Amarillo Uplift Research Articles

Detrital U-Pb zircon and 40Ar/39Ar muscovite geochronology from Middle Pennsylvanian strata in the Anadarko Basin, Texas Panhandle, USA

The Late Mississippian-Permian Anadarko Basin formed in Texas and Oklahoma, USA as the result of inversion of Neoproterozic and Cambrian rift structures. Subsidence was driven by flexural loading of the Amarillo-Wichita Uplift, and this uplift may represent the easternmost element of the Ancestral Rocky Mountains system. The northwestern part of this basin has generally been interpreted to have been filled by sediment derived from the Ancestral Front Range Uplift, ~475 km to the northwest during the early stages of basin filling. We test this model using U-Pb detrital zircon and 40Ar/39Ar detrital muscovite results from three subsurface samples of the Morrow B sandstone in the northwestern part of the Anadarko Basin. We provide a new maximum depositional age of 310.9 ± 4.9 Ma that indicates the age of the Morrow B to be no older than late Atokan to early Desmoinesian Age (Moscovian), ~10 Myr younger than previously interpreted. In contrast to some previous interpretations, we propose that the most likely source for the sediment in the Morrow B is the Amarillo Uplift to the south. Detrital zircon and detrital muscovite data have age peaks at 900–1300 Ma, 1370 Ma and 1600–1800 Ma corresponding to derivation from Grenville, Granite-Rhyolite and Yavapai-Mazatzal basement provinces, respectively. A dominant detrital zircon peak at 1370 Ma suggests that Mesoproterozoic granites in the Amarillo Uplift were exposed by Middle Pennsylvanian time, and detritus eroded from the Amarillo Uplift dominated the lowstand sediment budget of the Texas Panhandle during this time; small volumes of sediment were likely sourced from the Ancestral Front Range. This study presents the first detrital geochronology data from the subsurface Anadarko Basin and the first detrital muscovite data from late Paleozoic southwestern Laurentia. The results presented here highlight the interpretive power of combined detrital zircon and muscovite datasets.

Origin of helium and nitrogen in the Panhandle–Hugoton field of Texas, Oklahoma, and Kansas, United States

Helium and nitrogen variations in Panhandle–Hugoton field (PHF) gases are products of interaction between hydrocarbon gas from the Anadarko basin and at least two water masses with dissolved nitrogen and helium. The two most distinct water masses are from the Palo Duro basin (highest He/N2) and the Hugoton embayment (lowest He/N2). Geochemical data indicate several hundred million years of helium generation in porous rock. Helium migrated to the gas by diffusion through water-saturated rock and by west-to-east water flow. Sediment and basement helium generation and helium migration were modeled to validate timing and source of PHF helium. Models indicate a predominantly sedimentary helium source with some basement helium charge on the Amarillo uplift. Helium in the central and eastern PHF diffused from underlying rocks, whereas gases on the west and southwest sides were enriched in nitrogen and helium delivered by hydrodynamic water flow. Nitrogen in high-nitrogen gases was probably sourced as ammonium released from clays by cation exchange with brines derived from overlying salt units. The amount of mudrock (nitrogen and helium source) relative to other potential helium sources (arkose, radioactive dolomite) correlates to decreasing gas He/N2. The high helium concentrations in PHF gases result from multiple favorable circumstances. Old pore water accumulated dissolved helium during hundreds of millions of years of helium generation in sediment. High water/gas and low pressure favored higher helium concentrations in gas. Hydrodynamic flow delivered helium-rich pore water from basins west of the PHF.

A dynamic model for the Permian Panhandle and Hugoton fields, western Anadarko basin

