Abstract The Terry Sandstone Member, which lies within the middle part of the Upper Cretaceous Pierre Shale in the Denver basin of northeastern Colorado, has been interpreted as a marine-shelf depositional sequence. Such interpretation has been based mainly on the sedimentological character of the sandstone units within the depositional sequence and on their position seaward of correlative Lower Cretaceous shorelines. This study conducted a process sedimentological analysis of the Terry Sandstone sequence in an area the size of two townships approximately 8 mi downdip from the sandstone outcrop. Four core sequences, two of which are illustrated herein, and wire-line logs from over 200 wells were utilized to construct a series of subsurface maps and cross sections with which to evaluate sandstone body geometry, trend, and sedimentological character. The Terry Sandstone Member thickens over a distance of 11 mi from less than 60 ft thick in the northwestern corner of the study area to greater than 120 ft thick in the southeastern corner of the study area. An upper, sandy mudstone sequence forms a blanketlike unit that ranges from 50 to 70 ft thick throughout the study area. The lower sandstone unit of the Terry Sandstone is composed of two distinct northwest-trending, elongate, lens-shaped sandstone bodies (designated herein as the "northeastern" and "southwestern" sandstone bodies) that display a shinglelike overlapping relationship across a 1- to 2-mi wide, northwest-trending strip. The two units are separated by a shale permeability barrier unit (designated herein as the shale "notch"). Within the lower unit of the Terry Sandstone Member, the northeastern sandstone body displays a symmetrical lens-shaped cross section up to 70 ft thick and 4-5 mi in maximum width. It is characterized by a gradational base, a coarsening-upward vertical sequence, and a sharp upper contact with overlying marine mudstone. The shale "notch" appears to be composed of silt-poor clay shale; it "migrates" from the top to the base of the lower sandstone unit of the Terry Sandstone within 1.5 mi and effectively separates the two sandstone bodies. The Southwestern Sandstone Body displays an asymmetrical lens-shaped cross section (with more rapid thinning along the northeastern flank), is up to 50 ft thick and 5-6 mi in maximum width, and is characterized by a sharp, erosional basal contact, fining-upward vertical sequence, and gradational top. Evidence suggests that both sandstone bodies of the lower sandstone unit of the Terry Sandstone were deposited in a shallow marine-shelf setting. This evidence includes the general paleogeographic setting and features such as the extensively burrowed to bioturbated sandstone and mudstone sequences, the abundance of glauconite, and the extensive marine mudstone intervals of the Pierre Shale that completely encase the Terry Sandstone sequence. The northeastern sandstone body formed as an offshore marine sand bar, whereas the southwestern sandstone body apparently represents a channelized deposit composed of active channel-fill sand and overlying inactive channel-fill sandy mudstone. Of economic significance is the segregation of hydrocarbon types in the sandstone bodies; noncommercial volumes of gas are present in the northeastern sandstone body and commercial volumes of oil in the southwestern sandstone body.

Abstract Gas production from several, 6 to 23 ft. (2 to 7 m), single to multistory sandstone bodies of the Woodbine-Eagle Ford interval, 160 to 200 ft. (49 to 61 m) thick at 9,000 to 9,600 ft. (2,743 to 2,926 m) in the Damascas field has been developed since discovery in 1976. Subsequent offset drilling resulted in a few gas wells and several dry holes. In February 1979 the entire Woodbine-Eagle Ford interval was cored in the No. 7A Dorrance well. Sedimentologic core study generated a predictive depositional model which has guided field development of the subtle stratigraphic traps at a 5 to i well success ratio. Present gas reserves are 40 Bcf with 440,000 bbl of condensate. The productive area is located slightly southwest of the Sabine uplift and just uxJip from the Lower Cretaceous continental shelf edge. Seismic sections and foraminiferal paleoecology establish a middle-shelf depositional setting. Bioturbated, silty, shelf shales comprise the upper and lcier Woodbine-Eagle Ford interval. The middle is a complex of, (1) graded, medium to very fine-grained, massive to laminated sandstone beds; (2) contorted, softsediment-deformed intervals; (3) swirled and sheared siltstone beds; and, (4) thin diamict conglomerate beds. Genetic units indicate periodic rapid deposition by debris flows and low to high-concentration density currents. The several distinct productive sandstone bodies (porosities 9 to 14%; pemeabilities 2 to 10 md) are northward-thickening, dip-oriented lobes. The localized deposition in the shelf setting was controlled by delta development slightly to the north. Periodic major storms generated delta flooding which

