Early Pennsylvanian Time Research Articles

Timing and evolution of the Mississippian sedimentary system on southeastern Laurussia: Evidence from the Appalachian area, U.S.A.

This paper is a synthesis of Mississippian sedimentation on the southeastern, Appalachian margin of Laurussia. The Mississippian stratigraphic framework in the Appalachian area shows a large-scale, three-part, wedge-like, clastic‑carbonate-clastic sequence, which has been interpreted to be a likely flexural response to coeval Neoacadian Orogeny. Using an already established flexural framework and biostratigraphy, we have interpreted the approximate timing of flexural events based on biostratigraphy. These flexural events begin with active deformational loading that initiated rapid subsidence in the Appalachian foreland basin, during which the deeper-water, black Sunbury Shale and equivalents were deposited throughout the basin. Subsequent relaxational loading contributed to basin infilling through the deposition of a deeper-water, clastic succession, manifest as the extensive Cuyahoga-Logan-Borden-Grainger-Price-Rockwell-Pocono deltaic clastic wedge. By the time this clastic wedge had filled the foreland basin, clastic source areas in the orogen had apparently been lowered to the point that shallow seas migrated far eastward across former source areas. This brief period of equilibrium between the basin and source areas resulted in reduced clastic influx at a time when the basin entered subtropical latitudes, setting the stage for widespread carbonate deposition, represented by units like the Newman, Greenbrier, and Slade Limestones. Ensuing, rebound-related uplift, perhaps abetted by incipient, subsurface, Alleghanian thrusting, generated new sediment source areas and accompanying wedges of prograding marginal-marine and terrestrial sediment represented by the Pennington, Mauch Chunk and equivalent units. Marginal-marine sedimentation continued into Early Pennsylvanian time, when a combination of Alleghanian uplift and LPIA lowstand conditions generated a period of widespread, deep erosion that in many places across the Appalachian Basin destroyed substantial parts of the underlying Mississippian succession along the sub-Absaroka or “Mississippian-Pennsylvanian” unconformity. Although the basic Mississippian stratigraphic framework described above for the Appalachian Basin largely appears to be a consequence of Neoacadian flexural responses, major details regarding processes, timing and lithology were apparently controlled by concomitant interactions involving paleoclimate, eustasy, coeval Ouachita tectonism, and possibly early Alleghanian tectonism.

An integrated chemo-and sequence-stratigraphic framework of the Early Pennsylvanian deepwater outcrops near Kirby, Arkansas, USA, and its implications on remnant basin tectonics

The Jackfork Group in Arkansas, U.S.A. is a major ‘training ground’ for academics, students, and petroleum industry geoscientists interested in deep water exploration and development. Several superb outcrop sections are exposed and over the years numerous papers have been published on these rocks. The Kirby sections in western Arkansas provide a unique opportunity to build a complete and continuous sequence stratigraphic framework for Early Pennsylvanian time in the ancient Ouachita Basin. There are 25 outcrops, including 12 roadcuts on Highway 27, and 13 measured sections in the Baumgartner Quarry. All the measurements and interpretations on the outcrops were integrated with previous work using modern concepts of deepwater turbidite geology, including (1) regional tectonic and sequence stratigraphic framework of the Stanley Group, Jackfork Group, and Johns Valley Shale in the Ouachita Basin, (2) deepwater outcrop characterization and correlation from the DeGray Lake Section, Dierks Section and Big Rock Quarry which are on the trend of the depositional fairway, (3) chemostrata data from key shale layers which are considered as potential correlative condensed sections.Twenty-five key shale samples from Kirby, DeGray and Dierks sections have been tested using Inductively Coupled Plasma and Mass Spectrometry (ICP-MS) tests. An additional 525 samples were later collected for Handheld X-Ray Fluorescence (XRF) tests for both major and trace elements. Results of sequence and chemostrata analysis indicate that the entire Kirby sections consist of at least three 3rd order sequences from the bottom of the Jackfork Group to the middle of the Johns Valley Group, and over ten 4th order sequences caused by a combined effect of tectonic uplift, eustatic sea-level change, mixed sediment provenance and shifting of depositional fairways when the Ouachita Basin was transitioning from a passive margin to a remnant marine basin. Rare Earth and Trace Element results are compared with known tectonic data to further pinpoint the tectonic environment of the Ouachita Basin during early Pennsylvanian time, which is dominantly a continental arc setting.

