Meander-belt System Research Articles

Geomorphological evolution of the Group I fluvial system, AX field, Malay Basin

The AX Field is located 167km off NNE Kerteh, Terengganu of Peninsular Malaysia, southwest of Malay Basin. This study is focused on the 3D seismic analysis of the Lower Miocene fluvio-deltaic Group I. The objective of the study is to evaluate the geomorphological evolution of fluvial succession and interpret the possible intra-basinal and extra-basinal controls on the fluvial evolution. One hundred strata slices were generated in order to evaluate the evolution of the fluvial systems within the Group I interval. Seismic attributes such as Root Mean Square (RMS), sweetness and spectral decomposition were computed to map channel geomorphology. Different fluvial styles were identified in the different parts of the study area. Variation of fluvial style were identified within the Group I interval including the low sinuosity system with tributary channels, high sinuosity system with broad meander belt and braided system. Fluvial channel imaged within the studied interval can be divided into six (6) stages based on the channel morphology. Stage 1 (1875 – 1810 ms), the oldest stage, displays high sinuosity channels. Braided system associated with coal deposition were imaged in Stage 2 (1810 – 1765 ms). During stage 3 (1765 – 1710 ms), the fluvial system is characterized by low to moderate sinuosity channel features that varies in scale and geometry. Stage 4 (1710 – 1675 ms) display fluvial style characterized by low sinuosity-to-straight system with tributary channels. Stage 5 (1675 – 1615 ms) is represented by high sinuosity fluvial system with broad meander belt and point bar. During Stage 6 (1615-1536 ms) the fluvial system displays moderate-to-high sinuosity. The high sinuosity system in period 6 exhibit well-defined meander scroll bar morphology that reveals the extension and translation course of the fluvial system.

The technical challenges and outcomes of ground-penetrating radar: A site-specific example from Joggins, Nova Scotia

The Carboniferous Joggins Formation is known for its complete succession of fossil-rich, coal-bearing strata, deposited in a fluvial meanderbelt depositional setting. Hence, the Joggins Formation outcrop is an excellent analogue for studying the 2D geological complexities associated with meanderbelt systems. In this research, a conventional ground-penetrating radar system was tested with the intent of imaging near-surface, dipping, strata of the Joggins Formation (potentially with subsequent repeats as annual erosion provides new visual calibrations). The survey was unsuccessful in its primary goal, and for future reference we document the reasons here. However, the overlying near-surface angular unconformity was successfully imaged enabling mapping of the approximately 8 m of overlying glacial till. A successful outcome would have allowed observations from the 2D outcrop to be extended into 3D space and perhaps lead to an increased understanding of the small (e.g., bedform baffles and barriers) and large (e.g., channel bodies) scale architectural elements, meanderbelt geometry, and aspect ratios. The study comprises a 42-line, 3.46 km ground-penetrating radar survey using a Sensors and Software pulseEKKO Pro SmartCart system. It was combined with a real-time kinematic differential global positioning system for the georeferencing of survey lines. The 50 MHz antenna frequency, with a 1 m separation, was chosen to maximize the depth of penetration, while still maintaining a reasonable resolution. The results show that many of the lines are contaminated with diffraction hyperbolae, possibly caused from buried objects near or under the survey lines or surface objects near the survey lines. A total of thirteen unique radar reflectors are described and interpreted from this work. The thick clay-rich soil overlying the Joggins Formation probably contributed to significant signal attenuation and the nature of the Carboniferous strata (dip of the beds, pinching and swelling of the beds, bed thickness, etc.) also contributed to imaging difficulties.

Analysis of factors influencing the interpretation of a digitally examined fluvial meanderbelt system: Joggins Formation, Nova Scotia

Clastic reservoir exploration, development, and exploitation are inherently complex with recovery depending largely on the understanding of sand body architecture and interlayered clayey/silty baffles and barriers. Numerous data collection techniques and methods are now widely available for helping to enrich reservoir outcrop analogue data extraction from the well scale to the larger seismic scale. This integrated study uses the inherited, combined data from a localized light detection and ranging survey, measurements taken from a portable handheld spectrometer and air permeameter, in addition to total (or absolute) porosity measurements from thin sections to assist with the analysis of components influencing the interpretation of a digitally analyzed fluvial meanderbelt system outcrop. The purpose is not to perform a detailed reservoir characterization or to model a potential reservoir, but rather to study a section of a reservoir analogue and apply reservoir geology with integrated data collection techniques to highlight potential benefits and shortcomings of this type of approach. A point cloud survey generated from light detection and ranging, coupled with other tools including a portable handheld spectrometer and permeameter, supplements data from the light detection and ranging scan and increases the confidence of interpretations. Spectrometer measurements recorded at the outcrop are used to generate a pseudo-gamma log. Handheld air permeameter measurements give a sense of the permeability of corresponding lithologies, as well as the variability in permeability of the reservoir both laterally and vertically. Light detection and ranging also provides important information regarding rock properties. The high detail of the outcrop images is used for the assessment of reservoir characteristics. The reservoir data leads to an increased understanding of subsurface reservoirs, particularly of the fluvial meanderbelt type. This study shows the importance and drawbacks of a combined digital data collection approach for the analysis of a sedimentary outcrop.

