Abstract

New fusulinid and conodont biostratigraphic data demonstrate that the so-called “Wolfcamp Shale” basinal mudstone facies filling the Midland Basin actually ranges in age from early Missourian (early Late Pennsylvanian) to middle Leonardian (late Early Permian). Fusulinid foraminifera have traditionally been the primary index fossil for biostratigraphic correlations of Late Paleozoic marine strata in the Permian Basin and for defining regional chronostratigraphic boundaries. Fusulinids were benthic in habit and lived in shelf to upper slope settings, but were episodically transported downslope in slope-to-basin gravity-flow deposits. Conodont animals were pelagic, living up in the water column, and they were distributed rapidly during periods of transgression and sealevel highstand. As a result they are the primary index fossil used for definition and correlation of global chronostratigraphic stages. Conodonts occur in shelf to basinal facies, but biostratigraphically significant assemblages are generally most common in TST condensed horizons.

New biostratigraphic data are currently being analyzed from twenty-one Midland Basin cored wells, all of which have fusulinid data, and five of which have conodont data. Preliminary biostratigraphic interpretations are related below to the Wolfcamp A-D intervals, which are somewhat flexible petrophysical log signature zones. In most wells studied, the “Wolfcamp” basinal mudstones directly overlie lower or upper Strawn (early to late Desmoinesian) strata, as age-dated by fusulinids. The basal Wolfcamp D interval (Cline Shale, Penn Shale) is age-dated as early Missourian to early Virgilian (Kasimovian). The Wolfcamp C2 interval is middle to late Virgilian (Gzhelian). The Wolfcamp C1 interval is early Wolfcampian (Nealian, Asselian-early Sakmarian). The Wolfcamp B interval is late Wolfcampian (Lenoxian) to early Leonardian (late Sakmarian-Artinskian), with the Wolfcampian-Leonardian boundary in the upper part of the B interval. The Wolfcamp A interval is early to possibly middle Leonardian (latest Artinskian?-Kungurian). Additional conodont samples from a continuous core through the Wolfcamp A zone and Dean Sand are currently being processed.

Significantly, whereas Pennsylvanian-Lower Permian stratigraphic sections on the surrounding shelves are punctuated by hiatus breaks due to high-frequency glacioeustatic sealevel fluctuations, the deep-water basinal stratigraphy should consist of more complete sections. Studies in progress are attempting to: (1) better integrate conodont data with the more abundant fusulinid data, (2) correlate Midland Basin basinal stratigraphy to the well-known Eastern Shelf stratigraphy, and (3) correlate and interpret the relationships of basinal and shelfal sequence stratigraphy in the region.