Abstract: The “Wolfcamp Shale” of the Permian Basin in west Texas is a prime source of oil reserves within the USA. Despite its growing importance, definitive ages and stratigraphic correlations across the basin remain unclear. Fusulinid studies have assigned an approximate age of Late Pennsylvanian (Missourian) to early Permian (Leonardian) for the “Wolfcamp Shale,” but exact horizons for regional stage boundaries are not well defined. Analysis of conodont specimens from three cored basinal shale intervals of the Shell Stevens L1V well in the northern part of the Midland Basin (Lynn County, Texas) improves interpretation of regional stage boundaries within the “Wolfcamp Shale” and provides an accurate means of correlation to depositional cycles on the Eastern Shelf in central Texas. Conodont biostratigraphic markers were compared to log correlations and sequence stratigraphic correlations. Associated gamma-ray (GR) and resistivity (RES) logs show distinct log markers for the Pennsylvanian–Permian boundary and base of the Leonardian Stage that are traceable to immediately adjacent wells and possibly to portions of the southern part of the Midland Basin. Third- and fourth-order sequence stratigraphic cycles were delineated using core lithology, conodont abundances, and gamma-ray and resistivity readings. These sequences are linked to those observed in the Eastern Shelf stratigraphic sections and provide a useful secondary mode of correlation.

Abstract Lower Permian (Wolfcampian) carbonate platform margin depositional profiles and stratigraphic architecture were controlled primarily by the type of reef community that characterized the time, peak icehouse glacioeustatic sea-level fluctuations, and regional tectonic pulses from the paleocontinental collision along the southern margin of the basin. During transgressions, mound-shaped reef cores began growth in sub-wavebase, low-light (oligophotic), upper slope settings. The reef mound communities quickly diversified, and the reef mounds aggraded and shallowed upward through the euphotic zone, eventually reaching wavebase during early highstand, and graded upward into progradational crestal shelf-margin shoals during late highstand. Reef mound growth seaward of and downdip from the crestal platform margin shoals resulted in distally-steepened platform margin profiles, which persisted throughout Permian time. Wolfcampian reef mound facies are composed of bafflestones-boundstones with frameworks of phylloid algae, calcisponges and heliosponges, and fenestrate and ramose bryozoans, which were encrusted by Tubiphytes, laminar encrusting red algae ( Archaeolithoporella ), fistuliporid bryozoans, and microbialite. Peloidal cement matrices are common. Syndepositional to early marine botryoidal radial fibrous cements are generally sparse but are more common in windward margin buildups. Reef mound cores are surrounded by shallowing-upward skeletal packstone to grainstone flank beds. Crestal shelf-margin shoals are composed of medium-to coarse-grained grainstones with fusulinids, Tubiphytes, and robust dasycladacean algae, and contain small Tubiphytes patch reefs. Early Permian icehouse glacioeustatic sea-level fluctuations and episodic tectonic pulses created erosional paleotopography on unconformities near the base of the Wolfcampian and at the mid-Wolfcampian. On the eastern margin of the Central Basin Platform, Lower Hueco Group (lower Wolfcampian) carbonate bank complexes composed of reef mounds and packstone-grainstone flank beds grew along an erosional escarpment on the underlying Bursum Formation (uppermost Pennsylvanian), as seen at South Cowden (8790 Canyon) Field, Ector County, Texas. The escarpment depositional slope was >10°. Reservoir porosity in the Lower Hueco platform margin carbonate banks was greatly enhanced by subaerial exposure and meteoric dissolution at the overlying mid-Wolfcampian unconformity. Lithoclastic-skeletal debris flows are common in Wolfcampian forebank, deeper water, slope-to-basin facies. Outcrop analogs for the subsurface carbonate platform margin banks and forebank slope-to-basin facies are well exposed in the Hueco Group of the western outliers of the Hueco Mountains in far west Texas, which were paleogeographically located along the western (leeward) margin of the Diablo Platform and adjacent Orogrande Basin. Platform margin banks prograded over an irregular paleotopographic surface on the sub-Hueco Group erosional unconformity. The leeward upper slope to shelf-margin reef mounds have phylloid algal-dominated core facies with only sparse radial fibrous cements, which graded rapidly upward near wavebase into crestal shelf-margin grainstone shoals with Tubiphytes patch reefs. Proximal forereef, upper slope facies are composed of autochthonous upper slope crinoidal-fusulinid packstones, burrowed dark mudstones, and productid brachiopod wackestones and have interbedded allochthonous tongues of forereef grainflows, turbidites, and lithoclastic-skeletal packstone debris flows. More distal slope-to-basin facies are composed of autochthonous dark cherty nonburrowed mudstones-wackestones with interbedded allochthonous lithoclastic-skeletal packstone debris flow deposits and turbidites. Slope facies distributions reflect icehouse glacioeustatic sea-level fluctuations.

