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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 67 (1983)

Issue: 3. (March)

First Page: 422

Last Page: 422

Title: Importance of Shelf to Trough Black Phosphatic Shales in Mid-Continent: ABSTRACT

Author(s): Allan P. Bennison

Article Type: Meeting abstract


Pennsylvanian black shales containing radiolarian-rich phosphatic nodules, such as are now accumulating on outer shelves and upper slopes of modern tropical seas, are widespread throughout much of Kansas, Oklahoma, and other Mid-continent states.

Conodonts, inarticulate brachiopods, conularids, fish teeth, and radiolarians constitute the main biota of these black shales. Such shales characterize about half the 60 or more known Pennsylvanian cyclothems of Oklahoma and Kansas. These shales, individually approximating about 1 m (3 ft) in thickness in the extensively mined limestone and coal sequences of eastern Kansas and northeastern Oklahoma, have been described by Moore, Branson, Heckel, and others. However, much thicker, up to 8 m (26 ft), coeval zones in the deep Arkoma and Anadarko basins are surprisingly under-reported considering that such highly organic shales may have been the prime source of commercial oil and gas. Furthermore, those coeval black shale wedges that interfinger southward from the Arkoma trough into the r d bed- and conglomerate-dominated sequences flanking the southern tectonic borderlands are virtually unreported.

These are the "core shales" of Heckel, the deep stillstand or maximum transgressive facies. Representative black phosphatic shales crossing two or more tectonic provinces include the Desmoinesian sub-Verdigris, Anna, and Nuyaka Creek beds, and the Missourian Mound City and Stark beds.

Dysaerobic (low oxygen) and supposedly slightly shallower water, dark gray concretionary shales adjoin these black phosphatic shales. In many cyclothems the dysaerobic facies represent the deep stillstand facies where the latter is missing. Typically, it features a middle to outer shelf diverse molluscan community, including rapidly evolving goniatites useful for correlations.

These deep-water stillstand indicators are more useful for regional correlations than those for the low water stillstand that include coals, coaly shales, red beds, and paleosols. Carboniferous sea level fluctuations at times probably exceeded 200 m (656 ft), arising from interacting tectonic pulses and glacial-eustatic changes. Recent findings indicating that Carboniferous glacial maxima in southern Pangea exceeded Pleistocene maxima may account for the numerous regressive events on a global scale at that time.

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Copyright 1997 American Association of Petroleum Geologists