Panhandle-Hugoton, the largest North American gas field, has long been controversial because of extreme subnormal pressures, variable gas composition, and tilted fluid contacts, commonly attributed to hydrodynamic flow despite the absence of an effective updip aquifer. These anomalies are addressed in terms of a basin-scale petroleum system history, largely independent of the geographically underlying pre-Permian system.Hydrocarbons were already being generated in the deep Anadarko basin during the Early Permian, with efficient southward migration from all potential source rocks via bounding faults and Pennsylvanian–Permian alluvial fans. Giant Amarillo uplift drape structures trapped hydrocarbons immediately following Permian evaporite deposition. The pre-Laramide Panhandle field, at maximum burial depth and pressure, contained most of the oil and gas now found in mid-continent Permian reservoirs.The early Tertiary Laramide orogeny redistributed Panhandle field fluid columns, possibly spilling gas into the Hugoton embayment. Subsequent erosion of Permian reservoir facies in eastern Kansas allowed water discharge to outcrops at elevations below the regional hydraulic head. As regional pressure dropped in response, the Panhandle field gas cap expanded rapidly, forcing a late Tertiary–Quaternary mass movement of gas northward to fill Hugoton and associated fields.Panhandle-Hugoton pressures, upon discovery, were subnormal relative to drilling depth but normal relative to reservoir outcrop elevations in eastern Kansas, indicating that pressures are controlled by aquifer communication with the surface instead of burial depth. Variations in fluid contacts, pressure, and gas composition suggest that reservoir fluids are still moving, driven by decompression and the rapid volumetric expansion of a supergiant gas accumulation.

Controls on the regional-scale salinization of the Ogallala aquifer, Southern High Plains, Texas, USA

An extensive saline plume (>250 km 2) within the regionally important unconfined aquifer in the Neogene Ogallala Formation overlies the Panhandle oil and gas field in the Southern High Plains, Texas, USA. Relative to upgradient Ogallala water, the plume waters have δ 18O (−6.7 to −8.8‰) and δD (−42 to −88‰) values that tend to be depleted and have higher Cl (>150 mg/l) and SO 4 (>75 mg/l) concentrations. Various end-member-mixing models suggest that the plume composition reflects the presence of paleowaters recharged during Middle to Late Wisconsinan time rather than salinization associated with petroleum production. Paleowaters probably mixed with salt-dissolution zone waters from the underlying Upper Permian formations before discharging upward into the Ogallala Formation. Cross-formational discharge is controlled primarily by the geometry of the underlying units, as influenced by the Amarillo uplift, pinch-out of the laterally adjoining confined aquifer in the Triassic Dockum Group, variations in the saturated thickness of the Ogallala aquifer and the presence of potential pathways related to salt dissolution.

Late Miocene reactivation of Ancestral Rocky Mountain structures in the Texas Panhandle: A response to Basin and Range extension

Research Article| February 01, 1987 Late Miocene reactivation of Ancestral Rocky Mountain structures in the Texas Panhandle: A response to Basin and Range extension Roy T. Budnik Roy T. Budnik 1Bureau of Economic Geology, University of Texas, University Station Box X, Austin, Texas 78713 Search for other works by this author on: GSW Google Scholar Author and Article Information Roy T. Budnik 1Bureau of Economic Geology, University of Texas, University Station Box X, Austin, Texas 78713 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1987) 15 (2): 163–166. https://doi.org/10.1130/0091-7613(1987)15<163:LMROAR>2.0.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Roy T. Budnik; Late Miocene reactivation of Ancestral Rocky Mountain structures in the Texas Panhandle: A response to Basin and Range extension. Geology 1987;; 15 (2): 163–166. doi: https://doi.org/10.1130/0091-7613(1987)15<163:LMROAR>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Structural and stratigraphic evidence from the Ogallala Formation (Neogene) documents late Miocene tectonic activity within the Great Plains. Field and subsurface studies in the Texas Panhandle indicate that parts of the Amarillo uplift, a major element of the Pennsylvanian Ancestral Rocky Mountains, were elevated as much as 150 m during initial deposition of the Ogallala Formation. Reactivation of these basement structures occurred in response to Basin and Range extension and opening of the Rio Grande rift in central New Mexico and Colorado. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geology of McMordie and Hale Ranch Fields, Roberts County, Texas: ABSTRACT