Depositional settings and history of the Lower Miocene Fleming Group, Refugio County, Texas, as defined using seismic geomorphology

Abstract A study of the Lower Miocene Fleming Group in Refugio County, Texas from 3-D seismic and wireline-log data documents continuous strandplain and associated lower-coastal-plain fluvial systems along a wave-dominated shoreline. This study delineates and characterizes, through the interpretation of seismic attributes and seismic geomorphology maps from 15 horizons, three principal styles of stratal patterns: (1) parallel and subparallel, strike-continuous sets of beach ridge and swale deposits within a strandplain system that prograded at least 12 km, (2) sinuous channel belts in a coeval mixed-load meanderbelt system, and (3) acoustically dim, nearly featureless seismic attribute patterns that record homogenous and topographically subdued, shallow-marine mudstones associated with transgressive marine deposits and flooding surfaces. Holocene depositional analogs for these Lower Miocene sandy strandplain deposits include the Nayarit coastline in Mexico, whereas the lower coastal plain between Galveston and Corpus Christi, Texas serves as an appropriate analog for mixed-load meanderbelt systems. The Lower Miocene coastline in Refugio County consists of a regressive phase that was punctuated by two transgressive phases. These episodes of marine transgression correspond to the Anahuac Formation and the Marginulina A shale, respectively. Succeeding events record complex, amalgamated beach-ridge deposits and coeval fluvial-channel systems that exhibit patterns of channel migration, erosion, incision, and downdip fusion of channel belts. This study concludes that potential reservoir geometries and architectures in the Lower Miocene succession are much more complex than previously inferred, the result of rapidly fluctuating and evolving strike-fed shoreline systems and contemporaneous fluvial dip-dispersal systems, some manifested by shore-parallel trends confined in swales. Improved reservoir-development strategies in this prolific hydrocarbon-bearing play should be guided by distribution of sandy depositional elements and the paleogeographic setting.

Climate and base-level controlled fluvial system change and incision during the last glacial–interglacial transition, Roer river, the Netherlands – western Germany

AbstractThe fluvial development of the Roer river in the southeastern Netherlands and western Germany is presented for the Late Pleniglacial, Late-glacial and Early Holocene periods. Reconstruction of fluvial-style changes is based on geomorphological and sedimentological analysis. Time control comes from correlation to the pollen-based biochronostratigraphic framework of the Netherlands combined with independent optically stimulated luminescence (OSL) ages. At the Pleniglacial to Late-glacial transition a system and channel pattern change occurred from an aggrading braided to an incising meandering system. Rapid rates of meander migration, as established for the Late-glacial by optical dating, were likely related to the sandy nature of the substratum and the Late-glacial incision of the Meuse that resulted in a higher river gradient in the downstream part of the Roer. In the Roer valley the Younger Dryas cooling is not clearly reflected by a fluvial system response, but this may also be related to Holocene erosion of Younger Dryas fluvial forms. An important incision and terrace formation was established at the Younger Dryas to Early Holocene transition, probably related to forest recovery, reduced sediment supply and base-level lowering of the Meuse. The results of this study show a stepwise reduction in the number of channel courses from a multi-channel braided system in the Pleniglacial, to a double meander-belt system in the Late-glacial and a single-channel meandering system in the Early Holocene. The results show that the forcing factors of fluvial-system change in the Roer valley are climate change (precipitation, permafrost and vegetation) and downstream base-level control by the Meuse.

Late Quaternary floodplain development along the Stung Sen River in the Lower Mekong Basin, Cambodia

