Regional Stratigraphic Studies Research Articles

Geoscience Poster G8: Geomechanical rock properties of the Officer Basin

Poster G8 The Officer Basin spanning South Australia and Western Australia is the focus of a regional stratigraphic study being undertaken as part of the Exploring for the Future (EFTF) program, an Australian Government initiative dedicated to increasing investment in resource exploration in Australia. Despite numerous demonstrated oil and gas shows, the Officer Basin remains a frontier basin for energy exploration with significant uncertainties due to data availability. Under the EFTF Officer–Musgrave Project, Geoscience Australia acquired new geomechanical rock property data from forty core samples in five legacy stratigraphic and petroleum exploration wells that intersected Paleozoic and Neoproterozoic aged intervals. These samples were subjected to unconfined compressive rock strength tests, Brazilian tensile strength tests and laboratory ultrasonic measurements. Petrophysical properties were also characterised via X-ray computerised tomography scanning, grain density and porosity-permeability analysis. Accurate characterisation of static geomechanical rock properties through laboratory testing is essential. In the modern exploration environment, these datasets are a precompetitive resource that can simplify investment decisions in prospective frontier regions such as the Officer Basin. To access the poster click the link on the right. To read the full paper click here

Geomechanical rock properties of the Officer Basin

The Officer Basin spanning South Australia and Western Australia is the focus of a regional stratigraphic study being undertaken as part of the Exploring for the Future (EFTF) program, an Australian Government initiative dedicated to increasing investment in resource exploration in Australia. Despite numerous demonstrated oil and gas shows, the Officer Basin remains a frontier basin for energy exploration with significant uncertainties due to data availability. Under the EFTF Officer–Musgrave Project, Geoscience Australia acquired new geomechanical rock property data from forty core samples in five legacy stratigraphic and petroleum exploration wells that intersected Paleozoic and Neoproterozoic aged intervals. These samples were subjected to unconfined compressive rock strength tests, Brazilian tensile strength tests and laboratory ultrasonic measurements. Petrophysical properties were also characterised via X-ray computerised tomography scanning, grain density and porosity-permeability analysis. Accurate characterisation of static geomechanical rock properties through laboratory testing is essential. In the modern exploration environment, these datasets are a precompetitive resource that can simplify investment decisions in prospective frontier regions such as the Officer Basin.

Depositional model and sequential evolution of the upper Quintuco Formation in its type area: implications for paleogeographic reconstructions for the southern Neuquén Basin during the Valanginian

The Quintuco Formation (Berriasian-early Valanginian) in central Neuquén province comprise marine, transitional and continental deposits. These deposits were included in several regional stratigraphic studies since the 80’s but receiving little attention in terms of detailed facies analysis, palaeoenvironmental reconstructions and evolution. Understanding the evolutionary stage corresponding to cusp section of the Quintuco Formation and its relation with basal deposits of the Mulichinco Formation, is key to estimate the magnitude of associated change to the event of basin reconfiguration occurred in the early later Valanginian. In order to reconstruct the final stages of the evolution of the Quintuco Formation in its type locality (Sierra de la Vaca Muerta), a facial, architectural and sequence-stratigraphic analysis of the uppermost interval is presented. The study comprise the sedimentological description and interpretation of facies, combined with architectural analysis of key intervals. This allowed the identification of 7 facies associations, which represent the accumulation in prodelta (FA 1), delta front (FA 2 and FA 3), distal delta plain (FA 4 y FA 5) and proximal delta plain (FA 6 and FA 7). Subsequently, the spatial distribution of the different facies associations were analyzed and key surfaces related to significant changes in the depositional settings were identified across the investigated region. In this context, sequence-stratigraphic analysis of the studied interval was addressed and stacking patterns of the successions and vertical evolution of the interval are discussed. The upper interval of the Quintuco Formation represents the development of a deltaic system, where fluvial processes were dominant, but waves affected the off-axis parts of the system. The proximal areas were located towards the southwest with prodelta settings located toward the east. The delta system was built by successive shallowing-upward successions (15-40 m), bounded by regional transgressive surfaces, and with a long-term progradational staking. The reconstructed paleogeography for the last evolutionary stage of the Quintuco Formation suggest a well-established source area from the west-southwest, that would represent a new insight for paleogeographic settings for the southern Neuquén Basin during the Valanginian.

