Atokan Time Research Articles

Burial, Maturation, and Petroleum Generation History of the Arkoma Basin and Ouachita Foldbelt, Oklahoma and Arkansas

Removed overburden, burial, maturation, and petroleum generation analysis indicates that maturity in the Arkoma Basin and the Ouachita Foldbelt is explained effectively using simple burial models that account for the significant surface erosion that has occurred and assuming geothermal gradients similar to present-day gradients have been approximately constant through geologic time. Regional models, based on analysis at 115 well locations, indicate that from 5,000 to 15,000 ft (1.5–4.5 km) of section, differing with location from north to south and west to east, has been removed from the Arkoma Basin region, and as much as 25,000–40,000 ft (7.5–12 km) have been removed from areas of the Ouachita Foldbelt. Based on burial and thermal history reconstruction, increasing maturation from west to east across the basin is primarily the result of increasing overburden and subsequent surface erosion from west to east. The models predict most publicly available vitrinite reflectance data within a factor of 1.5 at two standard deviations. Comparison of model and measured reflectance-depth trends in six wells indicates that hydrothermal fluid movement should not have modified reflectance by more than approximately 20% in the center of the basin. Analysis indicates that most of the basin is overmature for oil production from intervals below the Spiro Sandstone, except to the north and northwest. Although thermal maturities are high, methane is stable throughout the basin. Except for the basal Arbuckle Group, all formations were thermally immature for oil generation prior to burial by the Mississippian and Morrowan in the Ouachita Foldbelt of Oklahoma and by the Atokan and Desmoinesian over most of the basin and study area. In the deeper part of the present basin, all strata entered and passed through the oil window during or within 10 My after Atokan time. Because no additional major quantities of hydrocarbons were generated after Atokan time, the hydrocarbons must have been emplaced and trapped during this brief time interval.

Genetic Stratigraphy and Reservoir Characterization of the Spiro Sandstone, Red Oak Field, Arkoma Basin, Southeastern Oklahoma: ABSTRACT

The Lower Atokan Spiro sandstone is a mixed carbonate-silicilastic reservoir that produces hydrocarbons from three discrete stratigraphic intervals at the Red Oak Field. Reservoir-quality sandstones develop in the seaward stepping sub-Spiro sequence (highstand system tract), landward stepping Foster [open quote]channel,[close quotes] and upper Spiro depositional sequences (transgressive and highstand system tract). The sub-Spiro and Foster [open quote]channel[close quote] sequences are separated by regional unconformity interpreted as a sequence boundary. Regressive marine shoreface cycles, genetically related to the sub-Spiro shale, comprise the lowermost producing interval. Fluvial/estuarine valley-fill (Foster channel) sandstones progressively onlap the sequence boundary overlying the regressive shoreface cycles and juxtapose reservoir-quality sandstones of different sequences, creating a complex reservoir architecture. Upper Spiro reservoir sandstones are developed within marine shoreface cycles that are deposited in a landward-stepping succession (highstand systems tract) following the drowning of incised paleovalleys. These aggradational / retrogradational successions downlap onto the valley-fill and sub-Spiro sequences representing the final stages of Spiro deposition prior to the high stand of sea level during Middle Atokan time. Regional stratigraphic correlations demonstrate progressive basinward truncation of the sub-Spiro regressive shoreface cycles by an erosional surface, creating a network of incised paleovalleys across the Pennsylvanian shelf. Based on core, well log,more » and outcrop interpretations, the magnitude of the facies offset across this sequence boundary indicates that a significant volume of reservoir-quality sediment has been partitioned basinward of the current producing areas.« less

Exploitation Utilizing 3D Seismic in the Red Oak Gas Field of the Arkoma Basin, Oklahoma, USA: ABSTRACT

Red Oak field,located in the Arkoma basin of Eastern Oklahoma, produces 200 Mmcfd of gas under pressure depletion drive with 2.6 TCF of gas recoverable. Structurally, the field occupied a position along the northern flank of the southward collapsing shear-margin formed during the Ouachita Orogeny. The basin flank is characterized by rapid subsidence and deposition of over 20,000 feet (6000 m) of shallow to deep marine shale and stacked sandstone in Atokan time (mid-late Carboniferous). This sequence culminates with a shoaling upward cycle and is structurally deformed by earliest Desmoinesian thrusting (280-265 Mya). Interpretation from an 18 mi{sup {center_dot}2} (47km{sup 2}) 3-D seismic survey was integrated with available with available well control and litho-facies mapping defining detailed structural irregularities and providing new drillsites while reducing economic risk. Resolution of data from the 3D seismic survey varied greatly. The one failed aspect of the original 3D survey design was to precisely map Red Oak sandstone. However, the survey was robust enough to provide excellent shallow and deep data, leading to identification of additional reservoir targets and multiple drilling proposals.

