Crevasse Splay Deposits Research Articles

Depositional and Diagenetic History of Hosston Formation (Travis Peak), Nuevo Leon Group, Trawick Field, Nacogdoches County, Texas: ABSTRACT

ABSTRACT Five major lithofacies were identified in the Hosston Formation of Trawick Field, Nacogdoches County, Texas. These were identified based upon vertical changes in sedimentary structures, textures, and lithologic composition. Facies identified in the Hosston Formation included: (1) a clean, crossbedded, tidal-channel facies, (2) a sandy, bioturbated, tidal-flat facies, (3) thin, rapidly deposited, crevasse splay deposits, (4) interdistributary bay sands, silts, and muds and (5) partially reworked distal deltaic sandstones. Distinct differences are observed between the upper and lower Hosston facies due to the transition from a lower, more continental fluvio-deltaic system to an upper, marginal marine delta-fringe system. In the lower Hosston, only the channel, crevasse splay, and interdistributary bay facies are significant. The regressive nature of the formation is documented through changes in color and detrital constituents as well as in the presence of prodelta muds and shallow carbonates found exclusively in the upper Hosston. The Hosston Formation has characteristically low porosity and permeability values. Variations in the reservoir quality of Hosston sands is controlled both by environment of deposition and diagenesis. Two diagenetic pathways are postulated. In the first, extensive silica and phyllosilicate cementation dramatically reduce reservoir potential and porosity values rarely exceed 2 percent. In the second, early hydrocarbon migration inhibits additional cementation and therefore suppresses further porosity reduction. Samples following this pathway have porosities of up to 30 percent. Late stage diagenetic events include cementation by calcite and anhydrite as well as isolated occurrences of intergranular albite cementation. Channel sands of the lower Hosston maintain the highest consistent reservoir qualities.

Submarine Fan Facies of Upper Cretaceous Strata, Southern San Rafael Mountains, Santa Barbara County, California: ABSTRACT

A 2900-m thick Campanian-Maestrichtian(.) turbidite sequence in Upper Mono Creek Canyon is interpreted to be a progradational submarine fan complex comprised of outer fan, middle fan, inner fan, and slope facies. The basal 600 m of the section consists of thinly bedded, laterally continuous fine sandstones, siltstones, and mudstones (mainly Mutti and Ricci Lucci facies D), interpreted to be outer fan interlobe and lobe-fringe deposits. These are punctuated by infrequent medium to very thickly bedded, flat-based, fine to coarse sandstones (facies C and B), which commonly coarsen and thicken upward, and are interpreted to be depositional lobes. Overlying these deposits are approximately 1400 m of middle fan deposits composed of frequent lenticular, commonly channelized and amalgamated, thickly bedded, fine to very coarse sandstones (facies C and B) organized in fining- and thinning-upward sequences, interpreted to be braided-channel deposits. These alternate with less common nonchannelized coarsening- and thickening-upward sequences suggestive of lobe-apical cycles. These multistory sand deposits are nested within thick intervals of fine sandstones, siltstones, and mudstones (facies C and D), interpreted to be levee, crevasse-splay, and interchannel deposits. Interfingered with and overlying these deposits are approximately 500 m of fining- and thinning-upward or noncyclic, erosionally based, commonly amalgamated, verymore » thickly bedded, medium to very coarse sandstones, pebbly sandstones, and conglomerates (facies A and B), interpreted to be inner fan deposits. Intercalated within this facies, infrequent, laterally discontinuous, thin to thickly bedded, fine to coarse sandstones, siltstones, and mudstones exist, interpreted to be interchannel, levee, and possibly channel-fill deposits.« less

Lakota Formation, Southern Black Hills, South Dakota: An Early Cretaceous Evolving Fluvial System: ABSTRACT

The fluvial, Early Cretaceous Lakota Formation consists of four spatially and temporally distinct sandstone units in the southern Black Hills and southeastern Powder River basin. Three of these units crop out in proximity to an area of uranium roll-front development (Edgemont mining district) where approximately 2300 wells were drilled and logged. Comparison of the resistivity logs of several of these wells with continuous cores of the Lakota Formation confirms their lithologic sensitivity. These logs (utilized to assist in subsurface facies interpretations where cores were not available), cores, and outcrops are the basis for the following facies interpretations. The discharge, sediment load, and resulting sinuosity of this fluvial system varied substantially throughout the time of Lakota deposition. The oldest unit consists of tabular deposits with complex internal architecture comprised of cross-cutting lateral accretion deposits. Upward-fining grain size, upward-decreasing scale of sedimentary structures, and the angular relationship between lateral accretion surfaces and overlying crevasse-splay deposits support this conclusion. The intermediate unit of ephemeral stream sediments is characterized by abundant pebble- and cobble-strewn erosional surfaces with up to 1.5 m relief, very poor clast sorting, and trough and planar cross-bedding with concave-upward foresets. The youngest unit has a predominance of tabular cross-bedding withmore » back flow climbing ripples and low dispersion of paleocurrent directions, suggesting a relatively straight, bed-load-type channel dominated by trains of sand waves.« less