Lithofacies classification in the Marcellus Shale by applying a statistical clustering algorithm to petrophysical and elastic well logs

Interpreting shale lithofacies is an important step in identifying productive zones in the Marcellus Shale gas play; the target reservoir can be less than 3 m (10 ft) thick with overlying shales that appear similar on certain petrophysical well logs and in the core. However, these nonreservoir-quality shales contain widely varying organic and mechanical properties. To distinguish between reservoir- and nonreservoir-quality shale facies, this classification method applies a pattern-recognition algorithm, expectation maximization, to a set of commonly available petrophysical and elastic well logs to create 1D shale facies models along the wellbore. The statistical algorithm classifies the petrophysical and elastic well-log data into defined facies using the theory that each well log contains a distribution of Gaussian curves for each facies. The method is applied to 12 wells across Pennsylvania and northern West Virginia, and it is able to discriminate between shale facies based on organic content and brittleness, characteristics which are not always evident in core. To verify the geologic accuracy of the facies models, the results are compared with the core data, well logs, mudlogs, and regional stratigraphic studies when available. In addition, we were able to designate a lithofacies-defined top to the Marcellus Formation in parts of the Appalachian Basin where the gradational contact with the overlying Mahantango Formation is poorly defined on petrophysical logs.

Geochemistry and petrogenesis of basaltic rocks from the Develidağ volcanic complex, Central Anatolia, Turkey

The Develidağ volcanic zone (DVZ) is a part of the Middle Miocene-Quaternary Cappadocian Volcanic Province (CVP) in central Anatolia. The DVZ is located at the eastern side of the Plio-Quaternary Sultansazlığı pull-apart basin, that opened along the Ecemiş left-lateral strike-slip fault. Towards the south, the volcanic rocks of the DVZ overlie the Paleozoic metamorphic rocks of the Taurus range. Regional stratigraphic studies indicate that volcanic rocks of the DVZ crop out in a ∼N–S trending rectangular-shaped exposure in the middle of which Middle Miocene andesitic rocks are found surrounded by Upper Miocene basaltic rocks. The purpose of this paper is to present basic geochemical data for the DVZ rocks and discuss possible processes of magma generation. The Develidağ basalts are characterized by low LILE (Rb, K, Ba, Th) and high HFSE (Nb,Zr,Hf,Y) contents, whereas the andesites generally have high LILE and HFSE values except for Nb and Zr. Variable abundances of Pb (3.11–12.09 ppm) and U (0.36–2.64 ppm) are associated with high Ba content within the rock suites. Although low Nb/La (0.6–0.7) and relatively high Ba/Nb ratios indicate crustal involvement for the basalts, high Zr/Ba (0.5), Zr/Hf (42–47) and Th/U (3.13–4.69) values imply contributions from an asthenospheric source component. Furthermore, the high Zr/Hf values (>36) are the diagnostic feature of metasomatized mantle (Dupuy et al., 1992; Rudnick et al., 1993). Moreover, multi-element patterns show that Develidağ basalts have similar trace element signatures to those of the US Cascades tholeiites. The ratios of Zr/Hf, Zr/Ba, Nb/Th and Sr/Ce indicate that basaltic rocks are derived from a MORB-like mantle, and calculated melting model reflects generation from a spinel peridotite source (3–4% melting), but the combined effects of melting and assimilation and fractional crystallization (AFC) processes seem to be partially responsible for the relatively evolved rocks. Typical tholeiitic-calk-alkaline associations are also observed among the volcanic rocks as a consequence of melting processes and crustal involvement, or due to the changes in the melting rate and degree. The lack of alkaline rocks that is conventionally observed during the final stage of many central Anatolian volcanic centers may be attributed either to the lack of extensional development or to a deficient amount of extension before the late Miocene.