ABSTRACT A new perspective regarding the genesis of the sand ridges on the storm-dominated New Jersey continental shelf has been developed by synthesis of two previous models. One model suggests the ridges formed as barriers during the last marine transgression. The other model suggests the ridges are post-transgressive features forming at the base of the shoreface. The bathymetric and textural data on the New Jersey nearshore and mid-shelf suggest two different groups of ridges: one group formed as barriers whereas the other group represents a post-transgressive origin. These ridge groups have distinctive orientations, wavelengths, morphology, and sediment textures. Intersecting the coast at 15° to 30° are coast-oblique ridges. Seaward of these is a 30-km-wide complex of ridges that are approximately coast-parallel. Farther seaward is a group of ridges parallel to the nearshore ridges. Cluster and spectral analyses produce similar groupings. Vibracores through the nearshore ridges reveal a medium to medium-coarse mean grain size with numerous occurrences of coarsening and fining (high-energy events). The vibracores through the mid-shelf coast-parallel ridges indicate two depositional zones. The upper approximately I m is similar to the nearshore ridges. Beneath this upper zone the grain size is fine to medium-fine, with fewer high-energy events. Also unique to the mid-shelf ridges are zones of interlayered mud and fine sand. The New Jersey mid-shelf coast-parallel ridges are interpreted as degraded barriers that were submerged by rising sea level. The nearshore and outer shelf coast-oblique ridges formed as post-transgressive, shoreface-connected ridges. Thus, the degraded barriers and seaward intersecting ridges are an earlier Holocene analog to the present barrier and shoreface-connected ridge complex.

ABSTRACT It has been proposed by Stubblefield and colleagues (this volume) that the sand ridges of the central new Jersey shelf contain a basal muddy sand stratum deposited as a lower shoreface facies during a period of coastal progradation. They conclude that the ridge morphology above the mid-shelf scarp is in part a relict strand plain. The coastal progradation hypothesis for the origin of the lower muddy sand facies is a reasonable one, but to date there is not enough evidence to discriminate among facies. Examination of the inner shelf surface above the mid-shelf scarp reveals topographic, stratigraphic, and grain-size patterns that may be interpreted as being in conflict with the relict strand plain model. We conclude that the ridge topography on the surfaces above the scarp on the New Jersey Shelf is a response to storm flows subsequent to transgression.

ABSTRACT Arguments proposed by Swift et al. (this volume) suggest that Stubblefield et al. (this volume) incorrectly infer that the New Jersey mid-shelf sand ridges represent a relict strandline. We suggest that the ridges are a product of three processes: barrier progradation, barrier degradation during marine transgression, and ridge aggradation by mid-shelf currents. Fauna and grain- size data argue strongly that the muds encased within the ridges were originally deposited as part of the open shoreface and not in either lagoonal or estuarine environments. Our topograhic and stratigraphic patterns are in conflict with a relict strand plain model. These patterns support a model for multiple-stage evolution.