Alluvial Architecture of Channel Belt Margins of Mississippi River, False River Region, Louisiana: ABSTRACT

The term alluvial architecture refers to the spatial arrangement and stratigraphic relationships between fixed channel belts (shoestring sands) and finer grained intervening flood-basin deposits. Overbank subenvironments associated with an active channel belt of the Mississippi River, including the levee, crevasses, and splay deposits, the upper point bar, and the abandoned channels, were cored using a standard vibracorer and Giddings Rig Soil probe. This study proposes that two end-member types of transition zones exist between channel belts and flood basins at an instant in time for a simple, sine-wave-shaped stream meandering in a fixed channel belt: (1) a levee to backswamp transition zone (type A), and (2) an upper point bar to backswamp transition zone (type B). T pe A transition zones, which consist of interstratified crevasse channel-fills, crevasse splay sheet sands, and fine-grained material of the natural levee, have greater potential for preservation than do type B transitions because crevasse splay deposits may extend for kilometers out into flood basins (type A) whereas preexisting point-bar deposits are destroyed during channel migration within the channel belt (type B). In type B transition zones, a sharp, erosional contact between the backswamp and upper point-bar subenvironments exists because of lateral migration of the point bar through preexisting facies within the fixed channel belt. End_of_Article - Last_Page 1420------------

Stratigraphy and Depositional Environments--Krebs Formation in Southeastern Kansas: ABSTRACT

End_Page 1317------------------------------The Krebs Formation (Middle Pennsylvanian-Desmoinesian) forms the lower portion of the Cherokee Group in the Cherokee basin of southeastern Kansas. The Krebs Formation near its outcrop in Cherokee and Crawford Counties consists of 78% shale and mudstone, 18% sandstone and siltstone, 3% coal, and 1% limestone, comprising a total thickness of 120 to 220 ft (37 to 67 m). Integration of data from continuous cores, outcrops, and geophysical logs provides a detailed stratigraphic framework and facilitates interpretation of depositional environments. Coal beds and associated seat-rock units, some having an areal extent of several thousand square miles, provide excellent stratigraphic marker beds for correlation of discontinuous reservoir sandstones. Radioactive dark-gray shale units and argi laceous limestone units often overlie coal beds and may be equally widespread. Net-sandstone isolith maps reveal the presence of a lobate deltaic complex in southwestern Missouri, characterized by both stacking and offset of major sandstone bodies. Coal beds commonly cap upward-coarsening, mud-dominated sequences consisting of dark-gray shale with occasional argillaceous limestones overlain by lenticular-bedded shale or wavy-bedded siltstones. This vertical transition of lithofacies is interpreted to result from the progradational infilling of large interdistributary bays. Coarsening-upward sandstone sequences--consisting of lenticular-bedded shale grading upward into wavy-bedded siltstone, flaser-bedded sandstone, and rippled or cross-bedded sandstone--represent distributary mouth-bar or crevasse-splay deposits. Fining-upward sequences--composed of a basal scou surface overlain by mud-clast conglomerates, large-scale cross-bedded sandstone, and rippled or flaser-bedded sandstone--are interpreted to be channel-fill or point-bar deposits. End_of_Article - Last_Page 1318------------

Rediscovering an Abandoned Reservoir: Lakota Formation at Lost Soldier Field, Fremont County, Wyoming: ABSTRACT

