Stream Deposits Research Articles

Sedimentologic Aspects, Mannville Group, Southwestern Alberta: ABSTRACT

Due to the relative paucity of data, the Mannville sediments located at the western margin of the plains (T10-20, R27W4 to W5) have not been subjected to detailed study. However, recent exploration activity in this area has resulted in the acquisition of much core and drilling data. Using these data, together with those obtained from outcrop studies in the Foothills and from all the available core toward Range 20, a regional appreciation of the stratigraphy, sedimentology, and diagenesis of southern Alberta is now possible. Three major phases of sedimentation are recognized. The first phase of Mannville sedimentation commenced with the deposition of coarse conglomerates in the Foothills (Cadomin) and chert-rich, often pebbly, sandstones in the plains (Basal Quartz, Cutbank, and Sunburst equivalents). These sediments were deposited as shallow braided or meandering stream deposits, often restricted to broad valleys on the Pre-Cretaceous unconformity. Fine-grained flood-plain deposits with well-developed pedogenic horizons are recognized within this interval. The second, a transgressive phase of sedimentation, followed, with the deposition of bentonitic shales and fossiliferous limestones (Ostracod Member) and quartzose (Glauconitic) sandstones. The areally extensive sediments, which were primarily deposited in a complex of shallow restricted marine(?) environments, have a distinctive mineralogy compared to the underlying and overlying units. Changes in sediment provenance can account for this mineralogic diversity. Thirdly, continental conditions returned to much of the area during post-Glauconitic time. This interval is represented by silts and shales intercalated with feldspathic and volcanic channel and crevasse sandstones. The textural and mineralogic immaturity of these sandstones is typical of the upper Mannville sediments of Alberta. Complexities arise to the east of Range 22 where deep-channels originating during Ostracod, ?Glauconitic, and Upper Mannville times have cut through the underlying sediments. Recognition of the initiation point of these channels is important because reservoir quality is largely controlled by sediment composition. The most prospective reservoirs are quartz-rich sandstones which were not subjected to extensive silica cementation, a consequence of deep burial. End_of_Article - Last_Page 585------------

Fan-Delta and Lacustrine Margin Sedimentation, Chinle Formation (Upper Triassic), Canyonlands National Park, Utah: ABSTRACT

Cyclic fluvial, fan-delta, and lacustrine margin depositional systems dominate the lower part of the Chinle Formation, Canyonlands area, southeastern Utah. Detailed facies analysis of the Moss Back Member and related strata document two alternating fluvial-lacustrine sequences. (1) During high fluvial output, coarse-grained meander belts built fan-deltas into shallow lakes. (2) During low fluvial output, braided streams waned before reaching the lakes, and algal and evaporatic mud flats formed on broad lacustrine perimeters. The high fluvial output phase comprises three depositional systems. (1) The coarse-grained meander belt system consists of point-bar and channel deposits. Abundant carbonate grains were cannibalized from upland caliche soils and lowland oncolite-bearing carbonate mud flats. (2) The fan-delta system contains sandstone and mudstone delta foresets that coarsen upward into delta-distributary deposits. (3) Blue-green, bentonitic, limy mudstone was deposited in the shallow lacustrine system. The low fluvial output phase comprises three depositional systems. (1) Braided stream deposits contain trough and planar cross-stratified calcarenite, calcirudite, and quartzarenite. (2) Micritic limestone, algal mats, and oncolites formed on swampy lake margins. (3) Shallow, warm, clear-water, lacustrine deposits are represented by continuous micritic limestone beds and bioturbated limy mudstone and sandstone. Three external controls produced the cycles. Tectonics associated with the adjacent salt anticline region affected source terrane (intrabasinal carbonate output) and fluvial discharge. Regional humid-arid cycles affected fluvial discharge and lake levels. Volcanic eruptions to the southwest produced periodic influx of volcanic debris that caused increased turbidity and sedimentation rates and decreased carbonate production. End_of_Article - Last_Page 550------------