Abstract The Kozhim carbonate bank, a 380-m-thick Upper Carboniferous-Lower Permian carbonate buildup exposed in the northeastern Komi Republic, Russia, grew on a topographic high near the margin of the Ural Trough. The carbonate bank is composed of intermixed radiaxial cement-rich biohermal facies and associated skeletal wackestones-packstones-grainstones. Traditionally the carbonate buildup has been referred to as a reef, but percentages of biohermal facies are uncertain and organic boundstones are not present, and in this study it is referred to as a carbonate bank. Bank growth was initiated in the Late Kasimovian-Early Gzhelian in response to subsidence of the Ural Trough. In the Gzhelian-Asselian lower part of the bank section, biohermal facies are composed of fenestrate bryozoan- Tubiphytes -radiaxial calcite cementstones. Within the bioherms and associated bioclastic facies, bryozoans, Tubiphytes, echinoderm ossicles, and brachiopods are common to abundant; and ammonoids, gastropods, and smaller foraminifera are sparse. Fusulinids, calcareous algae, and calcisponges occur locally at some horizons but are lacking through most of the section. These periodic occurrences of warmer-water biota probably indicate relative sealevel changes, but no evidence of subaerial exposure was found, and bank deposition appears to have been continuous. In the Late Asselian-Sakmarian upper part of the bank, the fenestrate bryozoan- Tubiphytes -radiaxial cementstones grade into and intermix with Palaeoaplysina bafflestones- cementstones. Palaeoaplysina -phylloid algal bafflestones-cementstones are the dominant facies near the top of the buildup. The Gzhelian-Asselian fenestrate bryozoan -Tubiphytes cementstone bioherms grew in a relatively cool-water deep-ramp to slope setting. The gradual transition in the upper bank from cooler-water facies to a warmer-water Palaeoaplysina -phylloid algal biohermal community suggests upward bank growth into shallower and warmer water. The upper Palaeoaplysina-rich part of the Kozhim bank is commonly cited as an outcrop analog for Upper Carboniferous-Lower Permian hydrocarbon reservoirs in the adjacent Timan-Pechora Basin that contain similar Palaeoaplysina bioherm facies. However, those reservoir facies are generally associated with shallow-water grainstone shoal deposits that often display evidence of repeated periods of subaerial exposure, and they did not undergo pervasive early diagenetic cementation. Palaeoaplysina bioherm complexes and associated flanking grainstone deposits exposed on Spitsbergen and Bjornoya in the Norwegian Barents Sea are probably better outcrop analogs for the Timan- Pechora reservoir sections.

