hydrographic surveys of the southern Canada Basin have been conducted in winter almost annually since 1979. Coverage has extended 100–200 km beyond the shelf to water depths of 2000–3000 m. Water properties in the upper 1500 m have fluctuated over intervals of a few years, and progressive changes have taken place during the 17 years of this study. At depths below the warm core of the Atlantic layer, water is warmer and fresher now than in the early 1980s. These changes reflect a thickening of the warm core at its lower boundary by more than 100 m. The maximum temperature of the Atlantic water has decreased by 0.06°C, and the salinity at the maximum has decreased by 0.027. Above the Atlantic layer the lower halocline (34.5 isohaline) has cooled by 0.12°C, a change which may indicate that ventilation of this level on the European side of the Arctic has been more effective in recent years. A good correlation between temperature fluctuations on the 34.5 isohaline and depth fluctuations of the lower 0°C isotherm suggests common forcing of both parameters. Variation in the flow through the Barents Sea into the Arctic is proposed as that common influence. The upper halocine has warmed by 0.15°C on the 33.5 isohaline. This trend could be a consequence of decreasing ventilation of the upper halocline over the broad shallow shelves on the Pacific side of the Arctic Ocean and may be related to the decrease in the strength of the Siberian High during the same time. The net result of changes in the hydrography of the Canada Basin has been an increase of 0.2 GJ m−2 in stored sensible heat, an amount sufficient to melt about 0.7 m of sea ice. The bulk of the increase results from the thickening of the Atlantic layer below 500 m.

Diving behaviour and foraging location of female southern elephant seals from Patagonia

Our aim was to describe the free‐ranging diving pattern and to determine the location of foraging of pregnant female southern elephant seals, Mirounga leonina, from Peninsula Valdes, Argentina. This colony is unusual in two respects: it is removed from deep water by a broad shallow shelf (345–630 km wide), and colony numbers have been increasing in recent years in contrast to numbers from other southern hemisphere colonies that are stable or in decline. Microprocessor controlled, geolocation‐time‐depth recorders were deployed on four females, recording a total of 15,836 dives (270 dive days) during the period February to April, 1992. Departing seals crossed the continental shelf quickly (54–5–62–1 h) and did not show signs of foraging until reaching deep water, due east of the colony in the South Atlantic Ocean. Diving was virtually continuous (93% of the time underwater) with overall mean (±S.D.) rates of 2.5±0.2 dives/h, mean dive durations of 22.8 ± 7.1 min (maximum dive duration = 79 min) with 1.6±0.6min surface intervals between dives, and dive depths of 431±193m (maximum dive depth = 1,072 m). The diving pattern of females from Patagonia is similar to that of seals from colonies where numbers are decreasing (Macquarie stock) or are stable (South Georgia Island). Our subjects did not, however, feed in or south of the Antarctic Polar Front, or in cold waters along the Antarctic coast, where seals from declining or stable colonies forage.

Relationship of Facies and Sequence Stratigraphy to Paleokarst and Fracture Overprints in the Carbonates of the Arbuckle Group in the Evaluation of Exploration and Horizontal Drilling Potential

The mid-continent region, especially in Oklahoma and Arkansas, contains thick dolomite Paleozoic carbonate sections with karstic character. These sections commonly exhibit strong structural overprints, including intense fracturing, due primarily to Pennsylvanian orogenies. The Arbuckle Group is compose of multiple parasequences that are the result of cyclic peritidal deposition on a broad shallow shelf. Sequence stratigraphy evaluation indicates that significant unconformities or disconformities occur within the Arbuckle Group along third-order sequence boundaries. Arbuckle carbonates were subjected to very early dolomitization (Ordovician through Devonian). Sequences in which intercrystalline porosity was developed with little or no vuggy porosity usually have low permeability and effective porosity. However, these dolomitic sequences are quite susceptible to fracturing and they may produce in structural traps. These fractured sequences in the Arbuckle also are particularly susceptible to karstification associated with multiple unconformities, especially in areas where there has been significant orogenic activity such as the Criner uplift in southern Oklahoma. Fracturing and subsequent karstification have significant influence on porosity development and can produce extremely heterogeneous reservoirs. Exploration for oil and gas in the Arbuckle Group has been a difficult task. Problems with seal and source along with complex diagenetic and structural histories still plague efforts to understand Arbucklemore » reservoir development. There is new evidence that dolomitization, fracturing and karstification may be related to particular sequence positions and their related facies and disconformities. Innovative advances in geologic concepts, such as sequence stratigraphy, and in techniques, such as horizontal drilling, may lead to new discoveries in the Arbuckle Group.« less