Middle Devensian deposits beneath the ‘Upper Floodplain‘ terrace of the River Thames at Kempton Park, Sunbury, England

Gravel pit sections in the ‘Upper Floodplain’ terrace exposed current-bedded gravel and sand containing a grey clayey silt lens. This silt has yielded fossil macroscopic plant remains, molluscs, ostracods and insects. Sedimentological and palaeontological results indicate that the silts were deposited in a slowly flowing river channel. The gravel and sand accumulated under cool, seasonally contrasting climatic conditions and represent braided stream deposits. A new lithostratigraphical unit, the Kempton Park Gravel, is proposed and its relation to the local terrace stratigraphy of the Thames is discussed. A radio-carbon date of 35,230 ± 185 BP indicates that the Kempton Park Gravel unit aggraded during the Middle Devensian substage.

Recognition of High Gradient Braided Stream Deposits, Sespe Formation (Tertiary), Ventura Basin, California: ABSTRACT

Lower Sespe arkosic sandstones and petromictic conglomerates of late Eocene and Oligocene age rim the Ventura basin in California and are interpreted as high-gradient, braided river deposits. Lower Sespe deposits contain a high ratio (over 20/1) of bedload/suspended load. Lenticular, poorly sorted, imbricated, bar-shaped conglomerate units interbedded with graded to parallel-laminated sandstone beds are typical of lower Sespe deposits. Graded and reverse graded conglomerate units, graded End_Page 603------------------------------ sets of avalanche cross-stratification, cuspate cut-and-fill structures, and antidune cross-stratification also characterize the lower Sespe conglomerate facies. These features all indicate a braided stream model for lower Sespe deposits. Steep gradients during lower Sespe deposition are suggested by: (1) clast size data (average maximum clast size = 41 cm), (2) a predominance of upper flow regime structures (85% of all structures measured), and (3) high consistency ratios (mean consistency ratio = 0.78) of paleocurrent data. End_of_Article - Last_Page 604------------

Early tertiary deltaic sedimentation around Kalka and Nahan, Punjab Himalaya, India

During the Early Tertiary epoch a basin of deposition covered almost all the present Himalayan foot hill zone. Large quantities of detrital sediments were transported from the northern source area to the depositional basin. The rivers carrying the sediment load formed large delta complexes. In the area studied at least two such delta complexes, one around Kalka and the other around Nahan, were formed; of these the one around Kalka contains the greatest volume of sediments. The recognition of the deltas is based on: (a) greater abundance of stream deposits than deposits of other environments; (b) lithologic criteria, particularly sedimentary structures, repetitive lithologic associations; (c) the general coarsening-upward nature of the sediments; (d) strong unimodal current direction; (e) the typical lenticular lithogenetic model, common in recent deltas. The rocks of the Nahan Group are divisible into three formations at Kalka and two formations at Nahan where central formation did not develop. Himalayan tectonism has largely controlled both delta formation and stratigraphy. The differential rates of sedimentation greatly influenced the rates of subsidence within the Nahan basin. The sediments of the Nahan Group are roughly bounded between two major phases of Himalayan uplift, namely Eocene and middle Miocene tectonic movements. The effects of intermittent tectonic pulses are well depicted in rocks of the Nahan Group. The intensity of the tectonism that controlled both the source-area rise and basin subsidence during deposition of the Nahan Group of sediments was nonuniform, both in vertical and lateral directions. Because of a generally higher rate of deposition compared to the rate of subsidence, the rocks of the Nahan group represent a normal regressive basin-filling sedimentation, forming thick detrital deposits due to progradation of younger deltas beyond the distal end of older deltas.