The Stung Sen River, the biggest tributary to Lake Tonle Sap in the Lower Mekong Basin in Cambodia, is characterized by large seasonal changes of water discharge under the Asian monsoon climate and seasonal changes in water level that reach at least 7m and are controlled by the water level of the lake. The Stung Sen River floodplain consists of two geomorphic units: meander belt along the river channel and backmarsh. Coupled observations of outcrops along the river channel and arrays of sediment cores across the floodplain north of Kampong Chheuteal village and Kampong Thom City, c. 150km and c. 70km, respectively, reveal that floodplain environmental changes at c. 11ka were possibly associated with the Holocene onset of the southeast Asian monsoon and probably with the emergence of Lake Tonle Sap. These observations also show that the present backmarsh–meander belt system was established about 5.5ka along with the unique Mekong–Tonle Sap connection, characterized by a reversal in flow direction during the monsoon season. The meander belt materials are replaced as the river channel shifts on a decadal to centennial timescale. Backmarsh sediments at sites Kampong Chheuteal and Kampong Thom had a constant accumulation rate of about 0.5mm/y during the Holocene, contrasting with rates of 0.1mm/y during the late Pleistocene. At around 11ka, a sand layer was deposited over all of the valley around Kampong Chheuteal, while wetlands enlarged around Kampong Thom, probably because of increased rainfall triggered by an enhancement of the Asian summer monsoon. This 11ka horizon has since been covered by clayey sediments keeping pace with the accumulation of lacustrine sediments in Lake Tonle Sap.

Sedimentary sequences, seismofacies and evolution of depositional systems of the Oligo/Miocene Lower Freshwater Molasse Group, Switzerland

ABSTRACTMagnetostratigraphic chronologies, together with sedimentological, petrological, seismic and borehole data derived from the Oligo/Miocene Lower Freshwater Molasse Group of the North Alpine foreland basin enable a detailed reconstruction of alluvial architecture in relation to Alpine orogenic events. Six depositional systems are recorded in the Lower Freshwater Molasse Group. The bajada depositional system comprises 200–400‐m‐thick successions of ribbon channel conglomerates and overbank fines including mud‐ and debris‐flows which were derived from the Alpine border chain. The alluvial megafan depositional system is made up of massive pebble‐to‐cobble conglomerates up to 3 km thick which reveal a fan‐shaped geometry. This depositional environment grades downcurrent into the conglomerate channel belt depositional system, which comprises an ≈2‐km‐thick alternation of channel conglomerates and overbank fines. The sandstone channel belt depositional system is bordered by the 100–400‐m‐thick overbank fines assigned to the floodplain depositional system. At the feather edge of the basin, 50–400‐m‐thick lacustrine sediments in both clastic and carbonate facies represent the lacustrine depositional system.The spatial and temporal arrangement of these depositional systems was controlled by the geometrical evolution of the Molasse Basin. During periods of enhanced sediment supply and during phases of stable sliding of the entire wedge, >2000‐m‐thick coarsening‐and thickening‐upward megasequences comprising the conglomerate channel belt, alluvial megafan and bajada depositional systems were deposited in a narrow wedge‐shaped basin. In the distal reaches of the basin, however, no sedimentary trend developed, and the basin fill comprises a <500‐m‐thick series of sandstone meander belt, floodplain and lacustrine depositional systems. During phases of accretion at the toe of the wedge, the basin widened, and prograding systems of multistorey channel sandstones extended from the thrust front to the distal reaches of the basin.The rearrangement of the depositional systems as a function of changing orogenic conditions created discordances, which are expressed seismically by onlap and erosion of beds delimiting sedimentary sequences. Whereas stable sliding of the wedge succeeded by accretion at the toe of the wedge is recorded in the proximal Lower Freshwater Molasse by a coarsening‐and thickening‐upward megasequence followed by erosion, the opposite trend developed in the distal reaches of the Molasse. Here, fine‐grained sandstones and mudstones were deposited during periods of stable sliding, whereas phases of accretion caused a coarsening‐ and thickening‐up megasequence to form.

Hierarchical Levels of Heterogeneity in a Mississippi River Meander Belt and Application to Reservoir Systems: Geologic Note (1)

Six hierarchical levels of sand heterogeneity that would be present in analogous reservoirs are recognized in a Mississippi River meander-belt system in southeastern Missouri: (1) meander belt, (2) meander scroll, (3) channel, point bar, and splay, (4) lobe sheet, (5) bedding unit, and (6) laminae. Heterogeneity level 1 is similar in size to an oil field and consists of (1) 9 million ac-ft (11.1 billion m{3}) of high-permeability channel sands in numerous meander scrolls and (2) 4 million ac-ft (4.9 billion m{3}) of low-permeability clay plugs (in numerous abandoned channels) that separate a field into isolated pools and reservoirs. Overbank flood-plain muds capping the meander-belt system are barriers to vertical migration of fluids. Heterogeneity level 2 is the size of pools within a field and has 1.2 million ac-ft (1.5 billion m{3}) of highly permeable, laterally accreted channel and point-bar sands, partly or wholly isolated from other sand bodies by low-permeability, abandoned-channel clay plugs. Abandoned mud-filled chutes impede lateral migration of fluids in the upper pa ts of the meander scroll. The third level of heterogeneity is composed of about 70,000 ac-ft (86.3 million m{3}) of individual, permeable channel, point-bar, and crevasse-splay sand bodies, with numerous thin sheets and lenses of low-permeability muds and silts (derived from chute fill and mud drapes) that impede vertical migration of fluids. Heterogeneity level 4 is the lobe sheet unit (reservoir pay zone about 450 ac-ft or 555,000 m{3} in volume) comprising channel, point-bar, and splay sand bodies. The fifth level of heterogeneity is the single bedding unit (about 10 ac-ft or 12,000 m{3}) at the scale of a reservoir flow unit or perforated interval. Permeable, cross-bedded sand bundles are separated by low-permeability, inclined and horizontal mud-silt layers and lenses present along bed-set boundaries. Heterogeneity level 6 is individual sand laminae (grain-flow lenses or grain-fall sheets) separated by textural variations (and hence permeability-porosity variations) and isolated mud-silt laminae. Permeabilities range from 90-160 d in sand; several millidarcys to 50 d in muddy sand, silt, and sandy silt beds from the levee, crevasse, and abandoned-channel fill; and less than 10 md in muddy silts and muds from abandoned-channel and chute-fill mud sheets and laminae. At all levels within a fluvial meander-belt system, these permeability heterogeneities are the principal controls on productivity throughout the life of a reservoir.

