Time-equivalent Rocks Research Articles

How are diagenesis and reservoir quality linked to depositional facies? A deltaic succession, Edgeøya, Svalbard

Abstract Middle to late Triassic strata exposed at Edgeoya, eastern Svalbard, represents an uplifted part of the northwestern corner of the Barents Sea. This interval is characterized by a predominantly mud-dominated deltaic depositional system where rocks with petroleum reservoir potential are expected both in delta front and channelized sandstone deposits. Recent drilling campaigns into time-equivalent rocks in the Barents Sea have however been disappointing, with porosity and permeability below expectation. This study improves the current understanding of reservoir potential within depositional systems of this kind by examining the link between different depositional facies, diagenesis and their impact on reservoir quality. Five depositional facies were mapped and correlated: channel, floodplain, shallow marine, prodelta and offshore. Our study suggests that diagenetic signatures that control the quality of reservoir rocks vary systematically with these depositional facies. Mechanical compaction was the main cause of porosity destruction in the channel, shallow marine and floodplain samples, while early carbonate cementation occludes the intergranular volume in the prodelta samples. Moreover, quartz cement reduced the porosity in the shallow marine depositional facies. The estimated maximum burial temperature of 124 °C indicates burial to several km depths. Depositional settings within a deltaic to shallow marine system do not exert a strong control on the abundance of chlorite, but it does impact whether the chlorite is pore filling or grain coating and porosity preserving. Efficient chlorite coating have aided in preserving porosities up to 32% in the very well/well sorted medium to fine grained sandstones deposited by fluvial dominated channels.

Stratigraphy and fossil plants from the Cutler Formation (Late Paleozoic) and their paleoclimatic implications, eastern Paradox Basin, Colorado

Exposures of the late Paleozoic Cutler Formation, near the town of Gateway, Colorado, have traditionally been interpreted as the product of alluvial-fan deposition along the western flank of the Uncompahgre uplift and within the easternmost portion of the Paradox Basin. The Paradox Basin formed between the western margin of the Uncompahgre uplift, a segment of the Ancestral Rocky Mountains, and the western paleoshoreline of the North American portion of Pangea. This part of Pangea is commonly thought to have experienced semi-arid to arid conditions and warm temperatures during the Pennsylvanian and Permian. We present stratigraphic and fossil plant evidence in this paper to support prior interpretations that the Cutler near Gateway, Colorado, was deposited by alluvial fans that hosted localized wetland areas. Our findings are consistent with the results of prior studies that have suggested the climate in the area was warm, semi-arid, and ice-free at the time the plants described in this paper were living. 
 Plant fossils collected from the Cutler Formation came from two sites in The Palisade Wilderness Study Area (managed by the U.S. Department of the Interior, Bureau of Land Management) of western Colorado. The stratigraphic sections at the sites were composed mostly of pebble to cobble conglomerate and sandstone, but the fossil plants were mainly preserved in fine-grained intervals (fine-grained sandstone to siltstone). The preservation of plant fossils in the proximal Cutler Formation is remarkable because the surrounding sections consist mostly of conglomerate and sandstone interpreted as fluvial and debris-flow deposits. The fine-grained strata containing the plant horizons must have been deposited in a wet and protected setting, possibly a spring-fed abandoned channel on the alluvial fan. The plants and their surrounding sediment must have been rapidly buried in order to allow for long-term preservation of the fossils. It seems likely that vegetation was abundant in and adjacent to low-lying wet areas on the fan’s surface, based on the abundance of plant fossils found at the two sites. The fossil plant assemblage includes Calamites, Walchia, and Pecopteris. The flora are interpreted to have lived near the apex of the alluvial-fan system. These fossils suggest that warm and at least seasonally and locally wet conditions existed in the area during the time that the plants were growing. More arid conditions during the late Paleozoic are suggested by the characteristics of some of the time-equivalent and near time-equivalent rocks exposed to the west of the study area in the central Paradox Basin.

Stratigraphy and fossil plants from the Cutler Formation (Late Paleozoic) and their paleoclimatic implications, eastern Paradox Basin, Colorado

Exposures of the late Paleozoic Cutler Formation, near the town of Gateway, Colorado, have traditionally been interpreted as the product of alluvial-fan deposition along the western flank of the Uncompahgre uplift and within the easternmost portion of the Paradox Basin. The Paradox Basin formed between the western margin of the Uncompahgre uplift, a segment of the Ancestral Rocky Mountains, and the western paleoshoreline of the North American portion of Pangea. This part of Pangea is commonly thought to have experienced semi-arid to arid conditions and warm temperatures during the Pennsylvanian and Permian. We present stratigraphic and fossil plant evidence in this paper to support prior interpretations that the Cutler near Gateway, Colorado, was deposited by alluvial fans that hosted localized wetland areas. Our findings are consistent with the results of prior studies that have suggested the climate in the area was warm, semi-arid, and ice-free at the time the plants described in this paper were living. Plant fossils collected from the Cutler Formation came from two sites in The Palisade Wilderness Study Area (managed by the U.S. Department of the Interior, Bureau of Land Management) of western Colorado. The stratigraphic sections at the sites were composed mostly of pebble to cobble conglomerate and sandstone, but the fossil plants were mainly preserved in fine-grained intervals (fine-grained sandstone to siltstone). The preservation of plant fossils in the proximal Cutler Formation is remarkable because the surrounding sections consist mostly of conglomerate and sandstone interpreted as fluvial and debris-flow deposits. The fine-grained strata containing the plant horizons must have been deposited in a wet and protected setting, possibly a spring-fed abandoned channel on the alluvial fan. The plants and their surrounding sediment must have been rapidly buried in order to allow for long-term preservation of the fossils. It seems likely that vegetation was abundant in and adjacent to low-lying wet areas on the fan’s surface, based on the abundance of plant fossils found at the two sites. The fossil plant assemblage includes Calamites, Walchia, and Pecopteris. The flora are interpreted to have lived near the apex of the alluvial-fan system. These fossils suggest that warm and at least seasonally and locally wet conditions existed in the area during the time that the plants were growing. More arid conditions during the late Paleozoic are suggested by the characteristics of some of the time-equivalent and near time-equivalent rocks exposed to the west of the study area in the central Paradox Basin.