Lost Soldier field (T26N, R90W) is one of several fields that parallel the Granite Mountains uplift. The subsurface structure at the Lakota level is a northwest-trending, doubly plunging anticline. The Lakota Formation at Lost Soldier field is a complex, fluvial-sand sequence, representing braided- and meandering-stream environments. Lakota channels have both podlike and sheetlike geometries, and grade laterally into crevasse splay and floodplain deposits. The Lakota reservoir was discovered in 1922. Between 1922 and the early 1950s, the reservoir produced about 5 million bbl of oil. The early Lakota wells were drilled mainly on the top of the structure and along lease lines. These early wells were all shut-in by the early 1950s when the Lakota reservoir, thought to be depleted, was abandoned. During the late 1970s, following Amoco's purchase of Lost Soldier field, Amoco geologists noticed high-resistivity anomalies in the Lakota Formation. In 1979 a well was recompleted in the Lakota with an IP of 65 BOPD and 22 BWPD. The successful recompletion proved that the early Lakota producers had not completely drained the reservoir. Since 1979 Amoco has completed or recompleted 22 Lakota producers. These wells have been drilled to define the oil-water contact for the Lakota, to develop a thick net-sandstone trend on the southern end of the structure, and to test the Lakota production potential in other parts of the field. It is estimated that the recent Lakota wells will produce about 1 million bbl of oil. The average IP for the wells is 67 BOPD and 104 BWPD. End_of_Article - Last_Page 865------------

Facies Analysis of Upper Jackfork Formation (Pennsylvanian), DeGray Dam, Arkansas: ABSTRACT

The DeGray Dam spillway cut displays perhaps the best exposed section of the upper Jackfork formation (Pennsylvanian) in the Ouachita Mountains of Arkansas and Oklahoma. Comprising more than 321 m (1,050 ft) of interbedded sandstone and shale, this deep-water succession was originally interpreted as a sequence of alternating proximal and distal turbidites, and subsequently as outer-fan depositional lobes. Recent detailed facies analysis, however, demonstrates that the succession represents a mid-fan association of channel and interchannel deposits. Channelized intervals consist of stacked thinning-upward and/or amalgamated packages characterized by Mutti and Ricci Lucchi facies A, B, C, and G. Associated interchannel intervals consist of facies C, D, E, and G beds that are randomly interbedded or form thickening-upward packages that superficially resemble depositional lobes. These lobe-like packages, which are generally less than 4 m (13 ft) thick, are interpreted as crevasse-splay deposits. A similar association of channel and interchannel deposits can also be observed in nearby outcrops of the underlying lower Jackfork. A longitudinal submarine fan system, analogous to the present-day Bengal fan, is visualized as the overall depositional setting for the Jackfork formation. End_of_Article - Last_Page 509------------

Sedimentology of Fraser River delta peat deposits: A modern analogue for some deltaic coals

On the Recent lobe of the Fraser River delta peat accumulation has actively occurred on the distal lower dilta plain, the transition between upper and lower delta plains, and the alluvial plain. Distal lower delta plain peats developed from widespread salt and brackish marshes and were not influenced appreciably by fluvial activity. Lateral development of the marsh facies were controlled by compaction and eustatic sea level rise. The resulting thin, discontinuous peat network contains numerous silty clay partings and high concentrations of sulphur. Freshwater marsh facies formed but were later in part eroded and altered by transgressing marine waters. The peats overlie a thin, fluvial, fining-upward sequence which in turn overlies a thick, coarsening-upward, prodelta—delta front succession. Lower delta plain—upper delta plain peats initially developed from interdistributary brackish marshes and were later fluvially influenced as the delta prograded. The thickest peats occur in areas where distributary channels were abandoned earliest. Sphagnum biofacies replace sedge-grass-dominated communities except along active channel margins, where the sedge-grass facies is intercalated with overbank and splay deposits. The peats are underlain by a relatively thin sequence of fluvial deposits which in turn is underlain by a major coarsening-upward delta front and pro-delta sequence. Alluvial plain peats accumulated in back swamp environments of the flood plain. Earliest sedge-clay and gyttjae peats developed over thin fining-upward fluvial cycles or are interlaminated with fine-grained flood deposits. Thickest accumulations occur where peat fills small avulsed flood channels. Overlying sedge-grass and Sphagnum biofacies are horizontally stratified and commonly have sharp boundaries with fine-grained flood sediments. At active channel margins however, sedge-grass peats are intercalated with natural levee deposits consisting of silty clay. These levees reduce both the number and size of crevasse splay deposits. Coal originating from peats of the different environments of the Fraser delta would vary markedly in character. Peats of the lower delta plain will form thin lenticular coal seams with numerous splits and have a high ash and sulphur content. Peats from the lower to upper delta plain will be laterally extensive and of variable thickness and quality. Basal portions of the seams will contain numerous splits and have a high sulphur content whereas upper portions will be of higher quality. Peats from the upper delta plain—alluvial plain will form thick, isolated and laterally restricted coal seams characterized by low ash and sulphur contents.