Pleistocene Meander-Belt Ridge Pattern near Houston, Texas: ABSTRACT

Meander-belt ridges are the most enduring depositional features of the Pleistocene outcrop near Houston, where those on the Beaumont Formation (seaward from Buffalo Bayou) are evident on most early aerial photographs and soils maps, especially the U.S. Geological Survey 1-ft contour-interval topographic sheets of Galveston and Harris Counties, surveyed in 1915-1926. These show the meander-belt ridges as drainage divides between the modern streams, which developed later on the intervening lowlands (except for upper Mustang Bayou). This systematic transposition of the drainage makes it possible to identify the eroded older ridges farther inland as well as those near the Gulf of Mexico. In the coastal sector from the Brazos River east to Galveston Bay and the San Jacinto River, the meander-belt ridges deflect eastward from successive fluvial terraces along the Brazos Valley and repeatedly branch downslope to the southeast. They belong to four major systems (one of which consists of three subsystems), within each of which the ridge pattern is internally consistent and distinct from contiguous systems. The major systems are separated by the sector's four longer streams, which head just outside the Brazos Valley and flow nearly east: Cypress Creek is the simplest example, and Buffalo Bayou the most complex. These streams flow almost parallel with the nearest meander-belt ridges on their south sides, whereas the ridges from the north approach at much higher angles and ar cut off abruptly by the stream. Each such stream evidently formed along the landward edge of an eastward-prograding increment of coastal plain, where it gathered the blocked drainage off the older plain to the north. As a result, these streams closely follow the unconformable contacts between successive coastal-plain increments, and so mark the approximate landward edges of a succession of distinct unconformity-bounded sequences of strata, each of which is expressed at the surface by its meander-belt ridge system. Substantially greater ages of the sequences farther inland are indicated by successive increases in the average slope of the ground and amount of erosion and weathering and, for the Beaumont units, by contrasting directional rates of meander-belt slope and quality of channel preservation. However, the boundaries of these formations as shown on the Geologic Atlas of Texas maps are not closely related to the corresponding eastward-flowing streams that fairly well define the stratigraphic sequences. Also, immediately west of the Brazos River and along the San Jacinto River some of the equivalent ridge systems are elongated in the dip direction, which makes the larger regional outcrop pattern much more complex than that described between these rivers. End_of_Article - Last_Page 1431------------

Stratigraphy and Depositional History of Coyote Creek-Miller Creek Trend, Lower Cretaceous Fall River Formation, Powder River Basin, Wyoming: ABSTRACT

The Coyote Creek-Miller Creek trend produces high-gravity, low-sulfur oil from a series of Fall River fields in an area generally characterized by west-southwestward monoclinal dip. The trend includes, from south to north, the Coyote Creek South, Coyote Creek, Donkey Creek, Kummerfeld, and Miller Creek fields. The Wood and West Moorcroft fields produce oil from very similar Fall River traps located several miles east and northeast, respectively, of Miller Creek. Only Donkey Creek includes structural closure; all of the other fields produce from purely stratigraphic traps. The reservoir sandstones are characterized by upward-fining sequences. These sequences locally replace and are generally easily distinguishable from two regionally correlative upward-coarsening sequences Analyses of cores and nearby outcrops indicate that the upward-fining sequences accumulated on point bars of a meandering river; the upward-coarsening sequences were deposited on the fronts of northwestward-prograding deltas. Detailed mapping of the fluvial and delta-front facies demonstrates that the Coyote Creek-Miller Creek trend, together with the Wood and West Moorcroft fields, represents a meander-belt system that was contemporaneous with the younger of the two delta-front units. Each of the stratigraphic-type fields occurs at a convexity along the eastern edge of the irregularly shaped meander belt; each consists of numerous point bars. Clay plugs, which resulted from infilling of abandoned meander loops, were preferentially preserved along the margins of the meander belt, where hey now serve as updip permeability barriers between the oil-bearing fluvial and water-wet delta-front sandstones. End_of_Article - Last_Page 864------------