Magnetic polarity stratigraphy and palynostratigraphy of the Mississippian-Pennsylvanian boundary interval in eastern North America and the age of the beginning of the Kiaman

Magnetic polarity data from the Maritimes Basin in eastern Canada reveal patterns of magnetic reversal through ∼17 m.y. of late Mississippian and early Pennsylvanian time. We collected samples from three sections on Cape Breton Island, a fourth that extends a section studied previously, and a fifth from a deep cored well on Prince Edward Island. We compared the polarity data from these sections to previous results from lower parts of the Joggins section of eastern Canada. All sections are constrained by new palynological data and collectively span the late Mississippian, Brigantian, Pendleian, and much of the Arnsbergian substages. A major unconformity at the Pennsylvanian-Mississippian boundary makes correlation difficult. The late Mississippian Arnsbergian substage is dominated by normal polarity, whereas the Pendleian and Brigantian are characterized by normal and reversed polarity intervals in about equal proportions. In the United States, the Mississippian-Pennsylvanian boundary is a major hiatus that juxtaposes strata of early Pennsylvanian age on Mississippian strata ranging in age from Brigantian to mid- to late Arnsbergian. Data for the Mauch Chunk Formation in the United States provide polarity reversal patterns for correlative sequences in the Central Appalachian Basin. Field mapping and magnetic polarity data show closely comparable Mississippian-Pennsylvanian relationships in the Maritimes Basin more than 1500 km northeast from the U.S. sections. The eustatic event that coincides with the Mississippian-Pennsylvanian boundary unconformity is late Arnsbergian to Yeadonian in both the central Appalachians and Maritime Basins. Basal Pennsylvanian strata of Yeadonian to Langsettian age on Cape Breton Island reveal normal paleomagnetic polarity through several hundred meters of section. These strata are overlain by a thick section showing only reversed polarity. We believe that this polarity boundary is the base of the Kiaman. An early Langsettian position (ca. 318 Ma) for the base-Kiaman superchron is indicated by these new data. In eastern Nova Scotia and New Brunswick, the sedimentary rocks just above the unconformity are of reversed polarity. It is clear that the sedimentation in the two areas is affected by the unconformity, and no magnetic or palynological correlation is possible. Above the unconformity, only saetosa zone sediments are present.

Drowning of the Upper Marble Falls carbonate platform (Pennsylvanian), central Texas: A case of conflicting “signals?”

Marble Falls and Smithwick Formation carbonates and shales were deposited on the western margin of the slowly subsiding Fort Worth Basin of central Texas during Early Pennsylvanian time. The Upper Marble Falls Member carbonate platform back-stepped towards the west–southwest as Smithwick outer shelf–slope black shales drowned platform margin areas to the east. The shelf-to-basin profile observed from subsurface data from the Fort Worth Basin and outcrops from the Llano Uplift area records platform drowning; however, the platform began to shoal upwards as it back-stepped towards the west–southwest, due to the presence of the Llano Uplift paleobathymetric high. The geometry of these facies tracts was controlled primarily by a combination of subsidence and second-order sea-level change, while the development of sequences and unconformities was controlled by third-order sea-level changes and environmental factors, and not subsidence. The termination of platform deposition in the Fort Worth Basin was caused by the interplay of third-order sea-level changes, environmental deterioration in advance of siliciclastic burial, and a lack of accommodation space. The depositional history of the Upper Marble Falls carbonate platform does not fit “classic” examples of drowned platforms described from other areas, but may be typical of platforms in slowly subsiding basins.

