Slope Shales Research Articles

Late Cambrian (Steptoean) sedimentation and responses to sea-level change along the northeastern Laurentian margin: Insights from carbon isotope stratigraphy

Carbon isotopes are applied as tools for stratigraphic correlation of poorly fossiliferous Upper Cambrian carbonate strata in the northern U.S. Appalachians. Upper Cambrian (Steptoean) marine carbonate rocks record a significant global positive carbon isotope excursion (δ 13C = +4‰ – 5‰ Vienna Peedee belemnite [VPDB]), the timing of which is well documented in fossiliferous sections elsewhere. The Steptoean excursion peaks at a sea-level lowstand that produced the Sauk II– Sauk III sequence boundary on the North American craton. In this study, this excursion is documented for the first time in the northern U.S. Appalachians in poorly exposed limestone debris flow and olistolith deposits interbedded within 20 m of continental slope shales of the Schodack Formation. These deposits contain the only reported pre– Elvinia zone Steptoean fauna in New York and record δ 13C values of up to +3‰ . The slope carbonate sediment was mainly derived from the shelf margin and is mixed with common coarse-grained silici-clastic material. These deposits reflect a seaward migration of the siliciclastic source area (exposed craton), suppressed carbonate platform sedimentation, and shelf bypassing during the Sauk II– Sauk III sea-level fall. Nonfossiliferous dolostones and dolomitic marbles of the proposed carbonate platform correlatives (the Pine Plains Formation in southeastern New York and the Stockbridge Formation from western Massachusetts) also contain common coarse siliciclastics ; however, sampled sections do not record elevated δ 13C values, indicating that these strata are probably not of Steptoean age. This suggests that Steptoean time is represented in the carbonate platform to slope succession of the northeastern (present-day) Laurentian margin by an extremely condensed stratigraphic interval or even a hiatus.

Identification of stratigraphic traps with subtle seismic amplitude effects in Miocene channel/levee sand systems, NE Gulf of Mexico

Abstract Stratigraphic traps created and preserved on the unconfined slope of the ancestral Mississippi submarine fan in the northeastern Gulf of Mexico have been found to contain substantial and profitable hydrocarbon reserves. Nearly 2 trillion cubic feet of gas (350 million barrels of oil equivalent) have been produced in the last 13 years in less than 1400 feet (425 m) of water. Drilling results have yielded 31 fields from 45 drilled prospects for a success rate of 69%. The main phase of exploration lasted eight years. The first 3D survey in 1994 sparked a major increase in drilling and success rates until the last field was discovered in 2001. This play has now essentially been exhausted of sizeable opportunities. The underlying Mesozoic section sources hydrocarbons to the play for middle Miocene slope sands. The unconfined (non mini-basin) slope sands are turbidites and debris flows that were deposited between 5 and 25 miles (8 and 40 km) from their coeval shelf margins. Stratigraphic traps in this environment are created at the margins of the levees as they interfinger with slope shales. Additional geometric modifications of the stratigraphic traps result from post-depositional erosion or ‘cannibalization’. The erosion left ‘monadnocks’, or remnant patches of porous sands, that are encased in low permeability shales. Turbidite channel/levee deposits are the dominant reservoir facies in these ‘patches’; however, they may consist of channel, levee, or debris flow facies. Detailed mapping of these dominantly gas charged erosional remnants shows little to no fit of seismic amplitude effects to structure and the hydrocarbon accumulations often appear as amplitude anomalies ‘floating in space’. This work was done to provide an analogue study, as well as to document any remaining prospects that were initially overlooked. Almost every discovery and dry hole in the trend was interpreted and included in this study. Integrating the well control into the geological model by using detailed seismic stratigraphy, whole core description, dipmeter logs, seismic to well tie synthetics, Gassmann fluid substitution and AvO analysis, provided the necessary insight to prosecute this play. Within the play area, seismic velocities of sands and shales are very similar. As a consequence of this, seismic reflections are generally weak and, due to the stratigraphic variability, they are discontinuous. Hence, standard sequence stratigraphic mapping techniques alone are not enough to define these subtle traps. The key to successful exploration is a complete understanding of the rock properties. All of the fields examined displayed subtle amplitude anomalies associated with the presence of hydrocarbons. The ability to understand and quantify both the amplitude is essential to increasing the probability of success in this play.