The McMordie Ranch and Hale Ranch fields produce from multiple stratigraphic traps representing several depositional environments. The fields, located in the southwestern Anadarko basin northeast of the Amarillo uplift, are traversed by the prominent northwest-southeast-trending Lips fault. Major down-to-the-northeast displacement along this fault during the Early Pennsylvanian affected deposition of the lower and middle Morrow. The McMordie Ranch field produces from multiple stratigraphic traps in the lower and upper Morrow, Cherokee granite wash, and Douglas sandstones on the downthrown side of the Lips fault. The producing lower Morrow sandstone, which occurs only on the downthrown side, reaches a maximum net-pay thickness of 34 ft (10.4 m). This sand facies appears to be a beach or offshore bar, 1 mi (1.6 km) wide and 2 mi (3.2 km) long, deposited parallel to a shoreline controlled by the Lips fault. In the upper Morrow, two distinct reservoir facies are recognized--point bars and stream mouth bars, which were deposited on a southeast-sloping delta plain. The lower Cherokee Group contains two productive granite wash sandstones, which were deposited near the distal north edge of a fan-delta complex sourced from the Amarillo Mountain uplift. The productive sections are each 8-14 ft (2.4-4.3 m) thick with variable porosity development. The shallowest reservoir in the field is the lower Douglas sandstone, an east-west-trending channel sandstone, 10-15 ft (3-4.5 m) thick and approximately 0.5 mi (0.8 km) wide. The one-well Hale Ranch field produces from the Lower Mississippian Kinderhook sandstone in a fault block on the upthrown side of the Lips fault. This 16-ft (5-m) thick sandstone averaging 10% porosity is interpreted as a northwest-southeast-trending barrier bar encased in shale. Characteristically, this sand shows an excellent SP deflection, despite a high gamma-ray reading due to the thorium content in the sand. End_of_Article - Last_Page 1316------------

Meteoric Burial Diagenesis of Pennsylvanian Arkosic Sandstones, Southwestern Anadarko Basin, Texas

Pennsylvanian arkosic sandstones at Mobeetie field were deposited in fan deltas that prograded onto a shallow shelf in the southern Anadarko basin. The detrital minerals of the sandstones reflect the composition of the Precambrian granites and granodiorites that were exposed in the nearby Amarillo uplift. Distal margins of some fan-delta lobes were reworked by marine processes, and carbonate fossil fragments and oolites were mixed with terrigenous clastics. The diagenetic history and, hence, reservoir quality of the distal, marine-reworked sandstones differ from those of the more proximal, nonreworked sandstones. The earliest diagenetic events--formation of chlorite ooids and precipitation of fibrous submarine Mg-calcite cement--took place in the depositional environment of the marine-reworked sandstones. On the basis of isotopic data, the remainder of the diagenesis evidently occurred in fluids of meteoric origin. Shallow, early meteoric diagenesis induced precipitation of iron-poor calcite spar in reworked sandstones; nonreworked sandstones lack calcite spar but contain early pore-lining chlorite cement. Dissolution of aragonite by fresh meteoric ground water created moldic porosity within reworked sandstones. As burial increased, porosity in both the reworked and nonreworked fan-delta sandstones was reduced by precipitation of authigenic quartz, feldspar, kaolinite, Fe-calcite, and ankerite. Oxygen isotope data suggest that these cements also precipitated from fluids of meteoric origin that became increasingly hot but remained relatively constant in isotopic composition. The last diagenetic events were influenced by the overlying Permian evaporites. Sodium-rich fluids caused partial albitization of detrital plagioclase, and sulfate derived from the evaporites precipitated as anhydrite and celestite.

Historical Seismicity of Texas--A Summary: ABSTRACT

A survey of the historical seismicity of Texas revealed more than 100 earthquakes of magnitude 3 or greater from February 1847 to December 1984. Of these, 82 earthquakes are known to have been felt, and 24 events are associated with reports of damage. The largest earthquake (magnitude = 5.9) occurred on August 16, 1931, near the town of Valentine in west Texas. Many buildings were damaged in the epicentral area, and the total felt area exceeded 1 million km2. Earthquakes in the Valentine area, as well as other events farther west near El Paso, are probably related to the Rio Grande rift system. Several large events in the Texas Panhandle may be associated with a zone of crustal weakness that follows the trend of the Amarillo uplift. Earthquakes in eastern Texas are associated with several fault systems in the Gulf coastal plain. A large earthquake on October 22, 1882, had previously been located in northeast Texas near the town of Paris; however, a reevaluation of the intensities suggests the epicenter was probably farther north in Oklahoma. Several other events in previous catalogs have been discarded or relocated. More recent Texas earthquakes may have been induced by oil field operations. Seismic activity in west Texas near the towns of Kermit and Snyder are probably caused by waterflooding projects. Several recent earthquakes in the Gulf coastal plain may be associated with fluid withdrawal from oil and gas fields in up-to-the-coast normal faults. End_of_Article - Last_Page 1419------------