Evolution of a Fluvial Clastic Wedge, Abo Formation (Wolfcampian), Sacramento Mountains, New Mexico: ABSTRACT

Outcrops of the Abo Formation (Wolfcampian) in the northern and central Sacramento Mountains of south-central New Mexico record the evolution of fluvial systems within a major clastic wedge. This wedge was derived from the Pedernal uplift and prograded westward into the Orogrande basin. Regionally, the Abo Formation thickens from 60 m (197 ft) in the central Sacramentos, to over 545 m (1,788 ft) in the northern portion of the range, a distance of approximately 29 km (18 mi). Superposed on the regional pattern are local thickness variations of up to 91 m (300 ft). These occur within distances as small as 0.4 km (0.25 mi), and represent local tectonic and paleotopographic controls on sedimentation. Clast-supported cobble conglomerates, associated with laminated mudstones and discontinuous coarse arkosic sandstones, are present near the base of the Abo Formation. The conglomerate lenses range from 0.5 to 20 m (1.5 to 66 ft) thick, and from 15 m (49 ft) to several kilometers wide. They exhibit poor stratification, occasional clast imbrication, and near horizontal to concave-upward erosional bounding surfaces. Locally, nodular CaCO3 occurs in beds or scattered throughout the associated mudstones. The central portion of the sequence is characterized by abundant conglomeratic medium to very coarse-grained arkosic sandstone lenses interbedded with laminated mudstones. These lenses range from 0.25 to 10 m (0.8 to 33 ft) in thickness, and 10 to 110 m (33 to 361 ft) in width. Commonly, several lenses will coalesce, forming laterally extensive belts. Both erosional and depositional (lateral accretion) bounding surfaces are well developed within individual lenses. Low-angle, large-scale trough cross-bedding is the dominant stratification type within these units, however, small to large-scale tabular foresets occur, as do planar, ripple, and small-scale trough stratification. Occasionally, individual units fine upward in grain size and/or scale of structures. This sandstone-rich horizo varies laterally in thickness, and grades upward into a mudstone-rich horizon at the top of the Abo Formation. The upper interval of the Abo sequence contains infrequent fine to coarse-grained arkose lenses enclosed in laterally extensive red mudstones. Paleosols and caliche horizons are present, but not common, in the mudstones. The basal conglomeratic horizon of the clastic wedge represents braided stream deposition at the distal end of arid alluvial fans associated with the tectonically active Pedernal uplift. Caliche and paleosols developed in the overbank mud drapes and levees of these deposits. The conglomerates grade into arkosic sandstones, reflecting the intense weathering and erosion of the granitic core of the uplift. The multilateral sandstone lenses characteristic of the mid portion of the Abo Formation were deposited in low-sinuosity, coarse-grained (bed-load) meander belts; however, higher sinuosity, mixed-load channel and crevasse splay deposits also occur locally. Finally, as the Pedernal source area was buried by its own debris, thick, broad flood-plain deposition predominated. Sandstones ass ciated with the red mudstones were deposited in shallow ephemeral channels and crevasse splays. End_of_Article - Last_Page 550------------

Depositional Setting of the Salt Wash Member of the Morrison Formation, Southwest Colorado

ABSTRACT The Jurassic Salt Wash Member of the Morrison Formation, southwestern United States, is host to extensive uranium and vanadium mineralization. The ores occur in sandstones deposited in a complex fluvial system. The depositional geometry of the member is that of a wedge-fan. Detailed facies analysis of an area of the mid-paleoslope part of the member (the Slick Rock Uranium District) documents three major sandstone facies interbedded within a sequence of mudstones, siltstones, and thin sandstones. A large-scale trough cross-bedded facies, characterized by broad and generally thin trough crossbeds which rest locally on giant crossbeds as much as 6 m thick and 50 m wide, is the dominant sandstone facies. Paleocurrent azimuths are unimodal and have low consistency ratios indicating little variation in paleocurrent direction. This facies represents deposition in northeast-flowing, low-sinuosity trunk streams. The low-sinuosity streams were confined between leveed banks and probably had low-braiding indexes. Course change was primarily accomplished by avulsion. The second most abundant sandstone facies is primarily characterized by medium- to small-scale trough crossbedded sandstone within lensoid, fining-upward cycles. A progressive vertical decrease in the scale of structures and ariable paleocurrent orientations are characteristic of these depositional units. This facies represents the point bars of meandering streams that, according to paleocurrent and net sand map evidence, acted as tributaries to the low-sinuosity trunk channels. The third sandstone facies, interpreted as crevasse-splay deposits, contains coarsening-upward sandstone packages with variable structures and a distinctive convex geometry. The mudstone, siltstone, and thin sandstone facies averages 50 percent of the member and is subdivided into four subfacies. Oxidized red mudstone with numerous thin, laterally persistent, sandstone and siltstone interbeds represents well-drained floodplain deposits. Less abundant are depositional units composed of calcareous, oxidized silty mudstone with minor sandstone. These units represent sedimentation in shallow ephemeral lakes. Deeper water carbonaceous, gray-green mudstones deposited under anoxic conditions and clay plugs, representing abandoned channel fills, are rare in the district. Detailed analysis of sedimentary facies, sandstone distribution, and regional facies relationships leads to the conclusion that the Salt Wash Member in the Slick Rock District is the product of sedimentation on a fan apron marginal to an alluvial fan or coalesced fan complex. Sandstone thicks that transect the district represent depositional axes of this fluvial system.