The Wilburton-Ouachita Trend--A Gas Giant for the 1990s

The Wilburton-Ouachita trend of southeastern Oklahoma runs roughly northeast-southwest from the town of Atoka near the Texas line to the town of Poteau and the Arkansas line. This 120-mi trend is a narrow system 25-35 mi wide with a strong horst-graben structure that separates the Arkoma basin in the north from the Ouachita basin hidden under the thrust system to the south. Recent seismic shooting shows that the basin was downwarped during the rift period of the Acadian orogeny. A fracture system of normal faults was formed in these lower Paleozoic carbonates. These faults were rejuvenated during the Morrow-Springer deposition, and subsequent Wichitan tensional faulting caused many of the horst blocks to move thousands of feet upward. During Atokan time massive shales were deposited into this faulted basin, causing differential loading and more movement. The final period of movement came in Late Pennsylvanian time with the compressional forces of the suturing of the South Oklahoma aulocogen and the thrusting of the Ouachita plate over the deformable Pennsylvanian shales. The author will examine how these forces formed the productive structures that are being drilled today in the Arkoma basin. A series of seismic lines crossing the Wilburton-Ouachita trend will be shownmore » along the extent of the play from Poteau to Atoka.« less

Paleogeographic Evolution of Late Paleozoic Taos Trough, Northern New Mexico: ABSTRACT

The Taos trough (or Rowe-Mora basin) of northern New Mexico was one of several tectonically active cratonic basins associated with the late Paleozoic Ancestral Rockies. As the basin and adjacent uplifts evolved, the depositional systems and paleogeography varied in conjunction with changing tectonic stability, fluctuating End_Page 687------------------------------ sediment input, evolution and integration of sediment dispersal systems, and varying water depth. Morrowan time was characterized by a widespread transgression by a shallow sea flanked by low-lying, muddy strand plains. At this time the surrounding uplifts were relatively low-relief features and supplied only fine-grained sediment to the basin. By Atokan time, vertical movements elevated the Uncompahgre uplift to the west of the trough, and coarse alluvial fan, braided stream, and fan-delta complexes prograded eastward into the basin. During this episode of intense diastrophism to the west, the eastern side of the trough remained fairly stable and a broad, shallow-marine shelf sloped gently off the Sierra Grande arch. Farther south, carbonates accumulated on the Pecos shelf. This general depositional and paleogeographic pattern continued into Desmoinesian time. However, to the north the Cimarron arch had become a major positive feature by earliest Desmoinesian time and several braided stream-fan-delta systems prograded southward and southwestward into the basin. By middle Desmoinesian time an extensive coastal plain had developed along the western margin of the trough, with low-sinuosity rivers feeding lobate deltas. A major middle to late Desmoinesian transgression inundated this coastal plain and shallow shelf conditions prevailed throughout most of late Desmoinesian time. In latest Desmoinesian time, southward prograding fluvial-deltaic systems began filling the northern part of the basin causing a net southward regression. By Early Permian time the basin was dominated by continental sedimentation with southward flowing braided streams. End_of_Article - Last_Page 688------------

Basin Evolution of Late Paleozoic Taos Trough, Northern New Mexico: ABSTRACT

The Taos trough (Rowe-Mora basin of earlier workers) of northern New Mexico was one of several tectonically active cratonic basins associated with the late Paleozoic Ancestral Rockies. The basin was an asymmetric, fault-bounded feature flanking the Uncompahgre uplift. The basin fill was controlled primarily by local tectonics. As the basin and adjacent uplift evolved structurally, the depositional systems likewise evolved in conjunction with changing tectonic stability, fluctuating sediment input, evolution and integration of sediment dispersal systems, and varying water depth. The onset of the first mild structural deformation and encroachment of the sea during Morrowan time was marked by deposition of a complex sequence of fine-grained sandstone, shale, coal, and limestone. These sediments were deposited in a variety of environments including mud flats, marshes, strand plains, and shallow-water carbonate banks. During Atokan time, tectonic activity of the Uncompahgre uplift increased, and subsidence of the basin accelerated. During this time, coarse alluvial-fan, braided-stream, and fan-delta complexes began to fill the basin along its western margin. By middle Desmoinesian time, an extensive coastal plain and a more continuous sediment supply had developed. Meandering rivers with coarse-grained bed-loads fed lobate and wave-dominated deltas. Associated wi h this delta platform were local algal-mound and cross-bedded shelf-edge carbonate accumulations. Basinward, a mixed carbonate and terrigenous slope system developed in conjunction with continued subsidence of the basin. End_of_Article - Last_Page 824------------

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.