Sequence Stratigraphy of the Upper Cretaceous Sego Sandstone Member Reveals Spatio-Temporal Changes In Depositional Processes, Northwest Colorado, U.S.A

Abstract The Upper Cretaceous Sego Sandstone Member of the Mesaverde Group has been extensively studied in the Book Cliffs area of Utah and Colorado, and has been the focus of stratigraphic reconstruction aimed at developing an understanding of the evolution of the Western Cretaceous Interior Seaway. The Sego Sandstone Member was deposited in a marginal marine, tide-influenced environment of the Cretaceous Seaway. This study documents the sequence stratigraphy of the Sego Sandstone Member in northwestern Colorado, just north of Rangely, and compares and contrasts it with equivalent strata in the Book Cliffs area in Utah. The Sego Sandstone Member in the study area contains three sequences characterized by progradational and aggradational stacking patterns. The stratigraphically lowest sequence consists of a prograding, tide-influenced delta overlain by marine mudstones, which represents a retrogradation and flooding surface. The second sequence is composed of multiple parasequences and consists of an incised valley filled with stacked tidal bars which then pass into a largely aggradational stacking pattern, composed of barrier-island deposits with back-barrier, flood-tidal-delta deposits, and wave-dominated-shoreface deposits. The third sequence is a broad, tide-dominated distributary-mouth system with a sharp, incisional basal contact. The three sequence boundaries documented in northwestern Colorado are consistent with the three main sequence boundaries identified in the Book Cliffs. However, whereas barrier islands and flood-tidal deltas are characteristic of the Sego Sandstone Member in northwestern Colorado, similar deposits are not as prevalent in the Book Cliffs of Utah, suggesting different depositional processes and paleogeography. Tidal and fluvio-deltaic processes are the dominant controls on deposition of the Sego Sandstone Member north of Rangely, Colorado. The transition from a tide-dominated fluvio-deltaic system to a mixed wave–tide-influenced coastline indicates a fundamental change in processes and depositional environment in the upper part of Sequence 2. Such change from a prograding fluvio-deltaic system to a more passive tide-modified coastline is not observed in the Book Cliffs, and may be the result either of large scale transgression or of relocation of the river system through large-scale avulsion, which is not observed in the Book Cliffs. Our study shows significant stratigraphic variability between rocks exposed in the Book Cliffs versus time-equivalent rocks exposed in northwestern Colorado in the Upper Cretaceous, which has implications for the regional basin architecture and stratigraphic correlations.

Stratigraphy and structural setting of Upper Cretaceous Frontier Formation, western Centennial Mountains, southwestern Montana and southeastern Idaho

Stratigraphic, sedimentologic, and palynologic data were used to correlate the Frontier Formation of the western Centennial Mountains with time-equivalent rocks in the Lima Peaks area and other nearby areas in southwestern Montana. The stratigraphic interval studied is in the middle and upper parts (but not uppermost) of the formation based on a comparison of sandstone petrography, palynologic age data, and our interpretation of the structure using a seismic line along the frontal zone of the Centennial Mountains and the adjacent Centennial Valley. The Frontier Formation is comprised of sandstone, siltstone, mudstone, limestone, and silty shale in fluvial and coastal depositional settings. A distinctive characteristic of these strata in the western Centennial Mountains is the absence of conglomerate and conglomeratic sandstone beds. Absence of conglomerate beds may be due to lateral facies changes associated with fluvial systems, a distal fining of grain size, and the absence of both uppermost and lower Frontier rocks in the study area. Palynostratigraphic data indicate a Coniacian age for the Frontier Formation in the western Centennial Mountains. These data are supported by a geochronologic age from the middle part of the Frontier at Lima Peaks indicating a possible late Coniacian-early Santonian age (86.25 ± 0.38 Ma) for the middle Frontier there. The Frontier Formation in the western Centennial Mountains is comparable in age and thickness to part of the Frontier at Lima Peaks. These rocks represent one of the thickest known sequences of Frontier strata in the Rocky Mountain region. Deposition was from about 95 to 86 Ma (middle Cenomanian to at least early Santonian), during which time, shoreface sandstone of the Telegraph Creek Formation and marine shale of the Cody Shale were deposited to the east in the area now occupied by the Madison Range in southwestern Montana. Frontier strata in the western Centennial Mountains are structurally isolated from other Cretaceous rocks in the region and are part of the Lima thrust sheet that lies at the leading edge of the Sevier-style overthrusting in this part of southwestern Montana and adjacent southeastern Idaho.

