2000 GSA Annual Meeting -- Reno, Nevada
2000 GSA Annual Meeting -- Reno, Nevada
Author(s): MCDONALD, Karen, and WAGGONER, Ben, Department of Biology, University of Central Arkansas, Conway, AR 72035-5003, benw@mail.uca.edu; HOTCHKISS, Frederick H. C., 26 Sherry Road, Harvard, MA 01451, HotchkissF@PAN0.Panametrics.com; HARPER, John A., Pennsylvania Geological Survey, 400 Waterfront Drive, Pittsburgh, PA 15222-4745, jharper@dcnr.state.pa.us.
Keywords: Chesterian, Mississippian, Ophiuroida, Echinodermata, taphonomy
The Imo Formation (Chesterian: Mississippian) of north-central Arkansas is the youngest Mississippian formation in the south-central United States, younger than the type Chesterian, and equivalent to the E2b-c zone of the upper Arnsberger Stage (lower Namurian). Its diverse biota, dominated by bivalves, cephalopods, gastropods, and crinoids, has been extensively studied by biostratigraphers. We describe a previously undocumented ophiuroid biota from the middle of the Imo Formation, which is a dark friable clayshale rich in siderite concretions. The ophiuroids are restricted to narrow (centimeter-scale) horizons within the clayshale. Two species have been found so far, both possibly new species: an ophiurid in the genus Aganaster, and an oegophiurid close to Schoenaster or Encrinaster. Both have close counterparts, if not conspecifics, in the underlying Pitkin Limestone. The echinoderm taphofacies resembles that of distal shelf siliciclastic paleoenvironments, such as the Hunsruck Slate; however, the facies was probably a low-energy, shallow-water, muddy paleoenvironment. Pyrite-rich horizons in the middle Imo Formation indicate transient episodes of anoxia, and these dysoxic episodes played a role in the preservation of these ophiuroids.
© Copyright 2000 The Geological Society of America. All rights reserved.