Atoka Formation (Pennsylvanian) Deposition and Contemporaneous Structural Movement, Southwestern Arkoma Basin, Oklahoma: ABSTRACT

Abstract The Atoka Formation along the southwestern margin of the Arkoma basin in Coal and Pontotoc Counties, Oklahoma, is predominantly a mudstone unit interbedded with thin (less than 30 ft), horizontally stratified, sandstones and rare, thin, sandy limestones. The Atoka was deposited primarily during a marine transgression on a shallow shelf that received minor amounts of sand-size sediment. The sandstones were deposited as the foreshore and shoreface portions of beach, barrier bar and/or longshore bar sand sequences during brief stillstands or minor regressions. The Clarita anticline was a rising, positive structural element during Atoka deposition. A thickened Atoka sequence is found to the north of this feature, and overlaps and thins across it. This was unrecognized in previous fusulinid studies which infer that Fusulinella occurs 100 ft above Profusulinella when the actual stratigraphic separation is over 1,000 ft. Movement of the Clarita anticline, an eastward extension of the Hunton anticline, indicates that the Arbuckle Mountains were first beginning to undergo structural uplift in early Atokan time. The Atoka sandstones, with updip pinchout and overlapping mudstones, can be stratigraphic traps for small hydrocarbon accumulations in the subsurface on the flanks of the Clarita anticline.

Stratigraphic Distribution, Pennsylvanian Fusulinids, Manzano Mountains, New Mexico: ABSTRACT

The Manzano Mountains, on the east edge of the Rio Grande Valley, are between Albuquerque and Abo Canyon, 40 mi south. Paleozoic rocks exposed in the range are chiefly of Pennsylvanian age. Rocks of Permian age crop out on the east flank of the mountains at the south end of the range. Pennsylvanian and lower Permian rocks have been subdivided into three major lithic types--one constituting the Sandia Formation and two constituting the Madera Limestone. The Sandia Formation contains a basal sandstone and conglomerate, and upper beds of sandstone and dark-colored shale with a few thin beds of fusulinid-bearing limestone. The Madera Limestone overlies the Sandia Formation with apparent conformity. The lower part of the Madera is dominantly limestone (commonly cliff-forming) and a few thin beds of shale and minor amounts of sandstone. The upper part, which overlies the lower part with no more than minor disconformity, consists of rhythmically alternating sandstone and conglomerate, shale, and marine limestone. This upper part has been subdivided for convenience into un ts B, C, and D. Fusulinids from the Sandia Formation indicate that these rocks were deposited during Atoka time. Fusulinids from the lower part of the Madera Limestone indicate deposition during Des Moines time. The upper part of the Madera is within the faunal zone of Triticites. The fusulinid fauna from unit B suggests deposition during Missouri time; that from unit C suggests deposition during early Virgil time; and that from unit D suggests deposition during late Virgil and early Wolfcamp times. Studies of insoluble residues from 52 samples of fusulinid-bearing limestones from 5 measured sections suggest that there is correlation between the amount of terrigenous material in the limestone and the relative abundance of the fusulinids. If the amount of terrigenous material is taken to be an index of water turbidity, tentative conclusions may be drawn concerning clarity of the water in which the fusulinids lived. End_of_Article - Last_Page 542------------

Dimple Limestone--A Turbidite Sequence: ABSTRACT

Recent work on the Dimple Limestone (Atokan) of the Marathon tectonic belt of West Texas indicates that it is predominantly a turbidite sequence deposited in relatively deep water below normal wave base. A narrow upperslope facies of nongraded, festoon cross-stratified grainstone, packstone, and chert conglomerate beds deposited in relatively shallow water occurs in the northwest part of the area. Wherever exposed, the formation is part of an overthrust complex which moved northwestward a distance of several miles or more. The basin facies to the southeast consists of interbedded pelagic and turbidite beds. The pelagic beds are dark gray to black terrigenous shale, calcilutite, and chert containing a sparse fauna of radiolaria and sponge spicules. The turbidite beds are graded lithoclastic fossiliferous limestones ranging in thickness from less than an inch to more than 9 feet. Lithoclasts are mostly angular fragments of Devonian and Ordovician cherts. The fauna is characteristic of shallow shelf and slightly deeper slope environments, and indicates redeposition by turbidity currents. Small quantities of ooliths, pellets, glauconite, and quartz sand grains occur commonly in the turbidite beds. The turbidite beds contain current direction features including aligned fossil fragments (particularly sponge spicules and brachiopod spines), cross laminae, ripple marks, and flame structures; and slope indicators such as convolutions and medium-scale slump structures. Sole markings are not evident. Delicate laminae in upper parts of graded beds commonly consist of siliceous sponge spicules aligned parallel with the current direction. Numerous measured sections of the basin facies indicate a record of continuous deposition from the flysch-like Tesnus Formation, through the Dimple Limestone, and into the overlying flysch-like Haymond Formation. Submarine slides containing large blocks of white Devonian chert locally produced erosional surfaces. A comparison of the Tesnus Formation, Dimple Limestone, and Haymond Formation with the Scaglia-Brecciola-Macigno sequence of the northern Apennines shows striking similarities. Current direction indicators, bed thickness, grain size, facies, and over-all formation thickness indicate that the Dimple Limestone was transported down a slope from the northwest. During early Atokan time, a carbonate shelf in the vicinity of the Glass Mountains provided shallow-water fossils and other carbonate debris for later redeposition in a relatively deep basin on the southeast. End_of_Article - Last_Page 372------------