ABSTRACT The continental shelf on the western margin of the Cretaceous western interior seaway was a muddy surface which bore abundant northwest-southeast trending sand bodies, as much as 20 m thick and many km long. Important examples are the Medicine Hat Sandstone, the Mosby Sandstone Member of the Belle Fourche Shale, the Shannon and Sussex Sandstone Members of the Cody Shale, and the Duffy Mountain sandstone and the Tocito Sandstone Lentil of the Mancos Shale. These deposits resemble the storm-built and tide-built sand ridges reported from the modem Atlantic Continental Shelf or from the Southern Bight of the North Sea. However, although modem sand ridges may protrude from the Holocene transgressive sand sheet through overlying Holocene mud deposits to be exposed on the present sea floor, no modem examples are known where sand ridges are completely encased in mud, as the Cretaceous examples seem to have been. Hydrodynamical theory suggests that special circumstances may allow the formation of sand bodies from a storm flow regime whose transported load consists of sandy mud. Under normal circumstances, such a transport regime would deposit little clean sand. The sea floor is eroded as storm currents accelerate, but erosion ceases when the boundary layer becomes loaded with as much sediment as the fluid power expenditure will permit (flow reaches capacity). Deposition of a graded bed occurs as the storm wanes and a storm sequence is likely to consist of thin clay beds with basal sand laminae. However, slight topographic irregularities in the shelf floor may result in horizontal velocity gradients, so that the flow undergoes acceleration and deceleration in space as well as in time. Fluid dynamical theory predicts deceleration of flow across topographic highs as well as down their lee sides. The coarsest fraction of the transported load (sand) will be deposited in the zone of deceleration, and deposition will occur throughout the flow event. Relatively thick storm beds (2 to 10 decimeters) can acccumulate in this manner. Enhancement of initial topographic relief results in positive feedback; as the bedform becomes larger, it extracts more sand from the transported load during each successive storm. Individual storm beds may tend to fine upward (waning current grading), but the sequence as a whole is likely to coarsen upwards, reflecting increasing perturbation of flow by the bedform as its amplitude increases. Stability theory suggests that the end product of these processes should be a sequence of regularly spaced sand ridges on the shelf surface. However, sandstone bodies within the Cretaceous shelf deposits are quite localized in stratigraphic position and lateral distribution. Upward-coarsening sequences are a widespread phenomenon in the western interior Cretaceous system, and the sandstone bodies appear to constitute localized sand concentrations within more extensive sandy or silty horizons. Especially widespread upward-coarsening sequences appear to be due to the close coupling between activity in the overthrust belt to the west and sedimentation in the foreland basin. Each thrusting episode increased relief as well as the load on the crust. Initially, the increased relief as well as the load on the crust. Initially, the increased relief resulted in a flood of sediment transported to the shelf on the western margin of the basin so that the shelf became shallower. As it did so, wave scour on the shelf floor increased the amount of bypassing and resulted in the deposition of increasingly coarser sediment. As relief in the hinterland waned, subsidence overtook sedimentation and the shelf subsided. Renewed thursting began the cycle anew. In a second mechanism for the formation of upward-coarsening sequences, tectonic uplift affect portions of the shelf as well. The initiation of Sevier or Laramide structural elements beneath the shelf and the remobilization of other, older structures created submarine topographic highs which caused slight sand enrichment over broad areas. The development of sand-enriched areas in the shelf floor by both mechanisms led to the flow-substrate feedback behavior that built large-scale, elongate bodies of clean sand.

ABSTRACT The Shannon Sandstone Member of the Gammon Shale in southeastern Montana is one of several Upper Cretaceous sandstones in the western interior of the United States which are interpreted as shelf sediments. Paleogeographic reconstruction suggests that the site of Shannon deposition in southeastern Montana was near the edge of a broad shelf more than 322 km (200 mi) from the shoreline. The Shannon is early Campanian in age and is equivalent to the Eagle Sandstone of central Montana. Exposures on the north flank of the Black Hills and subsurface data from oil and gas test holes reveal that the Shannon is made up of a hierarchy of sandstone bodies. From largest to smallest, the hierarchy is made up of five elements of different sizes: lithosome (level I), sheet (level II), lentil (level III), elongate lens (level IV), and small-scale facies packages (level V). The largest elements of the geometric hierarchy are documented in regional subsurface studies. The total sandstone lithosome is 121-182 m (400-600 ft.) thick, covers I 17,000 km 2 (45,000 mi 2 ), and is enclosed in sandy shale. In the center of the lithosome, there is a sandstone sheet which is 30 m (100 ft) thick and has an area of 18,200 km 2 (7,000 mi 2 ). The sheet is, in turn, composed of four coalesced lentils 15-23 m (50-75 ft) thick, each covering 1500 km 2 (600 mi 2 ). The sandstone in one of these lentils can be traced in closely spaced bore-holes and in nearby exposures. Detailed subsurface studies indicate that the lentil consists of at least two smaller elongate lenses; each lens is about 12-18 m (40-60 ft) thick and is about 52 km 2 (20 mi 2 ) in areal extent. In outcrop, a series of four lithologic units form a coarsening-upward cycle within a single elongate lens. Within the lenses, the uppermost sandstone units of the coarsening-upward cycle contain three different lithologies in a facies relationship. The sandstone facies are arranged in individual packages which are small-scale, covering areas of about .12 km 2 (.05 mi 2 ) and having thicknesses of 3-6 m (10-20 ft). The individual small-scale facies packages have a distinct geometry and are imbricate. Each of the five levels of the geometric hierarchy recognized in the Shannon shelf sandstone represents different formative processes. The larger elements of the hierarchy, specifically the lithosome (level I) and sheet (level II), were probably produced by long-term processes, such as episodic paleotectonism. The smaller elements may have been a response to short-term depositional processes on the ancient shelf. The lentils (level III), elongate lenses (level IV), and small-scale facies packages (level V) constitute an ancient response model that is very similar to a hierarchy of morphologic elements observed on modem continental shelves. Comparison with the modern hierarchical response model suggest that the lentils may be interpreted as complex fields of sand ridges, that the elongate lenses may be individual sand ridges, and the the small-scale facies packages may represent shelf sand waves located on the crest and flanks of the sand ridges.