Cratonal contributions to a “classic” molasse: The Carboniferous Pottsville Formation of eastern Pennsylvania revisited

The Carboniferous Pottsville Formation of eastern Pennsylvania in the central Appalachian foreland basin is considered a classic orogenic molasse, typical of other Acadian-Alleghanian clastic wedges along the eastern margin of Laurentia. Although the Pottsville conglomerates are thought to be derived entirely from early Alleghanian highlands, we have documented an unrecognized dramatic change in the sediment-dispersal pattern within the formation; northwest-directed paleocurrent indicators in older units (implying an orogenic provenance) are superseded abruptly by southwestward-directed paleocurrent indicators (implying a cratonal provenance). This alteration in the sediment-dispersal pattern is attributed to basin reorganization during Early Pennsylvanian time as tectonic loads migrated southward during orogenesis. The paleocurrent data warrant a revision of Early Pennsylvanian paleogeography and demonstrate that molasse facies can record a complex interplay between sediment dispersal systems along both active and cratonal margins of a foreland basin.

Three-dimensional controls on subsidence of a foreland basin associated with a thrust-belt recess: Black Warrior basin, Alabama and Mississippi

Tectonic transport from different directions and convergence of separate thrust belts at structural recesses impart three-dimensional variations to the pattern of foreland-basin subsidence. Assuming approximately uniform flexural rigidity of the lithosphere, subsidence is directly controlled by the geometry and density of the separate thrust loads, and flexural deformations caused by the loads interfere within the recess. For example, the Black Warrior foreland basin, which is located in Mississippi and Alabama, within a recess in the Appalachian-Ouachita orogen, formed in response to two separate thrust loads: (1) the Ouachita accretionary prism along the southwest side of the basin, and (2) thrust-imbricated passive-margin rocks in the Appalachian thrust belt along the southeast side of the basin. The subsidence history differs along two profiles, each perpendicular to the Appalachian and Ouachita thrust fronts. Subsidence of the foreland basin began in middle Mississippian (Meramecian) time in response to northeast-directed translation of the Ouachita accretionary prism. Subsequently, northwest-directed Appalachian thrusting began in middle Early Pennsylvanian (Morrowan) time. Subsidence curves from the profile perpendicular to the Ouachita prism show a clear progression of increasing subsidence rates and amounts toward the thrust front, whereas curves from the profile perpendicular to the Appalachian belt indicate out-of-plane effects from the Ouachita thrust front and a later increase in subsidence toward the Appalachian thrust front. The major implication of this work is that the three-dimensional geometry of a foreland basin within a thrust-belt recess includes diachronous interfering elements.

The Combined Effects of Eustasy, Tectonism, and Clastic Influx on the Development of Pennsylvanian Cyclic Carbonates, Southern Sangre de Cristo Mountains, New Mexico

Pennsylvania cyclothems are well documented on stable continental shelves and the cyclicity has generally been attributed to glacio-eustasy. As a contrast, Atokan-Desmoinesian cyclic carbonates of the southern Sangre de Cristo Mountains developed in a tectonically active foreland basin, formed by thrusting along the Picuris-Pecos fault during early Pennsylvanian time. Strata exposed in two sections (Dalton Bluff, 260 m; Johnson Mesa, 340 m) are characterized by (1) shallowing-upward cycles, (2) cycles of variable thickness (5-20 m), (3) incomplete cycles, (4) cycles interrupted by terrigenous clastic input, and (5) noncyclic intervals. Allocyclic mechanisms alone cannot fully explain these observations; the authors herein propose that a complex interplay among eustasy, tectonism, and clastic sediment supply were responsible for the observed cycles. Lithofacies analysis indicates that location within the foreland basin played a significant role in cycle attributes. In the deeper portions of the basin (e.g., Dalton Bluff), an idealized cycle, from base to top consists of (1) shale/marl facies, (2) brachiopod wackestone facies, (3) phylloid algal facies, and (4) marine clastic facies. No evidence for subaerial exposure of cycle caps is noted. In contrast, in shallow portions of the basin near the forebulge (e.g., Johnson Mesa) the marine clastic facies is substituted bymore » crinoidal grainstone/packstone facies that is capped by subaerial exposure surface. Each of the two cycles displays an overall grand (lower order) shallowing-upward cycle. This grand cycle developed as sediments infilled the initially starved foreland basin.« less

Depositional Environment and Mineralization of Late Mississippian Surprise Canyon Formation, a Carbonate-Clastic Estuarine Deposit, Grand Canyon, Arizona: ABSTRACT