Geology of Natural Gas in Arkansas Valley, Arkansas: ABSTRACT

The Arkansas Valley is an east-west trending synclinorium bounded on the north by the Ozark uplift and on the south by the Ouachita Mountain anticlinorium. The Valley is an eastward extension of the McAlester basin of southeastern Oklahoma. Pennsylvanian Atoka sediments form the principal outcrops in the Arkansas Valley. The Atoka measures 20,000 feet or more near the Ouachita front. Post-Atoka Pennsylvanian sediments (Hartshorne through Boggy) aggregate 3,000 feet in the western part of the Valley. Cambrian through Pennsylvanian Morrow sediments thicken southward from 5,000 feet or less in the Ozarks to an estimated 25,000 feet of correlatives in the Ouachitas. Little is known of the pre-Pennsylvanian section in the Arkansas Valley because of inadequate drilling densities. Morrow shales, sandstones, and subordinate limestones were deposited mainly under shallow-water marine conditions. Morrow facies changes occur southeastward in the subsurface and eastward on the outcrop. Meagerly fossiliferous dark shales and lenticular sandstones constitute the Atoka in the Valley. Regional Atoka facies changes have not been observed. Numerous east-west trending anticlines and synclines have surface expression in the Arkansas Valley. Many are faulted parallel with their axes. Steeply dipping flanks of the folds are indicated on the surface mostly by sharp ridges, and open synclines are expressed by flat-topped mountains. Folds in the southern part of the Arkansas Valley are tight and asymmetrical, with steeper north flanks. Dips of 50° or more are common. Progressively northward, the structures are less tightly folded and more nearly symmetrical. The folding was initiated during the Ouachita orogeny as early as Middle Pennsylvanian time and terminated after Boggy time. A major fault system marks the structural boundary between the Valley and the Ozarks. These faults are normal and predominantly downthrown toward the south with displacements up to 3,000 feet. The structural boundary between the Arkansas Valley and the Ouachitas is more difficult to define. Faults near the Ouachita front are predominantly downthrown toward the north. High-angle reverse faults and low-angle thrusts are the common types in that region. Major faulting in the Arkansas Valley was not initiated until Atoka time or later. Only dry gas is produced in the Arkansas Valley, chiefly from the Atoka but increasingly from the Morrow. The gas is sweet, generally high in methane and low in nitrogen. Currently there are 41 designated gas fields in the Arkansas Valley. Daily production averages 50 million cubic feet. Cumulative production since the first commercial well in 1902 totals 300 billion cubic feet. Reserves are in excess of 800 billion cubic feet. Pennsylvanian production is expected to be expanded both eastward and southward in the Arkansas Valley. Pre-Pennsylvanian sediments, especially the Mississippian End_Page 126------------------------------ and Arbuckle correlative sections, are prospective targets for oil or gas. End_of_Article - Last_Page 127------------