Drowning of an Ooid Shoal: Mississippian Greenbrier Limestone Near West Virginia Dome: ABSTRACT

The depositional setting for the Greenbrier Limestone of the central Appalachian basin included a continually subsiding basin, centered in southeastern West Virginia and adjacent Virginia, separated by a hinge zone from a broad shallow shelf located to the north and northwest. Oolitic limestones are thick and extensive in southern West Virginia; however, most of these units thin drastically and change in lithology to the north. An area of uplift, the West Virginia dome, developed along the hinge zone in north-central West Virginia. During early Greenbrier deposition, this area was exposed as an island. Later, the dome was submerged but remained as a submarine high throughout Greenbrier sedimentation. The northernmost occurrences of Mississippian oolitic limestones in West Virginia surround this feature.

Evidence for nonlocal forcing along the southeastern United States during a transitional wind regime

South Atlantic Bight (SAB) winds are typically well‐correlated over large alongshore scales and closely coupled with coastal sea level (CSL) fluctuations. However, this correlation is greatly reduced when meteorological dominance is changing between the Ohio Valley High of winter and Bermuda‐Azores High of summer. During the spring 1981 transition, strong persistent wind in the northern portion of the SAB (e.g., at Charleston) was highly coherent with local CSL (coefficient of determination, R2 = 0.70). In the southern SAB, however, a weak poorly organized wind regime was decoupled with local CSL. Daytona Beach CSL was uncorrelated with local alongshore wind, contrary to factional shelf models, but was coherent with alongshore wind stress and CSL at Charleston (R2 = 0.74), lagging events in the north by ∼15 hours. The results suggest that a CSL signal forced by wind in the northern SAB propagates southward with a phase speed of ∼700 km d−1. Barotropic shelf wave theory for an exponential shelf profile representing SAB topography predicts a phase speed of ∼500 km d−1. Thus the potential importance of shelf wave activity in the SAB during mixed wind conditions should be reassessed. Analysis of SAB wind‐CSL relationships provides, for the first time, direct evidence of remote forcing on the broad shallow shelf between North Carolina and Florida. Previous observations in the SAB from other seasons have not afforded evidence of nonlocal propagations, suggesting that conditions appropriate for their observation have a strong seasonal dependence.

Deep-Water Density Current Deposits of Delaware Mountain Group (Permian), Delaware Basin, Texas and New Mexico

The Guadalupian Delaware Mountain Group is a 1,000-1,600-m (3,281-5,250-ft) thick section of siltstone and sandstone deposited in a deep-water density-stratified basin surrounded by carbonate banks or reefs and broad shallow evaporite-clastic shelves. The most prevalent style of basinal deposition was suspension settling of silt. Laminated siltstone beds are laterally extensive and cover basin-floor topographic irregularities and flat-floored channels as much as 30 m (99 ft) deep and 1 km or more wide. Channels can be observed in outcrop at the basin margin and can be inferred from closely spaced wells in the basin. The channels are straight to slightly sinuous, trend at high angles to the basin margin, and extend at least 70 km (43 mi) into the basin. Sandstone beds, con ined to channels, form numerous stratigraphic traps. Hydrocarbon sealing beds are provided by laminated organic siltstone, which laterally can form the erosional margin where channels are cut into siltstone beds. Thick beds of very fine-grained sandstones fill the channels. These sandstones contain abundant large and small-scale traction-current-produced stratification. These sandy channel deposits generally lack texturally graded sedimentation units and show no regular vertical sequence of stratification types or bed thickness. Outcrop and subsurface evidence indicates Delaware Mountain Group sediments were deposited by saline density currents. Dense saline water originated on evaporitic shelves and spilled across the carbonate rim, down steep marginal slopes, and into the basin. Basinal waters were density stratified. Denser flows moved along the basin floor cutting channels or depositing sand in existing channels; less-dense flows moved along density interfaces in the water column and carried silt-size material far into the basin where it settled to the floor as thin alternating layers of detrital silt and organic debris. Little proximal to distal change occurred in the size or nature of the channels. Exploration predictions based on submarine fan models formed by turbidity currents would anticipate very d fferent proximal-distal changes in sandstone geometry and facies.