Structural geometry, style and timing of deformation in the Hawasina Window, Al Jabal al Akhdar and Saih Hatat culminations, Oman Mountains

ABSTRACT The Al Jabal al Akhdar and Saih Hatat culminations in the central Oman Mountains expose the complete mid-Permian to Late Cretaceous (Cenomanian) passive shelf and margin carbonate sequence beneath the allochtonous slope (Sumeini Group), basin (Hawasina complex), distal ocean-trench (Haybi complex) facies rocks, and the Semail ophiolite thrust sheets that were emplaced from NE to SW during the Late Cretaceous. Reconstruction of the pre-thrust sequences shows that time-equivalent rocks occur in successively stacked thrust sheets from shelf to slope to basin. The Al Jabal al Akhdar structure is a 60 km wavelength anticline plunging to the northwest beneath the Hawasina Window and with a fold axis that curves from WNW-ESE (Jabal Shams) to NNE-SSW (Jabal Nakhl). The structure shows little internal deformation except for minor intra-formational thrust duplexing within the Cretaceous shelf stratigraphy along the northern margin. The upper structural boundaries around the flanks of the shelf carbonate culminations have been re-activated as late stage normal faults. The Semail thrust formed a passive roof fault during late-stage culmination of al Al Jabal al Akhdar such that the ophiolite rests directly on Wasia Formation top-shelf with the entire Sumeini, Hawasina and Haybi thrust sheets displaced around the margins. NE-directed backthrusting and intense folding in the northern part of the Hawasina Window affects all allochtonous units and is related to a steep ramp in the Late Cretaceous shelf margin at depth. The Saih Hatat culmination is another 40 km half-wavelength anticline in the central Oman Mountains, but shows extreme deformation in the form of recumbent folds, sheath folds with NNE-trending axes and thrusting along the northern margin. High-pressure carpholite, blueschist and eclogite facies rocks are exposed at successively deeper structural levels, separated by high-strain normal sense shear zones. There is no evidence for a separate ‘North Muscat microplate’ or an intra-continental subduction zone, as previously proposed; all high-pressure units can be restored to show their pre-deformation palaeographic positions along the northern margin of the Arabian Plate. Both Al Jabal al Akhdar and Saih Hatat are Late Cretaceous culminations, folded after obduction of the Hawasina, Haybi and Semail ophiolite thrust sheets from northeast to southwest during the period Turonian to Campanian-Lower Maastrichtian. Maximum compressive stress along the central Oman Mountains was oriented NE-SW, parallel to the ophiolite emplacement direction, but a second compressive stress axis was oriented WNW-ESE, either concurrently or slightly later in time, resulting in a dome and basin structural geometry. The biaxial fracture pattern in the foreland, southwest of the Oman Mountains could be explained as a result of the WNW-directed emplacement of the Masirah ophiolite belt and Batain mélange during the Campanian-early Palaeocene. Both Al Jabal al Akhdar and Saih Hatat were positive topographic features at the end of the Cretaceous with Upper Maastrichtian and Palaeogene sediments onlapping both flanks. In Jabal Abiad, these Palaeogene sediments have been uplifted by at least 2 km since the Late Miocene-Early Oligocene associated with minor NNE-SSW compression. Tertiary shortening, folding and thrusting increases to the north in the Musandam peninsula where the first effects of the Arabian Plate-Eurasian Plate (Zagros belt) continent-continent collision are seen.

Late Neoproterozoic A-type granites in the northernmost Arabian-Nubian Shield formed by fractionation of basaltic melts

Geochemical, isotopic and age constraints support a comagmatic origin for Ghuweir Mafics and the Feinan A-type granites. The two rocks types, named collectively in this paper as the Feinan Ghuweir Magmatic Suite (FGMS), formed between 556 and 572 Ma ago according to Rb–Sr whole-rock dating. FGMS has low Sr initial ratios, which preclude a significant contribution of much older crust in the magma genesis. The FGMS has a wide range of silica contents, with a gap at 55–65 wt% SiO 2. It has a transalkaline to alkaline character; belongs to the medium to high K calc-alkaline series; it ranges from metaluminous to mildly peraluminous character and belongs to the alkali and alkali-calcic series. The Feinan granites and the Ghuweir rhyolites and rhyodacites are classified as A-type granites and belong to group A2 of Eby [Eby, N.G., 1992. Chemical subdivision of the A-type granitoids: petrogenetic and tectonic iplications. Geology 20, 641–644]. According to geochemical modeling the Ghuweir Mafics were derived from a subduction modified lithospheric mantle by 10% batch modal partial melting of a phlogopite-bearing spinel lherzolite. The intra-suite geochemical variations can be ascribed to fractional crystallization of olivine, pyroxene, and plagioclase. The accumulation of apatite in the most evolved samples is responsible for the high concentrations of REE. The Feinan granites and the Ghuweir rhyolites and rhyodacites were derived from the mafic magma by the fractional crystallization of ≈78% of the original magma to the mineral assemblage olivine+pyroxene+plagioclase+magnetite. The intra-suite geochemical variations in the Feinan A-type granites are due to the fractional crystallization of the mineral phases: amphibole +Na and K-feldspar+apatite +magnetite+zircon+allanite. The FGMS correlates with time-equivalent rocks in many parts of the Arabian-Nubian Shield and the surrounding areas.