Pennsylvanian Paleotectonics of Colorado Plateau

During the Pennsylvanian interval of time, the Colorado Plateau area underwent the most active period of tectonic activity of the Paleozoic era. Pennsylvanian paleotectonic relationships have been reconstructed from stratigraphic and lithologic data acquired from 264 control points. This was accomplished through the use of isopach maps, lithologic ratio maps, vertical sandstone distribution maps, field evidence, and paleontological data. Pennsylvanian tectonic elements of the Colorado Plateau area defined a northwest-trending parageosynclinal seaway. Greatest subsidence occurred in the north where the Paradox Salt basin formed as an asymmetrical half graben. On the south, depositional centers formed in the Pennsylvanian San Juan and Lucero basins. The seaway opened northwest onto the rapidly subsiding Oquirrh shelf, northeast into the northern Colorado trough, west into the Cordilleran geosyncline, southeast into the southern Colorado trough, and south and southwest into the Pedregosa basin and the Sonoran geosyncline. The seaway was bounded to the northwest by the Emery uplift, to the northeast and east by the Uncompahgre and San Luis uplifts, and to the west and southwest by the Kaibab-Zuni uplift. The Defiance upli t, a structural subsidiary of the Kaibab-Zuni lineament, projected north into the southern Paradox basin, where it strongly affected depositional patterns during Pennsylvanian time. Throughout most of Pennsylvanian time, the Emery uplift was a low, partly submerged, structural arch. A southeastern spur of tectonism, probably archipelagic in nature, affected depositional patterns from Desmoines to Wolfcamp time in the northwestern Paradox basin. Earliest and greatest Uncompahgre tectonism occurred in its southern part. Here it became a clastic source area in Atoka time, remained orogenically positive into the Permian, and during this time interval shed a tremendous coarse clastic delta to the southwest. The northwestern Uncompahgre responded to uplift later in the Pennsylvanian, but probably permitted marine access between the Eagle and Paradox evaporite basins during Desmoines time. Consequently, Paradox evaporites may be the composite product of two evaporite ba ins. The San Luis uplift, which was slightly positive in pre-Pennsylvanian time, did not respond to sharp uplift until late Pennsylvanian time. The Penasco uplift underwent early and late Pennsylvanian tectonism, but impressed only minor influence on the regional lithofacies. Although the Kaibab-Zuni uplift existed as a nearly continuous structural lineament during most of the Pennsylvanian, greatest and earliest tectonism occurred in the eastern, or Zuni segment. The Defiance uplift was probably positive in the Mississippian, and underwent successive tectonic pulses during Atoka-early Desmoines, late Desmoines, and late Pennsylvanian-early Permian times. Major hydrocarbon production in the Four Corners area displays close relationship to this tectonic element geographically and geologic lly. Because of sedimentary and tectonic barriers, a thick sequence of evaporites was deposited within the Paradox Salt basin. The Emery and Uncompahgre uplifts created restriction on the northwest, north, and east, and the Kaibab-Zuni uplift provided restriction on the southwest. The combination of a clastic barrier projecting into the Four Corners area from the northeast and the Defiance uplift projecting into the Four Corners area from the southwest formed nearly continous physical restriction across the southern Paradox basin and provided conditions necessary for cyclical evaporite deposition within the basin.

Structure of the Frontal Belt of Ouachita Mountains: ABSTRACT

The structure of the frontal belt of the Ouachitas is dominated by faulting. The faulting in general consists of a complex set of reverse faults roughly parallel with the Ouachita Mountain front and a related set of cross faults. Most of the reverse faults appear to dip at high angle and to have a horizontal component of movement that is of the order of magnitude of the vertical movement. Others, such as the Pine Mountain and Windingstair faults, appear to have had greater horizontal movement than vertical movement, and at least locally appear to dip at moderate angles. The Ti Valley fault and some minor faults appear to have a low angle of dip. The cross faults are of two types. In the northern part of the area they are characterized by strike-slip movement. South of the Windingstair fault the movement was dominantly upward. The minimum amount of movement on the reverse faults in the frontal belt appears to have been in excess of 50 miles, with the greatest part of that movement concentrated on the Ti Valley, Windingstair, and Pine Mountain faults. More or less simultaneous deformation seems to have occurred in an extreme frontal block, the block between the Ti Valley and Windingstair faults, and the block south of the Windingstair fault, with the deformation culminating in movement on the Ti Valley and Windingstair faults. Several lines of evidence suggest that the direction End_Page 1778------------------------------ of movement was generally northward and that greater movement occurred in the eastern part of the area than in the western part. Incompetent shale zones constituted gliding planes along which thrust movement took place, with the principal ones being the Womble shale, Springer formation, Caney shale, Stanley shale, and Johns Valley shale. One can postulate from indirect evidence the existence of an early period of faulting along the north margin of a late Mississippian-early-Pennsylvanian geosyncline. However, the structural development of the frontal Ouachitas started in Atoka time and continued until middle Pennsylvanian time and possibly as late as early Permian time. End_of_Article - Last_Page 1779------------

Basin-and-Range Structure Reflects Paleozoic Tectonics and Sedimentation: ABSTRACT