Submarine Fan Sedimentation, Ouachita Mountains, Arkansas and Oklahoma: ABSTRACT

More than 10,000 m (33,000 ft) of interbedded sandstones and shales comprise the Upper Mississippian and Lower Pennsylvanian flysch succession (Stanley, Jackford, Johns Valley, Atoka) in the Ouachita Mountains of Arkansas and Oklahoma. Deposited primarily by turbidity current and hemipelagic processes in bathyal and abyssal water depths, these strata form major submarine fan complexes that prograded in a westerly direction along the axis of an elongate remnant ocean basin that was associated with the collision and suturing of the North American and African plates. A longitudinal fan system is visualized as the depositional framework for these strata which were deposited in a setting analagous to the modern Bengal fan of the Indian Ocean. Facies analysis of the Jackfork Sandstone indicates that inner fan deposits are present in the vicinity of Little Rock, Arkansas; middle fan distributary channel and crevasse splay deposits occur at DeGray Dam, Arkansas; and outer fan depositional lobe deposits are present in southeastern Oklahoma. Basin plain equivalents are postulated to exist as far away as the Marathon region in west Texas. Boulder-bearing units (olistostromes) with exotic clasts were shed laterally into the Ouachita basin, primarily from its northern margin. These olistostromes occur throughout the fan succession in all facies (i.e., inner, middle, and outer fan). This relationship may serve as a useful criterion for recognizing similar fan systems in the rock record. End_of_Article - Last_Page 517------------

Two Depositional Models for Pliocene Coastal Plain Fluvial Systems, Goliad Formation, South Texas Gulf Coastal Plain: ABSTRACT

The Goliad Formation consists of four depositional systems--the Realitos and Mathis bed-load fluvial systems in the southwest and the Cuero and Eagle Lake mixed-load fluvial systems in the northeast. Detailed facies analysis indicates that Goliad bed-load and mixed-load fluvial systems represent two contrasting depositional environments. Five facies are recognized in the Realitos and Mathis bed-load fluvial systems: (1) primary channel-fill facies, (2) chaotic flood channel-fill facies, (3) complex splay facies, (4) flood plain facies, and (5) playa facies. Both channel-fill and splay deposits consist primarily of gravel, sandy gravel, and coarse to medium gravelly sand. Primary channel-fill deposits are characterized by large-scale accretionary foresets and trough cross-beds, localized scour-and-fill structures, and massive beds. Fining upward trends are crudely developed to absent, and no diagnostic vertical sequence of sedimentary textures and structures has been recognized. Chaotic flood channel-fill deposits are characterized by erratic interbedding of coarse and fine units, and of erosional and accretionary feat res. Individual beds have chaotic or poorly ordered textural profiles, and an absence of well-developed internal structures. Extensive flocks of stacked scour-and-fill structures are common. Complex splay deposits share characteristics of both crevasse and sheet splays. A model for Realitos-Mathis depositional environments shows arid-climate braided stream complexes with extremely coarse sediment load, highly variable discharge, and marked channel instability. Broad, shallow, straight to slightly sinuous primary channels were flanked by wide flood channels. These acted as part of the flood plain under most conditions, but as channels during high intensity flood flow. Flood channels passed laterally into broad, low-relief flood plains. Small playas occupied topographic lows near large channel axes. Three facies are recognized in the Cuero and Eagle Lake mixed-load fluvial systems: (1) channel-fill facies, (2) crevasse splay facies, and (3) flood plain facies. Channel-fill deposits consist of coarse to medium sand and gravelly sand, with a variety of large- and intermediate-scale sedimentary structures. Small-scale structures are common in finer grained beds. Fining-upward sequences are moderately well developed and commonly stacked. Crevasse splay deposits consist of medium to fine sand and silt, with abundant small-scale sedimentary structures. A model for Cuero-Eagle Lake depositional environments shows coarse-grained meander belts in a semi-arid climate. Slightly to moderately sinuous meandering streams were flanked by low, poorly developed natural levees. Crevasse splays were common, but tended to be broad and ill-defined. Extensive, low-relief flood plains occupied interaxial areas. While a number of models for coarse-grained meander belts exist in the literature, there are few models for arid-climate, gravel-rich braided stream complexes. These systems are likely to be characterized by extreme hydrodynamic complexity, which will be reflected in the resultant sedimentary package. The model proposed for the Realitos and Mathis fluvial systems may aid in recognition of analogous ancient depositional systems. In addition, since facies characteristics exercise broad controls on Goliad uranium mineralization, the proposed depositional models aid in defining target zones for Goliad uranium exploration. End_of_Article - Last_Page 484------------