Sedimentary Environments and Trace Fossils of Tertiary Oasis Deposits in the Central Namib Desert, Namibia

Dish-shaped lenses of dolomitic limestone commonly occur in the arenaceous Lower Tertiary and Quaternary eolianites of the Central Namib Desert region. They are interpreted as the deposits of vegetated playa lakes and, as such, have been described as fossil Regional stratigraphic studies have shown that there were two separate episodes during which these playa lakes formed. These are the Zebra Pan Carbonates of the Late Eocene-Early Miocene Tsondab Sandstone Formation and the Khommabes Carbonates in the partially consolidated Late Pliocene-Recent Sossus Sand Formation. In plan view, the dolomitic limestone beds have a roughly concentric distribution of lithofacies and trace fossils that reflect the water-table depth and the interaction of the vegetated lake margins with the surrounding dunes. The facies change laterally from outer arenitic dolocrete with branching unlined cylindrical Planolites burrows, some with indistinct meniscae (possibly Rutichnus) and ovoid Termitichnus nest structures to massive carbonate with desiccation, syneresis, and tepee structures containing abundant Taenidium burrows and reed-stem casts. The central facies comprises microlaminated carbonate with stromatolite mounds and algal matted surfaces with no burrowing infauna. The controlling factor over the stratigraphic distribution of paleo-playas is climate. Both intervals are linked to wet desert phases when the climate changed from an arid to relatively fluvial phase with perennial rivers and seasonal floods, through to semi-arid, with a more flashy fluvial discharge, and eventually returning to arid conditions. The spatial distribution of these assemblages is controlled by interdune topography and river-flood discharge in the case of the Zebra Pan-type end-point playas, and the elevation of groundwater table by bedrock highs in the case of Khommabes-type oases.

The onset of extensive loess deposition around the G/M boundary in China and its palaeoclimatic implications

Abstract In northern China the Red Clay-loess contact is palaeomagnetically defined at about the Gauss/Matuyama boundary. The Red Clay formation has recently been proposed as mostly of wind-blown origin, and can be interpreted as having been formed under climatic conditions of continuous warmth punctuated by small-amplitude oscillations. The shift from Red Clay development to extensive loess deposition exhibits a 3-fold abrupt increase in aeolian dust deposition rate, and marks the beginning of frequent and chaotic climatic fluctuations. This event implies a large-scale reorganization of the atmospheric circulation over East Asia, in particular the northward shift and intensification of the Siberian High, which may have resulted from a dramatic uplift of the Tibetan Plateau. Compared to another climatic shift recorded by the Chinese loess deposits, which occurred at about the upper boundary of the Olduvai subchron, the Red Clay-loess shift is much more pronounced and recognizable in the field. It appears that, in regional stratigraphic studies, the G/M magnetic reversal has a better correlation potential than the upper Olduvai.

Applying High-Resolution Sequence Stratigraphic Tools to the Texas Continental Margin to Fine-Tune Conventional Sequence Stratigraphic Models and Improve Reservoir Prediction: ABSTRACT

A regional sequence stratigraphic study of the Texas continental margin resulted in an improved understanding of Pleistocene-Holocene eustatic sea level fluctuations and their influence on depositional environments associated with several distinct systems: the Colorado/Brazos system, the Texas interfluve region, and the Rio Grande system. The dataset consists of approximately 7500 kilometers of high-resolution seismic profiloes, 200 borehole descriptions, and 12 sediment cores. The study involved analysis of three independent variables to better constrain Pleistocene-Holocene sea level history: coastal onlap derived from high-resolution seismic profiles; oxygen isotope data determined from selected sediment cores; and paleoenvironmental data developed from high-resolution micropaleontologic analyses. Combined lithologic data and seismic facies analysis is used to map the distribution of major depositional systems on the shelf during one complete glacial eustatic cycle. Our study provided anopportunity to fine tune conventional stratigraphic models with regard to the distribution of potential reservoirs relative to regionally mappable stratigraphic surfaces.