Two vertically stacked shelf-ridge (bar) complexes in the Shannon Sandstone member of the Cody Shale (designated upper and lower sandstones) crop out in the Salt Creek anticline of the Powder River Basin, Wyoming. The shelf-ridge complexes are composed primarily of moderately to highly glauconitic, fine- to medium-grained lithic sandstone and attain thicknesses of over 70 feet. The shelf-ridge complexes were deposited at least 70 miles from shore at middle to inner shelf depths by south to southwest-flowing shore-parallel currents intensified periodically and frequently by storms. Ridges in each sequence trend north-south, slightly oblique to current flow. A possible source of sediments for the shelf ridges was the Eagle Sandstone shoreline and deltaic deposits of southern Montana 200 miles to the northwest. Eleven facies were defined in outcrop on the basis of physical and biologic sedimentary structures and lithology. Vertical and lateral changes in facies are relatively abrupt where observed in closely spaced outcrop sections, and, in general, facies are stacked in coarsening-upward sequences with Central Bar Facies commonly immediately overlying Interbar Sandstone Facies. Porous and permeable potential reservoir facies include: Central Bar Facies, a clean cross-bedded sandstone; Bar Margin Facies (Type 1), a highly glauconitic, cross-bedded sandstone containing abundant shale and limonite (after siderite) rip-up clasts and lenses; and Bar Margin Facies (Type 2), a cross-bedded to rippled sandstone. These facies were formed by sediment transported and deposited in the form of medium- to large-scale planar-tangential troughs and sand waves on and across the tops of ridges by moderate to high energy shelf currents. Storm flow deposited Central Bar (planar laminated) Facies are rare. Finer-grained, non- to marginal-reservoir quality facies include Interbar Sandstone Facies (rippled to ripple-form bedded sandstone), Bioturbated Shelf Sandstone Facies, Bioturbated Shelf Siltstone Facies, Interbar Facies (interlaminated rippled sandstone and shale), Shelf Sandstone and Shelf Siltstone Facies (sub-horizontally laminated sandstone and siltstone). Interbar Sandstone Facies were most commonly deposited lateral to the higher energy portions of the ridges as well as near the base of the shelf ridges during their initial development. The two bioturbated facies most commonly occur near the base of the ridge complex and between the two vertically stacked ridge complex sequences, and probably represent periods of slow deposition. The Shelf Silty Shale Facies is actually a facies of the Cody Shale of which the Shannon Sandstone is a member. The most common vertical sequence of sandstone facies is one in which a coarsening-upward sequence is formed where Central Bar Facies overlie Interbar Sandstone Facies; this contrasts with the sequence observed at Hartzog Draw Field, 25 miles to the northeast, where the most common coarsening-upward sequence from bottom to top is Interbar Facies, Bar Margin Facies (Type 1) and Central Bar Facies. Relatively abrupt lateral changes in facies are observed in surface cross sections spaced from one-fourth to one mile apart. Thickness changes, but not facies changes, are readily observable on subsurface cross sections constructed using SP-resistivity logs. The association of two vertically stacked shelf-ridge complexes at Salt Creek is atypical compared to other Shannon sequences in the Powder River Basin in several respects. The lower sandstone is correlative with productive Shannon sandstones in many of the Powder River Basin fields (e.g., Hartzog Draw); the upper sandstone sequence is only locally developed in the area of the Salt Creek anticline. Also, the spacing between ridges and the length to width ratios of the ridges at Salt Creek are much smaller than those in other areas. These differences are particularly apparent in the upper sequence wherein the sandstone bodies appear to have oblate geometries very unlike the strongly linear geometries typical of most shelf sandstone ridges. These differences are attributed to the presence during Shannon time of an actively growing paleo-high in the area of the present day Salt Creek anticline which localized sand deposition and ridge formation. Similar early structural growth and its influence on shelf sedimentation has been well-documented for the Lost Soldier anticline area by Reynolds (J976). Baculites zones are commonly used for surface correlations in the study area and in Upper Cretaceous units throughout Wyoming. Subsurface cross sections paralleling surface sections at distances from one half to three miles away corroborate the surface sandstone correlations in the Salt Creek area. Bentonites above and below the Shannon form excellent subsurface correlation datums. Foraminiferal data indicate that the shelf-ridge complexes were deposited in water depths ranging from the middle shelf to the outer part of the inner shelf. A wide diversity in size, orientation, and type of burrow-fill material suggests a relatively hospitable environment for burrowers in portions of the shelf-ridge complexes. Rare Teichichnus, Thallasinoides, Chondrites , and plural curving tubes were identified. Common Cretaceous shoreline traces such as Ophimorpha, Asterosoma and Rhizocorallium were not observed. The Bioturbated Shelf Sandstone Facies and the Bioturbated Shelf Siltstone Facies range from 75 to 95% burrowed. Burrowing in the other facies averages from 5 to 27%. Glauconite is present throughout and is most abundant in association with shale rip-up clasts and limonite (after siderite) lenses and rip-up clasts in Bar Bargin Facies (Type I) Transport directions, determined by abundant high angle cross beds (mostly planar-tangential), indicate a south-southwest transport direction (188°) for current deposition of the high energy fades. The range of variation in transport direction at individual outcrops and overall is relatively small (60°). Most current ripples in the Interbar Sandstone Facies also indicate a southerly transport direction. Only very locally, in the top foot or two of some Central Bar and Bar Margin Facies, trough orientations indicate transport directions strongly oblique northeast to the general south southwest-flow direction. In both outcrop and in Hartzog Draw Field ridge complexes trend nearly north-south, slightly oblique to current flow, Detailed subsurface correlations of the Shannon sandridges throughout the Powder River Basin, using well developed bentonite markers, show the reservoir facies to “rise and fall” parallel to their elongation, indicating that the ridges were not deposited in layer-cake fashion. In the Salt Creek area, the reservoir facies generally gradually rise in section to the south parallel to the direction of current flow. In the Hartzog Draw area, theridges rise to the north, opposite to current flow; they also rise in paired fashion laterally east and west. These stratigraphic patterns of development of the higher energy shelf ridge-facies are inter preted to reflect sea-floor topography during their deposition.