Channeling in Paleocene Coals, Northern Powder River Basin, Montana: ABSTRACT

Interpretation of 1,200 geophysical logs in the northern Powder River basin, Montana, reveals the paleodrainages influencing coal deposition during the deposition of the Tongue River member (Paleocene, Fort Union Formation). Four channels with associated crevasse splay deposits are recognized: (1) an east-west Rosebud drainage near Colstrip, (2) a north-south Wall channel near Birney, (3) a north-south Dietz drainage near Tongue River Reservoir, and (4) a north-south Anderson channel in the vicinity of Moorhead. These channels support the concept of a major northeast-flowing drainage system during deposition of the Tongue River Member. Identification of these channels serves as a guide to future coal exploration. End_of_Article - Last_Page 1340------------

Depositional Environments and Reservoir Properties, Lonetree Field, Southern Denver Basin, Colorado: ABSTRACT

Significant accumulations of oil and gas are stratigraphically trapped in the Lower Cretaceous sandstone in the southern Denver-Julesburg basin of Colorado. Lonetree Field (T3S, R59W) provides a typical example of trapping mechanisms and reservoir properties. Examination of 5 cores and 75 wire-line logs within the field resulted in several conclusions. The deltaic J sandstone at Lonetree can be divided into three genetic units; a lower delta front (J-3), a middle delta plain sequence (J-2), and overlying destructional marine (J-1). Production at Lonetree is primarily from channel and crevasse splay sandstones of the J-2 interval. Traps are formed by the updip pinch-out of quality sandstone. Channel sandstones are characterized by a coarse to very fine-grained upward-fining sequence. Trough cross-beds predominate at the base, and ripples and rootlets at the top. Carbonized wood is ubiquitous throughout. These northeast-southwest-trending channels attain thicknesses of 15 to 50 ft (4.5 to 15 m) and widths up to 2,500 ft (760 m). Crevasse splay deposits show extreme lateral and vertical variation. Both coarsening and fining upward deposits are possible. Maximum thicknesses of 15 ft (4.5 m) are developed only in close proximity to channel sandstones. Rooted zones and ripple cross-stratification are common. Porosity in both channels and splays is secondary in nature, resulting from the dissolution of feldspars and calcite cement. This overprinting of secondary porosity on a complex depositional system has created numerous separate reservoirs within the field. Porosity in producing zones is commonly 13 to 20%, with permeabilities in excess of 75 md. Kaolinite is abundant in pore throats, and may present completion problems associated with brushpiling of fines during treatment. Little petrographic or petrophysical differences appear to exist between productive splay and channel sandstones. Typical cumulatives to date are 100,000 bbl of oil per well from splay sandstones, while channels contribute 125,000 bbl of oil per well. The 17 producing wells are expected to yield 2.4 million bbl of oil and 4 bcf of gas. End_of_Article - Last_Page 1335------------

Anastomosed River Deposits--Modern and Ancient Examples in Western Canada: ABSTRACT