A synopsis of PROBES

The Processes and Resources of the Bering Sea Shelf (PROBES) project is a 6‐year multi‐institutional (University of Alaska, Florida State University, University of Washington, Brookhaven National Laboratory, Southwest Fisheries Center, Bigelow Laboratory for Ocean Sciences) interdisciplinary study designed to understand the processes that contribute to the production of enormous numbers of animals (including crabs, fish, birds, mammals) in secondary and higher trophic levels in the vast Bering Sea continental shelf. The research plan is based on the hypothesis that the broad shallow shelf leads to an oceanographic structure of a semi‐permanent front‐interfront system in which phytoplankton primary production is coupled to a pelagic food web over the outer shelf and to a benthic food web in the middle shelf (see cover, this issue). The project has concentrated on the processes that control the survival of the early life history stages of the Alaska pollock (Theragra chalcogramma Pallas) as an example of mass and energy transfer in the pelagic system. PROBES began in 1976 and is sponsored by the Division of Polar Programs, National Science Foundation.

Upper Cretaceous Mosby Sandstone, Central Montana--Example of Thin, Widespread Storm-Generated Sandstone Cycles: ABSTRACT

The Mosby Sandstone Member of the Greenhorn Formation is composed of thin (less than 6 ft or 1.8 m), very fine-grained to fine-grained sandstone cycles separated by shale. The sandstones occur either as individual beds, generally less than 1 ft (9.3 m) thick and separated by interbeds of shale, or as amalgamated beds. The base of each cycle is a planar to undulating erosional End_Page 772------------------------------ surface, and the top is gradational with the overlying shale. The dominant sedimentary structures are parallel lamination and low-angle cross-lamination filling broad, shallow troughs. Wave-ripple cross-lamination and wave ripples are commonly developed in the upper parts of the cycles. Trace fossils are generally restricted to the rippled surfaces and consist of horizontal Ophiomorpha and Thalassinoides, Diplocraterion, and Chondrites. Shells, including gastropods, bivalves, and ammonites, occur as lenses near the base of the cycles and as concretions laterally where the sandstones are not developed. The thin sandstone cycles occur as elongate bodies that are a few tens of miles across and several tens of miles long. The bodies collectively occur in a southward-projecting lobe that covers an area of 40,000 sq mi (64,372 sq km) in central Montana. The sediments were transported as much as 700 mi (1,127 km) in a south-eastward direction from the Dunvegan delta in north-western Alberta. The sandstone cycles are interpreted to have been deposited by storm events on a broad shallow shelf. The sand was probably transported by intense wave action and storm-generated currents and deposited after erosion during the waning stages of the storm. End_of_Article - Last_Page 773------------