During Late Mississippian and Early Pennsylvanian time, a change in relative sea level caused marine invasion of ancient stream valleys cut into the top of the Redwall Limestone. These deposits are now exposed as isolated lens-shaped patches up to 120 m thick and 370 m wide in the walls of the western Grand Canyon. The basal unit of the valley fill consists of a fining-upward sequence from chert-pebble conglomerate to ripple-laminated and flat-bedded sandstone and siltstone, suggesting a transition from fluvial domination to tidal domination. The middle unit is a yellow-brown, cliff-forming skeletal limestone with a diverse marine fauna. This limestone unit has a large, distinct channel outline and commonly exhibits trough cross-strata having bimodal current directions. The dominant flow direction is upstream, suggesting a flood-tidal channel that filled with skeletal debris. Skeletal grainstone beds are commonly paired with a dark purple-brown sandy limestone, interpreted to be the ebb-tidal channel. Diagenetic patterns within the grainstone show good reserve possibilities within the flood-tidal channel. Above the limestone are mud-cracked and bimodally ripple-laminated sandstone and siltstone with interbedded algal, ostracodal limestone that suggest restricted intertidal conditions. Within the conglomerates and limestones are opaque hematitic clasts and two types of quartz grains thatmore » suggest early ferruginization and early silicification of evaporites on the newly exposed Redwall platform.« less

Plate Tectonics of Ancestral Rocky Mountains: ABSTRACT

The Ancestral Rocky Mountains were intracratonic block uplifts that formed in Colorado and the surrounding region during Pennsylvanian time. Their development related to the collision-suturing of North America with South America-Africa, which also resulted in the Ouachita-Marathon orogeny. During Early Pennsylvanian time, suturing was taking place only in the Ouachita region, and foreland deformation took place largely in the Mid-Continent. By Middle Pennsylvanian time, the length of the suture zone had increased, and it was active from the Ouachita to the Marathon region. At this same time, deformation of the craton also increased in intensity and in areal extent, culminating in the Ancestral Rocky Mountains. By Late Pennsylvanian time, suturing was taking place only in he Marathon region, and cratonic deformation decreased areally, spreading southward into New Mexico and west Texas and west into the Cordillera miogeocline. The Ancestral Rocky Mountains, and related features over a broad area of the western United States were formed while an irregularly bounded peninsula of the craton (including the transcontinental arch) was pushed northward and north-westward by the progressive collision-suturing of North America and South America-Africa. This intraplate deformation is, in some respects, similar to the deformation of Asia in response to the Cenozoic collision with India. End_of_Article - Last_Page 853------------

Changing patterns of Pennsylvanian coal-swamp vegetation and implications of climatic control on coal occurrence

Improved regional and interregional stratigraphic correlations of Pennsylvanian strata permit comparisons of vegetational changes in Euramerican coal swamps. The coal-swamp vegetation is known directly from in situ coal-ball peat deposits from more than 65 coals in the United States and Europe. Interpretations of coal-swamp floras on the basis of coal-ball peat studies are extended to broader regional and stratigraphic patterns by use of coal palynology. Objectives of the quantitative analyses of the vegetation in relation to coal are to determine the botanical constituents at the peat stage and their environmental implications for plant growth and peat accumulation. Morphological and paleoecological analyses provide a basis for deducing freshwater regimes of coal swamps. Changes in composition of Pennsylvanian coal-swamp vegetation are quire similar from one paralic coal region to another and show synchrony that is attributable to climate. Paleobotany and paleogeography of the Euramerican province indicate a moist tropical paleoclimate. Rainfall, runoff and evapotranspiration were the variable climatic controls in the distribution of coal-swamp vegetation, peat accumulation and coal resources. In relative terms of climatic wetness the Pennsylvanian Period is divisible into five intervals, which include two relatively drier intervals that developed during the Lower-Middle and Middle-Upper Pennsylvanian transitions. The climate during Early Pennsylvanian time was moderately wet and the median in moisture availability. Early Middle Pennsylvanian was drier, probably seasonally dry-wet; late Middle Pennsylvanian was the wettest in the Midcontinent; early Late Pennsylvanian was the driest; and late Late Pennsylvanian was probably the wettest in the Dunkard Basin. The five climatic intervals represent a general means of dividing coal resources within each region into groups with similar botanical constituents and environments of peat accumulation. Regional differences in basinal geology and climate were significant variables, but the synchronous control of paleoclimate was of primary importance.