Sediment Yield from a Mississippi Delta Cotton Field

AbstractSediment yield from flat (0.2% slope) delta fields was measured during a 33‐month study. Annual erosion rates were higher than anticipated, ranging from a maximum 12 metric tons ha−1 for a year of above normal rainfall to a minimum 5 metric tons ha−1 for a year of slightly below normal rainfall. Highest sediment concentrations in runoff occurred in the spring during the seedbed preparation and planting season when the soil was bare of vegetative cover, freshly plowed, and highly susceptible to erosion. Lowest concentrations occurred in September and October when vegetative cover was good.Sediment particle size data from selected storms showed that about 83% of the sediment removed from the fields was < 2 µm in diameter and nearly all, 98% was < 16 µm. Field soils were composed of 24% clay, 23% silt, and 53% sand, so considerable enrichment of fine particles occurred during the erosion process. This indicates that most sediment deposits in streams and lakes of the area are composed of fine materials and that gradual enrichment of coarse particles in the soils is occurring over long periods of time.

Geologic evidence for age of deposits at Hueyatlaco archeological site, Vasequillo, Mexico

Direct tracing of beds during excavation in May 1973, confirmed that the artifact-bearing layers at Hueyatlaco underlie 10 m of fine-grained, water-laid deposits that constitute part of the wide-spread Valsequillo gravels. Dissection of these deposits by the adjacent Río Atoyac has reached a depth of 50 m. The stratigraphic section at Hueyatlaco includes four distinctive tephra units. The oldest one occupies a small channel in a series of cut-and-fill stream deposits that have yielded bifacial tools. It lies more than a meter above flat-lying, fine-grained beds from which edge-retouched tools have been recovered. The three other tephra units occur higher in the section. Fission-track ages on zircon phenocrysts from two of the younger tephra layers (370,000 ± 200,000 and 600,000 ± 340,000 yr, 2σ) agree with concordant uranium-series dates for a camel pelvis that was found associated with bifacial tools at Hueyatlaco (245,000 ± 40,000 yr by 230Th and > 180,000 yr by 231Pa). These dates are compatible with the depth of burial and subsequent dissection of the Hueyatlaco deposits, as well as with the degree of hydration of volcanic glass shards and with the extent of etching of heavy-mineral phenocrysts from within the tephra layers. These findings suggest to us that further search for archaeological remains in deposits as old as those at Hueyatlaco would be warranted.

PETROLOGY, PROVENANCE, AND DEPOSITIONAL ENVIRONMENTS OF THE RESERVOIR SANDSTONES OF OSSU‐IZOMBE OILFIELD, IMO STATE, NIGERIA

Upper Eocene immature subarkoses derived from Cretaceous sedimentary rocks, older granites and metamorphic rocks, form the reservoir sandstones of the Ossu‐Izombe oilfield, Imo State, Nigeria. These sandstones accumulated as part of a deltaic deposit. The vertical section of deltaic facies consists, from base to top, of a progradational sequence (prodelta and delta front), an aggradational unit (delta plain, marsh and interdistributary bay), and an overlying transgressive shallow marine interval. This sequence was penetrated by nearly all of the 15 wells drilled in the area. Reservoir sandstones are present within the distributary mouth bars, point bars, braided stream deposits, and the shallow marine interval.Thin section studies, electric‐log analyses, and data from modern deltas have aided in establishing the environments of deposition of these sandstone bodies. The principal results of this study are: (1) the basal C sandstone represents distributary mouth bar sands deposited at water depths of about 10 m; (2) the intermediate B group consist of point bar and distributary channel sandstone deposits of the lower delta plain; (3) the A‐2 sandstone tops the deltaic cycle as a product of braided streams; and (4) a shallow marine deposit, the A‐1.1 and A‐1 sandstones, occurs at the top of the deltaic cycle.