Basin and Range-Age Reactivation of the Ancestral Rocky Mountains in Texas Panhandle: Evidence from Ogallala Formation: ABSTRACT

The Ogallala Formation (Neogene) is a widespread syntectonic alluvial apron that was shed eastward from the Rio Grande rift and related uplifts in Colorado and New Mexico during Basin and Range extension. In the Texas Panhandle, the Ogallala completely buried Ancestral Rocky Mountain (Pennsylvanian) structures. Renewed movement on these older structures during the Neogene influenced the thickness and facies distribution of the Ogallala. The Ogallala thickens into the Palo Duro, Dalhart, and Anadarko basins. Major distributary channels on Ogallala alluvial fans coincide with the axes of these basins, whereas major interchannel areas overlie intervening uplifts. Second-order structures subtly influenced the unit as well. For example, in the Carson basin, a Pennsylvanian rhomb graben along the Amarillo uplift, the Ogallala is over 250 m (820 ft) thick compared with 90 m (275 ft) in adjacent areas. Within the Palo Duro basin, local highs controlled the distribution of thin, interchannel fl od-basin and lacustrine deposits. Thicker, braided-stream channel deposits follow local lows. Later movement on the Amarillo uplift broadly folded the Ogallala. The southern high plains surface subtly reflects basement structure, with topographic highs overlying basement highs, suggesting post-Ogallala deformation within the Palo Duro basin. The Amarillo uplift is approximately perpendicular to the Rio Grande rift and parallel to the direction of Basin and Range extension. Thus, the stress field that produced the rift may have caused strike-slip movement and reactivation of the Carson basin along the Amarillo uplift. End_of_Article - Last_Page 459------------

Cenozoic Epeirogenic Uplift of Palo Duro Basin, Texas, and Its Influence on Structure, Salt Dissolution, and Topography: ABSTRACT

Sufficient data are available to interpret a general history of Cenozoic epeirogenic uplift and its influence on structure, salt dissolution, and topography in the Palo Duro basin. Much of the structural warping and deformation of Middle and Upper Permian rocks in the Palo Duro basin occurred during Cenozoic epeirogenic uplift. Cretaceous marine strata in the Texas and Oklahoma Panhandles and eastern New Mexico were uplifted 3,000-4,000 ft (914-1,219 m). The Tubb Sand (Permian) exhibits about 4,000 ft (1,219 m) of structural relief over the Amarillo uplift and Bravo dome. Differential uplift of the margins of the basin caused draping, fracturing, and faulting, which increased the amounts and rates of erosion and salt dissolution coincident with fault-bounded structures. Structural control of topography around the southern high plains is indicated by the coincidence of the Caprock escarpment and structural highs in Permian rocks, as well as the coincidence of many stream segments and segments of the Caprock escarpment with subsurface fault trends. End_of_Article - Last_Page 505------------

Interpretation of Underpressuring Using Pressure-Depth Profiles, Palo Duro Basin, Texas: ABSTRACT

Investigations of underpressured basins can be improved by identifying factors which impede accurate interpretation of pressure-depth (P-D) plots. The majority of P-D data from drill-stem tests are poor quality and are biased in distribution toward exploration and production; thus, true P-D trends, are obscured. Plotted P-D data may not only reflect flow conditions and underpressuring, but they can also be distorted by the hydrogeologic setting, i.e., surface topography, basin structure, and potentiometric surface. For example, surface topography can cause plotted data to appear more underpressured than it really is. Varying surface topography in conjunction with basin structure can cause P-D gradients to erroneously indicate potential cross-formation flow. If the effects of the hydrogeologic setting are not identified, P-D trends may be misinterpreted. In the Palo Duro basin, the problems of the hydrogeologic setting are reduced by delineating regions with little variation in surface topography and structure. The P-D plots for these different regions vary considerably. To evaluate the effects of the hydrogeologic setting on these P-D plots, theoretical models of hydrostatic conditions were computed for the hydrogeologic setting of each region. Comparison of regression lines through real P-D data to those through modeled data distinguishes true hydrologic conditions from the misleading effects of the hydrogeologic setting. P-D plots indicate regional underpressuring in Palo Duro basin. Variations in potential vertical flow within the underpressured basin are indicated by variations in P-D gradients across the basin. Potential upward flow is indicated along the western flanks of the Amarillo uplift, while potential downward flow is indicated in the southwestern and eastern parts of the basin. End_of_Article - Last_Page 514------------