Residual Shear Strength Mobilized in First-Time Slope Failures

This paper presents a review of long-term stability of stiff clay and clay shale slopes, and detailed reanalyses of 99 case histories of slope failures in 36 soft clays to stiff clays and clay shales. We analyzed 107 sections using the observed actual slip surface. In a first-time slope failure in clay or shale, part or all of the slip surface is unsheared prior to the occurrence of the landslide. Most stiff clays and clay shales contain stratigraphic discontinuities such as bedding planes and laminations. The fully softened shear strength is shown to be the lower bound for mobilized shear strength in first-time slope failures in homogeneous soft to stiff clays and on the slip surfaces cutting across bedding planes and laminations. For many of the first-time slope failures it appears that part of the slip surface is at the residual condition. For excavated slopes, the residual condition could be present before the final slope is formed, or it may develop in response to excavation by progressive deformation along nearly horizontal surfaces including bedding planes or laminations. In addition to the permeability dependent rise in porewater pressure, and softening, delayed first-time failure of slopes in stiff clays and clay shales is caused by propagation of the residual condition into the slope, on horizontal or subhorizontal surfaces including stratigraphic discontinuities. The residual condition is present on the entire surface of reactivated landslides.

Abstract: Bowman Road Gas Pool, A Subtle Structural Trap in the French Camp Gas Field, San Joaquin County, California 

Abstract The Bowman Road pool is an Upper Cretaceous Lathrop gas discovery drilled in September, 1994 by Nahama and Weagant Energy Company. It is located in the northernmost San Joaquin Valley, approximately five miles south of the city of Stockton. The pool, currently operated by Enron Oil and Gas Company, lies on the upthrown side of the Stockton fault one mile east of the large Lathrop gas field which has produced in excess of 350 Bcf from Lathrop sandstones. The Bowman Road discovery was based on regional studies and detailed subsurface geology tied to a one-half mile grid of CDP seismic. It targeted stacked Lathrop sandstones in a small faulted anticlinal closure. The exploration concept was a deeper pool test of the western structure of the French Camp gas field. This faulted, south-plunging anticline produced gas from the Second Starkey Sandstone and it was believed that deeper Lathrop sandstones had not been tested at a crestal position because of offset of the structural axis with depth. The crestal migration is caused by westward thinning of the section overlying prospective Lathrop sandstones. Nearly 500 feet of regional thinning occurs parallel to the Starkey shelf edge because of thicker Starkey shelf sand deposition to the east and greater compaction of coeval slope shale to the west. The interpretation proved to be correct and the well was completed from selected intervals between 7532′ and 7678′ at a rate of 3200 Mcf/D of 733 Btu gas on a 16/64″ choke. Production sales began in January, 1996 and development of the pool is currently underway.

Evaluation and Detection of Overpressures in a Deltaic Basin: The Sisi Field Case History, Offshore Mahakam, Kutei Basin, Indonesia: ABSTRACT

Widespread overpressure occurrences in the Mahakama delta area have caused a number of kicks and several blow-outs during earlier exploration. A review of pore pressure indications was conducted in conjunction with an extensive reinterpretation of seismic and well data basin wide. The iso-pressure lines were found to be broadly parallel to the facies change from sandy, delta-front deposits to outer shelf and slope shales, and are not associated with organic matter maturation or clay diagenesis. Overpressures are reached at depths ranging from 1000 m to more than 4000 m, depending on the facies. A 2-D numerical basin model calibrated on observed pressure profiles at wells indicated that the excess pressures were a direct function of the variable drainage efficiency of the formations by interbedded sands. The Sisi discovery located at the eastern, distal extremity of the upper Miocene deltaic sands provided a unique opportunity for a more detailed analysis. A hypothesis of widespread decoupling between reservoir and shale pore pressures was tested against well data during appraisal drilling. D-exponent plots and gas shows were carefully monitored to assess the excess pressures, and quantitative estimates of the shale pore pressures were computed from sonic logs. A regionally consistent calibration was achieved,more » which confirmed very large discrepancies compared to the pressures measured in interbedded permeable reservoirs. These conclusions have since been generalized to other areas of the basin, where they allowed safer drilling practices to be established as demonstrated by the observed reduction of lost time.« less

Low Stand Seismic Facies – A Carnarvon-Browse Basin Comparison

Similarities between eleven Mesozoic and Cainozoic supercycles in the Carnarvon and Browse Basins of Western Australia may be exploited in exploring for low stand turbidites, located just above sequence boundaries. Seismic and well sequence analyses are the tools used. Local well calibration is vital as geometrically similar units are being tested which are lithologically dissimilar, namely slumped shale slope units versus low stand clastic fans. Most low stand fans are associated with tectonically influenced base level drops, especially in the late Jurassic and Neocomian ? supercycles 3 and 4. These packages are the synrift phases of two major rift periods on the west coast. A particular boundary may have a low stand unit but its details can be quite different between basins.