Braided fluvial sedimentation in the lower paleozoic cape basin, South Africa

Lower Paleozoic braided stream deposits from the Piekenier Formation in the Cape Province, South Africa, provide information on lateral and vertical facies variability in an alluvial plain complex influenced by a moderate to high runoff. Four braided stream facies are recognized on the basis of distinct lithologies and assemblages of sedimentary structures. A lower facies, dominated by upward-fining conglomerate to sandstone and mudstone channel fill sequences, is interpreted as a middle to lower alluvial plain deposit with significant suspended load sedimentation in areas of moderate to low gradients. These deposits are succeeded by longitudinal conglomerate bars which are attributed to middle to upper alluvial plain sedimentation with steeper gradients. This facies is in turn overlain by braid bar complexes of large-scale transverse to linguoid dunes consisting of coarse-grained pebbly sandstones with conglomerate lenses. These bar complexes are compared with environments of the Recent Platte River. They represent a middle to lower alluvial plain facies with moderate gradients and no significant suspended load sedimentation or vegetation to stabilize channels. These bar complexes interfinger basinward with plane bedded medium to coarse-grained sandstones interpreted as sheet flood deposits over the distal portions of an alluvial plain with low gradients and lacking fine-grained detritus or vegetation.

Rise and Fall of Axum, Ethiopia: A Geo-Archaeological Interpretation

Civilizations represent human ecosystems amenable to systematic geo-archaeological analysis. The civilization of Axum, spanning the first millennium A.D., had its settlement core on the now-denuded, subhumid plateau of northern Ethiopia. Axum, a new city, began A.D. 100 as a ceremonial center, growing to over 10,000 people, as a prosperous emporium for international trade. Intensified land use led to mass movements in slope soils before A.D. 300, but a range of clayey stream deposits also implicates strong periodic floods and seasonally abundant moisture. The paleoclimatic ensemble suggests that stronger and more reliable spring rains allowed two crops yearly without irrigation, compared to only one with modern summer rains. Trade declined after 600 and Axum was essentially landlocked by 715. Intense land pressure and more erratic rainfall favored soil destruction and ecological degradation during the seventh and eighth centuries. Largely abandoned by 800 and pillaged by border tribes. Axum retained only symbolic significance as power shifted to the more fertile lands of humid central Ethiopia. Axum shows how the spatial and temporal variability of resources, and the interactions between a society and its resource base, can be fundamental in the analysis of historical process.

Predictive Depositional Model for Glacial Aquifers

ABSTRACTIn an investigation of the hydrogeology of bedrock valley fills in northeastern Wisconsin, it was determined that the yield and spatial distribution of glacial aquifers are controlled by bedrock topography and glacial geology. Lithologic control for identification and classification of aquifer units was obtained from several hundred drillers' logs calibrated by auger borings and interviews. Transmissivities of drift aquifers calculated from specific capacity tests using Jacob's equation agree well with estimates made from lithologic logs. Seismic refraction and gravity surveys were coupled with well log data to derive a bedrock elevation map. Vertical variability maps calculated from the lithologic logs show that bedrock topography strongly influences the distribution and character of glacial units, and clearly delineate the two glacial aquifer types present, valley and morainal units. Morainal units parallel the Late Wisconsin end moraine trends, while valley units are perpendicular to the moraines. The morainal units are small ice‐contact features such as kames, fans, and deltas. The valley units are stream deposits and may have been deposited subglacially. Valley aquifers are the most productive units. Using lithologic and geophysical data, and statistical analysis, a predictive depositional model can be proposed. This conceptual model allows knowledge of bedrock topography to be coupled with glacial geology to predict the spatial distribution of glacial aquifers.

Clastic Depositional Models: ABSTRACT

Depositional models of clastic rock sequences provide a valuable insight into many exploration and production problems in the energy business. The size, shape, lateral relations, and physical properties of sedimentary bodies have obvious and well-recognized relevance to searching for and developing petroleum, coal, and some ores. Acknowledgement of the value of depositional models by industry stimulated incisive research beginning more than 30 years ago and has sustained a very active and growing interest throughout the scientific community. As a result, today we face the blessing and the perlexity of a large and rapidly expanding fund of data. No person can now really cope with assimilating the literature, past and present, on clastic depositional models, especially a person busy searching for mineral resources. My goal is to review some of the important advances that have happened, may be happening now, or should be promoted. These views are certainly personal and biased. As examples of my opinion, marine shelf and slope models are very poorly understood; knowledge of modern processes, Holocene deposits, and ancient examples has not led to unifying principles that can bolster exploration predictions in these general depositional environments. At the other end of the information spectrum, stream deposits are well known from many studies of modern and ancient examples and considerable experimental data. Yet embarrassing gaps in understanding remain because of accidents of geography, climate, or scale. Clastic depositional models from outcrop and subsurface provide case histories of fluvial, marine-shelf, and basin-slope environments that illustrate the state of the art and applications to practical oil exploration problems. End_of_Article - Last_Page 1357------------