Tectonic History and Influence on Sedimentation of Rhomb Horsts and Grabens Associated with Amarillo Uplift, Texas Panhandle: ABSTRACT

The Amarillo uplift consists of an en echelon series of fault blocks separating the Anadarko basin from the Palo Duro basin. The uplift is part of a northwest-southeast zone of basement weakness that extends from the Wichita Mountains in Oklahoma to southeastern Colorado. Initial faulting, related to the opening of the southern Oklahoma aulacogen, took place from late Precambrian through Middle Cambrian time. Renewed movement in the Late Mississippian or Early Pennsylvanian, probably of a left-lateral transcurrent nature, broke the Amarillo uplift into a series of rhomb grabens and rhomb horsts. The Lefors basin, for example, in Gray County is a small rhomb graben 4 mi (6.4 km) by 8 mi (12.8 km) that contains in excess of 4,000 ft (1,200 m) of Pennsylvanian and Wolfcampian arkose (granite wash). The Amarillo uplift continued to subtly affect depositional patterns following its burial in Wolfcampian time. Salt beds in the Clear Fork Formation (Leonardian) are purer and thicker in grabens where salt deposition proceeded at a faster rate relative to horsts. Recurrent motion on the Potter County fault in northern Potter and northeastern Oldham County produced cumulative displacements of 1,600 ft (488 m) on top of the Pennsylvanian, 800 ft (244 m) on Wolfcampian strata, 600 ft (183 m) on top of the Clear Fork Formation, and 450 ft (137 m) on the Dockum Group (Triassic). Post-Permian displacements are the result of both salt dissolution and minor structural movement. There is no direct evidence for Quaternary faulting, although the uplift is seismically active. End_of_Article - Last_Page 511------------

Palo Duro Basin, An Exploration Frontier in the Texas Panhandle: ABSTRACT

Recent oil discoveries in the northwestern and central Palo Duro basin have renewed interest in this sparsely drilled area. Production in the northwest, in Oldham and Potter Counties, is from Pennsylvanian granite wash and carbonates. These fields are located in a highly faulted area south of the Amarillo uplift, and traps are structural. Discovery wells here produce oil at rates that range from 150 to 650 bbl per day. Oil production in the central basin, in Briscoe County, is from Pennsylvanian carbonate. The reservoir is probably a Strawn shelf-margin buildup or a debris flow into the basin from a younger Pennsylvanian shelf margin. The Palo Duro basin seems to contain potential reservoirs, traps, and source rocks; thermal maturity is probably the limiting factor for hydrocarbon production in the basin. The current geothermal gradient is relatively low, 1.1°F/100 ft (20°C/km), and it apparently has not been significantly different in the past. Vitrinite reflectance (Ro) measured in cores increases linearly with depth (temperature) by the relation: Ro = 0.00003 × depth (ft) + 0.36. Ro values seem to be in equilibrium with current depths and temperatures of the vitrinite. This suggests that (1) rocks in the basin are at or near their maximum burial depth, and (2) the geothermal gradient was not higher in the past. Shales of different ages that are at approximately the same depth have similar vitrinite reflectance values, an indication that increased time did not cause increased maturity in these Paleozoic samples. Deeply buried shales, 7,000 to 9,000 ft (213 to 2,743 m), from Pennsylvanian and Wolfcampian basin facies theoretically should have reached temperatures sufficient to generate hydrocarbons. Recent discoveries provide evidence that oil actually was generated in the Palo Duro basin. End_of_Article - Last_Page 454------------

Pennsylvanian Fan-Delta and Carbonate Deposition, Mobeetie Field, Texas Panhandle