Petrography of Neocomian Sandstones in Western Brooks Range, and Tunalik, Klondike and Burger Wells, Northwestern Arctic Slope-Chukchi Sea: ABSTRACT

Petrographic examinations were performed on Neocomian sandstones of the Ellesmerian sequence in outcrop in the western Brooks Range and from cuttings and cores from wells in the northwestern part of the National Petroleum Reserve in Alaska (NPRA) and adjacent Chukchi Sea that are probably coeval with the Kuparuk River Formation in the Prudhoe Bay area. Close similarities are recognized among samples from the 300 foot thick Tingmerkpuk Sandstone, which crops out in the western DeLong Mountains, and samples of apparently coeval intervals in the Tunalik, Burger, and Klondike wells. All are quartzose sublitharenities-quartzarenities that are dominantly fine to very-fine grained, with a few medium-to-coarse-grained sands. Grains are generally well-to very well-sorted, with minor detrital clays. Other framework grains include minor to trace amounts of cherts, argillaceous rocks, carbonate glauconite are commonly present, as rounded peloids, sometimes deformed between more rigid grains. Cements are principally quartz and/or carbonates, with diagenetic clays in a few samples. Notable porosity occurs in some core from the Tunalik well, but is not obvious in these cuttings or outcrop samples (other data indicate porosity in both other wells). Although petrographic characteristics alone do not establish a correlation among these widely spaced control points, the similarities supportmore » a common tectonic/stratigraphic framework for the Neocomian in all the areas. Regional stratigraphic studies of these sandstones suggest potential for significant reservoir quality associated with possible stratigraphic traps in the subsurface.« less

Shongaloo Field: A Recent Smackover (Jurassic) Discovery in the Arkansas-Louisiana State-Line Graben

ABSTRACT The new North Shongaloo/Red Rock/Haynesville/East Haynesville field (herein called Shongaloo) is the largest discovery in the mature Gulf Coast Jurassic Smackover play in the past 20 years. Field reserves are 20-30 MMBOE, developed from over 50 producing wells. The field is located within the Arkansas-Louisiana state line graben. The previously under-explored graben bisects the mature state line Smackover play, where exploration dates back to the 1940's. Shongaloo field is an upthrown fault trap within the graben. Vertical separation on the trapping fault ranges from 500 ft. to less than 200 ft. The trapping fault can only be resolved on modern, post-1982 seismic data. Shongaloo field pay is from the Smackover C sand and B carbonate. The C sand is overlain by a key transgressive marker horizon, the marine shale. The marine shale is overlain by prograding carbonate cycles including porous ooid grainstones of the B carbonate. The C sand is a tight gas-condensate bearing sandstone which requires hydraulic fracture stimulation for economic completion. B carbonate grainstone reservoirs have average porosity of 10% and average permeability of 60 md, ranging up to 1,800 md. The majority of the field reserves are from the B carbonate. Two technical methods were advanced in the discovery of the field. First, sub-surface correlations and core descriptions of Smackover facies documented that reservoir rocks follow paleo-shorelines across the graben in the Shongaloo area. Regional stratigraphic study indicates that the vast majority of porous grainstone facies within a mappable cycle of the Smackover B are parallel to and immediately down-dip (< 1.5 mi) of paleo-shorelines. The Smackover B facies in the state line area are associated with paleo-shorelines and are not greatly affected by Jurassic fault movements. Second, the critical trapping fault was resolved by shooting and interpreting modern proprietary seismic data. The discovery of Shongaloo Field demonstrates that significant reserves can be found in mature domestic plays by the application of modern technology and stratigraphic concepts.