Regional time-depth conversion of the Natih E horizon in Northern Oman using seismic stacking velocities

A method is presented for integrating stacking velocity data in the time-depth conversion of regional time horizons in North Oman (Fig. 1). The Natih E horizon has been chosen as the target for the depth conversion as it forms a well-defined seismic reflection in Northern Oman (Figs. 2 and 3). The Cretaceous Natih Formation is composed of a series of limestone beds, ranging in age from latest Albian to Early Turonian. Timeequivalent source rocks occur (Van Buchem et al., 1996) in addition to organic-rich sediments in older formations, making the hydrocarbon charge for the mapped traps less of a problem. Conventional well-based time-depth (TD) conversion methods have their limitations, especially when dealing with vast areas, irregular distribution of control points, and a complex subsidence history (local inversion tectonics; e.g. Veeken, and Van Moerkerken, in preparation). Most methods are only appropriate when enough well penetration is available to calibrate the data set. If too large an interpolation or extrapolation distance is given, then these methods become notoriously unreliable. In such cases, seismic stacking velocities may provide the only means to evaluate the subsurface structuration (Veeken, 2005). The velocity cube represents an excellent asset for detailed prospect evaluation (Schulz, 1999). Many essential reservoir attributes are based on seismic velocities (e.g. impedance contrasts) and therefore it is worthwhile expending some effort exploring all of its benefits.

A 3D outcrop analogue model for Ypresian nummulitic carbonate reservoirs: Jebel Ousselat, northern Tunisia

A three-dimensional high resolution sequence stratigraphic model of an Ypresian nummulitic carbonate ramp and organic-rich basin is presented based on outcrops in Central Tunisia. The sedimentation pattern is influenced by the interplay of different orders of variations in eustatic sea-level (third to fifth order), the pre-existing palaeotopography, and probably some synsedimentary tectonism (differential subsidence). Time-equivalent rocks deposited in a comparable structural and depositional setting along the northern margin of the African plate are hydrocarbon bearing (Tunisia and Libya). This example may thus serve as an outcrop analogue for this petroleum system, providing valuable information on the sub-seismic-scale distribution pattern, geometries and heterogeneities of both the reservoir and source rock facies. The studied outcrops cover an area of 10 by 20 km where present-day valleys provide three-dimensional access to the rocks. In addition, the transition from the inner/mid-ramp, with nummulitic reservoir facies, to the carbonate source rocks in the basin is exposed in continuous outcrops. This transition takes place in about 3 km, a distance generally beyond the resolution of well spacing. Based on the physical tracing of beds, and the recognition of three orders of depositional sequences (third to fifth) a high resolution time framework is constructed. The accumulation of large nummulites (best reservoir facies) is stratigraphically controlled, and occurs in the transgressive phases of the landward-stepping fourth order cycles (overall transgression). Carbonate production was at that time so high that aggrading geometries are observed during these transgressive pulses. Our observations show that size, morphology and reproduction strategy of foraminiferal assemblages and, particularly, nummulites and Discocyclina , are related to changes in water depth and, consequently, in accommodation space. A regional east–west cross-section shows significant thickness variations of the Ypresian succession that were probably controlled by synsedimentary differential subsidence. The detailed, sub-seismic-scale, geometrical information on stratal patterns and lateral facies change are quantified, and used in a 3D numerical stochastic modelling (HERESIM) of this petroleum system.

Are seismic/depositional sequences chronostratigraphic units?

Sequence Stratigraphic Analysis is claimed to be a “new globally valid system of stratigraphy … a precise methodology to subdivide, correlate and map sedimentary rocks” (Vail et al., 1991, p. 622). Sequence stratigraphic units, such as depositional sequences, depositional systems tracts, and parasequences, are time-equivalent rocks of specific durations controlled by cyclical changes in sediment supply related to eustasy. These units are bounded by regionally extensive unconformities with erosion beneath and onlapping strata above, or by physical surfaces separating either different patterns of stratal geometry or shoaling-up facies units. According to this school, precise correlations are based upon inferred time relations within depositional models.Several key concepts of sequence stratigraphy have their origins in early geological studies. For many years geologists have separated time-equivalent strata by regional unconformities related to changes in climate or sea level, e.g., J. Woodward, 1695 and T. C. Chamberline, 1909. Stratal surfaces, such as bentonites and limestone markers, have been used in place of fossils for time correlations since the first wells were drilled. Stratigraphic models have strongly influenced how we correlate strata since the time of William Smith.Two developments are, indeed, new and have sparked the current resurgence in stratigraphic research. One is the seismic technology to test the physical continuity of strata on a regional scale (50-100 km), and to test the stratal geometry of genetically related depositional packages. The second is the chart of global coastal onlap events and eustasy (Haq et al., 1988).Some key research problems are: (1) how to identify unique, time-significant stratal surfaces; (2) how to test their physical continuity; (3) how to test the time relations within depositional models; and (4) how to identify the unique, time-significant global events recorded in the stratigraphic record. These stratigraphic concepts can be tested by graphic correlation, which is a powerful technique of high precision, quantitative stratigraphy. Its application in Cretaceous sections of the Gulf Coast and Oman, and in the Plio-Pliestocene of the Gulf Coast aids the distinction between synchronous surfaces and diachronous boundaries.