Oblique Dunes of Oregon Coast: ABSTRACT

The oblique dunes of the Oregon coast are the largest (as high as 55 m) on the west coast of the United States. Where best developed, these perennial ridges have been migrating northeastward at an average rate of 4.4 m/year, as measured from aerial photographs dating from 1939 to 1975. The wind regime of the area can be characterized as obtuse bimodal, with sand-transporting winds from the north and northwest throughout the dry summers and between storms at other seasons, and from the south and southwest during fall, winter, and spring storms. Although the sand-transporting capacity of the southerly winds is reduced by accompanying rainfall, these winds are nevertheless strong enough to do more work than the northerly winds. Dune processes during the winter season of southerly storms include: (a) erosional smoothing of the south (stoss) slope; (b) erosion and formation of sawtooth remnants (yardangs) on the dune crest; (c) deposition by adhesion on the wet, low-angle north slope between the dune crest and the slip-face brink; (d) grainfall deposition on the west slip face, causing steepening to angles of 44° or more, followed by slumping; (e) local deposition by adhesion in wet interdune troughs; and (f) local subaqueous deposition in ephemeral interdune ponds and streams. Dune processes during the summer season of northerly winds include: (a) growth of small southeast-facing transverse dunes at the crests and on the south slopes of the oblique dunes, and in troughs between the oblique dunes; (b) pla tering of wind-rippled sand sheets against the base of the winter slip face, leading to the formation of a gently north-sloping apron; and (c) erosion of the upper part of the winter slip face, reducing the slope angle to 25° or less. End_of_Article - Last_Page 978------------

Depositional Environment and Reservoir Characteristics of Lower Cretaceous Paluxy Sandstones, Bolton Field, Hinds County, Mississippi: ABSTRACT

Bolton field produces oil from numerous lenticular, Lower Cretaceous sandstones at depths ranging from 8,350 to 11,800 ft (2,545 to 3,596 m). The field is on an elongated, faulted anticline in the northwest part of the Mississippi salt basin. It was discovered in 1954 as a result of seismic mapping. Two years after its discovery it had produced 1,475,179 bbl of oil. A core of the Lower Cretaceous Paluxy sandstone, and electric logs from the field area, were studied to determine the environment of deposition and reservoir morphology. The Paluxy sandstones were deposited within a fluvial system. Primary rock properties observed in the core indicate a braided stream deposit. Bedsets are thin, ranging from 0.5 to 9 ft (.15 to 2.74 m) in thickness. Sedimentary structures within bedsets consist of inclined laminae in the lower part to parallel laminae and ripples in the upper part, indicating deposition End_Page 1012------------------------------ from decreasing flow regime. This sequence of bedding, together with the vertical decrease in grain size observed, is typical of a fluvial sandstone. Sandstones (quartzarenite) are characterized by a high quartz content (85 to 98%), moderate matrix (5 to 10%), and a small amount of minor constiuents (muscovite and calcite). Electric log cross sections reveal lateral variation in sandstone thicknesses. Sand bodies are lenticular and discontinuous, characteristic of intermittant braided stream deposition. Reservoir sandstones have porosities ranging from 12 to 22% and permeabilities of up to 500 md. Recognition of the vertical sequence of primary rock properties is indicative of braided stream deposits, and the associated electric log characteristics may aid in future exploration and production efforts for similar reservoirs. End_of_Article - Last_Page 1013------------