Abstract The Upper Cretaceous (Cenomanian) Mosby Sandstone Member of the Belle Fourche Shale and correlative units form a large (greater than 300,000 sq km) lobe of sandstones that extend southeast from the Dunvegan delta in northeastern British Columbia and northwestern Alberta. The sandstones occupy the shallow, flat, western shelf of a north-south trending epicontinental seaway in Alberta, southwestern Saskatchewan, and central Montana. Sandstone bodies are concentrated on ancestral highs such as Bow Island Arch in southeastern Alberta, Bowdoin Dome in north-central Montana, and Central Montana Uplift that were episodically uplifted and that affected sedimentation. The sandstone bodies contain coarsening-upward cycles that indicate shoaling. The bodies occur as elongate northwest-southwest trending sand ridges that are tens of kilometers long. On the Central Montana Uplift, the postulated sand ridges have been redistributed around the shoals into still smaller bodies that are a few to several kilometers long. On the Central Montana Uplift, the Mosby is composed of thin (average 1.5 m) very fine-grained sandstone units that consist of individual beds intercalated with shale or amalgamated. The base of each bed is a planar to undulating erosional surface, and the top is gradational with the overlying shale. The dominant sedimentary structure is hummocky cross-stratification. The upward sequence of sedimentary structures suggests decreasing energy levels. Ripple cross-lamination and wave ripples are commonly developed near the top of beds or units where the beds are amalgamated. Trace fossils are restricted to the rippled surfaces and consist mainly of horizontal Ophiomorpha and Thalassinoides burrows. Unoriented shells, including gastropods, bivalves, and ammonites, are concentrated in lenses or concretions near the base of units and where the units grade laterally into shale. Sediments were transported by the interaction of waves and southward-flowing, geostrophic currents enhanced by wind forcing, as much as 1,100 km, from the Dunvegan delta and the sand was concentrated on positive features as coarsening- upward cycles. Regional and local characteristics suggest that the sediments forming the upper part of coarsening-upward cycles were subsequently redistributed by storm flows on the Central Montana Uplift. The sediments were suspended on the shoals and the bottom was irregularly scoured by passing storm flows. As the currents decelerated, the sediments were quickly dropped parallel to the undulating surface producing hummocky cross-stratification. Shells were concentrated by pressure gradients created by passing wave surges. The upward sequence of sedimentary structures within a single bed suggests deposition during the waning stages of a single storm event. Reworking of sediment by oscillatory waves, burrowing along rippled surfaces, and deposition of interbeds of finer-grained silt and mud indicate the return to fair-weather conditions. The morphology and lateral facies of individual bodies are controlled by bottom topography and by water depth.