DEPOSITIONAL HISTORY AND FACIES ANALYSIS OF THE UPPER JURASSIC SEDIMENTS IN THE EASTERN BARROW SUB-BASIN

Callovian to Tithonian syn-rift sediments in the eastern Barrow Sub-basin can be subdivided into five depositional sequences, each separated by regionally correlatable unconformities. Sequence boundary development can be closely related to periods of major changes in basin configuration associated with the sequential breakup of eastern Gondwanaland. Synchronism of major faulting with sequence boundary development during the early and late Callovian, mid Kimmeridgian, and mid Tithonian times supports tectonism being a dominant control on the development of Type 1 unconformities in the eastern Barrow Sub-basin.Upper Jurassic depositional sequences in the eastern Barrow Sub-basin, whether of tectonic or eustatic origin, consist primarily of lowstand systems tracts comprised, wholly or in part, of detached basin floor fan complexes, channelised and canyon-fed fan systems, slump deposits, outer shelf to slope deposits, and deep marine claystones. Inner shelf to shoreface sediments of the transgressive and highstand systems tracts are absent due to episodic, post-depositional uplift and erosion along the Peedamullah Shelf and Flinders Fault System during the Late Jurassic. The periods of uplift and erosion provided much of the sediment redeposited in basinal areas during lowstand times.Depositional models based on regional sequence stratigraphic studies can be integrated with local seismic stratigraphy to provide a mechanism for estimating likely reservoir quality, once controls on sedimentation (namely tectonics, eustasy, and sediment supply) are understood. This is demonstrated by the recognition of at least seven sandstone facies within the Upper Jurassic sedimentary section. Each sandstone has particular characteristics which can be related to the depositional setting. Reservoir quality is best developed in dominantly medium grained, moderate to well sorted sandstones, deposited as detached, basin floor submarine fan sands or interbedded turbidites. In contrast, reservoir quality is poorly developed in the remaining sand-prone facies deposited as slope fans, slumps, or distal turbidites.

Analysis of Some Seismic Expressions of Big Injun Sandstone and its Adjacent Interval

The Big Injun sandstone is an important oil and gas reservoir in western West Virginia. The pre-Greenbrier unconformity has complicated correlations, and hydrocarbon explorationists commonly have misidentified the Big Injun in the absence of a regional stratigraphic study. Paleogeologic maps on this unconformity show the West Virginia dome, with the Price/Pocono units truncated resulting in pinch-outs of different sandstones against the overlying Big Lime (Greenbrier Limestone). Drillers have named the first sandstone below the Big Lime as Big Injun, and miscorrelated the real Big Injun with Squaw, upper Weir, and even the Berea sandstone. In this report, an 8-mi (13-km) seismic section extending from Kanawha to Clay counties was interpreted. The study area is near the pinch-out of the Big Injun sandstone. A stratigraphic cross section was constructed from gamma-ray logs for comparison with the seismic interpretation. The modeling and interpretation of the seismic section recognized the relief on the unconformity and the ability to determine facies changes, too. Both geophysical wireline and seismic data can be used for detailed stratigraphic analysis within the Granny Creek oil field of Clay and Roane countries.

The Nature and Significance of Condensed Sections in Gulf Coast Late Neogene Sequence Stratigraphy: ABSTRACT

ABSTRACT Because it lends itself to predictable interpretations, sequence stratigraphy is gaining acceptance as a unifying concept in stratigraphy in general and in exploration in particular. Any regional seismic stratigraphic study must be predicated on a sound chronostratigraphic framework. Among the components of stratigraphic sequences and depositional systems, the condensed section is probably the most fundamental stratigraphic unit. Condensed sections are relatively thin, highly fossiliferous hemi-pelagic beds that form at the time of maximum transgression. Most reliable biostratigraphic datums occur within them, and, being regionally widespread, they are excellent chronostratigraphic markers. Since they are the only consistent tangible stratigraphic unit which ties deep water and shelf environments, they are of paramount importance in delineating sequence boundaries and systems tracts. This paper discusses various criteria for recognizing condensed sections, and documents 20 seismically resolvable condensed sections in the pre-Wisconsin Plio-Pleistocene to uppermost Miocene of the Gulf Coast. They are recognized by abundance and species diversity peaks, and are calibrated in terms of the foraminiferal and calcareous nannofossil datums within them. All of these condensed sections are associated with an increase in paleo-water depth. As indicated by the chronostratigraphic control provided by planktonic datums, they are not random events and can be shown to be regionally correlative. The scheme presented here has been developed in conjunction with seismic sequence studies and has been integrated with regional seismic grids. Sixteen of these condensed sections are maximum flooding events; four of them have been found to develop between lowstand slope fans and lowstand prograding wedges. The characteristics of condensed sections in various paleo-environmental settings are discussed, and the Miocene/Pliocene transition of the Gulf Coast is related to global eustatic events.