Cedar Creek--A Significant Paleotectonic Feature of Williston Basin: ABSTRACT

Cedar Creek is the major anticlinal structure demarcating the south-west flank of the Williston basin. This pronounced fold developed through a geologic history of recurrent tectonic movements along a northwest-southeast striking fault zone. The four major periods of tectonism documentable in the Cedar Creek area from early Paleozoic through mid-Tertiary affected the local and regional distribution, erosion, and/or preservation, and, though moderately, the depositional facies of sedimentary strata since Ordovician time. Post-Silurian-Pre-Middle-Devonian: Uplift and fault movement accompanied north and east tilting of the main Cedar Creek block. Several hundreds of feet of Silurian strata were eroded prior to Middle Devonian time, and a karst plain developed on the Silurian surface. Middle and Upper Devonian sediments progressively onlapped and infilled the uplifted, northwest plunging element. Late Devonian-Pre-Mississippian: During latest Upper Devonian--possibly earliest Mississippian--pronounced fault movement occurred along the major fault zone. The Cedar Creek block was uplifted and tilted north and east; maximum displacement along the zone appears to have occurred in the vicinity of the Pine-Gas City-Glendive oil fields. Extensive erosion resulted in the near peneplanation of the structure and significant truncation of Upper Devonian strata. Continued paleostructural influence during Mississippian time is indicated by the distribution and facies of early and middle Mississippian rocks. Late Mississippian (Chester) through Triassic: In late Mississippian (Chester) and early Pennsylvanian time, the central and northern portion of the Cedar Creek area underwent gentle downwarping, and periods of subsidence occurred with relative down-to-the-east fault movement along parts of the ancestral master and subsidiary faults. Similar fault movement(s) and subsidence which continued to occur during the Permian and Triassic periods significantly influenced the deposition and preservation of these evaporite rich, red-bed sequences. Relative tectonic stability was attained by the Middle Jurassic and essentially maintained until post-Paleocene time. Post-Paleocene: The Cedar Creek block underwent its greatest magnitude of uplift during the post-Paleocene; younger Tertiary beds are not present in the area. This uplifting was accompanied by major flexuring, the extensive linear belt of asymmetric drape-folding generally aligned with the ancestral fault zones, and deep fault adjustment. Northwest plunge along the crestal portion was significantly increased. The entire area was subsequently uplifted during epeirogenic phases of the mid-Tertiary in the northern Rocky Mountain region and about 1,500 ft (460 m) of Paleocene and Upper Cretaceous strata eroded along the axis of the present structure. End_of_Article - Last_Page 1332------------

Plate tectonics of the Ancestral Rocky Mountains

The Ancestral Rocky Mountains were intracratonic block uplifts that formed in Colorado and the surrounding region during Pennsylvanian time. Their development related to the collision of North America with South America–Africa, which produced the Ouachita-Marathon orogeny. In Early Pennsylvanian time, suturing was taking place only in the Ouachita region, and foreland deformation took place only in the mid-continent. By Middle Pennsylvanian time, the length of the suture zone had increased, and it was active from the Ouachita to the Marathon region. The extent of cratonic deformation also increased in intensity and in areal extent, culminating in the Ancestral Rocky Mountains. In Late Pennsylvanian time, suturing was taking place only in the Marathon region, and cratonic deformation decreased in extent and spread southward into New Mexico and West Texas. We suggest that the Ancestral Rocky Mountains, and related features over a broad area of the western United States, were formed while an irregularly bounded peninsula of the craton (including the transcontinental arch) was pushed northwestward by the progressive collision-suturing of North America and South America–Africa. This intraplate deformation is, in some respects, like the deformation of Asia in response to the Cenozoic collision with India.