Stratigraphy, Depositional History, and Petroleum Geology of Lower Cretaceous Fall River Formation, Powder River Basin, Wyoming: ABSTRACT

The middle Albian Fall River Formation, better known to petroleum geologists as the Dakota Sandstone, constitutes a northwestward-thinning wedge of predominantly sandy strata under and overlain by marine shale. Two major episodes of deltaic progradation can be recognized in the formation, permitting mapping of lower and upper deltaic members. Study of outcrops, cores, and subsurface relationships indicates that the Fall River consists predominantly of fluvial strata in the southeastern part of the Powder River basin; delta-front and delta-plain facies, which are cut out and replaced locally by northwest-trending meander belts, predominate in an area that trends northeastward across the central part of the basin; the delta-front facies pinches out into offshore marine sh le in the northwestern part of the basin. The large majority of Fall River stratigraphic trap-type fields produce oil and gas from sandy meander-belt deposits. The largest accumulations of hydrocarbons in traps of this type, as exemplified by the Powell-Mexican Springs trend (lower member) and the Coyote Creek-Miller Creek trend (upper member), occur in the more seaward parts of the deltaic members, near the seaward termini of meander-belt systems. Mapping of meander belts and of the surrounding deltaic deposits constitutes a necessary first step in exploration for stratigraphic traps within the Fall River Formation. End_of_Article - Last_Page 864------------

Reservoir Characterization of Mesaverde (Campanian) Bedload Fluvial Meanderbelt Sandstones, Northwestern Colorado: ABSTRACT

Reservoir characterization of Mesaverde meanderbelt sandstones is used to determine directional continuity of permeable zones. A 500-m (1,600-ft) wide fluvial meanderbelt in the Mesaverde Group is exposed as laterally continuous 3-10-m (10-33-ft) high sandstone cliffs north of Rangely, Colorado. Forty-eight detailed measured sections through 3 point bar complexes oriented at right angles to the long axis of deposition and 1 complex oriented parallel to deposition were prepared. Sections were tied together by detailed sketches delineating and tracing major bounding surfaces such as scours and clay drapes. These complexes contain 3 to 8 multilateral sandstone packages separated by 5-20 cm (2-8 in.) interbedded siltstone and shale beds. Component facies are point bars, crevasse splays, chute bars, and floodplain/overbank deposits. Two types of lateral accretion surfaces are recognized in the point bar facies. (1) Gently dipping lateral accretions containing fining-upward sandstone packages. Large scale trough cross-bedding at the base grades upward into ripples and plane beds. (2) Steeply dipping lateral accretion surfaces enclose beds characterized by climbing ripple cross laminations. Bounding surfaces draped by shale lags can seal vertically stacked point bars from reservoir communication. Scoured boundaries allow communication in some stacked point bars. Crevasse splays showing climbing ripples form tongues of very fine-grained sandstone which flank point bars. Chute channels commonly cut upper point bar surfaces at their downstream end. Chute facies are upward-fining with small scale troughs and common dewatering structures. Siltstones and shales underlie the point bar complexes and completely encase the meanderbelt system. Bounding surfaces at the base of the complexes are erosional and contain large shale rip-up clasts. End_of_Article - Last_Page 493------------

Transitions in a Fluvial System--Abo Formation, Southeastern Nacimiento Mountains, New Mexico: ABSTRACT

