Lower Pennsylvanian Ammonoids from the Bloyd Formation of Arkansas and Oklahoma

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Abstract
Approximately 8,000 well-preserved ammonoids from the Lower Pennsylvanian Bloyd Formation have been studied. They are from the type area of the Morrowan in Washington County, Ark., and from stratigraphic equivalents elsewhere in northern Arkansas and in Oklahoma. Representatives of the genera Proshumardites, Syngastrioceras, Pygmaeoceras, Pseudoparalegoceras, Retites, Branneroceras, Diaboloceras, Axinolobus, Cymoceras, and Pronorites are reviewed systematically. New species of Bisatoceras, Wiedeyoceras, and Gastrioceras are described. On the basis of ammonoid sequences in the type Bloyd Formation, it is concluded that numerous occurrences in Arkansas and Oklahoma can be assigned stratigraphically. Similarly, correlation can be made with the Namurian-Westphalian and Bashkirian-Moscovian sections of Eurasia. The ammonoid sequences from the type Brentwood serve to aid correlation with numerous localities throughout Arkansas and Oklahoma. This member contains the distinctive index species Branneroceras branneri as well as Proshumardites morrowanus, Bisatoceras secundum, Gastrioceras fittsi, Syngastrioceras oblatum, Pygmaeoceras morrowense, Pronorites arkansiensis, and Cymoceras miseri. The Union Valley Formation of southern Oklahoma appears, at least in part, to be a direct correlative of the Brentwood. The ammonoid fauna of the Brentwood indicates that it is a correlative of the Lower Bashkirian and Namurian C zone in the Eurasian section. The Woolsey Member of the Bloyd Formation is composed of terrigenous sediments and is either unrecognizable or absent in the adjacent stratigraphic sections in Arkansas and Oklahoma. The upper limit of the Woolsey Member is marked by the Baldwin coal. The “cap rock” of the Baldwin coal is the basal unit of the Dye Shale Member of the Bloyd Formation and contains distinctive zone ammonoids, Axinolobus quinni and A. modulus. Rare associates include a primitive form of Diaboloceras, D. neumeieri, as well as Proshumardites morrowanus, Wiedeyoceras n. sp., Gastrioceras n. sp., Syngastrioceras oblatum, Pseudoparalegoceras compressum, and Pronorites arkansiensis. The Gene Autry Shale and Wapanucka Limestone of southern Oklahoma are the apparent correlatives of the Dye Shale Member. This unit appears to be of the same age as the upper part of the Lower Bashkirian-Westphalian A zone in the Eurasian sequence. Directly overlying the Dye Shale Member is the Kessler Limestone Member of the Bloyd Formation. The Kessler Limestone fauna contains few ammonoids—sparse occurrences of Pseudoparalegoceras, Gastrioceras, and Axinolobus. The Otterville Limestone of southern Oklahoma is considered to be a correlative of the Kessler. The uppermost unit of the Bloyd Formation in northern Arkansas is the Trace Creek Shale Member, characteristically with large Diaboloceras neumeieri and Pseudoparalegoceras compressum. This unit also contains a few representatives of Bisatoceras n. sp., Gastrioceras n. sp., Boesites scotti, and Pronorites arkansiensis. As it is presently understood, the Trace Creek Member of the Bloyd Formation has few recognizable faunal correlatives in Arkansas and none in Oklahoma. However, with additional study, the uppermost portion of the Wapanucka Limestone may prove to be correlative with the Trace Creek Member; but at present the Wapanucka is restricted to the Middle Bloyd Formation. In most respects, the fauna of this member is gradational between that considered typical of the Bloyd Formation and that in the overlying Winslow Formation.

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