Tectonics of Western Permian Basin: ABSTRACT

The Permian basin of west Texas and southeastern New Mexico probably originated in late Precambrian time as a shallow geosynclinal trough on the southern edge of the North American craton. The extreme southern part of the region formed part of a subduction trough. By Late Cambrian time the seas in the southern part of the trough had advanced over part of the craton, and Paleozoic sedimentation began. Perhaps in late Precambrian a north-facing salient of the trough developed into an aulacogen with lateral dextral movement along steep faults in what later became the eastern part of the Delaware basin. During early and middle Paleozoic time the region was tectonically quiet. However by Middle Mississippian the Precambrian zones of weakness were reactivated and the Delaware and Midland subbasins began to form. At the same time the Marathon-Ouachita trough began to deepen. In early Middle Pennsylvanian time active tectonism with stresses from the northeast formed a small mountain chain between the two subbasins, on the site of the Central Basin platform. In later Pennsylvanian and Early Permian time renewed tectonism resulted in low-angle thrusting of early Paleozoic rocks northward from the Marathon trough. At the same time there was renewed vertical movement on the ancient lateral faults which formed the eastern edge of the Delaware basin. Middle and Late Permian was a quiet time tectonically with the only movement being slight subsidence, probably due to differential compaction in the basins. This regime continued through the early Mesozoic. By Early Cretaceous, however, a major change in paleogeography resulted in an advance of the sea over the region which was by then securely welded to the craton. The area was protected from severe deformation during the Laramide orogeny by the beam effect of the Matador and Amarillo uplifts on the north and the east-west folds of the Marathon belt on the south. Tectonic activity since the early Tertiary has been minimal. Tectonic style in the Delaware basin is characterized by eastward tilt in the Permian and younger rocks. The older rocks are folded and cut by steeply dipping faults. The Central Basin platform style comprises supertenuous draped structures in the younger rocks. These overlie closed folds with steeply dipping faults in the older rocks which trend north-northwest. The Midland basin contains en echelon faults and extension fracturing, reflecting moderate movement. The extreme southern part of the Permian basin consists of deep folded troughs bounded by thrust faults. The trend of these structures forms a large angle with the trends of the Central Basin platform. The long interval of tectonic stability in the region since the early Permian has undoubtedly contributed to the maturation and accumulation of the oil and gas resources of the basin. End_of_Article - Last_Page 762------------

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

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

Spectrum of Deltaic Systems in Pennsylvania Tectonically Active Cratonic Basin, North-Central New Mexico: ABSTRACT

The Pennsylvanian sedimentary fill of the Taos trough of New Mexico contains a variety of coarse-grained deltaic systems typical of tectonically active, cratonic basins. This delta spectrum reflects the interaction of a large number of variables including: water depth, substrate stability, basin energy, fluvial feeder system variations, and tectonic activity. The deltas evolve from small, coarse-grained fan-deltas into more extensive wave-modified, river-dominated, lobate deltas. This evolutionary trend results from an increase in water depth and basin energy. During Early Pennsylvanian time, vertical movements west of the Taos trough elevated the Uncompahgre uplift and coarse-grained fan-deltas prograded eastward into the basin. When these deltas prograded into water deeper than 4 m, frontal splays spread bedload sediment on the delta slope. Continued progradation gave rise to coarsening upward sequences 4 to 15 m thick capped by coarse channel deposits. When deltas built into very shallow water (<4 m), steep delta foresets developed. If the fan deltas advanced over thick mud sequences, syndepositional deformation occurred. As the delta complexes extended into the basin, a broad coastal plain was constructed, and the braided streams evolved into low-sinuosity, bed-load rivers. Water depth in the basin increased, and channel-mouth sand was reworked into extensive delta-front sheet sand bodies by the higher wave energy. On delta abandonment, delta-front sand was reworked into barrier islands with associated washovers, and inlets. In contrast, fan-delta abandonment is only represented by thin, bioturbated calcareous sandstone or limestone beds capping the progradational sequence. End_of_Article - Last_Page 688------------