Upper Paleozoic tectonism and sedimentation are the key factors which influenced the relation between valleys and ranges in northeastern Elko County, Nevada. The Mississippian orogeny which formed the Manhattan geanticline, a northeast-southwest welt in central Nevada, was also responsible for the formation of an east-west-trending positive feature in northern Elko County, Nevada. This feature, here named the Northeast Nevada high is probably an eastward extension of the Manhattan geanticline. The presence of the Manhattan geanticline divided the Cordilleran geosyncline into an eugeosyncline on the west and an unstable shelf on the east. Acting as a buffer zone for the compressive forces from the west, the shelf was gently folded into a series of essentially north-south-trending synclines and anticlines. Rapid erosion of the Manhattan geanticline and Northeast Nevada high fed coarse clastics east and south, depositing the Tonka and Diamond Peak formations on the shelf which was surfaced by the folded Mississippian Chainman shale. Maximum clastic deposition occurred in a foredeep parallel with the east side of the geanticline, while minor amounts from the Northeast Nevada high were being channeled southward into the folded geosynclinal shale basin. Contemporaneous with the clastic deposition in the synclinal folds, erosion was attacking the adjacent shale-capped anticlinal folds and many of the anticlines were eroded as deeply as the Ordovician Eureka quartzite and equivalents. Filling of the synclines and erosion of the anticlines continued through most of Morrowan time. By middle to late Morrowan time, erosion had reduced the anticlines to approximately the same level as the clastic-filled synclines. A veneer of chert and quartz gravels and grits remained as pediment deposits over this nearly peneplaned surface. By early Atokan time, the orogeny was limited to spasmodic unrest, supplying minor amounts of chert and quartzite pebbles into the newly formed limestone depositional basin (Moleen, Tomera, and Ely formations). In middle Des Moines time, the entire Great Basin was gently uplifted permitting erosion and non-deposition until at least middle Missourian time. By Virgil-Wolfcampian time, partial relaxation of positive forces in the Great Basin again permitted the seas to invade most of eastern Nevada and adjacent Utah (upper member of Oquirrh form tion, and equivalent carbonate formations). Leonardian time was characterized by quiescence and the deposition of a fusulinid-bearing carbonate formation over approximately 9,000 square miles of northeastern Nevada and adjacent Utah. This heretofore unnamed limestone sequence, dated lower Leonard to lower Guadalupian, is here named the Pequop formation. The Pequop formation is 1,525 feet thick at its type section in Sec. 3, T. 33 N., R. 65 E., Elko County, Nevada. Composed of purplish gray, irregularly bedded, platy, silty limestones with interbedded fusuline coquinas, the lower contact is placed at the red silt member which overlies the massive Wolfcampian limestones (unnamed formation), and the upper contact placed at the base of an unnamed massive dolomite sequence which is overlain by the Phosphoria formation. Subsidence continued, depositing the Phosphoria and Gerster formations, until uppermost Guadalupian time when once again the rejuvenated Manhattan geanticline and Northeast Nevada high shed minor amounts of limestone, chert, quartzite pebbles and gravel east and south into the western part of the Phosphoria depositional basin in Nevada. Episodes of tectonic unrest in northeastern Nevada occurred during Jurassic-Cretaceous (Nevadian) and Cretaceous-Tertiary time (Laramide). Not until post-middle Miocene was the area once again subjected to major compressive forces of the Cascadian (Basin-and-Range) revolution. The north-south-trending anticlines, eroded of their Mississippian, Devonian, Silurian and in some places Ordovician sediments, together with continued compressive forces from the west, acted, as the jaws of a vise on the thicker Paleozoic sedimentary piles within the synclines. Weaker sediments in the synclines were forced topographically higher than the adjacent anticlines with shearing occurring along the flanks of the anticlines. Thus, in northeastern Nevada, present ranges were born from a thick sedimentary section once protected in the synclines, while valleys were developed from eroded anticlines stripped of their lower and middle Paleozoic ection. End_of_Article - Last_Page 1105------------

Geologic Interpretation of Argillaceous Sediments: Part II. Clay Petrology of Upper Mississippian-Lower Pennsylvanian Sediments of Central United States

X-ray diffraction analysis of Upper Mississippian-Lower Pennsylvanian shales from the Ardmore and Anadarko basins indicates that the Meramecian Caney shales are characterized by the abundance of illite and the scarcity of chlorite and kaolinite, the Chester shales by the presence of illite and montmorillonite, the Springer shales by the presence of montmorillonite and the absence of illite, the Lower Pennsylvanian Morrow shales by an abundant illite content, and the Atoka shales by the relative abundance of mixed-layer clay. The Chester and Springer formations, which can be divided into three geographic provinces, are overlain by a 200±-foot mixed interval which consists of a mechanical mixture of the Morrow and underlying formation clays. The type of reworked Springer clay in the mixed interval can be used to identify the geographic province of the underlying Springer section. Both the Morrow and Atoka shales contain distinctive interfingering beds of reworked Springer shales which appear to be restricted to wells situated near the downthrown sides of faults. In the Ouachita geosyncline the Upper Mississippian Stanley shale is characterized by the presence of illite, chlorite, and feldspar, the Lower Pennsylvanian Jackfork and Johns Valley shales by the presence of mixed-layer clay and the relative scarcity of kaolinite and chlorite. Near the northern margin of the Ouachita area the lower Stanley shale is similar in composition to the Caney and Springer shales, indicating a transition between the two basins. The Stanley illite-chlorite shales and high-rank graywacke sandstones were deposited in a rapidly sinking eugeosyncline. The sediments were derived from a metamorphic-acid igneous complex on the south. The closely related Chester and Springer montmorillonitic shales and orthoquartzite-quartzose graywacke sandstones were deposited in a rapidly sinking intracratonic basin. They were probably derived from a broad, low-lying sedimentary-volcanic source north and south of the basin. The southern source area may have extended toward the east and partly separated the two depositional areas. The Pennsylvanian (Morrow) was ushered in by a major uplift and metamorphism in the southern part of the Ouachita eugeosyncline, and minor faulting along the southern flank of the Anadarko and Ardmore basins. The micaceous Jackfork shales (Ouachita basin) and orthoquartzite sandstones were derived from this Ouachita uplift. The micaceous Morrow shales (Ardmore and Anadarko basins) were derived in part from the Ouachita area and in part from the periodically uplifted montmorillonitic sediments along the south flank of the basins. At the beginning of Atokan time there was increased uplift along the south flank of the basins (the Wichita Mountains and Criner Hills) and as a result the Atokan shales are more montmorillonitic than the Morrow shales, and the sandstones are typical low-rank raywackes. Regionally the clay mineral suites in the Arkansas and Illinois basins and in the north Texas area can be related to those in Oklahoma. End_Page 272------------------------------