Late Cretaceous Fluvial Systems and Inferred Tectonic History, Central Utah: ABSTRACT

Upper Campanian nonmarine sedimentary rocks exposed between the Wasatch Plateau and the Green River in central Utah record a tectonic End_Page 1346------------------------------ transition from thin-skinned deformation in the thrust belt to basement-cored uplift in the foreland region. Thick sections of the Mesaverde Group in the Wasatch Plateau on the west and the Book Cliffs on the east are separated by the San Rafael swell, a basement uplift across which the group is erosionally thinned. Strata in the west (Castlegate Sandstone and Price River Formation) were deposited by east to northeast-flowing braided rivers. Time-equivalent eastern sections comprise a lower sequence of mixed braided fluvial deposits (Castlegate Sandstone and Bluecastle Tongue of Castlegate, coastal swamp and meander-belt deposits (Neslen Formation), and nearshore marine deposits (Buck Tongue of Mancos Shale and Sego Sandstone), and an upper sequence that coarsens upward from meander-b lt deposits (Farrer Formation) into pebbly braided river deposits (Tuscher Formation). Paleocurrent data indicate that rivers of the lower sequence flowed east, while those of the upper sequence flowed northeast. Figure Sandstones within the section consist of two distinct compositional suites, a lower quartzose petrofacies and an upper lithic petrofacies. The compositional boundary occurs at the top of the Bluecastle Tongue and can be correlated across the San Rafael swell. The quartzose suite contains mostly compositional quartzarenites and sublitharenites; the lithic suite is composed of litharenites and feldspathic litharenites. Lithic grain populations of the upper petrofacies are dominated by sedimentary fragments in sections of the Wasatch Plateau and volcanic fragments in sections near the Green River. The sedimentary lithic grains were transported generally eastward from miogeoclinal strata uplifted within the thrust belt. The volcanic lithic grains of the Farrer and Tuscher Formations were erived from more distal arc sources to the southwest, and transported through the thrust belt somewhere west of the Kaiparowits region, where time-equivalent sedimentary rocks are also rich in volcanic lithic fragments. Disappearance of volcanic lithics and appearance of pebbles at the top of the Tuscher Formation is interpreted to reflect a latest Campanian reorganization of drainage patterns that marked initial growth of the San Rafael swell and similar basement uplifts to the south of the swell. Contemporaneous fluvial systems that deposited the uppermost part of the Price River Formation in the Wasatch Plateau were apparently unaffected by the uplift and continued to flow northeast. Depositional patterns thus indicate that initial growth of the San Rafael swell was probably concurren with late deformation in the thrust belt. Depositional onlap across the Mesaverde Group by a largely post-tectonic assemblage of fluvial and lacustrine strata (North Horn Formation) indicates a minimum late Paleocene age for growth of the San Rafael swell and deformation within the thrust belt. End_of_Article - Last_Page 1347------------

Thrust emplacement of the Schoonover sequence, northern Independence Mountains, Nevada