The Abo Formation (Wolfcampian) in the southeastern Nacimiento Mountains, New Mexico, is an alluvial plain red-bed sequence which records the evolution of a fluvial system within the distal reaches of a large clastic wedge. Sediment was derived from the southern Uncompahgre uplift, with a paleoflow to the southwest. The Abo Formation consists of mudstone, feldspathic sandstone, and intraformational conglomerate. Three major lithofacies associations, representing distinct fluvial depositional settings, may be distinguished. The lowermost division (Unit A) (100+ m, or 330+ ft, thick) is mudstone dominant with isolated sandstone bodies showing cutbanks and lateral accretion surfaces. Ridge-and-swale features were noted on an exhumed point bar surface. The sandstones commonly show a progressive upward decrease in the scale of internal bedforms with a lower scour surface containing intraformational conglomerate. Trough cross-bedding is the dominant bedform in the sandstones with common planar-tabular and flat-bedded interbeds. The uppermost sandstones, characteristic of levee and crevasse-splay sands, are ripple cross-bedded with abundant bioturbation and some plant roots. The overbank mudstones contain numerous pedogenic horizons characterized by calcrete nodules (representative of all stages of evolution), vein networks, pseudo-anticlinal structures, slickensides, color mottling, and bioturbation. Vertebrate remains are found in the mudstones. The middle division (Unit B) (approximately 70 m, or 230 ft, thick) is characterized by thick multilateral channel sandstones and thinner mudstones. The sandstones are coarse grained and contain common intraformational conglomerate. Large-scale trough cross-stratification is the dominant bedform. Pedogenic horizons are present but less common than in unit A. The uppermost division (Unit C) (approximately 30 m, or 100 ft, thick) is entirely composed of reddish-orange, very fine-grained sandstone. Flat bedding is the dominant bedform. High aggradation is indicated by abundant dish structures in the sandstones and aggradational ripple cross-bedding. Mudstone is rare and preserved as thin laminae. End_Page 466------------------------------ The fluvial sequence from Unit A to Unit B is thought to represent the transition from a fine-grained to a coarse-grained meander-belt system. The transition to Unit C reflects a change to flash-flood, ephemeral-stream controlled sedimentation. Allocyclic phenomena are suggested as probably cause of these fluvial transitions. End_of_Article - Last_Page 467------------

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------------

Depositional Systems and Lignite Prospecting Models--Wilcox Group and Meridian Sandstone (Eocene), Northern Mississippi: ABSTRACT

ABSTRACT The first year of a three year Mississippi Mineral Resources Institute project to map the surface and subsurface terrigenous clastic depositional systems and lignite seams of the Eocene Wilcox-Meridian stratigraphic interval in Mississippi has been completed. Data from 620 oil and water well electric logs, 65 sample logs, and 35 surface exposures have been utilized to determine the distribution of the principal sandstone bodies in the northern third of the state. Evidence from eight regional cross sections indicates that the combined Wilcox and Meridian study interval can be subdivided into a minimum of four distinct units: (1) a basal Wilcox progradational interval, (2) a lower Wilcox fluvial-deltaic unit, (3) an upper Wilcox fine-grained fluvial unit, and (4) a Meridian coarse-grained fluvial unit. In extreme northwestern Mississippi the highest 200 feet of the Wilcox is made up of massive sandstone and is genetically related to the overlying Meridian Sandstone (Claiborne Group). This is the Meridian-Upper Wilcox Aquifer System of hydrologists. Hence, in the northern-most counties of the study area, the upper Wilcox and Meridian become a single, very massive, coarse-grained fluvial unit. Sandstone-body geometry and lignite distribution within the Wilcox-Meridian interval have been evaluated using net-sandstone isolith, net sandstone percent, thickness of most massive sandstone unit, and lignite isopleth maps. Vertical curve shapes for individual wells furnish additional information for interpreting specific environments of deposition. Dendritic, contributory sandstone belts of low to moderate sinuosity comprise the dominant geometry displayed by these rock bodies, even for the basal Wilcox progradational interval. Nine distinct sandstone belts 6 to 10 miles wide are noted in the lower Wilcox; seven of these same belts are also present in the upper Wilcox. The Meridian has a much higher proportion of sandstone, averaging approximately 70 percent, but the 90 percent sandstone contour line highlights 8 sandstone belts occupying roughly the same trends as the sandstone maxima for the lower Wilcox. A down-dip bifurcating (distributary) net-sandstone pattern characteristic of the upper delta-plain facies has been mapped in the basal Wilcox, lower Wilcox, and upper Wilcox units in southern Bolivar, Yazoo, Sharkey, Issaquena, and Washington Counties. Lignite isopleth maxima in central Washington County of 17 and 18 for the lower and upper Wilcox lend further support for identifying the proximal delta plain. The principal distal deltaic sandstone bodies of the Wilcox high-constructional Holly Springs Delta System, however, have been mapped by other researchers as being 75 to 125 miles downdip from this area. Lignite prospecting in the Wilcox-Meridian section of North Mississippi can be greatly aided through an understanding of the geographic and stratigraphic distribution of the major depositional systems. Regarding fluvial systems, organic debris preferentially accumulates with fine-grained sediment. Significant lignite deposits most likely formed in overbank deposits away from active stream channels. As a result, commercially exploitable seams associated with ancient fluvial systems should be sought in interstream basins of fine-grained meanderbelt systems where values of lower than 50% sandstone occur. Such areas are present in the lower Wilcox of Calhoun, Webster, and Choctaw Counties and in the upper Wilcox as far north as Marshall County. Coarse-grained meanderbelt and incised valley-fill fluvial systems represent much less satisfactory settings for lignite formation, inasmuch as most of the fine-grained material is transported downdip without being deposited. Consequently, the Meridian coarse-grained meanderbelt system throughout North Mississippi is barren of lignite. High-constructive deltas furnish optimum conditions for the accumulation of organic material. In North Mississippi, the thickest individual seams and the greatest number of seams occur at the junction of the alluvial plain and proximal delta plain. This junction is the position where channels first bifurcate to exhibit a distributary pattern and is well demonstrated by lower and upper Wilcox sandstone bodies in Washington, Sharkey, and Yazoo Counties. End_Page 283------------------------

