Reworked Cretaceous elasmobranch teeth and provenance of the Paleocene Hanna Formation (Hanna Basin, Wyoming)

Abstract

The Hanna Formation, exposed in the northeastern Hanna Basin, Wyoming, represents deposition from late early Paleocene into earliest Eocene time in alluvial, floodplain, and lacustrine environments. A 600-m-thick section that yields abundant vertebrate fossils begins 975 m above the local base of the formation. This section has been dated as latest Torrejonian through middle Tiffanian using mammalian index fossils. The terrestrial mammals are accompanied by numerous elasmobranch teeth, representing species thought extinct since the end of Cretaceous time. They are species known from two locally widespread marine Cretaceous units, the Wall Creek Member of the Frontier Formation and the Steele Shale. These units are broadly exposed on flanks of the Sweetwater arch, which define the northern margin of the Hanna Basin. The elasmobranch teeth from the Hanna Formation range in size from a few millimeters to over three centimeters and display transport-induced abrasion not seen on their in situ Cretaceous counterparts. Enameloid of cutting edges, crown points, and cusplets is rounded or sometimes broken, and bony bases commonly are etched or dissolved away. These teeth were reworked and transported from the Wall Creek Member and Steele Shale during uplift and erosion of the Sweetwater arch in middle Paleocene time. Lithic clasts from Paleozoic and Mesozoic strata derived from that arch also occur in association with the elasmobranch teeth in the Hanna Formation. The Cretaceous clasts and teeth indicate a local, northerly source for part of the Hanna Formation in the northeastern Hanna Basin.