Depositional facies of two Canadian modern anastomosed river systems, the upper Columbia River and lower Saskatchewan River, occur in intermontane and plains settings, respectively. Both systems contain low-gradient, multiple, interconnected, laterally stable sand-bed channels, with adjacent splay, levee, and shallow wetland deposits, all aggrading in accordance with channel sedimentation. While aggrading cross-valley alluvial fans or subsidence tend to control sedimentation rates in intermontane valleys, basin subsidence and/or regional tilting controls deposition rates in plains settings. Deposits in the upper Columbia River valley (120 × 1.5 km) consist of low-sinuosity multistoried stringers (textural cycles) with planar tabular cross-bed sets of channel sands and numerous sandy crevasse-splay deposits. Channel deposits are laterally contained by deposits of levee silt and lacustrine mud, and when buried are vertically mud encased. Aggrading at an average rate of 60 cm/100 years over the past 2,500 years, the anastomosed system is very dynamic, exhibiting many avulsions and channel End_Page 631------------------------------ fills. Deposits in the lower Saskatchewan River valley (120 × 80 km), a much wider basin with much slower aggradation rates (± 15 cm/100 years), occur as channel sands flanked by laterally extensive (1 km) sheets of overbank levee deposits of fine sandy silt, which grade into even more laterally extensive thick deposits of mud or peat. With time, dominant channels become sinuous, thus causing increased flow resistance, major avulsions upriver, and eventual channel filling and abandonment. Many channel sand-fill deposits form cross-section geometries, ranging up to 15 cm thick by 120 m wide. Facies differences of the two anastomosed river systems are believed to be caused by both the rate of sedimentation and width of the sedimentary basin. Other than size differences of similar sedimentary environments, Columbia River channels are less sinuous, avulse more frequently, and contain coarser grained sand. In the Saskatchewan, some crevasse-splay (sheet sands) and associated avulsions are laterally extensive (10 × 30 km) and very complex. Wetlands in the Columbia are dominated by marsh (organic-rich mud) and lacustrine silt, whereas thick (up to 3 m) laterally extensive peat bogs dominate in the Saskatchewan system. Within the upper Mannville Group of the Lloydminster area there exists a large-scale mappable complex of fluvial channel-fill sandstones that exhibit an anastomosed pattern. The complex has areal dimensions of 250 km (width) by 700 km (length). Channel sandstones are thick (up to 35 km), narrow (300 m), can be traced for several kilometers, and are stratigraphically variable. The channel fills are multistoried, with the predominant sedimentary structures consisting of plane beds, cross-beds, and climbing current ripples. Interchannel sediments consist of interbedded sheet sandstones, siltstones, mudstones, and coals. The predominant sedimentary structures of the interchannel sandstones are the same as those found within channel sandstones. From a compare-and-contrast approach, it is concluded that meandering, sandy braided, valley-fill, deltaic, or tidal origins cannot account for the observed sand-body geometries and facies distribution. The modern model that best explains the sediment and facies distributions within the upper Mannville is the anastomosed fluvial model in which narrow, vertically accreting channels are bordered by extensive aggrading interchannel wetland deposits with interbedded crevasse-splay sands. Hydrocarbon distributions within the upper Mannville are stratigraphically controlled and oil quality can be directly related to depositional facies. Common trapping mechanisms consist of updip shale-filled channels, structural closure formed by differential compaction, and lateral sandstone pinchouts. End_of_Article - Last_Page 632------------

Effects of overbank deposition on the quality and maceral composition of the Herrin (No. 6) coal (Pennsylvanian) of Southern Illinois

A detailed study of the effects of overbank deposition on the quality and maceral composition of the Herrin (No. 6) Coal Member (Carbondale Fm., Middle Pennsylvanian) in southern Illinois was done using proximate, total-sulfur and maceral analyses. Channel samples were taken of the Herrin (No. 6) Coal and descriptions of the rocks overlying the coal were made at nine locations. the Herrin (No. 6) Coal in the study area is known to 3ave a high ash content toward the Walshville Channel (a channel sandstone possibly representing a distributary channel that existed during Herrin peat deposition). Sulfur content is knwon to in crease markedly where marine or marginal marine rocks directly overlie the coal and decrease where overlain by thick non-marine shale. Three types of overbank deposits are found in the study area accounting for the changes in coal quality and composition. Overbank flooding as sheet flow off the levee of the Walshville Channel introduced suspended material into the peat swamp. The suspended material either produced numerous shale laminae or intermixed with the peat, producing an increased ash content of the resultant coal close to the Walshville Channel. Contemporaneous with the introduction of clastic material by overbank flooding, partial oxidation of the peat occurred. Oxygenated flood water was able to oxidize the peat, increasing the pseudovitrinite content of the resultant coal. Fire splay deposits forming the partings, occurred when the swamp was flooded after a dry period or a swamp fire. The build-up of crevasse splay deposits forming the Energy Shale helped decrease sulfur content of the Herrin (No. 6) Coal in portions of the study area. The non-marine, shaly distal crevasse splay facies formed an impermeable barrier to downward movement of sulfate ion-bearing marine water during deposition of the marine units above the Energy Shale. High-sulfur coal is associated with the sandy crevasse distributary channel-fill facies and bay-fill facies of the Energy Shale or where the Energy Shale is not present. The crevasse distributary channel-fill facies allowed marine water to permeate through it, reaching the peat and resulting in high-sulfur coal. High-sulfur coal found where the bay-fill facies or other marine units directly overlie the coal is a result of marine water being able to permeate the peat directly.