Upper Campanian and Lower Maestrichtian Depositional Systems and Gas Production, Southern Sacramento Basin, California: ABSTRACT

Upper Campanian and lower Maestrichtian strata of the southern Sacramento basin include four west- and southwest-prograding submarine-fan/slope/delta systems. The Winters, Lathrop, Tracy, and Blewett formations consist of submarine-fan and related slope/basin-plain deposits that were fed by various deltaic complexes of the Starkey Formation. Four major basinwide transgressive shale units (Sacramento Shale, Sawtooth Shale, Ragged Valley Shale, and H and T Shale) help intrasystem correlations. The Winters, Tracy, and Blewett fans are small, radial, coalescing sand-rich systems that contain the following principal facies: (1) sandstone-filled inner fan channel deposits, (2) mudstone-dominated inner fan interchannel deposits, (3) middle-fan amalgamated suprafan-type sandstone-rich channel deposits, and (4) mudstone-dominated outer fan deposits. The Lathrop fans are larger, elongate, mixed-sediment systems that contain basin-plain, outer fan lobe, middle fan-channel, levee, interchannel, and inner fan channel facies. The Sierran-derived fluvio-deltaic Starkey Formation can be divided into six sand-rich deltaic cycles that can be subdivided on the basis of log signaturres and spatial distribution into prodelta, delta-front, lower delta-plain, and upper delta-plain/fluvial facies. More than 50 gas fields produce from these systems. Stratigraphic traps include updip pinchouts of submarine canyon/gullies and inner fan channels into slope shale, especially in the many overlapping and coalescing sand-rich systems. Lateral pinchoutsmore » of outer fan lobes and middle-fan suprafan-type bodies are also productive. Structural traps generally characterize production from deltaic deposits because of the more continuous nature of these bodies.« less

A new technique of locating turbidite fans: Candeias formation (lower cretaceous), Recôncavo Basin, Brazil

The reservoir package in the Candeias oil field, the largest stratigraphic accumulation of hydrocarbons in the Recôncavo Basin, has been reinterpreted as consisting of sublacustrine turbidite fans interbedded among Candeias shales, with the best quartzose arenite reservoirs corresponding to channeled mid-fan lobes. The occurrence in the turbidites of reworked lacustrine carbonate clasts with subaerial exposure features indicates that the Candeias submarine fans originated from an intrabasinal shallow platform and followed NNW-SSE fault-controlled channels on their way to final deposition. Thick and irregularly shaped lobes of massive fine sandstones (Pitanga Member) are always located immediately behind (that is, updip from) the relatively thin and coarse turbidite fans and overlap them diachronically. The petrographic composition of the massive sandstone expresses an extrabasinal origin from low-grade metamorphics of the Precambrian basement, and their sedimentary structures indicate large-scale fluidized sediment flow depositional processes. However, the admixture of a small amount of shallow water lacustrine carbonate clasts reveals that Pitanga sediments also crossed the carbonate platform before being deposited as isolated or coalescent lobes. Candeias turbidite fans had a stationary source area, the carbonate platform, and reached the distal portion of the shale slope. Pitanga fluidized flows were deposited in a proximal position of the shale slope and had a deltaic source that gradually prograded from NNW to SSE across the Tucano Basin. The constant geometric relationship between Pitanga lobes and Candeias turbidite fans originates from the fact that both depositional systems followed the same NNW-SEE fault-controlled channels; this becomes, therefore, an exploration tool for the prediction of oil-producing Candeias fans that are at the limit of resolution of seismic reflection techniques. Three operational steps are required: structurally analyzing the carbonate platform to locate potential transverse faults to which feeder channels might correspond; outlining the lobes of Pitanga massive sandstones related to such channels using seismic reflection profiles; and drilling exploratory wells for Candeias turbidite fans beneath the frontal portions of Pitanga lobes.

Drought and Survival in a Self-Perpetuating Pinus pungens Population: Equilibrium or Nonequilibrium?

