Late Pennsylvanian Time Research Articles

Dust and loess as archives and agents of climate and climate change in the late Paleozoic Earth system

Abstract Palaeo-loess and silty aeolian-marine strata are well recognized across the Carboniferous–Permian of equatorial Pangaea. Aeolian-transported dust and loess appear in the Late Devonian in the west, are common by the Late Carboniferous, and predominate across equatorial Pangaea by the Permian. The thickest loess deposits in Earth history – in excess of 1000 m – date from this time, and archive unusually dusty equatorial conditions, especially compared to the dearth of equatorial dust in the Cenozoic. Loess archives a confluence of silt generation, aeolian emission and transport, and ultimate accumulation in dust traps that included ephemerally wet surfaces and epeiric seas. Orogenic belts sourced the silt, and mountain glaciation may have exacerbated voluminous silt production, but remains controversial. In western Pangaea, large rivers transported silt westward, and floodplain deflation supplied silt for loess and dust. Expansion of dust deposition in Late Pennsylvanian time records aridification that progressed across Pangaea, from west to east. Contemporaneous volcanism may have created acidic atmospheric conditions to enhance nutrient reactivity of dusts, affecting Earth's carbon cycle. The late Paleozoic was Earth's largest and most long-lived dust bowl, and this dust represents both an archive and agent of climate and climate change.

Late Pennsylvanian vegetation changes in the Donets Basin: syndynamic aspect

The purpose of the article. The article is devoted to the study of one from forms of syndynamics in Late Pennsylvanian time, namely phytocoenogenesis. The study of the phytocoenogenesis of ancient plant communities, i.e., the formation of new types of palaeophytocoenoses, is based on the research of vegetation diversity that is reflected in the ecological-floristic classification of the Late Pennsylvanian phytocoenoses of the Donets Basin by the Brown-Blanquet method. Material and research methods. The phytocoenogenetic researches consisted of the analysis of the changes of the species and syntaxonomic composition of vegetation cover throughout the Late Pennsylvanian. The peculiarities and main factors of phytocoenogenesis have been identified on the palaeosyntaxa in rank of the orders of the following vegetation types: wetland forests and woodlands of coastal lowlands, wetland forests and woodlands of deltaic plains, seasonally dry woodlands of river valleys and lagoon coasts. The palaeophytocoenotic data are based on the plant fossil assemblages from lacustrine, lacustrine-swamp, lacustrine-deltaic, floodplain-lacustrine, floodplain and lacustrine-lagoon deposits in more than 17 localities and 11 boreholes located within the Bakhmutska and Kalmius-Toretska troughs. Research results. Based on the analysis of the phytocoenogenetic processes (progressive and regressive development of plant communities) and the manifestations of phytocoenogenesis (formation of new communities and loss of old communities) in the development of the Late Pennsylvanian vegetation, three models of the formation of new communities and one model of the loss of communities from vegetation cover have been proposed. Each of the models reveals the relationship between the phytocoenogenetic processes and environmental changes. The evolution-progression model reflects the progressive development of palaeophytocoenoses that is expressed by the appearance of evolutionary new coenopopulations in the conditions of the expansion of landscape types (expanding biotopes). The migration-progression model conforms to the progressive development of palaeophytocoenoses as a result of the migration of plants from decreasing landscape types to expanding ones (expanding biotopes). The substitution-regression model is consistent with the regressive development of palaeophytocoenoses along with structure simplification and dominant substitution of plant communities due to the contraction of certain landscape types (decreasing biotopes). The elision-regression model of the loss of communities reflects the regressive development of palaeophytocoenoses that is expressed by the decrease in the number of coenopopulations and the number of individuals in remaining coenopopulations in the conditions of the contraction of some landscape types (decreasing biotopes). The scientific novelty. For the first time, the Late Pennsylvanian vegetation dynamics (syndynamic) have been studied. The proposed three models of the formation of new communities and one model of the loss of communities reveal the main directions in evolution of palaeophytocoenoses and give the first ideas about phytocoenogenesis in late Palaeozoic times.