Research Article| October 01, 1981 Thrust emplacement of the Schoonover sequence, northern Independence Mountains, Nevada ELIZABETH L. MILLER; ELIZABETH L. MILLER 1Department of Geology, Stanford University, Stanford, California 94305 Search for other works by this author on: GSW Google Scholar JAMES BATESON; JAMES BATESON 1Department of Geology, Stanford University, Stanford, California 94305 Search for other works by this author on: GSW Google Scholar DAVID DINTER; DAVID DINTER 1Department of Geology, Stanford University, Stanford, California 94305 Search for other works by this author on: GSW Google Scholar JAMES R. DYER; JAMES R. DYER 1Department of Geology, Stanford University, Stanford, California 94305 Search for other works by this author on: GSW Google Scholar DWIGHT HARBAUGH; DWIGHT HARBAUGH 1Department of Geology, Stanford University, Stanford, California 94305 Search for other works by this author on: GSW Google Scholar D. L. JONES D. L. JONES 2U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94325 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1981) 92 (10): 730–737. https://doi.org/10.1130/0016-7606(1981)92<730:TEOTSS>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation ELIZABETH L. MILLER, JAMES BATESON, DAVID DINTER, JAMES R. DYER, DWIGHT HARBAUGH, D. L. JONES; Thrust emplacement of the Schoonover sequence, northern Independence Mountains, Nevada. GSA Bulletin 1981;; 92 (10): 730–737. doi: https://doi.org/10.1130/0016-7606(1981)92<730:TEOTSS>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Preliminary paleontologic data from the northern Independence Mountains, Nevada, suggest that at least part of the Schoonover sequence is time equivalent to the lithologically and structurally similar upper Paleozoic Havallah sequence. The base of the Schoonover is a major thrust fault that juxtaposes paleogeographically distant but, in part, time-equivalent rocks. Along the west side of the range, the Schoonover sequence structurally overlies Late Mississippian nonmarine to shallow-marine rocks that unconformably overlie deformed Ordovician strata of the Roberts Mountains allochthon. This nonmarine to shallow-marine sequence is correlative to overlap assemblage sequences elsewhere in Nevada. Along the east side of the range, the Schoonover is thrust over shallow-marine to slope-facies Permian rocks that unconformably overlie deformed lower Paleozoic miogeoclinal rocks as well as the Roberts Mountains allochthon.The allochthonous Schoonover sequence is a heterogeneous assemblage of deformed deep-water sedimentary rocks and greenstone units. Isoclinally folded chert in the structurally lower half of the Schoonover yields ages based on radiolaria that range from earliest Mississippian to possibly latest Devonian at the very base of the sequence to as young as Early Pennsylvanian in structurally higher parts of the sequence. Clastic rocks within the lower part of the sequence contain debris derived from erosion of Roberts Mountains allochthon-type rocks. Rare volcaniclastic and volcanogenic units in the Schoonover indicate an andesitic-dacitic source. The structurally higher part of the allochthon contains a thick succession of silty-limestone turbidite deposits that represent yet a third, carbonate source terrane for the Schoonover sequence.Structural data indicate southeast-directed thrusting of the Schoonover sequence. Thrusting postdates deposition of Late Permian rocks and is inferred to predate intrusion of Late Jurassic plutons present to the north of the map area. Similarity of structural style and constraints on the timing of thrusting in the Independence Mountains strongly suggest that the Schoonover sequence may represent the northeasternmost exposures of the Golconda allochthon.The age of at least some of the pelagic sediments in the Schoonover sequence may overlap the age of the youngest rocks known in the Roberts Mountains allochthon as well as the age of the Antler Orogeny. This suggests that the Schoonover sequence was deposited, at least in part, in a location that was paleogeographically distant from the edge of the Nevadan portion of the North American shelf during the Antler Orogeny. No volcanism occurred on the continental shelf during the emplacement of the Roberts Mountains allochthon; therefore, the presence of tuffaceous turbidites within the Schoonover also suggests deposition far away from the continental margin. On the other hand, the composition of coarse clastic rocks within the Schoonover is compatible with their derivation from the eroding Roberts Mountains allochthon, suggesting proximity to the North American shelf. If sedimentary rocks from dual sources (volcanic arc and Roberts Mountains allochthon) are indeed interbedded in the Schoonover, rather than tectonically interleaved, then this might argue in favor of deposition of the Schoonover-Havallah sequences in a back-arc basin setting. This content is PDF only. 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Sedimentation and Trapping Mechanism in Upper Miocene Stevens and Older Turbidite Fans of Southeastern San Joaquin Valley, California

Most of the stratigraphic oil production in the southern San Joaquin Valley comes from the sandstones of the upper Miocene turbidites. Updip, time-equivalent rocks of the shallow-marine environment are second in productivity. Third in importance are the deep-marine, fractured, siliceous shales deposited beyond distal margins of the fan complex. Many underexplored areas remain in the San Joaquin Valley despite many years of field development and wildcat drilling. The Miocene of the Bakersfield arch is representative of the classic arcuate type of turbidite deposits characteristic of modern submarine fans. Further paleogradient of the fans has been accentuated and not reversed through geologic time; stratigraphic oil fields are numerous. As in most California deep-marine troughs, the turbidite facies most easily reached by the drill, and in which most of the oil reserves have been developed, are in the mobile zone of basin-margin wedging. Two additional trapping aspects related primarily to depositional stratigraphy within the midfan facies are (1) intrabasin contemporaneous faults (analogous to Gulf Coast growth faults) resulting in expanded section and reverse drag on the downthrown block, and (2) compaction anticlines formed by post-sediment ry accommodation over submarine channel sands resulting in structural inversion with depth. South of the Bakersfield arch is an earlier (pre-late Mohnian) turbidite basin, virtually unexplored and referred to informally as the Maricopa subbasin, where drilling depths to potential Miocene pays range from 12,000 to 20,000 ft (3,600 to 6,000 m). Basic stratigraphic relations recognized in the Bakersfield arch may be applied in the subbasin to develop new hydrocarbon reserves.

Early Cretaceous Stuart City Shelf Margin of South Texas: Its Depositional and Diagenetic Environments and Their Relation to Porosity: ABSTRACT