Evolution of the San Juan Basin, Colorado, and New Mexico Basin Analysis Case Study: ABSTRACT

During 1985 to 1989, the US Geological Survey conducted a basin analysis study that investigated the sedimentological and structural history of the San Juan basin and vicinity. The San Juan basin is a large asymmetric basin in southwest Colorado and northwest New Mexico near the southeastern edge of the Colorado Plateau. It formed as a result of northeast-directed compression coupled with a slight northeastward shift of the Colorado Plateau during the Late Cretaceous to Oligocene Laramide orogeny. Although the basin assumed its final configuration during the Laramide orogeny, regional stratigraphic and sedimentologic studies show that uplifts active during the Laramide orogeny were also active in the geologic past. Analysis of 1,000 mi of reflection seismic data defines a rectilinear pattern of northwest- and northeast-trending faults that involved the Precambrian basement in basin development. Middle Pennsylvanian to Late Permian, Middle to Late Jurassic, Early Cretaceous, Late Cretaceous-Oligocene, and Miocene were major times of movements on the faults and/or bounding uplifts. Preserved Mesozoic and older sedimentary rocks in the basin accumulated as part of widespread regional depositional systems. Pennsylvanian strata, consisting of a thick sequence of clastic and carbonate rocks, and evaporites deposited in both open and restricted marine environments contain significantmore » oil and gas resources. Mesozoic strata deposited in a variety of widespread continental and marine environments host 50% of the country's uranium reserves. Five marine transgressive-regressive cycles during the Cretaceous resulted in a thick sequence of intertonguing marine and continental rocks containing large resources of coal, oil, and gas.« less

Sequence Stratigraphy of Plio-Pleistocene Strata in the South Additions: Offshore Louisiana: ABSTRACT

TGS Offshore Geophysical Company and GECO Geophysical Corp. have conducted a regional sequence stratigraphic study of the Plio-Pleistocene in offshore Louisiana south additions. Strata are deposited along an unstable progradational continental margin and systems tracts differ greatly from those described for stable progradational continental margins. Delineation of these units required iterative analysis of seismic, wireline, and biostratigraphic data. Condensed intervals were interpreted from 45 wells. Lowstand slope-fans exhibit concave-upward reflections associated with reflections that downlap away from them, chaotic reflections, and thin parallel units. Reflections in lowstand prograding-wedge systems tracts exhibit divergence toward downthrown sides of growth faults. Unlike stable passive margins, progradational patterns are not observed. Both transgressive and highstand systems tracts generally exhibit concordant reflections. Oblique progradational patterns, common in highstand strata along stable progradational margins, are rare. These systems tracts also exhibit characteristic patterns on SP and resistivity well-logs. Basin floor-fans commonly exhibit well-defined blocky sandstones and excellent reservoir potential. Good reservoir sands are also present in the lowstand prograding wedge system tract, characterized by sand-rich coarsening-upward sequences. Lithology in the lowstand slope-fan systems tract is highly variable. However, prolific reservoirs are present in channel facies, which exhibit blocky or fining-upward sequences. Reservoir quality is generally poor more » in the transgressive systems tract and highstand systems tract. This approach allows systems tracts to be correlated throughout the Gulf Coast Basin and effectively used to time geologic events in the basin and to predict location of reservoir and seal strata. « less

Evidence for Unconformities in the Cenozoic of the Central and South Texas Continental Shelf from Regional Electric Log and Seismic Cross Sections: ABSTRACT