Abstract The Viking Sandstone in much of central Alberta, including the Joffre-Joarcam area, consists of a series of overlapping sediment sheets that become progressively younger westward toward the Paleoshoreline. During a profound regression at the beginning of Viking deposition, streams flowed across the former shelf surface depositing sand in deltas, as now evidenced by the irregular-shaped reservoirs of eastern Alberta. An ensuing trangression, punctuated by minor regressions or stillstands, which reworked shoreline sediment (supplied to shorelines) into linear shelf sand bodies, formed the linear reservoirs (e.g., Joffre and Joarcam fields) west of a series of irregular-shaped sandstone reservoirs. During the transgression, the retrograda- tional nature of the sediments sheets, which contain the linear sand bodies, was formed. Well log cross sections show, in addition to large-scale shoreward shingling, that the Viking thickens westward, pinches out westward, and that each sediment sheet produced during the overall transgression contains several northwest-trending, elongate “cleaner” sandstone bodies. Cores of these elongate sandstone bodies and their underlying beds commonly exhibit a coarsening-upward succession of: (1) silty marine shale, (2) intercalated silty shale and rippled sand, which in some places becomes a structureless bioturbated clayey sand, (3) glauconitic crossbedded sandstone, and (4) polymictic pebble conglomerate. Conglomerate also occurs randomly within the sequnce. Final deposition of the elongate bodies on a shelf tens of miles from the paleoshoreline is documented by: (1) marine shale enclosing the Viking, (2) absence of consistent landward-seaward facies changes, (3) abundant glauconite, (4) an “offshore” trace fossil assemblage, and (5) distant eastward location with respect to contemporaneous strandline facies. Sparse evidence, such as coal partings and plant fragments in the irregular-shaped sand bodies, supports the interpretation that they are drowned delta complexes. All evidence, including the shingling relationship of the sediment sheets containing the elongate sand bodies, can be explained by retrogradational shelf sedimentation. Modem sand ridges of the New Jersey Shelf may be analogous in many respects to the linear “cleaner” sandstone bodies of the Viking.

Abstract Economically important sandstone bodies enclosed in marine shales have been described from several areas of the Western Interior region in Utah, Colorado, Wyoming, and New Mexico. Many of these sandstone bodies are believed to have been deposited in open marine, mid- to outer-shelf settings. This study in northern Colorado demonstrates that many of these sand bodies were deposited originally in nearshore and shallow marine settings and that their present occurrence and distribution was largely dependent upon eustatic sea level changes and minor transgressive-regressive phases of local origin. Along the western margin of the Upper Cretaceous seaway in Utah, nearshore sedimentation rates apparently were high due to uplift in nearby source areas, and shelf areas were broad and shallow. These factors resulted in discontinuous seaward migration of strandlines into former shelf areas of western Colorado. Basinal areas lay to the east in areas where subsidence was greatest. Discrete sandstone members of the lower Mesaverde Group and Mancos and Pierre shales in northern Colorado include the following depositional environments: estuarine or distributary channels, reworked distributary mouth bars, distributary mouth bars, delta front sheet sands, pro-delta sand sheets, shoreface, and mid- to outer-shelf sand ridges. Variations in texture, sorting, bioturbation, thickness and bedding of these units reflect variations in rate of deposition and in wave and current energy. Evidence of transgressive reworking is present locally at the tops of some units, as well as in discrete sandstones that are completely encased in marine shale. Although some sandstone members (e.g., the Hygiene Sandstone of the Pierre Shale) have been correlated previously over distances exceeding 80 kms, our examination of cross-bedding patterns and composition indicates that several distinct environments of deposition are present, including nearshore to outer shelf environments. Cross-stratification (medium-scale tabular) and lithology of the easternmost exposures of the Hygiene are interpreted to be similar to those in modem sand ridges on the U. S. Atlantic, Bering Sea, and North Sea continental shelves, whereas massive to cross-bedded sandstones to the west were deposited in delta front environments. The sand was derived from the west, transported eastward, and then redistributed by southward-flowing storm and oceanic currents.

ABSTRACT Graded sandstones separated by finer-grained sediments of a shelf and shoreface environment have been widely attributed to sporadic storm activity, with sand transport related to storm-surge ebb. According to this classic storm-surge ebb model, barrier- lagoon coasts are prerequisite. However, re-examination of hurricane data, as well as coring on the Texas shelf, suggests that emplacement of these storm-graded sediments was related to high energy bottom-retum flow produced by wind forcing during the height of storms. Seaward runoff from backbarrier areas is interpreted to have had negligible influence on shelf sedimentation in the units studied. Graded sandstones of the Lower Cretaceous Grayson Formation in northeast Texas, which bear close resemblance to the storm beds in the cored units, are interpreted to have been deposited off a coast dominated by small lobate deltas that lacked fringing barrier islands. Nearshore sandstones are relatively thick, vertically amalgamated, and dominated by hummocky cross stratification produced by the interaction of unidirectional currents and storm waves. In contrast, thinner sandstones deposited below storm wave base commonly include a basal shell layer with transported and aligned gastropods probably oriented by strong bottom currents. These sandstones show parallel lamination or rare low-angle foresets that grade upward into siltstone and mudstone with local intervening ripple cross lamination. Burrowing is restricted to the upper parts of the graded units and to the overlying fair-weather mudstones; however, burrowing is surprisingly sparse, possibly because of high rates of sedimentation.

