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Volcaniclastic rocks of Cretaceous age make up the Alisitos group, which is exposed over large areas of northern Baja California, Mexico, and is inferred to have been deposited within an island arc. Intimately intermixed penecontemporaneous intrusive, extrusive, and fragmental volcanic rocks characterize many of the intra-arc deposits within the Alisitos. Peperites, formed by mixing of magma and wet sediment, are useful for demonstrating contemporaneity of volcanism and sedimentation. Peperites were recently discovered at the type locality of the Alisitos group at Punta China, only 25 km south of Ensenada, Mexico. These intrude a 50-m thick section, dominated by mafic lithic lapilli tuff breccias and lava flows, that accumulated in a nearshore to beach environment. Near Punta Maria (about 100 km north of Guerrero Negro), coarse-grained volcaniclastics interpreted to have been deposited on a north to northwest-facing submarine slope are exposed in sea cliffs. Ammonite (Pervinquieria.) and belemnite fossils suggest a Cretaceous age and marine deposition for the Punta Maria sequence. Horizontal burrows and lack of wave reworking indicate subwave-base depth for most of the section. The most common rock types in the sequence are andesitic lithic lapilli tuff and tuff breccia, deposited from nonturbulent high-concentration sediment gravity flows (density-modified grainmore » flow). Comparison of the Punta Maria and Punta China sections suggests that hydrostatic pressure and depositional slope play important roles in controlling the magma/sediment interaction. At Punta China, a shallow-water association was intruded by small sills, resulting in a highly dispersed peperite (explosive interaction). The Punta Maria section, comprising deeper water slope deposits, was intruded by a larger magma body that partly ingested a limestone and produced a thick carapace of hyaloclastite (nonexplosive interaction).« less

Tabular sandstone beds, hummocky cross-stratified sandstone (HCS), and trough and planar tabular cross-stratified sandstone (St/Sp) are characteristic facies of coarsening- and thickening-upward Cretaceous marine strata in the Rocky Mountain region. All contain sharp and/or erosional basal surfaces and consist of normally graded, planar, parallel and/or concordant laminae, indicating episodic erosion and deposition from suspension, with very little or no bed-load deposition. Sharp-based tabular sandstone beds are normally graded overall (sandstone-to-mudstone) and contain normally graded planar parallel laminae. Similarly, HCS sets consist of normally graded concordant to slightly asymptotic laminae that overlie hummocky and swaley erosional basal surfaces. Sets of St/Sp are composed of unidirectional dip-oriented cross-strata bound by reactivation surfaces, which commonly have a sigmoidal shape. Internally, these cross strata consist of normally graded concordant to asymptotic laminae. Normally graded laminae of all three facies typically fine upward from sand to silt or clay with abundant carbonaceous trash and/or mica flakes, indicating deposition from suspension. Tabular sandstone beds record punctuated deposition from suspension below storm wave base, but HCS records erosion and deposition from suspension within storm wave base. St/Sp record erosion and deposition from suspension within storm wave base and under the influence of a unidirectional current, perhaps reflectingmore » longshore drift in the upper shoreface zone. Dip-oriented outcrops reveal that sharp-based tabular sandstone beds can be traced up depositional dip into the erosional bases of HCS. St/Sp commonly build downcurrent from hummocks of HCS. These features suggest that erosion and deposition of all three facies are genetically related to the same erosional and depositional event.« less

The palynoflora of the Fossil Butte Member of the Green River Formation was studied to relate the flora and climate to the genesis of oil shale in ancient Fossil Lake. Samples were collected from measured sections representing open and marginal lacustrine environments. These samples were analyzed using standard palynologic and visual kerogen techniques. Evidence from the palynoflora suggests that during deposition of the Fossil Butte Member, the climate was in transition between humid, subtropical to a cooler, drier, warm temperate one with moderate fluctuations during various episodes of deposition. Additional evidence indicates that moist lowlands and flood plains existed around Fossil Lake, with upland forests on the surrounding ridges and mountains. Streams originating in the highlands supplied water to Fossil Lake and the surrounding vegetation. The age of the palynoflora along with K-Ar age determinations indicate that deposition of oil shale in Fossil basin was contemporaneous with deposition of the evaporite facies of the Wilkins Peak Member in the Green River basin. This suggests that factors affecting oil shale deposition i the two basins were locally, as well as regionally, controlled. The dominance of amorphous kerogen in the kerogen samples suggests an algal origin for the majority of the oil shales. The palynoflora, the kerogen type, and the stratigraphic relationships indicate that the deposition of oil shale in Fossil basin was due to local climatic and environmental conditions. End_of_Article - Last_Page 1208------------