Stratigraphy and paleogeographic significance of a Late Pennsylvanian to Early Permian channeled slope sequence in the Darwin Basin, southern Darwin Hills, east-central California

The complex stratigraphy of late Paleozoic rocks in the southern Darwin Hills consists of regionally extensive Mississippian and Early to Middle Pennsylvanian rocks overlain by latest Pennsylvanian to Early Permian rocks, herein called the Darwin Hills sequence. Deposition of this latter sequence marked the beginning of the Darwin Basin. In Mississippian time, a carbonate platform prograded westward over slightly older slope deposits. In the Late Mississippian this platform was exposed to erosion and siliciclastic sediments were deposited. In Early to Middle Pennsylvanian time the area subsided, forming a west-facing ramp that was subjected to deformation and erosion in Middle or early Late Pennsylvanian time. Later this area was tilted westward and deep-water sediments were deposited on this slope. In latest Pennsylvanian to earliest Permian time, a major channel was cut through the older Pennsylvanian rocks and into the Upper Mississippian strata. This channel was gradually filled with increasingly finer grained, deep-water sediment as the area evolved into a basin floor by Early Permian (Sakmarian) time. Expansion of the Darwin Basin in Artinskian time led to a second phase of deposition represented by strata of the regionally extensive Darwin Canyon Formation. The geology in this small area thus documents tectonic events occurring during the early development of the Darwin Basin.

Late Paleozoic tectonics and paleogeography of the ancestral Front Range: Structural, stratigraphic, and sedimentologic evidence from the Fountain Formation (Manitou Springs, Colorado)

Sedimentologic, stratigraphic, and structural data indicate that the Fountain Formation at Manitou Springs consists of three tectonostratigraphic units, herein termed the lower, middle, and upper Fountain Formation. Both the lower and middle Fountain Formation were deposited in a fan-delta setting adjacent to the active ancestral Ute Pass fault. The lower is thin and predominantly characterized by depositional stability as recorded by abundant well-developed, deeply rooted paleosols. The middle Fountain Formation reflects fan progradation and highland rejuvenation as a result of increased activity of the ancestral Ute Pass fault. This activity was likely the result of a basinward splay of the ancestral Ute Pass fault with reverse dip-slip motion as indicated by (1) exhumation of the lower Paleozoic Sawatch Formation and (2) fault-parallel folding. The upper Fountain Formation reflects deposition in a NW-SE–trending, braided-river system and appears to postdate movement on the ancestral Ute Pass fault. Lithostratigraphic correlation allows for a Latest Pennsylvanian–Early Permian age for the upper Fountain Formation, thus constraining cessation of the ancestral Ute Pass fault to Middle to Late Pennsylvanian time. The Fountain Formation was deposited within a NW-SE–oriented structural trough (i.e., Woodland Park trough) that separated the ancestral Front Range into a northern block (i.e., ancestral Front Range block) and a southern block (i.e., Ute Pass uplift). The Woodland Park trough was bounded on its southern margin by the ancestral Ute Pass fault, which was active throughout deposition of the lower two Fountain tectonostratigraphic units, during which time sediments were shed northward across the ancestral Ute Pass fault into a marine environment. By the time of deposition of the upper Fountain Formation, movement on the ancestral Ute Pass fault had ceased or dramatically decreased. During this time, the Fountain Formation records predominantly axially oriented, braided-stream deposition. Comparison of the timing and kinematic history of the ancestral Ute Pass fault with other documented reverse and strike-slip faults of the ancestral Rocky Mountains suggests that: (1) cessation of faulting within the ancestral Rocky Mountains displays a slight east-to-west younging, but the crude age resolution of adjacent basin fill also allows for the possibility of relatively synchronous cessation of faulting, and (2) kinematics of all faults examined are consistent with a NE‑SW–oriented maximum horizontal compressional stress field. These inferences contrast with the proposed Laurentian–Gondwanan diachronous closure models for the ancestral Rocky Mountains, which require rotation of the maximum horizontal stress orientation through time and pronounced east-to-west younging of faulting. Rotation of the horizontal stress field may be recorded in multistage, kinematic histories of faults, but multiple faults require study to assess this model. Age constraints on post-tectonic units of the ancestral Rocky Mountains, such as the upper Fountain Formation tectonostratigraphic unit defined herein, are poor and further refinement would greatly aid understanding of the kinematic timing and tectonic driving force of these ancient mountains.

