Lower Clear Fork Research Articles

Morphology and histology of dorsal spines of the xenacanthid shark Orthacanthus platypternus from the Lower Permian of Texas, USA: palaeobiological and palaeoenvironmental implications

Detailed studies on Carboniferous species of the xenacanth Orthacanthus have shown that the xenacanth dorsal fin spine can be used for skeletochronological analyses and provides valuable information about development, growth and environmental life conditions of those extinct sharks. We report here for the first time the histology and skeletochronology of Permian specimens, dorsal spines of Orthacanthus platypternus from the Craddock Bone Bed (lower Clear Fork Formation; Early Permian, Leonardian age) of northern Baylor County (north-central Texas, USA). Twelve dorsal spines of O. platypternus preserve a highly vascularized wall mainly composed of centrifugally growing dentine in a succession of dentine layers, probably deposited with an annual periodicity. As expected, spines of individuals with 1–2 dentine layers, presumably juveniles, present the smallest sizes. However, spines of individuals showing at least 3–4 dentine layers and interpreted to be subadults/young adults, are distributed in two spine-s...

Mississippi Valley-Type Sulfide Mineralization, Sheffield Channel, and Implications for Delaware Basin Evolution: ABSTRACT

A zinc-lead deposit of Mississippi Valley-type character occurs in the subsurface (1312 m) Sheffield channel, in basal Leonardian (Permian) dolomites along the southern Central Basin platform, Pecos County, Texas. The host dolomites represent shelf-marginal facies that pass landward into an inner shelf-carbonate/evaporite province (Lower Clear Fork), and abruptly seaward adjoining a prominent basement fault into equivalent basinal facies of the Bone Spring formation. Zoned sphalerite, accessory gypsum, and abundant pyrite are the major sulfide species present in the deposit. Sulfur isotope and fluid inclusion data, in conjunction with reconstructions of Delaware basin geothermal history, suggest that sulfide mineralization occurred during or subsequent to a burial-temperature maxima in the late Mesozoic to early Tertiary. Mineralizing fluids appear to have been derived from basinal rocks older than the Bone Spring (3355-m depths), and migrated upward along the platform bounding fault system. Sulfide precipitation at temperatures of 96/sup 0/ +/- 10/sup 0/C resulted from mixing of basinal brines and ambient host fluids that were charged with sulfur derived locally. Burial paragenesis sequentially involved a major period of pre-ore dolomitization, minor carbonate dissolution, complex sulfide-mineral emplacement, and post-ore dolomite and calcite cementation. Similar carbonate diagenetic sequences found elsewhere in the Delaware basin may bemore » evidence of a common regional hydrodynamic history of the basin.« less

Stratigraphy and Depositional Mosaics of Lower Clear Fork and Wichita Groups (Permian), Northern Midland Basin, Texas

The stratigraphy, depositional environments, and reservoir potential of the Wichita Group and overlying Lower Clear Fork (Permian) in the northern Midland basin are described on the basis of seismic interpretations, log correlations, and lithologic analyses. The Wichita in this area is divided into a lower member of slope to basinal limestones and shales, and an upper member of shelf dolomites, varicolored shales, and evaporites. Only a starved lower Wichita section is present in areas to the south. Fusulinid evidence and physical correlations suggest that the Wolfcampian-Leonardian boundary occurs within the Wichita and ascends stratigraphically toward the north. These data, in conjunction with the upward-shoaling aspect of the Wichita, establish a progradational relatio between shelf facies of the upper member and slope to basinal facies of the lower member. The Lower Clear Fork beneath the Tubb member records continued southward progradation of similar shelf facies. These lithologies are represented in the basin by limestones and shales beneath, and possibly including lowermost Dean sandstones. Depositional systems of the Wichita and pre-Tubb Clear Fork sections are carbonate-dominated, rimmed shelves adjoining deeper basins. Periods of rapid progradation and offlapping shelf-edge accretion were cyclic, being coincident with periods of low sea level. The Tubb section records a change to a mixed siliciclastic and carbonate, rimmed shelf system. Deposition of shelf Tubb and basinal Dean facies was related to a mosaic of cyclic, reciprocal siliciclastic-carbonate sedimentation. Rapid shelf outbuilding occurred at low sea-level stands during the siliciclastic phases of sedimentation. The formation of the shallow North platform was more a consequence of constructional-sedimentary processes during the late Wolfcampian and early Leonardian than of structural influences. Over much of the northern Midland basin study area, potential stratigraphic hydrocarbon reservoirs are in Wichita and Lower Clear Fork strata. Such reservoirs occur as separate offlapping shelf-marginal (dolomitized) oolite grainstone shoals rather than organic buildups as was described in earlier studies.

Oils from Yeso Reservoirs and Their Basinal Equivalents: ABSTRACT

Yeso rocks contribute a high percentage of the oil produced in the Permian basin. The Yeso was deposited as three distinct facies in late Leonardian time. The shelf deposits of slightly anhydritic dolomite are divided into the Glorieta and the upper and lower Clearfork rock units; the shelf-edge deposits around the basin margins consist of massive reef-like dolomite; and the basin deposits are predominantly End_Page 194------------------------------ shale, sandstone, siltstone, and limestone of the Spraberry Formation. The basin sediments appear to be ideal petroleum source rocks and limited fracture development restricts the occurrence of basin oil to rock of Permian age. Shelf or platform oil may be mixed Permian and pre-Permian due to structural deformation, faulting, fracturing, and deposition across truncated surfaces. Correlations of oil analyses with reservoir rock types suggest that differences in the chemical composition of basin and shelf oils may be related to the amount of evaporites in the reservoir rocks. Oil groupings can be explained geologically through differences in sources, environment, paths of migration, or mixing of oils. End_of_Article - Last_Page 195------------

Embar Field, Andrews County, Texas: ABSTRACT

From its discovery, May 14, 1942, to January 1, 1963, the Embar field produced 26,543,022 barrels of oil from pays in the Grayburg, upper Clear Fork, lower Clear Fork, Devonian, Fusselman, and Ellenburger. San Andres production on the Embar structure is not included in this figure since it is incorporated with the Martin San Andres field. Up to 1,500 feet of sandstone and red shale of Tertiary, Cretaceous, and Triassic age cover the Permian which in turn is about 6,400 feet thick and directly overlies a major unconformity. Beds ranging in age from Pennsylvanian to Precambrian are present beneath this unconformity, but within the limits of the Embar Ellenburger field, beds no younger than Precambrian, Ellenburger, or basal Simpson are found. The pre-Permian structure of the Embar field is an anticlinal fault block which is part of a much larger anticlinorium. The structure was peneplaned following its uplift in late Mississippian or early Pennsylvanian time and again in late Pennsylvanian or earliest Permian. This structure is faintly reflected through the Permian pay formations because of drape or renewed uplift along the old axes. The Devonian is productive on the south flank of the Embar structure in updip pinchouts complicated by faulting. Although analyses reveal that oils from the different formations in the Embar field are similar, waters are distinctive. This does not damage the concept that the oil from the upper and lower Clear Fork, Devonian, and Ellenburger are probably from the same source. This source may ha e been dark shales of Pennsylvanian and Mississippian age which were preserved in structurally lower areas. It is theorized that migration may have begun in post-Clear Fork time when all the present reservoirs were covered. From the source beds the oil may have followed the unconformity at the base of the Permian strata, then fault planes, until it reached the Ellenburger and Devonian. From these formations the oil may have gradually migrated vertically to the lower and upper Clear Fork beds. End_of_Article - Last_Page 2077------------