Fan deltas prograded repeatedly onto a shallow, carbonate-shelf environment in the Anadarko basin during Late Pennsylvanian (Missourian) time. These fan-delta sandstones were part of a thick wedge of coarse clastics which were derived from the Amarillo uplift. Similar fault-bounded basement uplifts found throughout the west-central United States at this time formed an intracratonic tectonic setting in which alluvial-fan and fan-delta sedimentation was common. Missourian reservoirs at Mobeetie field consist of three alternating limestone and sandstone units. Limestones accumulated along strike in elongate, phylloid-algal mounds that separated the open shelf from a more restricted lagoonal environment. Algal-mound cores consist of algae-foraminifer wackestone and packstone, and mounds are capped by coral boundstone and encrusting algae-foraminifer wackestone. Progradation of coarse-grained terrigenous clastics onto the carbonate shelf halted carbonate production. Braided streams breached low areas along the algal-mound trend and deposited fan-delta sediments on the shelf basinward of the mounds. Distal-fan-delta deposits in the two younger sandstones were reworked along strike into spits and bars. Carbonate deposition recommenced following w ning of clastic influx and subsidence of the fan-delta sediments. Algal mounds were reestablished preferentially on shallow platforms formed by terrigenous clastics of the previous cycle. The principal trapping mechanism in Mobeetie field is structural draping of Pennsylvanian beds over an underlying horst block, forming a doubly plunging anticline. However, within the area of closure, stratigraphy controls reservoir quality and production. Updip fluvial sandstones and undisturbed distal-fan-delta deposits have good porosity and permeability, and produce oil. Reworked fan-delta deposits are not productive, even within the area of closure, because of the loss of porosity by calcite cementation.

Pennsylvanian Fan-Delta Deposition, Mobeetie Field, Texas Panhandle: ABSTRACT

A prism of Pennsylvanian and Lower Permian arkosic sandstone and conglomerate (granite wash) was deposited in alluvial fans and fan deltas north of the Amarillo uplift in the Anadarko basin. Deposits adjacent to the main bounding fault are as much as 1,500 m thick but thin to 30 m within approximately 60 km of the uplift. Mobeetie field, in northwest Wheeler County, is located 16 km north of the basement uplift, at the northern limit of upper Missourian granite-wash sedimentation. Oil and gas are produced from a sequence of Missourian granite wash and interbedded carbonate rocks. The oldest Missourian carbonate unit and the overlying classic unit form a representative depositional cycle. In the carbonate unit, a series of elongate, phylloid-algal mounds composed of algae-foram wackestones and packstones developed along strike and separated the open shelf from a more restricted lagoonal environment. Progradation of coarse-grained clastics onto the carbonate shelf halted carbonate production. End_Page 922------------------------------ Braided streams breached low areas along the algal-mound trend and deposited distal fan-delta sediments on the shelf basinward of the mounds. Coarse conglomeratic sandstones were deposited directly on carbonates, but do not show a progradational, coarsening-upward sequence. Carbonate deposition recommenced following waning of clastic influx and subsidence of the fan delta. Algal mounds developed preferentially on shallow platforms built by the previous cycle of clastic deposition. Distal parts of the two youngest fan-delta cycles were reworked along strike, and terrigenous grains were oolitically coated in high-energy oolite shoals. End_of_Article - Last_Page 923------------

Promising Morrow Sandstone Discoveries in Southeastern Colorado: ABSTRACT

Eight new Morrow oil and gas fields have been discovered in southeastern Colorado during the past two years. The latest discovery resulted in a well flowing oil from a Morrow sandstone at an initial rate of 700 bbl of oil per day and 750 Mcf of gas per day. The trapping mechanism in this well, as in most, is a stratigraphic and structural combination. The depositional history of Morrow sandstones in southeastern Colorado was strongly influenced by regional tilt into the Anadarko basin, established at the close of Mississippian time. Mississippian rocks were subject to truncation in the area of the Las Animas arch and development of karst topography on the erosion surface. Transgression of the sea from the southeast in Morrowan time resulted in the deposition of a sandstone and shale sequence. The Morrow series consists of a basal transgressive sandstone member (Keyes sandstone) overlain by at least three sandstone units deposited during regressive pulses in an overall transgressive sequence. These Morrow sandstones generally trend from northwest to southeast toward the Anadarko basin; their source is inferred to have been the ancestral Rocky Mountain End_Page 672------------------------------ uplift for the upper sequence and the Amarillo uplift for the basal Keyes member. The environment of deposition appears to have been fluvial delta plains and associated tidal channels which were alternately created and destroyed by regression and transgression of the Morrowan seas. Distribution of these elongate sandstones is generally erratic and meandering; individual sand bodies have an estimated width of up to 1 mi (1.6 km) and a maximum thickness of 42 ft (12.8 m). The nature and distribution of these sandstones leave many areas essentially unexplored. Southeastern Colorado has had substantial Morrow discoveries which justify a closer look at the area. End_of_Article - Last_Page 673------------