Reevaluation of Depositional Environments of Salt Wash Member of Morrison Formation, Uravan Mineral Belt, Southwest Colorado: ABSTRACT

The Uravan mineral belt of southwestern Colorado has proved to be a significant source of uranium and vanadium. Since 1948, 63 million lb of U3O8 and 332 million lb of vanadium have been produced. About 90% of the ore has come from the upper sandstone ledge of the Salt Wash Sandstone Member of the Morrison Formation. It has long been recognized that the Salt Wash was the product of fluvial sedimentation. More recently most authors have concluded that the Salt Wash was deposited as a braided stream system and some have proposed that the system was part of a large alluvial fan complex because of the arcuate pattern of the belt. However, results of a detailed sedimentologic analysis of this sequence in the Slick Rock district suggests that the entire Salt Wash including the uraniferous upper ledge was deposited in a fine-grained meander belt system. Evidence for this interpretation is based on the high percentage (up to 54%) of fine-grained bioturbated and/or rooted flood-plain sediments; associated coarsening-upward crevasse splay or overbank splay deposits; and the abundance of fining-upward point bar sequences. The fine to medium-grained sandstones of the point bar deposits crop out as a series of discontinuous ledges numbering between 3 and 6 throughout the district. Each ledge consists of a number of abbreviated and complete point bar sequences ranging in thickness from a few feet to over 25 ft. These point bar deposits grade laterally and vertically into levee, abandoned channel, and crevasse splay assemblages and are interbedded with thick sequences of overbank mudstones, siltstones, and thin sandstones. End_of_Article - Last_Page 796------------

Flood-Plain Sequences of Fine-Grained Meander-Belt System, Lower Wasatch and Upper Fort Union Formations, Central Powder River Basin, Wyoming: ABSTRACT

Models of fine-grained meander-belt systems generally emphasize the coarser sandstone facies of channel origin and neglect the relatively fine-grained overbank sediments of the flood plain. This emphasis is unfortunate because of the volumetric importance of the flood-plain sequences in many ancient stratigraphic successions such as the Tertiary coal and uranium-bearing rocks of the northern and central Rocky Mountain and Great Plains provinces. An appreciation of the processes of formation, and lateral and vertical successions of these deposits can provide valuable information for mining and reclamation activities. Macroscopic and microscopic studies of 18 continuous cores through the lower Wasatch and upper Fort Union Formations in a 4-sq mi (10 sq km) area of southeastern Campbell County, Wyoming, allow recognition of six flood-plain environments associated with point-bar sequences. These flood-plain sequences, recognized on the basis of primary and secondary sedimentary structures, presence and type of bioturbation, organic content, and presence or lack of preferred vertical and lateral successions, include lacustrine, lacustrine delta fill, well-drained and poorly drained swamps, crevasse splay, levee, and abandoned channel. Recognition End_Page 831------------------------------ is based in part on comparison with criteria developed for Holocene sequences of the Atchafalaya flood plain. Lacustrine deposits consist of highly organic, parallel-laminated clay with some silty laminations. Burrow structures are quite common and nodules of pyrite are present. Calcium carbonate is common along bedding planes. Lacustrine delta fills form coarsening-upward sequences which grade upward from lake deposits into interlaminated silts, clays, and sands with abundant ripple laminations, steeply dipping spill-over foresets, and loading features. Burrowing and pyrite nodules are present in the lower part and coarse organic debris and roots are common in the uppermost part. Poorly drained swamps are best recognized by their high content of organic fragments and root bioturbation that destroys most depositional structures. Pyrite and siderite are common as nodules. Well-drained swamps, n contrast, are relatively high in silt content and contain few organic fragments. Roots and root bioturbation structures are well preserved and stratification is only vaguely discernible. Iron oxide nodules are common, especially as crusts surrounding roots. Calcium carbonate nodules also are present in these deposits. Crevasse-splay deposits are similar to those of well-drained swamps except that stratification is better preserved and thin (less than 1 m) coarsening-upward sequences are common. These sequences contain ripple laminations, shallow scour-and-fill structures, and may be capped by rooted beds of sand. Natural levees are recognizable as fining-upward sequences with a well-stratified, burrowed, sandy lower part grading upward into a finer grained, root-bioturbated upper part ith iron oxide and calcium carbonate nodules. Finally, abandoned channel-fill sequences consist of a complex interlayering of swamp, lacustrine, lacustrine-fill, and crevasse-splay deposits lying on, and often below, point-bar sequences. End_of_Article - Last_Page 832------------