The Stuart City trend, South Texas, represents a climax biogenic development along the Early Cretaceous (late Aptian, Albian, and early Cenomanian) shelf margin. Landward of this trend, a wide variety of shallow-water shelf carbonate sediments accumulated on a broad, relatively flat platform. Seaward, the entire section consists of dark planktonic foraminifer-bearing argillaceous carbonate sediments. The sediments of the Stuart City trend make up the Stuart City limestone, which attains a total thickness of 2,000 to 2,500 ft. Time-equivalent rocks which crop out in central Texas are the Glen Rose and Edwards Formations. Between 1954 and 1961 many wells ranging in depth from 11,000 to 20,000 ft were drilled with the Stuart City Formation as their final objective. Of the 19 wells from which cores were obtained for this study, 12 were considered gas wells with initial production ranging from 1.5 to 36.5 MMCFGD. Six of these wells still produce gas. Depositional facies and environments and their relation to the diagenesis and porosity development provide a model for further hydrocarbon exploration along the Stuart City and the deeper Sligo trends. The Stuart City carbonate rocks have been assigned to five major environments of deposition: shelf lagoon, shelf margin, upper shelf slope, lower shelf slope, and open marine. The shelf-lagoon facies include miliolid wackestone, mollusk wackestone, toucasid wackestone, and mollusk-miliolid grainstone. These facies accumulated in generally low-energy condition in water depths from 0 to 20 ft. In contrast, the narrow band of shelf-margin carbonate rocks is made up of coral-caprinid boundstone, requienid boundstone, and rudist grainstone, all of which accumulated in moderate to high-energy conditions and in less than 15 ft of water as a complex of reefs, banks, bars, and islands. Seaward of the shelf margin, the upper shelf-slope environment comprises the caprinid-coral wackestone and co al-stromatoporoid boundstone facies, the lower shelf slope, the intraclast-grainstone, echinoid-packstone, and echinoid-mollusk-wackestone facies. Farther seaward in water depths greater than 60 ft, the open-marine environment is represented by the planktonic-foraminifer wackestone basins. Porosities in the carbonate rocks of the Stuart City trend are divisible into two main types, those which are fabric related and End_Page 2206------------------------------ those which are nonfabric related. Primary fabric-related types consist of intraparticle and interparticle. Primary intraparticle porosity, openings within the body chambers of the rudists, is present in the caprinid-coral wackestone, coral-caprinid boundstone, and requienid boundstone. Primary interparticle porosity was originally very high (greater than 30 percent) in the rudist grainstone facies but cementation soon after deposition--submarine, phreatic, and meteoric--reduced the porosity to less than 10 percent, and late subsurface cementation filled the remaining porosity. Primary interparticle porosity now is present only in a few very thin intervals. Secondary fabric related porosity consists of solution-enlarged interparticle and moldic. Both occur in the boundstone and grainstone facies but in very thin restricted units. The poor development of solution enlarged interparticle and moldic porosity reflects the minor role that subaerial exposure played during the development of Stuart City trend. Nonfabric related porosity consists of vertical fractures. This type of porosity is present in abundance in several studied wells in the form of open, nonlined, vertical fractures. The low porosities along the Stuart City Trend are the result of two processes--(1) lack of significant periods of subaerial exposure for development of secondary porosity types and (2) massive cementation which destroyed primary porosity. Further exploration along this trend should be aimed at identifying areas which may have been exposed soon after deposition and developed secondary porosity or areas which subsided more rapidly and have preserved primary porosity. End_of_Article - Last_Page 2207------------

Sedimentary Models, Cycles, and Deltas, Upper Cretaceous, Wyoming

Sedimentary models developed from modern sedimentation alone are of limited usefulness to the petroleum geologist because of (1) lack of variation resulting from deposition during a consistent trend of sea-level change; (2) poor control in the vertical dimension, particularly in intermediate and deeper water environments; and (3) the lack of direct association with petroleum accumulation. More widely useful models can be derived from certain ancient rock sequences, utilizing principles based on modern sedimentation. Good outcrops, abundant well control, bentonite marker beds, and dominantly stratigraphic oil and gas production make the Upper Cretaceous formations of Wyoming particularly useful in providing models pertinent to both industry and academic needs. The Ericson Sandstone of southwestern Wyoming and its nonmarine and marginal-marine equivalents can be combined with marine elements of the model from eastern Wyoming to produce a complete wide-shelf model consisting of fluvial, paludal, barrier-island, shelf, slope, and basinal facies. Significant differences between the resulting model and previously proposed models are the podlike configuration, the presence of thick slope deposits, and the significant submarine topography at the epicontinental slope. A narrow-shelf model can be derived from the Lewis Shale, Fox Hills Sandstone, and Lance Formation of the Red Desert basin. The complete sedimentary cycle consists of the same units as the complete model. Only regressive sedimentary cycles have been recognized. The maximum marine transgressive shift is reflected in a basinal facies directly overlying the paludal facies of the Parkman in southwestern Powder River basin. The complete model includes a distinct fluvial facies deposited at the mouth of a river and is, by definition, a delta. An incomplete sequence consisting of paludal, barrier-island, and a thin shelf facies is interpreted to be interdeltaic. Several deltas can be delineated on this basis from the Ericson and time-equivalent rocks of Wyoming and adjacent states. When these deltas are related to sequences interpreted as deltaic in older and younger Upper Cretaceous rocks of the area, a shifting pattern of deltaic sedimentation similar to that of the modern delta complex is suggested. If this pattern of shifting loci of deposition is correct, some stratigraphic concepts may require reexamination.

Oil Source Beds in Cretaceous Mowry Shale of Northwestern Interior United States

Presence of oil source beds in the Mowry Shale and other marine time-equivalent rocks is established by organic geochemical analyses. Critical factors in the depositional and postdepositional history of these strata influenced the oil-generation process. Depositional conditions controlled the distribution of organic matter deposited in the sediments. High average concentrations of organic matter were preserved in the extensive central basin area, where fine-grained sediments accumulated at relatively slow rates. Postdepositional burial and concomitant thermal history appear to have been critical factors in the conversion of indigenous organic matter into crude oil. Samples with progressively deeper burial histories tend to be relatively enriched in hydrocarbons, and these hydrocarbons tend to be increasingly like those produced from reservoirs associated with the Mowry and time-equivalent rocks. Hydrocarbon mixtures in the shales comparable with those in the reservoirs are essentially restricted to those samples which have been buried at least as deeply as 7,000 ft. Study of the depth of burial history, therefore, provides a valuable means for anticipating areas where source beds are most likely to have been developed in the Mowry Shale and equivalent strata. Within the study area all known oil fields in reservoirs associated with the Mowry and its equivalents are located either within, or updip and adjacent to, areas where source beds have been found in these strata. Thus, regional distribution of oil accumulations appears to be predictable with knowledge of the source-bed distribution.