Sequence stratigraphy is concerned with making predictions about reservoirs ahead of the drill, however, little attention has been paid to the configuration of organic-rich facies of source rock quality. We suggest that preservation of source rock type facies in clastic systems is mutually exclusive and time successive. The main database is a collection of cores and other samples through the Holocene Rhone delta. The early Holocene Transgressive Systems Tract (TST) contains five levels of channelization. The most significant peat bed is located immediately landward of the shoreline of maximum transgression (SMT). The Highstand Systems Tract (HST) consists of two parasequences, containing mostly laterally continuous strandplain complexes without peat. In addition to sufficient accommodation space, an important control on formation of fresh-water peats and organic-rich shelf muds is availability of river-induced nutrients. Peat quality, however, is best without riverine clastics. In a delta plain, a balance between these two controls may be reached when river-fed nutrients are trapped there indirectly. The potential for such a condition arises in a TST setting. On the shelf, eutrophication of marine habitats is also controlled by river-fed nutrients, but excess river clastics are detrimental to marine source rock quality. A balance between these two controls more » may be reached in HST settings where fine-grained riverine clastics are forced onto the shelf rather than in the delta plain. In this case, nutrient supply to the shelf results in large quantities of marine biomass. This biomass becomes sufficiently concentrated due to moderate fine-grained riverine sedimentation which guarantees burial and preservation. Thus, varying river-water and nutrient supply in TST and HST settings seems to control large-scale preservation patterns of both continental and marine organics. This hypothesis suggests further potential for using sequence stratigraphy for source rock occurrence. « less

Cyclic fluvial, fan-delta, and lacustrine margin depositional systems dominate the lower part of the Chinle Formation, Canyonlands area, southeastern Utah. Detailed facies analysis of the Moss Back Member and related strata document two alternating fluvial-lacustrine sequences. (1) During high fluvial output, coarse-grained meander belts built fan-deltas into shallow lakes. (2) During low fluvial output, braided streams waned before reaching the lakes, and algal and evaporatic mud flats formed on broad lacustrine perimeters. The high fluvial output phase comprises three depositional systems. (1) The coarse-grained meander belt system consists of point-bar and channel deposits. Abundant carbonate grains were cannibalized from upland caliche soils and lowland oncolite-bearing carbonate mud flats. (2) The fan-delta system contains sandstone and mudstone delta foresets that coarsen upward into delta-distributary deposits. (3) Blue-green, bentonitic, limy mudstone was deposited in the shallow lacustrine system. The low fluvial output phase comprises three depositional systems. (1) Braided stream deposits contain trough and planar cross-stratified calcarenite, calcirudite, and quartzarenite. (2) Micritic limestone, algal mats, and oncolites formed on swampy lake margins. (3) Shallow, warm, clear-water, lacustrine deposits are represented by continuous micritic limestone beds and bioturbated limy mudstone and sandstone. Three external controls produced the cycles. Tectonics associated with the adjacent salt anticline region affected source terrane (intrabasinal carbonate output) and fluvial discharge. Regional humid-arid cycles affected fluvial discharge and lake levels. Volcanic eruptions to the southwest produced periodic influx of volcanic debris that caused increased turbidity and sedimentation rates and decreased carbonate production. End_of_Article - Last_Page 550------------