A 10-year follow-up study of recruitment and survival of table mountain pine (Pinus pungens) growing in shallow soil on a granite dome in western North Carolina showed that the population, which perpetuates itself without fire or other disturbance, has experienced long intervals of high survival and recruitment interrupted by brief episodes of low survival and recruitment probably caused by extreme drought. For these pines growing in shallow soil over bedrock, the Palmer Drought Severity Index was poorly correlated with episodes of low survival. A more appropriate drought index for the edaphic situation was the maximum number of consecutive rainless days in each 5-year age-class germination interval. Analysis of age-specific survivorship for the 1977-1986 decade showed that table mountain pines less than 30 years of age experienced lower survival rates during drought than older pines. INTRODUCTION In 1976 an unusual population of table mountain pine (Pinus pungens Lam.) was found growing on the rounded SW shoulder of Looking Glass Rock, a 1-km2 granite dome near Brevard, in western North Carolina (Barden, 1977). The age structure of these pines, derived from increment cores, showed that this population was an edaphic climax which had perpetuated itself since the most recent fire in 1889 (Barden, 1977). Zobel (1969) had predicted that such a stand of Pinus pungens might be found on rock outcrops or shale slopes where hardwood species grow poorly. McIntosh (1950) found pine populations (Pinus strobus, P resinosa, and P banksiana) which, on the basis of size structure of the stands, appeared to be perpetuating themselves on sandstone and limestone bluffs in southwestern Wisconsin. However, the table mountain pine population on Looking Glass Rock is the first reported pine climax, based on known age structure and disturbance history, in the eastern United States (Barden, 1977). In 1976 I interpreted the close fit of a log-linear curve to the age-structure data as evidence that the population was approaching equilibrium, that is, having relatively constant and approximately equal mortality and recruitment rates. Harper and White (1974) suggested an expanding population as an alternative interpretation for such data, but this explanation seemed unlikely because of the number of dead pines found in varying stages of decomposition (Barden, 1977). The carrying capacity of the site appeared to be set by the availability of pockets of soil deep enough to support droughttolerant pines but too shallow to support hardwoods during summer droughts (Barden, 1977). In 1986 I recensused the same trees, using detailed maps and fixed-point photographs from the 1976 study to relocate all members of the population. The purpose of the new study was to determine whether the population continued to exhibit the loglinear age structure after 10 additional years of recruitment and mortality. My reinterpretation of the cohort data, based on 5-year rather than 20-year age classes and a more critical analysis of weather records, differed sharply from conclusions of the earlier study, reflecting a new understanding of the importance of sporadic high morality in the population dynamics of the stand. STUDY AREA AND METHODS Looking Glass Rock is an oblong granite dome, 2.0 km by 0.5 km, which rises 300 m above the surrounding terrain to a maximum elevation of 1200 m. It is located 10 km NW of the nearest permanent weather station at Brevard, North Carolina, where

Source Rock Potential of an Eocene Carbonate Slope: The Armancies Formation of the South-Pyrenean Basin, Northeast Spain: ABSTRACT

The Armancies Formation is an Eocene carbonate slope succession in the Catalonian South Pyrenean basin. It ranges from 500 to 700 m in thickness. The first 200 m are made of a thin-bedded facies of wackestones alternating with dark pelagic fauna of miliolid, ostracods, bryozoans, and planktonic foraminifers and show significant bioturbation. They also show a low organic content (< 0.5% TOC). The lime-mudstone beds show a massive structure or planar millimeter laminations. They may contain sparse pelagic fossils of planktonic foraminifers, ostracods, and dinoflagellates; they do not show any bioturbation, and have high TOC values, which can reach individual scores of about 14%. They qualify, therefore, as a typical oil shale. Rock-Eval Pyrolysis analysis affords a mean S{sub 2} value of 25 mg HC/g. Mean S{sub 1} value is around 1.0 mg HC/g. As is typical of an initial oil window, T{sub max} maturity parameter ranges from 432 to 440{degree}C (mean = 434{degree}C). This degree of evolution is in accordance with the very low value of carbonyl and carboxyl groups, as determined by IR spectrometry and NMR on Fischer assay extract. The proton NMR shows an aromatic/aliphatic hydrocarbon ratio of 1:4, as expected in earlier stages of catagenesis. N-alkanemore » gas chromatography profiles show n-C{sub 15} to n-C{sub 19} prevalence and that neither even nor odd carbon numbers prevail. This distribution perfectly matches that of typical sediments of marine origin and also agrees with obtained hydrogen index values (mean HI = 500 mg HC/g TOC). Sedimentological and geochemical results indicate an autochthonous marine organic matter and the potential of these slope shales is good oil-prone source beds.« less

Evolution of Permian Carbonate Shelf and Foreshelf Detrital Systems, Midland Basin, Texas: ABSTRACT

A major phase of shelf progradation and consequent filling of the Midland basin began in the Early Permian. Prior to this time, shelf carbonate systems were mostly ineffective in generating coarse detritus in quantities sufficient to infill adjoining basins. Beginning in the Early Permian (Wolfcampian), however, the character of shelf and basin deposition was abruptly modified, in part because of faunal changes - proliferation of rock-forming biotic communities and the consequent potential for rapid shelf-marginal oversteepening - and the effects of sea level fluctuations and periods of shale influx. Shelf systems in the northern Midland basin and Eastern shelf evolved from ramps in the lowermost Wolfcamp to steep rimmed platforms by the lower Leonard. Shelf progradation into the Midland basin was cyclic, with extensive carbonate shelves having developed during sea level highstands and massive shale wedges deposited during alternate lowstands. Coarse megabreccia with markedly little sand dominates in the lower to middle Wolfcamp systems, despite the occurrence of ramps in this section. Such an anomaly can be explained by invoking widespread shelf wastage caused by the inherent instability and failure of the shale slopes on which these ramps were deposited. In certain middle Wolfcamp zones, thin shelf-ramp sections may be more » recognized only because any thick shelf-marginal buildups that may have been present have been displaced into adjoining basinal tracts. The evolution to rimmed-platform shelves by the lower Leonard strata, foreshelf debris occurs mostly as complexes of megabreccia wedges and aprons, anastomosing megabreccia, and sand-channel and lobe deposits derived from adjoining rimmed-platforms. Major variations exist in the gross geometries of foreshelf units and component reservoirs even laterally within rocks of the same age. « less