The Cottage Grove fault system (Illinois Basin): Late Paleozoic transpression along a Precambrian crustal boundary

The Cottage Grove fault system in southern Illinois has long been interpreted as an intracratonic dextral strike-slip fault system. We investigated its structural geometry and kinematics in detail using (1) outcrop data, (2) extensive exposures in underground coal mines, (3) abundant borehole data, and (4) a network of industry seismic reflection profiles, including data reprocessed by us. Structural contour mapping delineates distinct monoclines, broad anticlines, and synclines that express Paleozoic-age deformation associated with strike slip along the fault system. As shown on seismic reflection profiles, prominent near-vertical faults that cut the entire Paleozoic section and basement-cover contact branch upward into outward-splaying, high-angle reverse faults. The master fault, sinuous along strike, is characterized along its length by an elongate anticline, ∼3 km wide, that parallels the southern side of the master fault. These features signify that the overall kinematic regime was transpressional. Due to the absence of suitable piercing points, the amount of slip cannot be measured, but is constrained at less than 300 m near the ground surface. The Cottage Grove fault system apparently follows a Precambrian terrane boundary, as suggested by magnetic intensity data, the distribution of ultramafic igneous intrusions, and patterns of earthquake activity. The fault system was primarily active during the Alleghanian orogeny of Late Pennsylvanian and Early Permian time, when ultramafic igneous magma intruded along en echelon tensional fractures.

A cratonic Middle Jurassic paleopole: Callovian‐Oxfordian stillstand (J‐2 cusp), rotation of the Colorado Plateau, and Jurassic North American apparent polar wander

Paleomagnetic results have been obtained from two sequences of the upper Middle Jurassic (upper Callovian) Summerville Formation which are located on the North American craton (north central New Mexico). The thicker of the two sections (Trujillo) exhibits a well‐defined dual‐polarity remanence, forming a magnetostratigraphy of at least 12 relatively short polarity intervals. The high reversal frequency is consistent with that recorded by the oceanic crust at this time. The second section (Romeroville) is thin, gypsiferous, and poorly indurated; consequently, only a short interval (5 m) was successfully sampled. The characteristic magnetization was wholly normal polarity. In the rapid reversals of the thicker section, that thickness of normal polarity is unique and thus forms the basis of magnetostratigraphic correlation between the two sections. A cratonic late Middle Jurassic paleomagnetic pole at 148.2°E, 57.3°N (k = 30.3, α95 = 6.0°) was obtained from the larger Trujillo sequence. Despite large confidence limits, the small sample population of the Romeroville sequence displayed the same pole. Throughout the western United States, the Summerville Formation is overlain with little or no discordance by the Morrison Formation, although the two stratigraphic units are commonly reported to be separated by an unconformity, designated J‐5. The New Mexico paleopoles of the Summerville and overlying lower Morrison Formations are statistically identical at the 95% confidence level; a similar situation exists between Summerville and lower Morrison paleopoles of the Colorado Plateau. The presence of the same geomagnetic field direction in both the Summerville and the lower Morrison Formations suggests that the J‐5 unconformity which separates them may encompass little time, perhaps representing only a minor break in deposition. The two newly investigated Summerville sites are located east of the Rio Grande Rift, hence representative of the North American craton; their paleopoles are displaced counterclockwise relative to those of the Colorado Plateau, and in amounts consistent with other coeval craton‐Colorado Plateau pole pairs. Together, six coeval paleopoles indicate that the Colorado Plateau has rotated clockwise by about 9.0 ± 3° since Late Pennsylvanian time. The cratonic Summerville paleopole also removes the uncertainty over the Jurassic North American apparent polar wander path. The statistically identical Summerville and lower Morrison paleopoles form a substantial data set, five paleopoles, which indicate that NA APW traced a path roughly along 60° latitude. The five paleopoles stand in stark contrast to the paleopole from the slightly older (5–8 Ma) Moat Volcanics. Spreading rates in the central Atlantic do not allow NA plate motion that would include both paleopoles positions. Because the weight of the data, five paleopoles from widely spaced localities and different tectonic blocks, the Moat Volcanics pole cannot be representative of North America in the Middle Jurassic.