Early Pennsylvanian Braided Stream Sedimentation, Northwest Arkansas: ABSTRACT

The Bloyd Formation (Morrowan) of northwest Arkansas is a heterogeneous succession of sandstone, limestone, shale, and siltstone units that accumulated in shallow-marine and nonmarine environments during Early Pennsylvanian time. Sedimentation occurred on a gentle paleoslope inclined to the south. The basal Brentwood Limestone Member is overlain in extreme western Arkansas by coal-bearing shale and siltstone strata of the nonmarine Woolsey Member. Eastward, Woolsey strata pass into a thick and laterally extensive sandstone body characterized by abundant sets of tabular cross-strata, medium to coarse-grain size, and beds of quartz-pebble conglomerate. The unit is bounded below by a marked unconformity that displays up to 6 ft (1.8 m) of erosional relief on the underlying B entwood Member. Erosion surfaces occur throughout the unit and bound genetically significant intervals characterized by a consistent succession of sedimentary structures. Quartz-pebble conglomerates resting directly on erosion surfaces are overlain by sets of large-scale tabular cross-strata. Thin intervals of ripple-laminated sandstone containing well-developed ripple bed forms with occasional clay drapes overlie the tabular sets and are bounded above by erosion surfaces. These genetic intervals are repetitive throughout the unit and reflect sedimentation by initially competent but rapidly waning current systems. Paleocurrent measurements derived from tabular cross-strata indicate that the sediment was emplaced by south-flowing unidirectional currents characterized by low dispersion. The succession of sedimentary structures contained within genetic sequences and paleocurrent data indicates that the middle Bloyd sandstone body accumulated in several south-flowing braided stream systems in northwest Arkansas. End_of_Article - Last_Page 2120------------

Structural Evolution of Michigan Basin and Its Petroleum Potential: ABSTRACT

Well data suggest that the Precambrian basement of the Michigan basin is highly faulted with vertical tectonics exerting a dominant control over basin structures. A single deep test on the mid-Michigan gravity high penetrated at least 5,000 ft (1,500 m) of Precambrian(?) red beds, suggesting the presence of a rift or graben. An embryonic form of the Michigan basin was present in Cambrian time. The basin, in approximately its present dimensions, was created during the Middle Ordovician. Two phases of maximum subsidence occurred during the Paleozoic: 4,000 ft (1,200 m) of Silurian sediment, topped by several major unconformities, is followed by 3,800 ft (1,140 m) of Devonian strata. Variations in thickness and sedimentary facies in the Silurian End_Page 450------------------------------ and Devonian support the concept of two intrabasin platforms: one in southwestern Michigan (Allegan) and the other in southeastern Michigan (St. Clair). A generalized depocenter lies directly northwest of Saginaw Bay. There were important deviations from this depocenter, particularly during the Ordovician and Middle Devonian. The major northwest-trending folds in the central part of the Michigan basin were generated during Late Mississippian or Early Pennsylvanian time, and are probably the result of the regeneration of dominantly vertical movements along basement faults. Major petroleum production in Michigan is from (1) Ordovician dolomitized fault zones; (2) Silurian pinnacle reefs; and (3) Devonian anticlinal traps with marked variations in porosity. Future possibilities lie primarily with the discovery of additional dolomitized fault zones in Ordovician carbonate rocks or in Devonian stratigraphic traps. Cambrian production is a possibility, but there is inadequate control to define its potential. End_of_Article - Last_Page 451------------

Subsurface Study of Atoka (Lower Pennsylvanian) Clastic Rocks in Parts of Jack, Palo Pinto, Parker, and Wise Counties, North-Central Texas

A lithofacies analysis of the subsurface Atoka clastic rocks in parts of Jack, Palo Pinto, Parker, and Wise Counties, north-central Texas, was designed to provide a meaningful interpretation of depositional environments that existed during Atokan time, as well as the most favorable areas for production of hydrocarbons. The Atoka clastic rocks are the product of the Ouachita orogene interacting with an epeiric sea on the west during Early Pennsylvanian time. They are a series of sediments deposited between the Caddo Limestone of Strawn age and the top of Marble Falls Limestone of Morrowan age. The study area is in the west-central part of the Fort Worth basin which is a pericratonic or retro-arc basin in plate tectonic terminology. The deepest part of the basin is adjacent to the folded and thrust-faulted Ouachita belt on the southeast. The subsidence of the Fort Worth basin during Atokan time accommodated the eroded detritus from the rising Ouachita system on the southeast. Limestones were accumulating in the western area where more stable conditions existed on the Concho platform. The investigation provided valuable information that may augment hydrocarbon exploration programs throughout the basin; the procedure of investigation can be applied to similar basins beyond the confines of the immediate area.

