Microfaunal Assemblages Of The Placid Shale (Missourian, Upper Pennsylvanian), Brazos River Valley, North-Central Texas

Abstract

The Pennsylvanian and lowermost part of the Permian strata in North-Central Texas is assigned to the Strawn, Canyon, and Cisco Groups. The Canyon Group is approximately equivalent to the middle-upper part of the Missourian (Upper Pennsylvanian) and crops out in the Brazos River valley in a generally northeast-southwest strip across Palo Pinto and Jack counties. The Canyon Group is subdivided into seven formations that contain several cycles of well-known clastic and carbonate units. The Placid Shale, one of the clastic units within the Canyon Group, consists of two cycles of gray, brown, olive sandstone/siltstone/mudstone with few thin limestone beds. It is bounded below by the Winchell Limestone and above by the Ranger Limestone. In the Possum Kingdom Lake area in Palo Pinto County, three localities of the Placid Shale were chosen for study of their microfossil content. Closely spaced samples were taken from the two dominantly marine sequences of the two cycles, one in the lower part and one in the upper part of the formation. The dominant microfaunal elements present are holothurian sclerites, ostracodes, conodonts (Streptognathodus firmus and S. pawhuskaensis), fusulinids (which fall within the Triticites newelli zone) and a number of distinctive species of small foraminifers. The foraminifers are the main focus of the present study because of their diversity and biostratigraphic significance. The primary goal of this project is to describe and update the taxonomic assignments of the various microfaunal elements from two cycles of the Placid Shale.