Depositional Facies and Uranium Occurrence, Triassic (Dockum Group), Texas Panhandle: ABSTRACT

Late Triassic (Dockum Group) rocks accumulated in relict Paleozoic basins bound by the Amarillo uplift on the north and the Glass Mountains on the south. Basins were reactivated by late Paleozoic or early Mesozoic tectonic activity that created the Gulf of Mexico. More than 2,000 ft (600 m) of terrigenous clastics, derived mostly from older sedimentary rocks, accumulated within the basin. Source areas were in Texas, Oklahoma, and New Mexico. The Dockum Group accumulated in a variety of depositional systems including (1) braided and meandering streams, (2) alluvial fans and fan deltas, (3) highly constructive lobate deltas, (4) lacustrine systems including ephemeral and relatively long-lived lakes, and (5) mud flats. Dockum sedimentation was cyclic, a reflection of alternately humid and arid climatic conditions. During humid climatic conditions lake level was relatively stable. Meandering streams supplied sediment to high-constructive lobate deltas in the central basin area; braided streams and fan deltas were dominant depositional elements within southern and northern basin areas. During arid climatic conditions base level was lowered, stream valleys evolved, and small fan deltas developed along ephemeral lake margins; evaporites, calcretes, silcretes, and soils developed on floors of ephemeral lakes and on delta platforms. Uranium occurs within about 25 depositional facies. Highest uranium values are in lacustrine facies which developed under arid climatic conditions. Channel-lag facies of meander-belt systems generally exhibit consistently higher uranium values than other depositional facies. Crevasse-channel and crevasse-splay deposits locally contain mineralized carbonized wood. Delta-front sandstones of high-constructive lobate deltas contain uranium. Radioactive minerals are present within conglomeratic parts of the valley-fill sequence. Although a relation exists between uranium occurrence and depositional facies, prediction of uranium occurrence is difficult because of a complex groundwater history. End_of_Article - Last_Page 835------------

Fluvial, Deltaic, and Slope Reservoir Sandstone in West-Central Texas: ABSTRACT

More than 300 Lower Permian sandstone oil and gas fields are present in an area of about 7,000 sq mi on the Eastern shelf and slope of the Midland basin. All these fields appear to be in stratigraphic traps in fluvial, deltaic, and slope depositional environments. Two Lower Permian Cook fluvial sandstone systems extend in a downdip direction across the Eastern shelf to the shelf edge, where progradational Cook sandstone facies form two high-constructive deltas. These are the proximal parts of a thick terrigenous clastic wedge of sediments covering the eastern slope. Outcrops of Cook channel-fill deposits grade upward from conglomerate and quartz sandstone, to gray shale. Downdip from outcrop to the shelf edge, Cook sandstone is present in one meander belt system 82 mi long. The upper 30 mi is about five mi wide and bounded by shale and two thin lenticular limestone beds. The belt narrows to about one mi where adjacent limestone beds are thicker and were more resistant to lateral channel erosion. About 58 mi downdip from outcrop, the meander belt divides into two belts. Each belt is about one mi wide and bounded by shale and thick limestone beds to the shelf edge. Point bars (with characteristic electric-log SP curves), crevasse splays, and/or natural levees are discernible in the meander belts. Many stratigraphic traps are present in point bars, in sandstone belts perpendicular to regional dip, and in belts of sandstone that overlie buried reefs, hills, structures, and other channel sandstones. Paleocreekology is applicable in locating topographic features on the paleoshelf. Both Cook sandstone deltas, nearly joining, are in an area of 295 sq mi. Delta-plain facies consist of interbedded dark gray shale, fine-grained, lenticular sandstones, and thin coal lenses underlain by thick, medium-grained sandstone facies in distributary channels. Electric-log SP curves of the channel sandstones are characteristic. Delta front distributary mouth bar and sheet-sandstone facies overlie thick prodelta shale and thin, lenticular sandstone. The Group 4000 Cisco sandstone field is the only oil field of any significance in the two deltas. It produces from delta-plain sandstone facies. West of the shelf edge, the Cook wedge of shale and sandstone lenses reaches a maximum thickness of about 1,300 ft at midslope and thins basinward. The base is a maximum gradient of about 3° at midslope. The Northeast Bloodworth Canyon sandstone field produces from Cook sandstone, probably a turbidite and slump deposit, evidently deposited in a submarine channel. The Jameson Strawn sandstone field, about 35 sq mi in area, produced from Cook lower slope sandstone lenses, probably part of a submarine fan. End_of_Article - Last_Page 420------------