Abstract A combination of stratigraphic and diagenetic events has trapped oil in thin-bedded, clayey sandstones of the Upper Cretaceous Woodbine-Eagleford Formations. Five sandstone units occur in Kurten Field and are designated from top to bottom as “A” through “E”. Foraminifera and nannofossils indicate these units to be late Tiironian. The <L C” and “D” units are elongate north to south, 4.5 miles wide, over 10 miles long, and 40 feet thick. The “B” and ‘E” units are thinner and trend northeast to southwest. Grain size coarsens upward in the “B”, “C”, and “D” units, averaging 0.14 mm and ranging from 0.09 mm to 0.18mm. Grain size fines upward in the “E” unit. The sandstone's average composition is 66% quartz, 1% feldspar, 2% rock fragments, and 28% matrix. Sedimentary structures in the “B”, “C”, and “D” units grade upward from laminated and bioturbated siltstones to clean sandstones with flaser cross-beds. The “E” unit consists of repeated bedsets about I foot thick of massive to faintly laminated sand, overlain by wavy to undulatory laminated sand, which is overlain by marine shale. Bedsets commonly show sharp to irregular basal contacts. Sedimentary structures, bioturbation, and microfauna indicate that the units are offshore bars which have been generated by a combination of river-mouth by-passing, storm-generated sheet flows, and longshore currents. The porosity is largely diagenetic and occurs in the clayey beds. It appears to have been formed by fresh water leaching along an erosional unconformity overlain by the Austin Chalk. Permeability becomes progressively poorer away from the unconformity, and a permeability barrier ultimately forms a poorly defined updip limit for the field, making Kurten a combination diagenetic and stratigraphic trap. Relatively widespread occurrences of offshore bars suggest that similar traps may be fairly common in ancient shelf sediments.

ABSTRACT The Wishart Formation of the Early Proterozoic Labrador trough has the sedimentary structures of a high-energy shelf deposit. It is divisible into six facies designated A through F: (A) very fine to fine-grained sandstone in cm-thick, laminated to rippled layers and lenses; (B) fine-grained sandstone in decimeter-thick layers displaying internal flat lamination to hummocky cross-stratification and capped by a veneer of ripples; (C) fine to coarse-grained sandstone with dm-thick trough crossstratification and biomodal paleocurrents; (D) thin, laterally persistent beds of coarse conglomerate in which the larger clasts are all slabs of penecontemporaneously-cemented sandstone; (E) crudely stratified graywacke containing pseudonodules and softsediment folds; and (F) stromatolitic to thinly laminated dolomite. Ninety percent of the aggregate thickness of the Wishart Formation consists of facies A, B, and C. For about half of that thickness, they are grouped into upward-coarsening cycles similar to ones in various Phanerozoic marine shelf deposits; elsewhere, they appear to be randomly interstratified. Facies A and B, which form the lower parts of the cycles, are interpreted as storm deposits which accumulated in deeper areas of the shelf. Facies C, which forms the upper parts of the cycles, is interpreted as tidal sands deposited in shallower areas. The cycles average 12 m in thickness and are attributed to shelf aggradation during pulses of more rapid sedimentation. All of the cycles are capped by erosional surfaces. Many are overlain by a thin lag conglomerate (facies D) formed by the winnowing of partially cemented sands during episodes of sediment starvation. Where cycles are absent, facies shifts presumably reflect long-term and possibly random variations in tidal currents, storms, or other factors. Facies E and F are not incorporated into cycles. Facies E formed by either slumping or foundering of previously-deposited strata (probably facies A). The dolomites of facies F indicate that siliciclastic sedimentation was outpaced by carbonate accumulation for a time. Meager evidence suggests that some of the carbonates are cryptalgal, but given the Early Proterozoic age of the Wishart. they could have been deposited anywhere from the intertidal zone to the lower limit of the photic zone. Hence, facies E and F add little to the paleoenvironmental picture already gleaned from facies A, B, and C. The characteristics of the Wishart can all be explained in terms of processes taking place on modem continental shelves, supporting a uniformitarian interpretation of Early Proterozoic shelf environments. However, this analysis also serves to highlight certain shortcomings in our understanding of the stratigraphic record those processes leave. In particular, it cannot be determined with the available evidence (which is considerable) whether or not the Wishart in the Schefferville area was deposited near a coastline. Likewise, the reasons for the variability of the sandstones attributed to storm deposition are obscure. Lastly, the interpretation of the Wishart as a high-energy shelf deposit supports a marine origin for the superjacent Sokoman Formation, one of the largest of the cherty iron formations.