Shifts in late Paleozoic atmospheric circulation over western equatorial Pangea: Insights from pedogenic mineral δ18O compositions

The d 18 O values of pedogenic calcites, phyllosilicates, and iron (oxyhydr)oxides from fossil soils throughout the southwestern United States show systematic paleolatitudinal and temporal trends that indicate a significant change in soil moisture conditions and atmospheric circulation patterns over southwestern Euramerica throughout latest Pennsylvanian and Early Permian time. A progressive depletion of as much as 6‰ in mineral 18 O values with increasing distance northward (to 108) of the paleoequator indicates a weakening or disruption of zonal easterly flow over the study area by latest Pennsylvanian time. Furthermore, elevated d 18 O values suggest a proximal source of paleoprecipitation over the study area, perhaps due to initiation of reversed equatorial flow over tropical southwestern Euramerica. Coupling of the geographic and temporal trends defined by pedogenic mineral d 18 O values with those defined by climatically sensitive paleosol pedotypes in the study area, along with overall elevated mineral d 18 O values, provides some of the strongest evidence that Northern Hemisphere monsoonal circulation was well established over Pangea by Early Permian time.

A Petroleum System for the Salina Basin in Kansas Based on Organic Geochemistry and Geologic Analog

The Salina Basin historically has been an “exploration desert”—a home of dryholes. Although this basin, which underlies much of north-central Kansas, may never be a prolific source of hydrocarbons, recent research into the maturation and geochemistry of organic matter and oils in Kansas can provide guidelines for a new exploration strategy. The Salina Basin is similar to the oil-productive Forest City Basin in northeastern Kansas in many ways. Both basins originated as a single large basin (i.e., the North Kansas Basin) prior to the rise of the Nemaha Uplift in Late Mississippian-Early Pennsylvanian time. Their Paleozoic stratigraphy thus is similar and the axes of both basins are presently at approximately the same depth. Thermal maturation modeling and available organic-matter maturation data indicate that the lower Paleozoic rocks in the axes of both basins are in the early stages of oil generation. In the Forest City Basin the Ordovician Simpson Group is the deepest known hydrocarbon source-rock—oil-reservoir interval, and by analogy, exploration tests in the Salina Basin, at a minimum, should penetrate through this stratigraphic interval. Ordovician Simpson Group shales in the Forest City Basin are the source rocks for a geochemically distinct oil, which also occurs in Ordovician reservoirs in the extreme southern end of the Salina Basin. To increase the odds of success in an exploration program in the Salina Basin, wildcat wells should be drilled where thermal maturation is greatest. The broad NW–SE-trending basin axis is the most logical area. Exploration tests along this axis in the northern end of the basin may have an extra advantage as organic matter in the Simpson Group may be more thermally mature because of greater burial depth during the Cretaceous. Along the eastern margin of the nearby Central Kansas Uplift and Pratt Anticline, several Paleozoic geologic structures, some of which contain major oil fields, are attributable to tectonic reactivation along the western margin of the Precambrian Central North American Rift System (CNARS). Prospective structural trends in the Paleozoic section of the Salina Basin are anticipated to be associated with this underlying tectonic boundary. The western margin of the CNARS trends NNE–SSW where it passes under the axis of the Salina Basin in northeastern Lincoln and southeastern Mitchell counties. This area is sparsely drilled, with less than two tests per township. If an exploration program can define lower Paleozoic structural closures in this region, these structures may represent the best chance for future petroleum discoveries.

Chloroplast and nuclear gene sequences indicate late Pennsylvanian time for the last common ancestor of extant seed plants.

We have estimated the time for the last common ancestor of extant seed plants by using molecular clocks constructed from the sequences of the chloroplastic gene coding for the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (rbcL) and the nuclear gene coding for the small subunit of rRNA (Rrn18). Phylogenetic analyses of nucleotide sequences indicated that the earliest divergence of extant seed plants is likely represented by a split between conifer-cycad and angiosperm lineages. Relative-rate tests were used to assess homogeneity of substitution rates among lineages, and annual angiosperms were found to evolve at a faster rate than other taxa for rbcL and, thus, these sequences were excluded from construction of molecular clocks. Five distinct molecular clocks were calibrated using substitution rates for the two genes and four divergence times based on fossil and published molecular clock estimates. The five estimated times for the last common ancestor of extant seed plants were in agreement with one another, with an average of 285 million years and a range of 275-290 million years. This implies a substantially more recent ancestor of all extant seed plants than suggested by some theories of plant evolution.

Chloroplast and nuclear gene sequences indicate late Pennsylvanian time for the last common ancestor of extant seed plants.