Pennsylvanian Geology of Northern Arkansas: ABSTRACT

Rocks of Pennsylvanian age in the Ozark Highlands and Arkansas Valley are essentially the same as the sequence representing earlier Pennsylvanian deposition in eastern Oklahoma. Outcropping in the Arkansas Valley are the Atoka, Hartshorne, McAlester, Savanna, and Boggy formations; in the Ozark Highlands only the Morrow group and the Atoka formation are exposed. There is in general a gradual thickening of beds toward the south from the outcrop toward the Arkansas Valley, an eastward extension of the McAlester basin. A marked eastward thickening in the lower Morrow and Mississippian has recently been demonstrated by Maher and Lantz who tentatively assign these thickened units to the Jackfork sandstone and Stanley shale, respectively. This thickening apparently begins along north-south line through Pope County, which also approximates the eastern limit of outcrop of post-Atoka Pennsylvanian rocks. The significance of this facies change in terms of petroleum possibilities can not yet be determined. The Arkansas Valley, bounded on the south by the Ouachita Mountains and on the north by the Boston Mountains of the Ozark Highlands, is characterized structurally by long east-west-trending anticlines and synclines, many of which are faulted parallel with the axis. This folding had its beginning as early as Atoka time but the folding and faulting were not completed until after Boggy time. Along part of its northern margin the Arkansas Valley is separated from the Boston Mountains by a system of normal faults with a total vertical displacement as much as 3,000 fee . Pennsylvanian rocks in the Boston Mountains show a moderate regional dip south. Local folding is not as sharp and is not as common as in the Arkansas Valley. Much of the folding in the area had its inception in pre-Pennsylvanian time but almost all of the recognized faulting is post-Atoka. The anticlines of the Arkansas Valley have long been prospected for petroleum and to a slightly greater extent than those of the Ozark Highlands. The recorded production End_Page 424------------------------------ of the Valley for the past fifty-three years totals about ¼ billion cubic feet, essentially all from Atoka sandstones. It has been estimated that the present proved reserves amount to about four-fifths this amount. Much of the Arkansas Valley and most of the Ozark Highland area have not yet been adequately tested. During the past year an 8-million-cubic-foot well was brought in at 4,000 feet to establish the easternmost commercial production. It is in Pope County, half way between the Oklahoma line and the western margin of the Mississippi River Embayment. End_of_Article - Last_Page 425------------

Regional Geology of the Pennsylvanian in the Mid-Continent Area: ABSTRACT

A large proportion of the Pennsylvanian sediments in the Mid-Continent area are dark shales, which tend to grade eastward into sandstones, and westward into limestones. Most of this dark shale is believed to have come from a land mass (Llanoria), located to the southeast. The large part of the rocks in this land mass consisted of closely-folded, faulted and uplifted shales, sandstones, and limestones previously deposited in geosynclinal troughs, during Mississippian and earlier Paleozoic periods. The Pennsylvanian is divided into six series--Springer, Morrow, Atoka, Des Moines-Strawn, Missouri-Canyon, and Virgil-Cisco. Several maps show regional conditions of deposition, the location of land masses, and the source of sediments during each of these series. Since it is impossible at this time to satisfactorily show the Springer, Morrow, and Atoka on separate maps, they are presented together on one map. End_Page 1671------------------------------ At the beginning of the Pennsylvanian, during Springer time, the sea was quite restricted and sediments of that age were deposited only in southern Oklahoma, in adjacent parts of Arkansas and North Texas, and in the Marathon region of West Texas. During Morrow time, the Pennsylvanian sea expanded somewhat and dark shales and limestones were deposited over a larger area. During Atoka time, conditions changed markedly in the Mid-Continent area, and the Wichita period of orogeny occurred. In addition to the continued uplifting of the Llanoria or Ouachita Mountain land mass to the southeast, three other different kinds of uplift occurred. One set of the new uplifts formed large anticlinal islands, such as the Wichita Mountains and Uncompahgre Uplift, trending northwest-southeast in the ex anding Pennsylvanian sea. These large islands were eroded rapidly and clastics accumulated on their flanks. Some of the other uplifts were smaller and were rapidly buried by later sediments. The sea expanded to form new marine basins between uplifted land-masses. During Des Moines-Strawn time, the Pennsylvanian sea probably extended to its maximum limit in this region. Thick sands which have produced much oil were deposited on the eastern side of the area and many limestones accumulated in the western area. During Missouri-Canyon time, there was relatively little uplift or structural movement. Conditions in the seas were most favorable for reef building by calcareous organisms, particularly in West Texas. During Virgil-Cisco time, another period of orogeny, the Arbuckle, occurred in southern Oklahoma and extended into the Panhandle of Texas. The Arbuckle and Wichita mountains were again uplifted and the thick series of Pennsylvanian shales and sandstones in the Ardmore and Anadarko basins were folded and faulted to form traps that produced many prolific oil fields. End_of_Article - Last_Page 1672------------