Cretaceous-Tertiary Boundary, San Juan Basin, New Mexico and Colorado: ABSTRACT

The Cretaceous-Tertiary boundary in the San Juan basin, New Mexico and Colorado, has been variously placed at the base, at the top, and somewhere within the Ojo Alamo Sandstone. New evidence bearing on the location of this boundary includes palynologic analyses, vertebrate paleontology, paleomagnetism, and trace-element analyses. The Ojo Alamo Sandstone is a series of fluvial, high-energy, braided stream deposits consisting of from one to several conglomeratic sandstone sheets separated by overbank or floodplain shales. The conglomerates contain pebbles and cobbles of quartzite, jasper, and andesite on the west side of the San Juan basin. The pebbles and cobbles diminish in size eastward and are rare to nonexistent on the east side of the basin. There is a major unconformity at the base of the Ojo Alamo. The Cretaceous-Tertiary boundary, as determined earlier from palynologic criteria, is below the base of the Ojo Alamo Sandstone in the uppermost part of the underlying Kirtland Shale on Mesa Portales. New evidence from the Star Lake area, New Mexico, tends to confirm that boundary location. Other new data based on vertebrate remains, also from the Star Lake area, indicate that the boundary is within the Ojo Alamo Sandstone. Recently published paleomagnetic data and trace element studies now underway may also help in solving the Cretaceous-Tertiary boundary problem. End_of_Article - Last_Page 559------------

Depositional environments of some Pleistocene coastal terrace deposits, southwestern Oregon—case history of a progradational beach and dune sequence

Pleistocene coastal terrace deposits exposed in sea cliffs near Gold Beach, Oregon can be divided into four stratigraphic units: a basal gravelly unit and three overlying sandy units, each with mud beds, a paleosol, or the modern soil in its uppermost part. The gravelly unit consists of gravel and sand in its lower part, sand, in part pebbly or cobbly, in its middle part, and mud and sand in its upper part. Black sand and transported pieces of wood are common in the middle part of the unit, and wood is common in the mud. This unit is interpreted as a progradational deposit including environments ranging from lower forebeach at the base to backbeach flats and streams at the top. The main sandy parts of the sandy units are made up of a crossbedded sand facies, the dominant structure in which is medium-scale crossbedding, and an irregularly bedded sand facies, which is locally pebbly and is dominated by scour-and-fill structures. Deciding between shallow marine and eolian interpretations of the sandy units proved exceptionally difficult until modern analogues were found in the fine details of the internal structures. Largely on the basis of such structural details, the crossbedded sand facies is interpreted as the product of small eolian dunes, and the irregularly bedded sand facies is interpreted as deposits of interdune ephemeral streams, ephemeral ponds, and wet to dry subaerial flats. The mud beds and paleosols at the tops of the sandy units represent times of temporary stabilization of the dune field.

Permian intermontane basin sedimentation in southern Scotland

Towards the end of the Carboniferous period, several isolated basins were eroded into Carboniferous sediments lying within the Lower Palaeozoic Southern Uplands massif. Eruption of basaltic lava flows ensued, followed by deposition of thin pediment breccias as desert conditions became established, leading to the development of extensive aeolian sands which interfinger with alluvial fan breccias at the basin margins. Such intermontane desert basins have rarely been documented in the stratigraphic record and the Permian deposits are here compared to similar deposits and processes operating in Recent intermontane desert basins. A number of breccia types have been distinguished, some attributable to secondary reworking of deflation areas on alluvial fans, and the deposits of streamfloods, braided streams, sheetfloods and desert-floor ephemeral streams have been recognised.