Depositional Facies and Uranium Occurrence, Triassic (Dockum Group), Texas Panhandle: ABSTRACT

Late Triassic (Dockum Group) rocks accumulated in relict Paleozoic basins bound by the Amarillo uplift on the north and the Glass Mountains on the south. Basins were reactivated by late Paleozoic or early Mesozoic tectonic activity that created the Gulf of Mexico. More than 2,000 ft (600 m) of terrigenous clastics, derived mostly from older sedimentary rocks, accumulated within the basin. Source areas were in Texas, Oklahoma, and New Mexico. The Dockum Group accumulated in a variety of depositional systems including (1) braided and meandering streams, (2) alluvial fans and fan deltas, (3) highly constructive lobate deltas, (4) lacustrine systems including ephemeral and relatively long-lived lakes, and (5) mud flats. Dockum sedimentation was cyclic, a reflection of alternately humid and arid climatic conditions. During humid climatic conditions lake level was relatively stable. Meandering streams supplied sediment to high-constructive lobate deltas in the central basin area; braided streams and fan deltas were dominant depositional elements within southern and northern basin areas. During arid climatic conditions base level was lowered, stream valleys evolved, and small fan deltas developed along ephemeral lake margins; evaporites, calcretes, silcretes, and soils developed on floors of ephemeral lakes and on delta platforms. Uranium occurs within about 25 depositional facies. Highest uranium values are in lacustrine facies which developed under arid climatic conditions. Channel-lag facies of meander-belt systems generally exhibit consistently higher uranium values than other depositional facies. Crevasse-channel and crevasse-splay deposits locally contain mineralized carbonized wood. Delta-front sandstones of high-constructive lobate deltas contain uranium. Radioactive minerals are present within conglomeratic parts of the valley-fill sequence. Although a relation exists between uranium occurrence and depositional facies, prediction of uranium occurrence is difficult because of a complex groundwater history. End_of_Article - Last_Page 835------------

Depositional Systems and Petroleum Potential of Lower Permian Strata, Palo Duro Basin, Texas: ABSTRACT

Lower Permian (Wolfcampian) strata of the Palo Duro basin consist of thick, terrigenous clastic and carbonate facies which were deposited in (1) fan-delta, (2) high-constructional delta, (3) carbonate-shelf and shelf-margin, and (4) slope and basinal systems. Lateral and vertical facies sequences across the basin indicate that these strata are regressive deposits and they document the first episodes of Permian marine retreat from the Texas Panhandle. Terrigenous clastic sediment was derived from highlands which surrounded part of the Palo Duro basin. Exposed Precambrian granite in the Amarillo uplift, Sierra Grande uplift, and Bravo dome yielded large quantities of arkosic sand (granite wash) to fan-delta systems which emptied into shallow-marine environments in the northern part of the basin. Along the basin's southeastern margin, high-constructive deltas prograded westward from the Wichita Mountains, depositing quartz-rich sand and mud across the shelf. Seaward of the clastic facies belt, a carbonate-shelf-margin complex, averaging 1,000 to 1,200 ft (300 to 365 m) in thickness and facing south toward the Midland basin, dominated Wolfcampian deposition in the Palo Duro basin. The western shelf margin consists of a superposed sequence of carbonate strata exhibiting limited basinward progradation. Contrarily, the eastern shelf margin is composed of several superposed, progradational carbonate sequences, individually averaging several hundred feet in thickness. During early to middle Wolfcampian time, the eastern shelf margin prograded westward 10 to 30 mi (16 to 48 m) while the western margin remained stationary. Shelf margins shifted in response to deposition of slope sediments in front of the shelf and in feeder channels, creating a f undation for subsequent carbonate buildups. Potential hydrocarbon reservoirs are thick zones of shelf-margin dolomites, delta-front sandstones, and fan-delta arkoses. Porosity figures in those facies are as high as 10 to 20%. Each potential reservoir facies is juxtaposed with potential source beds and nonporous sealing beds. End_of_Article - Last_Page 1426------------