Abstract Pennsylvanian and Lower Permian organic carbonate buildups mostly lack large massive reef-building organisms, but rather were constructed by relatively small organisms, such as calcareous algae, calcisponges, and bryozoans, Microbial encrustation and submarine cementation often played significant roles in the construction of the buildups. These buildups consist of a spectrum of mound and reef types whose organic and inorganic compositions are controlled mainly by seawater temperature and light availability, which in turn are related directly to paleolatitude and/or water depth, and growth position. Lower Upper Carboniferous (Bashkirian-lower Moscovian, Morrowan-lower Desmoinesian), shallow shelf to shelf-margin, organic carbonate buildups are mostiy broad low banks and mounds constructed by calcareous algae (e.g., Dvinella/Donezella, Komia, Cuneiphycus , phylloid algae), Chaeleles sponges, and microbialites, In Japanese seamount reefs, which were located in high-energy open sea settings, rugose corals, tabulate corals, and chaetetids were the important constructors. Steep platform slopes during this time were apparently supported mainly by microbialite and synsedimentary cementation. Upper Upper Carboniferous (Desmoinesian-Virgilian/“Bursumian”, Moscovian-Gzhelian/Orenburgian) tropical shallow-water shelf and shelf-margin carbonate buildups are predominantly bafflestones composed mostly of erect udoteacean phylloid algae. In contemporaneous, slightly deeper-water, mound-flank to intermound areas, communities of calcareous sponges, laminar encrusting red algae, and bryozoans built smaller boundstone reefs. In Lower Permian (Wolfcampian, Asselian-Sakmarian) tropical paleolatitudes, those two buildup communities merged and constructed bafflestone-boundstono reefs in shelf-margin settings that were composed of a framework (baffler guild) of erect phylloid algae, calcareous sponges, and fenestellid and ramose bryozoans, which were encrusted by (binder guild) laminar red algae ( Archaeolithophyllum), Archaeolithoporella, Tuibiphytes , and fistuliporid bryozoans (Chiorosponge association). Many tropical shelf-margin buildups are rich in originally aragonitic botryoidal radial fibrous cements, and in some, heliosponges and specialized Permian reef brachiopods are locally abundant. Such Lower Permian aigal-calcisponge-cement reefs are known throughout the Permian circum-equatorial belt, from present-day southwestern and midcontinent North America, to the circum-Mediterranean region, to southeast and East Asia. That integrated reef-building community radiated and persisted through the rest of Permian time, reaching its acme of development in Middle and Upper Permian tropical reefs, such as the Capitan Reef of the Permian Basin. At more subtropical to warm temperate paleolatitudes, upper Upper Carboniferous to Lower Permian shallow-water shelf to shelf-margin buildups were constructed mainly by Palaeoaplysma , with varying proportions of intermixed phylloid algae, and sometimes botryoidal radial fibrous cements (Chloroforam association). Paleoaplysina buildups are known from around the northern margin of the paleocontinent Laurasia, which includes the present-day northwestern United States, Arctic Canada, the Barents Sea region, and the Russian Ural Mountains region. As seen in Arctic Canada and Russia, contemporaneous, somewhat deeper-water, upper slope buildups were constructed by fenestrate bryozoans and Tubiphytes , and those buildups commonly have pervasive originaily calcitic radiaxial cements tones (Brynoderm to Brynoderm-extended association). The Palaeoaplysina -phylldalgal algal and fenestra tebryozoan-Tubiphytes reef communities intermixed in buildups at intermediate water depths, and the latter buildup type sometimes shallowed upward into the former buildup type. In adjacent deeper-water, lower slope to basinal settings, siliceous sponges constructed organic carbonate buildups (Hyalosponge association) that commonly included intermixed bryozoans and brachiopods at intermediate depths. Most shallow-water, tropical and subtropical buildups were composed predominantly of originally aragonitic organic and inorganic elements (e.g., phylloid algae, Palacoaplysina , calcareous sponges, botryoidal radial fibrous cements), whereas buildups in deeper and/or cooler water were dominated by Mg-calcite constituents (e.g., bryozoans, Tubiphytes , radiaxial cements). Many of the cooler- water biotic elements were also contributors to warm-water reefs, but there they were usually greatly diluted by the more prolific tropical biota.