Mobile Basin-Shelf Border in Northeast Oklahoma During Desmoinesian Cyclic Sedimentation: ABSTRACT

Desmoinesian coal cycles and marine limestone bank episodes in northeastern Oklahoma are related to the mobility of the basin-shelf border area. This mobility may reflect the northward passage of a long-term crustal wave generated either by the Carboniferous convergence of the North American and African plates and/or the intense activity along the orogenic arcs comprising the now buried Llanoria complex in the northern Gulf of Mexico area. The shelf-edge carbonate banks (Wapanucka Limestone), located along the Choctaw fault in Early Pennsylvanian time, had moved 80 mi (128 km) north to Tulsa County by Middle Pennsylvanian (Marmaton) time and then another 80 mi (128 km) north to southern Kansas by Late Pennsylvanian time. Other tectonic and corresponding stratigraphic elem nts also show a broad northward shift. These include an isostatic compensated couplet between basinal foredeep and orogenic arc (zeugogeosyncline of Kay), a usually subsiding paleobathymetric axis and the southward-tilting slope and shelf of the craton border area. Complicating this simple picture is the overprint of eustatic rise and fall of sea level perhaps caused by waxing and waning of Gondwana glaciers that persisted throughout much of the Late Carboniferous. Rising sea level on the northern shelf was marked by coals and limestones giving way to gray to black organic clay shales; and later, stagnant to lowering sea level was marked by progradational delta and associated silty to sandy deposits and finally a deep soil profile. Together these constitute typical cyclothems which are numerous throughout the shelf areas in northeast Oklahoma and adjoining states. These include some of the more productive Oklahoma oil and gas sandstones such as the Cleveland, Skinner, Red Fork, Bartlesville, and Booch. The thicker basinal deposits of the southern Arkoma basin also contain some coals, but sandstones rather than shale or limestone commonly overlie such coals, perhaps owing to the greater ruggedness and tectonism of its source area in the east and south. End_of_Article - Last_Page 2115------------

Genesis and Distribution of Desmoinesian (Pennsylvanian) Sandstone Reservoir, Sleepy Hollow Field, Red Willow County, Nebraska

Sleepy Hollow oil field, Red Willow County, Nebraska, discovered in 1960, has produced nearly 30 million bbl of oil from a "basal" Desmoinesian (Pennsylvanian) sandstone. The trap for this substantial oil accumulation is complex, related to (1) sandstone characteristics and distribution, and (2) gentle late Paleozoic structure. Examination of reservoir core samples from 23 Sleepy Hollow field wells reveals sedimentary characteristics which, when combined with observations of the stratigraphic sequences in and adjacent to the sandstone, suggest that the following sequence of sedimentary events governed the distribution of this reservoir rock. The Precambrian surface that had been exposed at least since Early Pennsylvanian time was incised by fluvial channels that were choked with granite detritus (quartz and feldspar in various stages of weathering). During the Middle Pennsylvanian marine transgression of the Precambrian surface, quartz fragments derived from the granite wash were reworked by westward longshore drift in the shallow sea but rarely were transported more than a few miles from the drainage system from which they were derived. Accumulation of relatively pure quartz sand is presumed to have occurred as a result of the removal of feldspar through weathering of the exposed crystalline rocks and by winnowing by shallow-marine currents. After several episodes of shoreline and shallow-marine sand deposition and accompanying winnowing of clay, lobate sand bodies coalesced to form the present continuous reservoir. Sand deposition did not occur north and south of Sleepy Hollow because of the absence of a nearshore high-energy environment for clastic deposits. Sand deposition did not occur east of Sleepy Hollow because of the relative position of the fluvial channel and the direction of longshore drift.