The name "Great Falls lineament" is given to a northeast-trending zone of diverse geologic features that can be traced northeastward from the Idaho batholith in the Cordilleran miogeocline of the United States, across thrust belt structures and basement rocks of west-central and southwestern Montana, through the cratonic rocks of central Montana, and into southwesternmost Saskatchewan, Canada. The zone is well represented in east-central Idaho and west-central Montana where geologic mapping has outlined northeast-trending, high-angle faults and shear zones that: (1) extend more than 150 km (93 mi) from near Salmon, Idaho, northeastward toward Anaconda, Montana; (2) define a nearly continuous zone of faulting that shows recurrent movement from middle Proterozoic to Holocen time; (3) controlled the intrusion and orientation of some Late Cretaceous to early Tertiary batholithic rocks and early Tertiary dike swarms; and (4) controlled the uplift and orientation of the Anaconda-Pintlar Range. Recurrent movement along these faults and their strong structural control over igneous intrusions in this region suggest that northeast-trending faults represent a fundamental tectonic feature of the region. Figure Geologic features that are similar to those mapped in the Salmon-Anaconda region are present to the southwest and the northeast. In central Idaho, these structures include numerous northeast-trending faults and pronounced topographic lineaments that cut across the southern part of the Idaho batholith, and a northeast alignment of Tertiary igneous rocks that cut the Idaho batholith and adjacent rocks. East and southeast of the Anaconda-Pintlar Range, subparallel, high-angle faults and topographic lineaments are present in the Highland, Pioneer, Ruby, and Tobacco Root Mountains. High-angle faults may have in part controlled the orientation of the northeast-elongate Boulder batholith. Northeast-trending structures are not easily traced across the thrust belt of western Montana or across he Lewis and Clark line. In the central Montana plains, northeast of the disturbed belt, however, a broad zone of colinear, northeast-trending structures is present, and includes: parallel, buried basement highs that in part controlled depositional patterns of some Paleozoic and Mesozoic sedimentary rocks; major physiographic features, such as the remarkably straight, 175-km (109-mi) long segment of the Missouri River, and equally long, buried river channels in southwestern Saskatchewan; a northeasterly alignment of highly differentiated igneous rocks and a belt of ultrabasic intrusions and related diatremes; End_Page 1350------------------------------ and a well-defined pattern of northeast-trending gravity and aeromagnetic anomalies underlying this part of central Montana and southwesternmost Saskatchewan. Taken together, all these geologic features define a broad, northeast-trending zone at least 150 to 200 km (93 to 125 mi) wide and more than 1,000 km (620 mi) long. The zone is approximately colinear but not demonstrably continuous with the well-exposed boundary in eastern Saskatchewan and Manitoba between the Archean Superior and the Proterozoic Churchill provinces of the Canadian Shield. This boundary is also characterized by: high-angle faults, shear zones, and topographic lineaments; pronounced linear gravity and magnetic anomalies; igneous intrusions; and fault controlled depositional patterns and mineralization. That the Great Falls lineament is controlled by a similar Precambrian boundary between the Archean Wyoming province of southwestern Montana and early Proterozoic terrane to the north is speculative; however, the geologic features found along the Great Falls lineament share many common characteristics with features present along the Archean-Proterozoic boundary in Canada. End_of_Article - Last_Page 1351------------

Detailed mapping and stratigraphic studies in the Ridgway area, along the northern flank of the San Juan Mountains in Colorado, indicate that the lower Mancos Group (Upper Cretaceous) can be subdivided into four formations. From oldest to youngest, they are the Benton Formation, Juana Lopez, Sage Breaks Shale, and Niobrara Formation. These formations have lithologic and paleontologic content similar to their equivalents in the Denver and San Juan basins and Black Hills. The underlying Benton Formation consists of 470 ft (143 m) of black shale that includes the Greenhorn Limestone Member (12 ft or 3.5 m thick) lying 300 ft (91 m) above the base. The overlying Juana Lopez is 40 ft (12 m) thick and consists of alternating layers of calcareous siltstone and black shale. The succeeding Sage Breaks is a 155-ft (47 m) calcareous shale unit. The overlying Niobrara Formation is an 85-ft (26 m) resistant, cliff-forming, calcareous shale. Subsurface and outcrop data in the south indicate that the study area straddles the margin of the Pennsylvanian Uncompahgre uplift. Structural evolution has been controlled largely by three east-west trending Precambrian fault blocks--the Ouray graben, the Orvis block, and the Uncompahgre block. Recurrent movement of these fault blocks from the Paleozoic through the Cenozoic affected both sediment thickness and facies distribution. A cross section restored to the base of the Dolores Formation (Upper Triassic) indicates that the Permian End_Page 572------------------------------ Cutler Formation increases 1,050 ft (320 m) in thickness across the Orvis fault, which had down-to-the-south movement. During the Laramide orogeny, a significant reversal in movement occurred along the fault to cause the north-dipping monocline now present at the surface. End_of_Article - Last_Page 573------------