ABSTRACT Regional stratigraphic studies conducted by the Minerals Management Service, Gulf of Mexico, Outer Continental Shelf Region, on the offshore Texas shelf have identified at least eight major and several minor unconformities within the Cenozoic sediments. The identified erosional features associated with these unconformities include structural truncation, valley incision, and submarine canyon formation, which are the characteristics of a Type I unconformity as defined by Vail et al. (1977). There is also evidence for onlap by the overlying beds. The major unconformities are observed on the upthrown and downthrown sides of regional growth fault systems which are roughly parallel to the Texas coast. Where data quality permits, the major unconformities are identified over most of the Texas shelf. The minor unconformities are observed in the expanded sections on the downthrown blocks of major growth faults only. Although these erosional surfaces may also be Type I unconformities, no evidence of erosion is observed on the upthrown sides of the major growth faults. The unconformities of Oligocene and lower and middle Miocene age were initially interpreted from electric log correlations and subsequently verified on seismic data. The upper Miocene and younger unconformities were first recognized through seismic stratigraphy studies and then tied to electric log correlations. Most of the unconformities are observed near partially eroded diapiric ridges located along what was the outer shelf/upper slope area at the time of the erosion. A major upper Miocene unconformity is observed in the inner shelf/transitional environment of deposition in Matagorda and Mustang Island areas. This erosional surface was most likely formed by wave erosion during the subsequent transgression after deposition of the Discorbis 12 age sediments. There is also evidence of submarine scour by turbidites or massive slumping on this upper Miocene unconformity in the area of lower slope deposition. No evidence of erosion of outer shelf or upper slope sediments is apparent. Mapping in the lower Miocene trend delineated shale ridges plunging in a basinward direction perpendicular to regional strike. Electric log correlation of regional markers, which represent flooding surfaces, facilitated correlation of the unconformities on the seismic sections across these ridges. Occasionally very detailed seismic stratigraphy studies were necessary to establish which flooding surface should be used as the regional marker across large faults and ridges. End_of_Record - Last_Page 51--------

Regional Stratigraphic and Depositional Study of Rock Units in Upper Garden Gulch and Parachute Creek Members of Green River Formation, Piceance Creek Basin, Colorado: ABSTRACT

The Eocene Green River Formation in the Piceance Creek basin, Colorado, is well known for its thick sequence of rich oil shale and associated saline minerals. In the subsurface, rocks of economic interest extend from the upper part of the Garden Gulch Member to the top of the Parachute Creek Member. These rocks are lacustrine deposits that represent open-and near-shore sedimentation in ancient Lake Uinta. The depocenter of the lake formed in the northern part of the basin where rates of subsidence exceeded rates of sedimentation. The Garden Gulch and Parachute Creek Members can be divided into a series of laterally gradational rock units that can be traced from the eastern part of the Piceance Creek basin westward into the south-central part of the Uinta basin, Utah. Alon the margins of the Piceance Creek basin, the lower and middle units consist of fissile argillaceous shale of low organic content, silty claystone, and siltstone. Toward the depositional center of the basin, these rocks grade into a thick sequence of carbonate-rich, kerogenous shale and lean marlstone that is interbedded with a saline facies composed of nahcolite, dawsonite, and halite. The gradational boundary between these facies represents the contact between the Garden Gulch and Parachute Creek Members. In the subsurface, the contact can be recognized by a change from a low to high resistivity log response which reflects the transition from clay-rich to carbonate-rich rock. The regional thickness distribution pattern of individual units suggest that, during the middle Eocene, Lake Ui ta gradually expanded from a shallow, relatively fresh, semirestricted body of water to a saline, alkaline lake that occupied a closed basin. A pronounced thickening of the lowermost units along the southern margin of the basin is attributed to streams that prograded northward into the basin from the southern and southwestern margin of the lake. The middle and upper units, however, thin toward the basin edge, suggesting the lake gradually expanded to the south. During the late Eocene, open-lacustrine sedimentation shifted from the Piceance Creek basin westward into the Uinta basin, Utah, due to a large influx of siliciclastic sediment (Uinta Formation). Structural analysis of individual units indicates that present day intrabasinal tectonic features were not in existence during Parachute Creek time. End_of_Article - Last_Page 535------------

Mineralogy and phase chemistry of Mount St. Helens tephra sets W and Y as keys to their identification