Engineering geology mapping of depositional environments in the lower mississippi alluvial valley

Floodplains are highly utilized for agriculture, industry, and urban centers with the rivers serving as water transportation routes. Because of these activities, sites for structures must be selected, channels maintained for navigation, and the area must be protected from flooding. Alluvial depositional environments are valuable indicators of natural conditions that can assist the engineer for project planning, design, and construction in floodplains. Principal depositional environments (exclusive of deltas) are meander belts (natural levees, abandoned channels, abandoned courses, and point bar deposits), backswamp, and braided stream deposits. Each depositional environment or feature has predictable characteristics that can be related to its behaviour as engineering materials. These features are readily recognizable and can be mapped from their unique patterns formed on the land surface. The value of identifying these features has been proven by the U. S. Army Corps of Engineers where major drainage basins in the Lower Mississippi Alluvial Valley have been mapped at a scale of 1∶62,500. These maps have applied to engineering problems concerned with erodibility of river banks, areas most susceptible to underseepage, foundation conditions, suitable sources of borrow material for levee construction, and others.

Latest Jurassic--Early Cretaceous Regressive Facies, Northeast Africa Craton

Nonmarine to paralic detrital deposits accumulated in six large basins between Algeria and the Arabo-Nubian shield during major regression in latest Jurassic and Early Cretaceous time. The Ghadames (northwestern Libya and adjacent Tunisia and Algeria), Sirte (north-central Libya), and Northern (Egypt) basins lay along the cratonic margin of northeastern Africa. The Murzuk (southwestern Libya and northern Chad), Kufra (southeastern Libya and nearby Sudan), and Southern (Egypt) basins lay in the south within the craton. Data for reconstructing distribution, facies, and thickness of relevant sequences are adequate for the three northern basins only; reconstructions of the southern ones are suggestive at best. High detrital influx near the end of Jurassic time and in mid-Cretaceous time produced regressive nubian facies composed largely of low-sinuosity stream and fan-delta deposits. In the west and southwest the Ghadames, Murzuk, and Kufra basins were filled with a few hundred meters of detritus after long-continued earlier Mesozoic aggradation. The three northeastern basins were filled with 1,000 to 2,000 m of detritus. In northern Egypt the regressive sequence succeeded earlier Mesozoic marine sedimentation; in the Sirte and Southern basins correlative deposits accumulated on Precambrian and Variscan terranes after earlier Mesozoic uplift and erosion. Waning of detrital influx into southern Tunisia and adjacent Libya in the west and into Israel in the east initiated an Albian to early Cenomanian transgression of Tethys. By late Cenomanian time it had flooded the entire cratonic margin, and spread southward into the Murzuk and Southern basins, as well as onto the Arabo-Nubian shield. Latest Jurassic--earliest Cretaceous, mid-Cretaceous, and Late Cretaceous transgressions across northeastern Africa recorded in these sequences may reflect worldwide eustatic sea-level rises. In contrast, renewed large supply of detritus during each regression and a comparable subsidence history of intracratonic and marginal basins imply regional tectonic control.

Sedimentology and Paleogeography of Cambrian Transgression in Wyoming: ABSTRACT

Middle and Upper Cambrian deposits in Wyoming represent a classic transgression or onlapping sequence of clastic and non-clastic rocks. However, sedimentologic and paleontologic criteria indicate deposition in a variety of shallow-marine, nearshore environments rather than in a simple progressively deepening marine setting. The basal, coarse-clastic Flathead Sandstone was deposited in fluvial and nearshore marine environments as evidenced by sedimentary and biogenic structures. Braided stream deposits are characterized by alternating sets of planar cross-stratified and horizontally-bedded conglomerate and sandstone. Intertidal deposits consist of ripple cross-stratified sandstone and silty mudstone. Herringbone cross-stratification and desiccation features are common. Subtidal deposits consist of tabular and lenticular sandstone with both large-scale compound cross-stratification and small-scale trough cross-bedding. Younger, fine clastic and carbonate sequences (Gros Ventre and Gallatin Formations) represent more offshore environments where ooids and stromatolites formed on carbonate shoals. The shoreline was oriented north-south throughout the Cambrian and was characterized by numerous embayments and islands of Precambrian basement. A number of local, as well as regional, regressions are recorded within this easterly transgressive sequence. End_of_Article - Last_Page 750------------