The transition from slope to basinal facies generally is not so well exposed or understood as it is in the Pennsylvanian of New Mexico's Pedregosa basin and in the Liassic of Switzerland's Piemontais geosyncline. The shape of the Pedrego basin is controlled by basement faulting; its trend is oblique to the Ouachita-Marathon geosyncline. During periods of stable sea-level stand, carbonate sedimentation prevailed in shelf areas, and bioherms built up at the shelf edge. Bioclastic foresets and huge slumps of brecciated material from these bioherms extended from the shelf edge into the basin, where deep-water carbonates were deposited. During periods of low sea-level stand, rivers cut through the shelf deposits and shelf-edge bioherms and eroded deep valleys on the slope. As sea level rose, the basin was filled mainly with shales, and valleys on the slope were filled with sand. The geometry of these sand bodies at the base of the slope is probably fan- or cone-shaped. Normal faults in the Liassic slope sediments of the Piemontais geosyncline reflect the structural tension that persisted during early geosynclinal stages. Thick beds of structureless breccias accumulated on the downthrown side of these faults. In the downslope direction, the breccias grade into turbidite facies; farther downslope, thick wedges of turbidite thin at their distal edges. No evidence was found of channels or canyons that would serve as point sources for sediment dispersion down the slope. The turbidites grade upward into radiolarian chert. End_of_Article - Last_Page 804------------

The Norte de Monagas trend is located in the Eastern Venezuelan Basin and is characterized by high temperature, high initial pressure and production rates, variable PVT properties, GORs, and asphaltene content, and the presence of a tar mat at the base of the oil column. These reservoirs were petrophysically characterized through the integration of core and log data. Productive sands are composed of very coarse- to medium-grained sands deposited primarily in deltaic to shallow marine environments. These sands are quartz-rich, with clay matrix averaging 3% in the Oligocene and 6% in the Cretaceous. The main difference is the presence of up to 30% feldspars in the Cretaceous. Clay mineralogy is dominated by illitic material in the Cretaceous and by kaolinite in the Oligocene. Post-depositional changes dominated by cementation, mechanical compaction and subsequent dissolution processes have resulted in marked improvement or degradation of the productive characteristics of these reservoirs. Rocks were divided into four petrophysical. categories based on grain size, pore throat radius, porosity and permeability. Mercury saturation vs. mercury saturation over capillary pressure plots show the dominant interconnected pore system which controls flow rates is best defined by 50% saturation. Plots constructed using R50 pore aperture correlation indicate the poremore » throats to be macro to mega porous. Average log calculated water saturation is 14% compared with estimated from capillary pressure data of 12% at 1200{prime} above free water level, and 15% from relative permeability data. Relative permeability data indicate the rocks behave preferably as water-wet.« less

The Buntsandstein facies outcrops along a 12 km long, 150 m thick cuesta near Ayllon (Central Spain). The outcrop study is based on vertical sedimentological sections and continuous photo paneling, and demonstrates the presence of two depositional systems: an alluvial fan system in the lower half of the outcrop, and a straight and braided river system in the upper part of the outcrop. This overall evolution is probably related to base-level fall to base-level rise cycle, in which the reservoir architecture is linked to genetic units stacking pattern: during the base-level fall, the alluvial fan is prograding over sand flat and sandy alluvial plain deposits. Coarse and pebbly proximal sandsheets are interbedded with finer reddish distal deposits. Reservoirs units are laterally continuous, but silty alluvial plain deposits constitute vertical permeability barriers, during base-level stillstand, erosive channels and sandsheets are vertically amalgamated. Reservoirs units are laterally continuous and vertically connected, during the base-level rise, alluvial fan deposits are overlapped by straight river deposits. Reservoirs units are laterally connected but silty argillaceous alluvial plain horizons are preserved, at the end of the base-level rise, braided and straight river deposits are amalgamated. Fully connected, these reservoirs units have a very large lateral extension.more » A lithofacies database is compiled on this outcrop, and variograms, horizontal and vertical proportion curves are completed. Each stage of the base-level cycle is then quantitatively characterized by a specific heterogeneity pattern. The outcrop study will improve the prediction of reservoir extension and architecture in subsurface gas storage of the Paris basin.« less