Comment on “Possible Effects of Erosional Changes of the Topographic Relief on Pore Pressures at Depth” by J. Tóth and R. F. Millar

Water Resources ResearchVolume 21, Issue 6 p. 895-898 CommentariesFree Access Comment on “Possible Effects of Erosional Changes of the Topographic Relief on Pore Pressures at Depth” by J. Tóth and R. F. Millar C. E. Neuzil, C. E. NeuzilSearch for more papers by this author C. E. Neuzil, C. E. NeuzilSearch for more papers by this author First published: June 1985 https://doi.org/10.1029/WR021i006p00895Citations: 8AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL References Blair, B. E., Physical properties of mine rock, 3, Rep. Invest. U.S. Bur. Mines Invest., 5130, 1955. Blair, B. E., Physical properties of mine rock, 4, Rep. Invest. U.S. Bur. Mines Invest., 5244, 1956. Brace, W. F., Some new measurements of linear compressibility of rocks, J. Geophys. Res., 702, 391– 398, 1965. Bredehoeft, J. D., Response of well-aquifer systems to earth tide, J. Geophys. Res., 7212, 3075– 3087, 1967. Dejong, J., Foundation displacements of multistory structures, Ph.D. thesis,, 251 pp.,Dept. of Civ. Eng., Univ. of Alberta,Edmonton,1971. Fatt, I., Compressibility of sandstones at low to moderate pressures, Bull. Am. Assoc. Pet. Geol., 428, 1924– 1957, 1958. Gibson, R. E., The progress of consolidation in a clay layer increasing in thickness with time, Geotechnique, 84, 171– 182, 1958. Hendron Jr., A. J., G. Mesri, J. C. Gamble, G. Way, Compressibility characteristics of shales measured by laboratory and in situ tests, Determination of the In Situ Modulus of Deformation of Rock, ASTM Spec. Tech. Pub., 477, 137– 153, American Society for Testing and Materials, Philadelphia, Penn., 1970. Jacob, C. E., On the flow of water in an elastic artesian aquifer, Trans. AGU, 21, 574– 586, 1940. Lutton, R. J., D. C. Banks, Study of clay shale slopes along the Panama Canal, Report 1, East Culebra and West Culebra Slides and the Model SlopesTech. Rep. S70-9, 285U.S. Army Eng. Water. Exp. Sta., Vicksburg, Miss., 1970. Matheson, D. S., S. Thompson, Geological implications of valley rebound, Can. J. Earth Sci., 106, 961– 978, 1973. Meinzer, O. E., Compressibility and elasticity of artesian aquifers, Econ. Geol., 233, 263– 291, 1928. Neuzil, C. E., D. W. Pollock, Erosional unloading and fluid pressures in hydraulically “tight” rocks, J. Geol., 912, 179– 193, 1983. Peterson, R., Rebound in the Bearpaw shale, western Canada, Geol. Soc. Am. Bull., 699, 1113– 1124, 1958. Peterson, R., Discussion of “The elastic properties of a dense glacial till deposit” by E. Klohn, Can. Geotech. J., 22, 140, 1965. Pettijohn, F. J., Sedimentary Rocks, 718, Harper and Row, New York, 1957. Serota, S., R. A. J. Jennings, The elastic heave of the bottom of excavations, Geotechnique, 92, 62– 70, 1959. Toth, J., R. F. Millar, Possible effects of erosional changes of the topographic relief on pore pressures at depth, Water Resour. Res., 196, 1585– 1597, 1983. van derKamp, G., J. E. Gale, Theory of earth tide and barometric effects in porous formation with compressible grains, Water Resour. Res., 192, 538– 544, 1983. Vaughn, R. V., H. J. Walbancke, Pore pressure changes and delayed failure of cutting slopes in overconsolidated clay, Geotechnique, 234, 531– 539, 1973. Citing Literature Volume21, Issue6June 1985Pages 895-898 ReferencesRelatedInformation

A Delta-Slope-Submarine Fan Model for Maestrichtian Part of Great Valley Sequence, Sacramento and San Joaquin Basins, California