Preparation and analysis of coal seam data utilizing paleoenvironment modeling, hazard #7 coal, Eastern Kentucky

Pennsylvanian strata of the Hazard coalfield, Eastern Kentucky, contain fluvial, upper-delta plain facies characterized by thick localized coals, bay-fill shales, levee silt- and sandstones, and channel-fill sands and gravels. Although the deltaic nature of these sediments has long been established, mining and exploration activities in the district require a thorough understanding of small areas within the delta environment. Coal quantity and quality trends in the Hazard #7 seam, the major producer in the area, have been examined in detail. The #7 coal accumulated in a peat swamp restricted laterally by a major fluvial channel. Three types of non-coal parting are recognized. Thin, tabular, fine-grained partings resulted from periods of increased terrigenous influx into the swamp. Lenticular crevasse-splay deposits locally split the coal. Wedge-shaped, fine- to coarse-grained partings, of probable levee origin, are found along the channel margin. Post-swamp deposits consist of thick bay-fill shales, thin shales, silt- and sandstones deposited on floodplains, and channel-fill sandstones and gravels. The modeling technique discussed provides guidance for mine development and regional exploration by prediction of coal seam quantity and quality trends from local geologic features. The thickest #7 coal is split by thin tabular partings and is overlain by thick shale sequences. Coal overlain by silt and sandstone is thinner and unsplit, and typically of higher heat value. Regional seam thinning due to channel scour is recognized. Factors which control the configuration of the coal seam include position within the peat swamp, proximity to the fluvial channel, swamp burial processes, and paleochannel sinuosity.

Overbank deposits of the Narrator Brook, Dartmoor, England

SummaryOverbank deposits of the Narrator Brook, exposed by the excavation of a drainage ditch, are described. The deposits, which consist of coarse (sand and gravel), fine (silt and clay) and peat units, were laid down in a floodbasin environment. The coarse units are interpreted as crevasse-splay deposits, rather than being classified as floodbasin sediments along with the fine units. Three stages of development in the evolution of the floodbasin are identified. The first stage is represented by the development of peat at a time when the area was not subjected to flooding, the second by deposition of the fine and coarse sediment units and the third by an upper peat horizon. No precise date for the formation of the deposits is at present available, but it is thought that they may be the result of man's activity in the catchment.

Sedimentologic Description of Part of Coal-Bearing Carboniferous Sequence Exposed Near Joggins, Nova Scotia: ABSTRACT

A 300-m section of the upper Cumberland Group exposed south of M'Cairn's Brook near the village of Joggins, Nova Scotia, includes five lithologies, including sandstones, siltstones, claystones, carbonaceous shales, and coal. The sandstones and siltstones display one or End_Page 730------------------------------ more of the following sedimentary structures: trough cross-beds, ripple laminations, horizontal laminations, convoluted laminations, or massive bedding. Additional features present include vertical accretion deposits, lateral accretion deposits, and crevasse splay deposits. A statistical analysis of the several facies and the contacts between them resulted in interpretation of the environments of deposition as a meandering river and associated flood plain. The lower half of the section, including a significant number of coal beds and carbonaceous shales, was deposited primarily within the flood-plain basin, whereas the upper part of the section, with coarser grained siltstones and sandstones, implied deposition closer to the channels. In addition to the sedimentologic analysis, a petrographic study of the coal beds was conducted. Results of a maceral analysis indicate a probable freshwater origin for the coals. Identification of the plant fossils found over the coal beds reveals a freshwater flora, and the mode of preservation indicates deposition in a crevasse splay deposit. End_of_Article - Last_Page 731------------

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