Abstract This volume is a collection of papers which, for the most part, were included in a symposium of Shelf Sandstone deposits sponsored by SEPM at the Annual Meeting in Denver in 1980. A variety of techniques are useful in documenting shelf depositional process and sand body geometries. Among these are sedimentary structures observed in outcrops and cores, biogenic data including trace fossils, micro- and macro-fauna, detailed seismic sections, and detailed subsurface correlations. These types of data are all readily available, at least locally, for Mesozoic rocks. In studying older shelf sandstones such as those from the early Paleozoic and Precambrian, some techniques cannot be used; trace fossils are rare to absent, and it is difficult to establish the relationships of the shelf sand-bodies to the shoreline of the broad shallow seas that were common at that time.

Abstract The SEPM core workshop on deep-water clastic sediments was organized to provide participants with an opportunity to view cores from a variety of deep water depositional settings and to demonstrate the application of process sedimentology in the interpretation of depositional enviroments from the study of cores and associated subsurface data. The studies assembled for presentation in the workshop have dealt with sedimentary sequences which have been interpreted as having formed by deposition of non-calcareous, clastic sediment in relatively deep water (generally slope basins and greater depths). These studies also have been concerned principally with coarser deep-water sediments (usually fine-grained sandstone or coarser) of such stratigraphic sequences because of their potential as hydrocarbon reservoirs with primary and (or) diagenetically modified inter-granular porosity and permeability. Obviously those coarser parts of such deep-water sequences are anomalous in that they represent transport and deposition by processes that did not operate most of the time in the overall relatively quiet depositional settings. The probable processes of transport and deposition of such anomalous coarse clastic sediment, and overall models for dispersion and accumulation of such sediment, therefore have been considered in some detail in the studies included in the core workshop. However, this is not a course on sedimentary mechanics; it is a course in comparative stratigraphic and sedimentologic analysis. Six core sequences, which have been the subject of detailed sedimentological study, will be on display during the core Workshop. Two studies pertain to deep-water, Upper Cretaceous sandstones (mainly the Winters Sandstone) of the Sacramento Valley

Abstract The detailed sedimentological analysis of whole-diameter cores is greatly enhanced by the core being in good condition; ideally, cores should be complete (100% recovery) and well labeled with all pieces in correct order. Considerable information loss occurs when cores are mishandled; therefore, specific procedures should be followed. At the well site care should be exercised to preserve correct orientation of individual core segments and the core should be properly marked. Hügel orientation grooves cut in the core will facilitate core reconstruction. Commercial core analysis laboratories usually generate the greatest amount of information loss; therefore, special care should be used during subsampling for fluid-saturation analyses and porosity-permeability measurements. A core gamma-ray scan should be obtained whenever possible to aid in later core-to-log correlation. Finally, prior to detailed examination, it is recommended that the core be slabbed, lapped and photographed. X-ray radiography may be needed to enhance subtle sedimentary structures in massive-appearing sandstones and mudrocks. For proper sedimentological core analysis we recommend a process-sedimentology approach which emphasizes detailed lithologic description and the recognition of genetic units within the vertical sequence. A continuous, detailed sketch should be made and a description made using a check list of important lithologic features. An understanding of Walther’s Law is required for maximum use of the vertical sequence. At least four types of genetic units can be delineated to interprete the physical, biological and chemical processes responsible for generating the sedimentary rock product of the core. Those types are: sedimentation unit, ichnogenetic unit, soft-sediment-deformation (s-s-d) unit, and diagenetic unit. Finally, the sedimentological core analysis should be used to calibrate the wire-line logs and associated sursurface data. The main steps in such calibration are to first determine the core-to-log depth correction and then to determine the level at which genetic units and lithofacies can be recognized on the logs. Such calibration leads to better correlation of sedimentological information to nearby non-cored wells and allows for lateral extension of predictive sedimentological models throughout the subsurface study area.