Chloroplast and nuclear gene sequences indicate late Pennsylvanian time for the last common ancestor of extant seed plants.

Stable isotopes in Late Pennsylvanian brachiopods from the United States: Implications for Carboniferous paleoceanography

Stable isotopic analyses have been performed on nearly 500 nonluminescent brachiopod shells from Kansas, New Mexico, and Texas to evaluate temporal and geographic variability in the carbon and oxygen isotopic record for late Pennsylvanian time. Brachiopod specimens were collected from Missourian and Virgilian shales and examined in thin section for preservation of microstructure and absence of cathodoluminescence as a primary test for shell preservation. Regional variations are observed in the δ18O and δ13C values of nonluminescent brachiopod shells of the same genera. Average δ18O values for the three genera analyzed ( Crurithyris , Composita , and Neospirifer ) are highest in Kansas (≈-1.9‰), intermediate in Texas (≈-2.3‰), and lowest in New Mexico (≈-3.6‰). Vital effect on the δ18O of these genera appears minimal. δ18O data and other evidence suggest warmer temperatures for the shallow New Mexico localities and slightly higher salinity for the Kansas sea relative to the Texas sea. The δ13C values of Composita average about 1‰ higher than those of co-occurring Crurithyris and Neospirifer , suggesting microhabitat differences or vital effects. Preservation of this species effect argues for preservation of original δ13C values. Average δ13C values are highest in Texas, intermediate in Kansas, and lowest in New Mexico. Although these values range from 2.6‰ to 4.9‰, for individual genera the regional variation in δ13C averages less than 1‰. Data for nonluminescent brachiopods and marine cements reveal a mid-Carboniferous δ13C increase of 2‰ in Paleotethyan sea water. This increase is not seen in samples from the North American epicontinental seas, which opened to the Panthalassa ocean. This regional difference in δ13C appears to be due to changes in ocean circulation associated with the closing of the equatorial seaway and formation of Pangea.

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

Thrust-ridge paleodepositional model for the Upper Freeport coal bed and associated clastic facies, Upper Potomac coal field, Appalachian Basin, U.S.A.

Two differential depositional sequences are recognized within a 37-m-thick lowermost section of the Conemaugh Group of Late Pennsylvanian (Westphalian D) age in the southern part of the Upper Potomac coal field (panhandle of Maryland and adjacent West Virginia). The first sequence is dominated by the Upper Freeport coal bed and zone (UF); the UF consists of a complex of interfingered thick coal beds and mudrocks. The UF underlies the entire 500 km 2 study area (approximately 40 km in a NE-SW direction). The second sequence is dominated by medium- to coarse-grained sandstone and pebbly sandstone. They were deposited in channel belts that cut into and interfingered laterally with mudrock and fine- to medium-grained sandstone facies of floodbasin and crevasse-lobe origin. Thin lenticular coals occur in the second sequence. Nowhere in the study area does coarse-grained sandstone similar to the sandstone of the channel belts of the second sequence occur within the UF. However, 20 km north of the study area, coarse channel belts are found that are apparently synchronous with the UF (Lyons et al., 1984). The southeastern margin of the study are is bounded by the Allegheny Front. Between it and the North Mountain thrust (75 km to the southeast), lie at least eight other thrusts of unknown extent (Wilson, 1887). All these thrusts are oriented northwest; Devonian and older strata are exposed at the surface between the Allegheny Front and the North Mountain thrust. A blind-thrust ridge model is proposed to explain the relation of the two markedly depositional sequences to the thrusts that lie to the southeast of the Upper Potomac coal field. This model indicates that thrust ridges diverted coarse clastics from entering the swamp during a period when the thick Upper Freeport peat accumulated. Anticlinal thrust ridges and associated depressions are envisioned to have developed parallel to the Appalachian orogen during Middle and early Late Pennsylvanian time. A blind thrust developed from one of the outboard ridges, and it was thrust farther outboard ahead of the main body of the orogen. Sediment derived from the orogen was diverted into a sediment trap inboard of the ridge (Fig. 1). The ridge prevented sediment from entering the main peat-forming swamp. Sediment shed from the orogen accumulated in the sediment trap was carried out of the ends of the trap by steams that occupied the shear zone at the ends of the blind-thrust ridge (Fig. 1). Remnants of blind-thrust ridges occurs in the Sequatchie Valley thrust and the Pine Mountain thrust of the southern Appalachians. The extent, parallel to the orogen, of the thick areally extensive UF coal is related to the length of the blind-thrust ridge that, in turn, controlled the spacing of the river-derived coarse clastics that entered the main basin from the east. Further tectonism caused the thrust plane to emerge to the surface of the blind-thrust ridge. Peat accumulation was then terminated by the rapid erosion of the blind-thrust ridge and by the release of trapped sediment behind it. The peat was buried by sediments from streams from closely spaced channel belts] with intervening floodbasins. The model was implications for widespread peat (coal) deposits that developed in tropical regions, a few hundred kilometers inland from the sea during Pennsylvanian time (Belt and Lyons, 1989).