Shoaling of the Great Valley fore-arc basin, which began in the northern Sacramento basin during the Campanian Stage, shifted southward in the Maestrichtian to the southern Sacramento and northern San Joaquin basins, where a regressive sequence of submarine fan, slope, and deltaic deposits aggregates 6,500 ft (2 km) in thickness. Water depth decreased to sea level as ponded submarine fans aggraded the basin floor, and slope and deltaic systems prograded basinward over them. The central part of the fore-arc basin was filled by the end of the Cretaceous; the southern part was not filled until the middle Eocene. Deltas prograded westward and southwestward from the Sierran magmatic arc. The Starkey delta system evolved from cuspate/lobate wave-dominated deltas with extensive upward-coarsening delta-front sand beds to highly lobate fluvial-dominated deltas with moderately developed delta-plain facies. This system was succeeded by elongate fluvial-dominated deltas with thick delta-plain/fluvial sequences in the Bethel Island fluvial-deltaic system. Thin, fine-grained, black, glauconitic shale beds were deposited during intervals of regional transgression and deltaic abandonment. Several progradational-transgressive cycles are recorded. The Roberts Island slope system comprises several thick shale sections deposited during progradational events. Benthic foraminifera grade upward from abundant, varied, lower bathyal species to sparse upper bathyal or neritic species. Clinoform beds that offlap from east to west are evident in electric-log cross sections and seismic profiles. Lenticular, channeled sand beds, transverse to the slope, appear to be incised slope channels that funneled sand from the delta front to submarine fans on the basin floor. Shelf-edge slumps can locally be recognized by flow folds, mudstone blocks, and anomalous thicknesses and bedding attitudes, particularly near growth faults. The Union Island submarine fan system consists of several fans; most are elongate parallel to the southeasterly trend of the basin. Fans consist of upper, middle, and lower segments. Thick-bedded, channelized, upward-fining, coarse-grained sand beds characterize the upper fan. Medium to thick-bedded, channelized, massive fine to medium-grained sand beds predominate on the midfan. Lower fan deposits are not extensive but consist of symmetrical or upward-coarsening sequences of shale and thin to medium-bedded fine to very fine sand. Channeling is less common on the lower fan. Most fans have multiple lobes, each lobe having been fed by a different delta. Gas is produced from deltaic and submarine fan deposits. Delta-front sand beds form very small reservoirs where growth faults provide subtle structural traps. Distributary-channel sand beds are highly productive, owing mainly to stratigraphic pinch-outs but locally perhaps to structural traps. Submarine fan traps have so far been located mainly in midfan deposits, either in the structurally high suprafan mounds or along lateral fan margins near the base of the slope where fan deposits onlap and pinch out against slope shale. Exploration in lower and upper fan deposits has barely begun.

Depositional Relations of Cretaceous and Lower Tertiary Rocks, Northeastern Alaska

Analysis of depositional environments, new paleontologic data, and analogy with depositional patterns observed in areas to the west all indicate the need for revision of Cretaceous and lower Tertiary stratigraphy in northeastern Alaska. In the Sadlerochit Mountains area of the Arctic National Wildlife Refuge, the northern-derived (Ellesmerian), late Neocomian Kemik Sandstone Member and organic-rich pebble shale member of the Kongakut Formation unconformably overlie Jurassic and Triassic rocks. The unconformity, which is of mid-Neocomian age, is present throughout northernmost Alaska and passes southward into a conformable shelf sequence. After pebble shale deposition, the depositional pattern is simply one of progradational basin filling from a southern (Brookian) provena ce. This pattern is represented in vertical sequence initially by deep-marine basinal deposits succeeded by prodelta slope shales, and ultimately by deltaic deposits that prograded to the east or northeast in a predictable fashion over most of the area. The better known, thick, shallow-marine and nonmarine units in the central North Slope grade to thin prodelta or basinal turbidites; some units are entirely missing owing to nondeposition on the south-dipping flank of the basin. No evidence exists for subaerial erosion to explain these marked stratigraphic changes. In the Sadlerochit Mountains area, which may be an extension of the Barrow arch, the pebble shale (Neocomian) is overlain by about 1,600 ft (500 m) of deep-water deposits of Late Cretaceous age; Aptian and Albian rocks are either absent (by nondeposition) or are represented by a thin, condensed section. Here the Upper Cretaceous to lower Tertiary section consists of basinal shale, bentonite, and thin-bedded prodelta turbidites. Two noteworthy features in this sequence are an interval of organic-rich shale of probable Turonian to Coniacian age just above the pebble shale, and the placement of the Cretaceous-Tertiary boundary within deep-marine deposits. Unlike the western part of the North Slope, where deltaic deposition occurred in the Early Cretaceous, prograding deltaic deposition d d not reach the western part of the Wildlife Refuge until early in the Tertiary. The consistent pattern of easterly deltaic progradation is complicated at Igilatvik (Sabbath) Creek (in the north-central Wildlife Refuge) by the occurrence of thick, regressive, dominantly nonmarine deposits of early Tertiary age, which apparently prograded into the basin from the south or southeast.