Voluminous and widespread tephras were produced frequently during the last 36,000 yr of volcanic activity at Mount St. Helens. Numerous tephra sets have been defined by D. R. Mullineaux, J. H. Hyde, and M. Rubin (1975, U.S. Geological Survey Journal of Research, 3, 329–335) on the basis of field relations, FeMg phenocryst assemblage, and 14C chronology and are valuable marker beds for regional stratigraphic studies. In this study modal abundances and mineral compositions were determined (via petrographic and electron microprobe techniques) for numerous samples of individual layers within tephra sets W and Y to evaluate the degree of compositional variability within and between tephra layers and criteria by which to distinguish among Mount St. Helens and other Pacific Northwest tephras. Although individual layers within a set (e.g., We, Wn) cannot be distinguished from each other on the basis of mineralogic characteristics examined, mineral compositions allow distinction among layers W and Y and other Pacific Northwest tephras (e.g., Mazama, Glacier Peak). FeTi oxide compositions and T- f O 2 estimates derived using coexisting magnetite-ilmenite are especially useful due to the compositional homogeneity of these minerals both within and between samples of a given unit over a wide geographic area. The silicates show more compositional variability than the oxides, but iO 2 Al 2O 3 contents in hornblende and Fe/Mg ratios in hypersthene aid in distinguishing among Pacific Northwest tephras.

Sedimentation on a Passive Continental Margin (Late Cambrian-Early Ordovician), Central Appalachians: ABSTRACT

Environmental reconstruction of rocks of the Richmond slice, the tectonically highest slice at the eastern end of the Hamburg klippe in Pennsylvania, indicates that these rocks were deposited on a northwest-facing passive continental margin in Late Cambrian to Early Ordovician time. Detailed sedimentologic studies suggest that they can be divided into four main lithologic types, each bearing the imprint of one or more processes involved in its deposition. These include: (1) thin to thick-bedded, massive to graded, parallel and cross-laminated grainstone or calcarenite (high density turbidity currents; fluidized flows); (2) thin to medium-bedded, massive to structureless, graded and cross-laminated black lime mudstone and wackestone rhythmically interbedded with very thin o thin-bedded black limy mudstone and shale (low density turbidity currents and suspension); (3) thinly laminated graphitic black shale interlayered with irregular (lag) concentrations of fine sand and silt (redistribution of sediment by bottom currents); and (4) very thick-bedded, sand-matrix carbonate-clast conglomerate (gravelly high density turbidity currents and cohesive debris flows). The proposed depositional processes form a continuum of mechanisms that were in operation in the slope environment. Regional stratigraphic studies suggest that the carbonate rocks of the Richmond slice were deposited on a depositional margin or ramp characterized by a gentle slope (1 to 2°) that decreased in gradient basinward. Only the lower slope portion of the continental margin has been preserved. End_of_Article - Last_Page 1171------------

Middle and Upper Devonian Stratigraphy in Northwestern West Virginia: ABSTRACT

End_Page 1577------------------------------Regional stratigraphic studies of the Middle and Upper Devonian clastic sequence of West Virginia have been based on gamma-ray-density logs, cores, and sample studies. In northwestern West Virginia, the Upper and Middle Devonian unit is characterized by interbedded gray and black shales, with the darker shales recognized on gamma-ray logs as zones of increased radioactivity. The overall thickness of the section decreases toward the west whereas the proportion of black shale increases toward the west. The proportional increase in black shale occurs in two ways; there is an increased number of black shale beds in the west, and the thickness of the interbedded gray shales decreases. The study area extends across the western boundary of the Rome trough, with the presence of this tectonic feature manifested in two ways in the sediments accumulated over the trough. First, there is a pronounced thickening of the sediments into the trough, and second, the Rhinestreet Shale Member of the West Falls Formation and the Huron Member of the Ohio Shale undergo a facies change from black shale west of the trough to gray shale in the trough area. Further, cross sections and isopach maps provide additional evidence that the Rome trough controlled basin configuration in the region and thus influenced sedimentation during the deposition of the Upper and Middle Devonian clastic sequence in northwestern West Virginia. End_of_Article - Last_Page 1578------------