Atoka Series in North Central Texas: ABSTRACT

During the Mississippian and Pennsylvanian periods in North Texas, the area of thickest accumulation shifted westward and the tectonic activity, as shown by the character of the sediments and their rate of thickening, decreased. In the sequence of gradually changing depositional environments from linear geosynclinal type to continental basin type, the Atoka and Des Moines epochs are transitional. The depositional pattern of Atoka sedimentation suggests a modified geosynclinal environment. Sediments of early Atoka time indicate that the Atoka basin formed under conditions of considerable crustal mobility. Thousands of feet of interspersed shale and dirty sandstones attest the rapid burial of the geosynclinal sediments. The Atoka series thins rapidly across a hinge area, where the sediments are predominantly shales with some interbedded sandstones, to the shelf area, where limestones and dark shale were deposited. The different sedimentary types, controlled by environment, were deposited contemporaneously. The marked divergence of time and lithologic units makes it advisable to apply terminology only to facies types. The Marble Falls formation in North Central Texas, as generally considered, includes the Pennsylvanian-Mississippian unconformity. Although the Muenster Arch was not uplifted until post-Atoka-pre-Des Moines time, conditions existed in the general area which inhibited the rate of downwarping relative to the surrounding area. The thinner Atoka sediments, which were deposited over the Arch, were later removed by erosion. End_of_Article - Last_Page 1673------------

Jesse Pool, Pontotoc and Coal Counties, Oklahoma

The development of the Jesse pool, located in the southeast part of the Franks graben, followed shortly after the discovery of the Fitts pool. Both the stratigraphy and the geological history of these pools are closely related. The Jesse pool lies on a large anticline, which is faulted on the south side. The faulting ordinarily occurs as a series of step faults, and is regarded as the controlling factor for trapping the oil. This anticline is almost as large as the Fitts anticline, but generally, the porosity and permeability of the reservoir rocks are not as good. Consequently, the productivity will be considerably less in the Jesse pool than in the Fitts pool. The major structural movements of the Arbuckle Mountains are reflected in the Jesse anticline; the first major tructural uplift occurred at the close of Atoka time (early Pennsylvanian), and the second major uplift occurred in the late Pennsylvanian time. Oil or gas is produced from rocks of Pennsylvanian, Siluro-Devonian, and Ordovician age, at depths ranging from 1,800 feet to 4,700 feet. The most prolific production is from the Wilcox sand at the crest of the anticline.

Pennsylvanian Sedimentation in Arkansas Coal Field

About 13,000 feet of Pennsylvanian strata (of upper Pottsville and lower Allegheny age) are present in the southern part of the Arkansas coal field. The strata thin northward at such a rapid rate that they are only about half as thick at the north side of the coal field as equivalent strata at the south side. The sediments in the Pottsville portion of the section consist dominantly of shale, sandy shale, and sandstone, and the overlying strata of Allegheny age consist of these lithologic types together with a number of coal beds. The major source of the sediments appears to have been farther south, in and south of the Quachita Mountains, but some sediments in the lower part of the section probably came from the east. Deposition of the Pennsylvanian sediments took place in a basin: (1) that was progressively warped downward and whose north margin migrated northward across the area but lay across the central part of the coal field during most of early Atoka time; (2) that underwent minor deformation by lateral pressure from the south during the deposition of the sediments; (3) that stood close to sea-level throughout the greater part of the time of deposition of the sediments; (4) that received no invasion of marine waters of sufficient depth or duration to leave a perceptible record in the stratigraphic column; and (5) that received the bulk of the sediments under fluviatile conditions. In consequence of these and other conditions the Arkansas coal field is characterized by (1) northward thinning o the strata, part of which is due to progressive northward overlap at the base of the Atoka formation and part to northward thinning of individual beds; (2) a sharp change in the structural pattern along an east-west line which probably marks the zone of greatest change in thickness of the coal-basin strata; (3) limited lateral extent of lithologic units; and (4) no apparent symmetry of recurrence of lithologic types in the stratigraphic column.