Sunbury Shale of Central Appalachian Basin--Depositional Model for Basinal Black Shales: ABSTRACT

End_Page 1003------------------------------A model for the deposition of basinal black shales and associated base-of-slope turbidites is established in the central Appalachian basin. The model is based on an outcrop and subsurface stratigraphic study of the thin, but extensive Lower Mississippian Sunbury black shale. This model is termed the Sunbury cycle and it provides an explanation for the geometrid relations observed between basinal black shales and laterally adjacent gray/green slope shales and siltstones. Two genetic types of black shales are recognized--transgressive and regressive. The transgressive black shale is the basal unit that initiates the cycle. Characteristically thin and widespread, it was deposited in the anaerobic zone of a stratified water column. Its sharp basal contact represents the rapid migration of the anaerobic environment over base-of-slope and slope deposits. This is caused by an increase in the subsidence rate of the active basin, decrease of clastic influx, and a minor rise in sea level. The regressive black shale overlies the basal unit. It is thicker, more laterally restricted, and represents the distal facies of base-of-slope turbidites formed by a progressive increase of clastic influx and a decrease in the subsidence rate of the basin floor. The thick re ressive black shale and laterally adjacent non-black clastics represent facies of a continuous unit (base-of-slope turbidites) containing varying amounts of preserved organic material. The degree of organic preservation is the result of deposition in zones of varying oxygen content caused by the intersection of the stratified water column with the basin floor. End_of_Article - Last_Page 1004------------

Unstable Progradational Clastic Shelf Margins: ABSTRACT

Ancient shelf margins have generally been overlooked in some progradational clastic systems such as the northwestern Gulf of Mexico and the Niger delta. Apparently the contemporaneous structural deformation, particularly growth faulting, obscures depositional dips and foreset-topset geometry, making recognition of shelf breaks from these criteria virtually impossible. Nonetheless, their positions can be estimated from their association with characteristic micro-faunal assemblages, with initiation of growth faulting, with facies changes, and with geopressure. Rapid subsidence of progradational shelf margins results primarily from three processes: isotatic depression of the basement due to sedimentary loading, extensional thinning of the sedimentary wedge due to gravity tectonics, and compaction. Instability of the continental slope causes substantial basinward mass transport by deep-seated gravity sliding. This is manifested as down-to-basin listric growth faults originating at the outer shelf and upper slope (extensional regime), and shale and salt ridges and domes originating at the lower slope (compressional regime). The rapidly subsiding shelf margin acts as a major sediment trap, leading to accumulation of thousands of feet of shallow-water sediments, including deltaic sandstones, along a growth-faulted trend that may be hundreds of m les long. Shelf-margin deltas differ substantially from shallow-shelf deltas in that they show thicker and better differentiated progradational units and steeper clinoforms. Sand geometry of shelf-margin deltas is influenced by two competing factors: absence of a broad shelf to attenuate wave energy, thus favoring wave dominance, and high sand continuity, versus rapid subsidence, which prevent lateral reworking and thus favor river dominance and low sand continuity. Rapid downfaulting of shelf-margin deltaic sandstones against dewatering slope shales leads to the accumulation of excess fluid pressure in deep fault-bounded reservoirs. Mapping of geopressure trends can therefore provide a generalized picture of shelf-margin progradation in Cenozoic basins. End_of_Article - Last_Page 1008------------

Pennsylvanian-Early Permian Depositional Systems and Shelf-Margin Evolution, Palo Duro Basin, Texas

The Palo Duro basin of the Texas Panhandle is filled primarily with Pennsylvanian, Permian, and Triassic strata that record the depositional history of a shallow cratonic basin. Regional deformation during Early Pennsylvanian time across a belt encompassing the southern Oklahoma and Delaware aulacogens resulted in the formation of the basin. Rapid basin subsidence and marine transgression dominated Pennsylvanian depositional history but was followed by marine regression and rapid filling of deeper parts of the basin during Early Permian (Wolfcampian) time. Pennsylvanian and Lower Permian strata consist of four major facies assemblages or depositional systems. An extensive fan-delta system is composed of arkosic sandstones eroded from Precambrian highlands flanking the bas n and deposited by braided streams along the margins of the basin. In the southeastern part of the Palo Duro basin, westward prograding, high-constructive deltas dispersed sediment across a shelf environment and into basin and slope environments. A thick, massive sequence of limestone accumulated seaward of the deltas in a carbonate-shelf and shelf-margin system that encircled most of the basin. The slope and basin system consists of terrigenous clastics that were funneled downslope into deep basinal environments by feeder channels that formed along shelf margins. Interplay between basin subsidence and local sedimentological controls determined the depositional style and resulting facies patterns. Rapid Pennsylvanian subsidence combined with invasion and deposition of terrigenous clastics across carbonate-bank environments caused parts of the northwestern shelf margin to retreat westward toward shallow, clear water. During Early Permian time subsidence rates slowed and sedimentologic controls dominated basin evolution. Thick slope wedges, which were formed by deltas that prograded to shelf edges and debouched sediment into the slope environment, created shallow foundations for subsequent carbonate-bank development and progradation. Porous shelf-margin dolomites, delta-front sandstones, and fan-delta arkoses are considered potential reservoirs for oil and gas. Potential source rocks may be present in adjacent, thick basinal and slope shales.