Sedimentary Models, Cycles, and Deltas, Upper Cretaceous, Wyoming: ABSTRACT

Sedimentary models developed from modern sedimentation alone are of limited usefulness to the petroleum geologist because of (1) lack of variation resulting from deposition during a consistent trend of sea-level change; (2) poor control in the vertical dimension, particularly in intermediate and deeper water environments; and (3) the lack of direct association with petroleum accumulation. More widely useful models can be derived from certain ancient rock sequences, utilizing principles based on modern sedimentation. Good outcrops, abundant well control, bentonite marker beds, and dominantly stratigraphic oil and gas production make the Upper Cretaceous formations of Wyoming particularly useful in providing models pertinent to both industry and academic needs. The Ericson Sandstone of southwestern Wyoming and its nonmarine and marginal-marine equivalents can be combined with marine elements of the model from eastern Wyoming to produce a complete, wide-shelf model consisting of fluvial, paludal, barrier-island, shelf, slope, and basinal facies. Significant differences between the resulting model and previously proposed models are the podlike configuration, the presence of thick slope deposits, and the significant submarine topography present at the epicontinental slope. A narrow-shelf model can be derived from the Lewis Shale, Fox Hills Sandstone, and Lance Formation of the Red Desert basin. The complete sedimentary cycle consists of the same units as the complete model. Only regressive sedimentary cycles have been recognized. The maximum marine transgressive shift recognized is reflected in a basinal facies directly overlying the paludal facies of the Parkman in southwestern Powder River basin. The complete model includes a distinct fluvial facies deposited at the mouth of a river and, by definition, a delta. An incomplete sequence consisting of paludal, barrier-island, and a thin shelf facies is interpreted to be interdeltaic. Several deltas can be delineated on this basis from the Ericson and time-equivalent rocks of Wyoming and adjacent states. When related to sequences interpreted as deltaic in older and younger Upper Cretaceous rocks of the area, a shifting pattern of deltaic sedimentation similar to that of the modern delta complex is suggested. If this pattern of shifting loci of deposition is correct, some stratigraphic concepts may require reexamination. End_of_Article - Last_Page 602------------

Revised Stratigraphic Nomenclature of Pennsylvanian System, Paradox Basin

Pennsylvanian rocks of the Paradox basin provide an excellent example of rock-stratigraphic units occurring in a clear-cut relation with time-stratigraphic units. These rocks have had many names applied to them; some are confusing and contradictory. A system of simple, formal rock-stratigraphic and time-stratigraphic terms is proposed that is usable even in difficult transition areas where evaporite facies grade into shelf carbonate and terrigenous clastic rocks. The Hermosa Group includes rocks that overlie the Molas Formation of Late Mississippian and Early Pennsylvanian age and underlie the Permian Cutler Group, except in the northwestern part of the basin where the Elephant Canyon Formation of Early Permian age disconformably overlies lithologically similar Pennsylvanian rocks. The Hermosa Group consists of three distinctive formations. The oldest is the Pinkerton Trail Formation consisting mainly of marine carbonate rocks and shale. The overlying unit is the Paradox Formation, a sequence of evaporites, carbonate rocks of restricted marine origin, and dark-colored shale. The Paradox Formation is restricted to strata containing evaporites; time-equivalent rocks containing no evaporites are excluded from the formation. Outside the area of ev porite deposition, the Hermosa Group is called the Hermosa Formation undifferentiated because the individual units become indistinguishable. The youngest formation of the group is the Honaker Trail Formation consisting of carbonate, shale, and sandstone. The Honaker Trail Formation disconformably underlies the Permian rocks in the greater part of the Paradox basin. The group contains rocks of Morrowan(?), Atokan, Desmoinesian, Missourian, and Virgilian Series, identified for the most part by abundant fusulinids. A formal time-stratigraphic nomenclature is proposed for further subdivision of the Paradox and Honaker Trail Formations. This system uses the pay zones of the petroleum geologists. These units are bounded at top and base by dark-colored shales of regional distribution which are approximate time-marker beds according to fusulinid determinations. These time-stratigraphic shale units can be traced through many thousands of square miles, even from the evaporites into non-evaporitic facies of the Paradox basin. The shale markers are 10-25 feet thick in most of the basin, but are thicker toward the east. A formal time-stratigraphic unit named the Four Corners Stage is proposed as a subdivision of the Desmoinesian Series. The Ismay, Desert Creek, Akah, and Barker Creek Substages are included in the new stage. It is suggested that these units (formerly the pay zones) be called substages when used as time-stratigraphic units in the Paradox basin. Alternate type sections are proposed for some Pennsylvanian formations: (1) an alternate (subsurface) type section for the Pinkerton Trail Formation, (2) a subsurface type section for the Paradox Formation, and (3) an alternate type section for the Honaker Trail Formation.