Brushy Canyon Formation, Texas: A Deep-Water Density Current Deposit

In mid-Permian time, the Delaware basin was a nearly circular deep, about 160 km in diameter. It was ringed by banks or reefs, which were surrounded in turn by very broad shallow shelves, lagoons, sabkhas, and alluvial plains. Broad tectonic down-warping caused deposition of about 1,000 m of sediment in the basin and neighboring shelves in both the Leonardian and Guadalupian Series (an average of 75 m/m.y.), whereas distant areas received only a fraction of those thicknesses. Although the basin and adjacent shelves accumulated nearly equal sediment thicknesses, appreciable slopes existed at the basin margins throughout mid-Permian time. Thus, the basin waters are estimated to have been more than 100 m deep in Leonardian time and as much as 600 m in late Guadalupian time. Simultaneously, bordering banks were very shallow or even emergent, and they prograded basinward several kilometers. The basin margin is spectacularly revealed by a transverse fault in the Guadalupe Mountains. Here the relations of basin, slope, and shelf beds can be clearly seen. The quartzose siltstone and sandstone of the Brushy Canyon Formation (the lower one-third of the Guadalupian Series), where they wedge out at the basin margin, are the topic of this study. These beds have been variously interpreted as shallow marine or deep turbidity current deposits. In my estimation, they are neither. Rather, they have unusual features that suggest deposition in relatively deep water by saline and cold density currents. Surrounding shelves provided these dense water masses. Numerous basinward-trending channels are one product of these density currents. The channels are commonly 20 to 30 m deep, a kilometer or more wide, and extend far into the basin. Channel floors are flat, and the sides commonly slope 20° to 30°. The channels are filled in a special, though unordered, way. Finely laminated coarse-or medium-grained siltstone beds mantle channel floors, walls, and interchannel areas. Fine-grained, locally conglomeratic sandstone beds are confined to channel floors and abut the walls. Chaotic debris beds locally fill channels near the steepest basin slopes. The basin waters were apparently density stratified. Dense shelf water, spilling through channels in surrounding banks, flowed down marginal slopes and along the basin floor. These denser flows cut channels or deposited sandstone beds confined to channels. At other times, less dense shelf water spread over more dense, stagnant basin layers, raining suspended silt over the irregular basin floor. Characteristics that distinguish these density current deposits from the more common turbidity current deposits include (1) less evident proximal-distal changes (the same rock types and channel relations exist near the basin center as in outcrops 50 km away), (2) coarser porous sandstones confined entirely to channels (some channels have much sand and others little), (3) no levees or overbank deposits that would serve as channel proximity indicators, and (4) mostly ungraded sandstone beds that contain little finer matrix. Similar density current deposits have not yet been recognized in other areas or geologic systems. However, they may be anticipated wherever basin waters were restricted sufficiently to become density stratified and where broad evaporitic shelves or lagoons bordered such basins.

Petrology of Pennsylvanian Carbonate Bank and Associated Environments, Azalea Field, Midland County, Texas: ABSTRACT

The Azalea carbonate biogenic bank developed on a broad shallow shelf during early Strawn deposition. Study of cores and well cuttings defined 3 major facies. The deeper micrite is dark brown with scattered crinoid fragments and was deposited in water from approximately 50 to 200 ft deep. The biogenic bank facies is composed of coarse fossil fragments including crinoids, Bryozoa, pelecypods, brachiopods, fusulines, and platey algae. The sheltered micrite is light brown with small fossil fragments consisting mainly of bivalves and Foraminifera deposited behind the bank. Bank development was confined to the west edge of an Atokan structural terrace where oscillation waves were impinging upon the rising sea floor. Moderately high energy, shallow water and the associated supply of nutrients provided necessary ingredients for prolific growth of organisms which formed the biogenic bank. Hydrocarbon production has been obtained from the bank over 13 mi of its length. Recently production has been extended 1 3/4 mi on the south end and 1½ mi on the north end. There is a good possibility that additional biogenic banks have developed on the broad Strawn shelf. End_of_Article - Last_Page 157------------

Petrology of Pennsylvanian Carbonate Bank and Associated Environments, Azalea Field, Midland County, Texas: ABSTRACT

The Azalea carbonate biogenic bank developed on a broad shallow shelf during early Strawn deposition. Study of cores and well cuttings defined 3 major facies. The "deeper water" micrite is dark brown with scattered crinoid fragments and was deposited in water from approximately 50 to 200 ft deep. The biogenic bank facies is composed of coarse fossil fragments including crinoids, Bryozoa, pelecypods, brachiopods, fusulines, and platey algae. The "sheltered" micrite is light brown with small fossil fragments consisting mainly of bivalves and Foraminifera deposited behind the bank. Bank development was confined to the west edge of an Atokan structural terrace where oscillation waves were impinging upon the rising sea floor. Moderately high energy, shallow water and the associated supply of nutrients provided necessary ingredients for prolific growth of organisms which formed the biogenic bank. Hydrocarbon production has been obtained from the bank over 13 mi of its length. Recently production has been extended 1 3/4 mi on the south end and 1½ mi on the north end. There is a good possibility that additional biogenic banks have developed on the broad Strawn shelf. End_of_Article - Last_Page 157------------