Silurian stratigraphy and facies distribution in Washington Land and western Hall Land, North Greenland

A new lithostratigraphic scheme is erected for the Silurian rocks of Washington Land and western Hall Land, North Greenland. The lowermost Silurian sediments are grouped with underlying Ordovician rocks in the Morris Bugt Group. The remaining Silurian platform carbonate sediments are included in the Washington Land Group (new) whilst basin slope rocks are assigned to the Peary Land Group (new). Following the Morris Bugt Group Silurian sediments were deposited in two main environments, platform and basin slope. Sediments in these settings are completely different, the platform is characterised by carbonates and the basin slope by mudstones, cherts and resedimented conglomerates. The hinge between platform and basin slope is characterised by abrupt and complex facies changes and in some areas carbonate buildups and megabreccias. The platform succession is at maximum 2 km thick whilst the basin slope sediments are about 600 m thick. The platform region subsided very unequally and two major periods of platform subsidence are recognisable; late Cincinnatian to early Llandovery and late Llandovery to early Wenlock. Four formations are included in the Morris Bugt Group, of which the uppermost Aleqatsiaq Fjord Formation straddles the Ordovician – Silurian boundary. The Washington Land Group is divisible into 10 formations including: Adams Bjerg Formation (new) composed of marginal marine stromatolitic dolomites with a carbonate buildup – Early(?) to Middle Llandovery Age; Pentamerus Bjerge Formation (new) a carbonate buildup, fringing reef, complex – Middle(?) Llandovery to Wenlock Age; Kap Godfred Hansen Formation (new) composed of submarine fan calcarenites and megabreccias – Middle (?) Llandovery to Wenlock Age; Petermann Halvø Formation (new) composed of marginal to open marine dolomites – Early Llandovery Age (?); Bessels Fjord Formation (new) a carbonate buildup complex – Middle (?) to Late Llandovery age; Offley Island Formation composed of shallow platform biostromes – Late Llandovery Age; Kap Lucie Marie Formation (new) composed of offshore to slope calcarenites and shales – Late Llandovery Age; Kap Morton Formation (new) composed of offshore platform to basin slope (peri-platform) lime mudstones – Wenlock to Ludlow Age; Kap Maynard Formation(new) divisible into Dolomite and Limestone Members (new) composed of offshore platform to basin slope (peri-platform) dolomites, shales and lime mudstones – Wenlock to Ludlow Age; Hauge Bjerge Formation (new) divisible into Cape Tyson Member and Kap Independence Member (new) a carbonate buildup complex – Middle (?) Llandovery to Wenlock Age. The Peary Land Group is divisible into two formations including: Cape Schuchert Formation composed of basin slope and starved basin, calcarenites, cherts and black limestones – Middle to early Late Llandovery Age; Lafayette Bugt Formation (new) composed of basin slope shales and conglomerates – Middle Llandovery to Ludlow Age.

A New Species of Lomatium (Apiaceae) from Utah

A new species ofLomatium belonging to theCynomarathrum group is described and its relationships to other species in the group are indicated.Lomatium junceum appears to be restricted to Emery Co., Utah, on barren clay and shale slopes in the San Rafael Swell and adjacent eastern base of the Wasatch Plateau.

Palo Duro Basin Analysis: ABSTRACT

A comprehensive stratigraphic analysis of the Palo Duro basin, emphasizing facies and depositional systems, provides a framework for resource analysis and exploration. A pre-Pennsylvanian section consists of thin, basal, Cambrian sandstone overlain by Ordovician and Mississippian shallow-shelf carbonate rocks. Pennsylvanian and Lower Permian rocks consist of 1,000 m of basin and slope shales, massive shelf-margin carbonate rocks, and deltaic sandstones. Facies distributions define a major episode of basin subsidence and transgression (Pennsylvanian) with shelf-margin retreat, followed by regression (southward), and basin filling (Early Permian). In Late Permian time, the basin was an extensive sabkha-shelf platform on which salt (upper sabkha), anhydrite-gypsum (lower sab ha), dolomite (subtidal to intertidal shelf), and red beds were deposited. Two continuous 1,220-m cores drilled to the base of the evaporites provide a unique opportunity for calibration of well logs with petrographic studies, and for resource evaluation (fluid tests, uranium and copper analyses, etc). The subsurface analysis has been coordinated with geomorphic and hydrologic studies in the same area (climate, slope, stream, and eolian process monitoring and field surveys of selected drainage basins) to demonstrate surface and subsurface interrelations. For example, locations of salt solution zones in the evaporite section coincide with surface erosional features, suggesting control of solution rate and position by drainage. High solute loads of area streams are probably derived from subsurface evaporite solution and result from regional surface discharge of groundwater circulation systems. Lack of proven oil and gas reserves in the basin may be attributable to a limited volume of thermally mature source rocks. End_of_Article - Last_Page 511------------