Sea-level control of mixed-carbonate-siliciclastic, gravity-flow deposition: Lower part of the Keeler Canyon Formation (Pennsylvanian), southeastern California

Depositional cycles present in a mixed carbonate and quartz gravity-flow sequence exposed in the Death Valley-Owens Valley of southeastern California may record the response of this system to third-order (∼0.7-1.4 m.y.) sea-level changes during Late Pennsylvanian time. Time-equivalent outer-shelf and basin-margin deposits elucidate the following key elements of this depositional system: (1) a distally steepened ramp profile, (2) an unchanneled basin-margin sequence which accumulated as a line-sourced apron system, and (3) penecontemporaneous deposition of carbonate and fine-grained quartz sediment in both the shallow- and deep-water regimes. Basin-margin facies consist of mudstones, turbidites, debris flows, and megabreccias organized into thickening- and coarsening-upward packages. Lime mudstone intervals (3-7 m thick) alternate with mixed-carbonate-siliciclastic, gravity-flow intervals (10-30 m thick). Time-equivalent outer-ramp facies show a similar organization of alternating silty, bioclastic wackestones/grainstones (10-40 m thick) containing a diverse open-marine fauna and rhythmically bedded lime mudstone (15-60 m thick) which is barren of fauna. An upward increase in debris flows and megabreccias within basin-margin cycles indicates the involvement of large-scale ramp processes and cannot be explained by simple progradation and abandonment of depositional lobes. The absence of megabreccia caps on many of the cycles argues against autocyclic progradation and collapse of the platform, although larger scale coarsening- and thickening-upward trends within the basin-margin succession are attributed to autocyclic processes. Instead, we propose that cycles develop in both the shallow- and deep-water regimes by changes in relative sea level. Drowning of the outer shelf during relative highstands (rhythmically bedded lime mud stone) would yield starved-basin deposits (lime mudstone). As sea level falls, both carbonates and siliciclastics prograde closer to the shelf edge (silty, bioclastic wackestones/grainstones), thus activating the mass-transport system (mixed-clastic, gravity-flow packages). Increased storm and wave influence on the outer ramp/upper slope triggers debris flows and occasional large-scale collapses, producing megabreccias. Most models of turbidite deposition have stressed reciprocal sedimentation with sea-level change: bioclastic turbidites during highstands and silicilclastic turbidites during lowstands. Instead, our model for mixed-clastic deposition emphasizes the importance of the build-up profile, the position of sea level relative to the shelf margin, and the rate of change of sea level.