Exploration Potential of Pennsylvanian-Permian Carbonate-Shelf Margins and Deltaic Sandstones, Palo Duro Basin, Texas: ABSTRACT

Potential hydrocarbon reservoirs occur in Pennsylvanian and Lower Permian (Wolfcampian) carbonate-shelf margins, fan-delta arkosic sandstones, and deltaic sandstones in the Palo Duro basin. Thick basinal shales, which are stratigraphically equivalent to shelf carbonate rocks and sandstones, may have served as hydrocarbon source beds, although present thermal gradients are inadequate for liquid hydrocarbon generation. During the Pennsylvanian, a carbonate-shelf-margin complex with 200 to 400 ft (60 to 120 m) of depositional relief developed around a narrow embayment that opened southward into the Midland basin. The position of local shelf margins shifted through time. Following initial construction, shelf margins retreated shelfward throughout Pennsylvanian and earliest Permian time. During later Wolfcampian, shelves prograded westward and southward into the basin, filling it by late Wolfcampian time. Potential hydrocarbon reservoirs are thick zones of secondary porous dolomite within the shelf-margin complex. Dolomite porosity is commonly greater than 10%. The distribution of porous dolomite along shelf margins may indicate dolomitization was related to (1) early postdepositional, mixing-zone diagenesis in islands present along the shelf margin, or (2) dewatering of basinal shale, leading to montmorillonite-illite conversion and release of magnesium during burial diagenesis. Pennsylvanian and Permian fan-delta deposits of arkosic sandstone (granite wash) shed off the Amarillo uplift are potential clastic reservoirs. Production occurs from stratigraphically equivalent, granite wash deposits on the northern side of the uplift in the Anadarko basin. Porosity in granite wash sandstones averages 15%. Pennsylvanian quartzose sandstones are also interbedded with thick sequences of basinal shale, suggesting that the sand entered the basin through passes in the shelf margin. Geometry of sandstone bodies suggests deposition as distributary-mouth-bar fingers of high-constructive elongate deltas. Porosity in these sandstones reaches 18%. End_of_Article - Last_Page 461------------

Lower Permian Facies Tracts and Evolution of Carbonate-Shelf Margins, Palo Duro Basin, Texas Panhandle: ABSTRACT

Lower Permian (Wolfcampian) strata of the Palo Duro basin consist of 1,000 to 2,000 ft (300 to 600 m) of terrigenous clastic and carbonate sediments which were deposited in basin, slope, shelf-margin, shelf, and deltaic environments. Lateral and vertical sequences of facies throughout the basin indicate that these strata are regressive and document the first episodes of Permian marine retreat from the Panhandle region of Texas. Terrigenous clastic sediment was derived from highlands which surrounded part of the Palo Duro basin. Exposed Precambrian granite in the Amarillo uplift, Sierra Grande uplift, and Bravo dome yielded large quantities of arkosic sand (granite wash) to fan-delta systems which emptied into shallow-marine environments in the northern part of the basin. Along the basin's southeastern margin, high-constructive deltas prograded westward from the Wichita Mountains depositing quartz-rich sand and mud across the shelf. Seaward of the clastic-facies belt, an arcuate, carbonate-shelf-margin complex, averaging 1,000 to 1,200 ft (300 to 360 m) in thickness and facing south toward the Midland basin, dominated Wolfcampian deposition. The western shelf margin consists of a superposed sequence of carbonate strata exhibiting limited basinward progradation. In contrast, the eastern shelf margin is composed of several superposed, progradational carbonate sequences, individually averaging several hundred feet in thickness. During early to middle Wolfcampian time, the eastern shelf margin prograded westward 10 to 30 mi (16 to 48 km) but the western margin remained stationary. Shelf margins shifted in response to deposition of slope sediments in front of the shelf, creating a foundation for subsequent carbonate b ildups. Slope deposits consist of (1) hemipelagic mud, (2) fine-grained clastic sediments transported downslope End_Page 461------------------------------ through submarine canyons incised in the shelf margin, and (3) redeposited carbonate sediment derived from shelf-margin buildups. End_of_Article - Last_Page 462------------