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

Paleozoic evolution of southern margin of Permian basin

Prior to Late Pennsylvanian time, the Permian basin of West Texas was a part of the southern margin of the North American craton and was the site of shallow, warm-water carbonate deposition. As the South American margin of Gondwana gradually moved northwestward during late Paleozoic time, the oceanic floor deposits and overlying turbidite fan deposits between North America and South America became parts of large accretionary wedges of semiconsolidated sediments. In a series of steps, these wedges were folded and thrust toward the northwest against, then finally onto, the southern margin of North America. These compressive steps caused the repeated piling up of the accretionary wedges to form rapidly eroding highlands on the cratonic margins. Repeated loading on the North American margin formed a series of elongate, deep-water, depositional troughs (fore-deeps) immediately north and west of the accretionary wedges, and these received thick accumulations of turbidites. The youngest of these deep basins include the northwestern part of the Val Verde basin to the north and the Marfa basin to the west which received basinal deposits from within Desmoinesian into early Late Permian time. Loading of the cratonic margin also resulted in renewed faulting along generally northwest- to north-trending older zones of weakness. These zones are apparently of Precambrian age. On the craton, the late Paleozoic faults mainly had high-angle to vertical fault planes and commonly had large components of horizontal displacement. This fault system outlines the major uplifts and basins in the Permian basin region. The joining together of Gondwana and Euramerica across the Marathon salient of the orogenic belt was essentially completed by mid-Wolfcampian time. After that time, the southern margin of the Permian basin, represented by the Dugout and Marathon allochthons and the Diablo Platform, became a relatively stable tectonic area. The Glass Mountains expose a series of north- and northwest-dipping cuestas of Permian carbonate shelf and shelf-edge deposits that accumulated on a platform constructed from these two allochthonous accretionary wedges. These Permian deposits prograded north and northwestward into the narrow Hovey Channel, which connected the Marfa basin with the southern end of the Delaware basin. Carbonate-reef growth finally restricted the inflow of marine water into the Delaware basin near the end of late Guadalupian time.

Pennsylvanian deposystems and paleoclimates of the Appalachians

Central Appalachian basin rocks indicate major changes in source areas, base level, and climate for Late Mississipian through Late Pennsylvanian time. Through time, dominant source areas change from craton and northeastern orogen to southeastern orogenic mountains. The Mississipian—Pennsylvanian uncomformity correlates with a worldwide eustatic event. Early Pennsylvanian rocks contrast markedly with underlying Mississippian Mauch Chunk redbeds in composition and texture yet the source areas and alluvial plain setting were similar. Seas transgressed northeastward during Middle and earliest Late Pennsylvanian time along axes of the Pocahontas and Dunkard basins while fluvial-dominant deltas prograded northwestward. This progradation (1) progressively extended the coastal plain, (2) provided coastal swamp environments for peat accumulation, and (3) eventually cut off the narrow seaway of Dunkard basin from the Central Interior by Late Pennsylvanian and transformed it into a large shallow freshwater lake. Redbeds, calcium carbonate concretions and gilgai paleosol structures of Late Mississippian and Late Pennsylvanian rocks suggest seasonally dry tropical climate whereas the lithologies of the earlier Pennsylvanian rocks are indications of everwet tropical climates with their quartz-rich fluvial sandstones, high quality coals, and aluminum-rich clay deposits. Streams change from predominantly bedload to suspended load generally from Early to Late Pennsylvanian, in response to changes in paleoslope gradient, distance from source area, unvegetated highland, and change from everwet to intermittently dry tropical climate. Greater dryness is suggested for increased alkaline conditions observed in paleosols, carbonates, and coal beds of Late Pennsylvanian time, but sufficiently wet to prevent evaporite deposition.

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------------

A Fresh Look at the Marfa Basin, West Texas: ABSTRACT

The lack of production in the Marfa basin remains an enigma. Although covered with approximately 3,000 ft of thick volcanics, the basin has Paleozoic stratigraphy and lithologies similar to the prolific Delaware basin to the east. In a geologic and geophysical review, we analyzed the overall hydrocarbon potential for this basin. The lower Paleozoic (Cambrian through Devonian) rocks, which are conducive to hydrocarbon generation in other Permian basins, have many undrilled structures, as revealed by seismic mapping, but have also pervasive fresh water and dead oil. Several fault systems are mapped on the surface. We believe that the rocks were uplifted in Late Mississippian and Pennsylvanian time and in certain areas were exposed to meteoric water. Hydrocarbons leaked out along with induction of fresh water into the formations. Recharge areas have been identified from isopach maps. In certain parts, the lower Paleozoic rocks are overmature and offer further obstacles to exploration. During the Early Permian, arkosic sediments were deposited in a rapidly subsiding east-west graben. Tilted to the south, the graben accumulated as much as approximately 9,000 ft of Lower Permian sediments that have occasional hydrocarbon shows. Small stratigraphic traps due to pinchouts, truncations, and facies changes are to be expected in these rocks. Possible reef growths in the Late Permian offer further opportunities for trap development. The rocks were also exposed to meteoric water during nondeposition in the Triassic, Jurassic, and post-Cretaceous periods. However, the subtle Permian traps, being small, could have escaped destruction and freshwater flushing and thus might offer some potential for future exploration. End_of_Article - Last_Page 141------------

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------------