Abstract: Biogenic Mounds and Associated Trace Fossils: Wolfe City Formation (Upper Cretaceous), North-Central Texas

William C. Dawson, D. F. Reaser

Small, conical, sandstone mounds occur in some outcrops of the lower Wolfe City Formation (Upper Cretaceous) in Ellis and Navarro counties, north-central Texas. These biogenic mounds resemble the ichnogenus Chomatichnus sp., originally described from Carboniferous limestones of Lancashire, England (Donaldson and Simpson, 1962). Chomatichnus also occurs in the Dakota Sandstone (Upper Cretaceous) of northwestern New Mexico. This ichnogenus represents well-defined conical mounds having a central vertical burrow. Petrographically, Wolfe City Chomatichnus consist of calcite-cemented, glauconitic, bioclastic, argillaceous, fine-grained, quartz arenite; these mounds are more glauconitic than the surrounding lower Wolfe City sandstone. Average dimensions of Wolfe Ci y specimens are diameter, 11 cm, and height, 7 cm. The central vertical burrow is lined with shell debris and is clay filled. Striae surround the central burrow. Biogenic mounds are common in modern marine carbonate environments (e.g., Callianassa mounds), but are rare in the geologic record.

Thalassinoides, Gyrolithes, and Pseudobilobites occur in lower Wolfe City strata with Chomatichnus. However, Chomatichnus has not been observed to connect with any of the associated ichnofossils. The upper Wolfe City contains a profuse ichno-assemblage of Thalassinoides; Pseudobilobites; Gyrolithes; U- and star-shaped trace fossils; large-diameter, sinuous, horizontal burrows; and tripartite digitate burrows. Pelecypods and gastropods are typically abundant in Wolfe City outcrops, but are scarce in strata containing abundant ichnofossils. The Wolfe City ichno-assemblage records the activities of infaunal crustaceans in a low-energy shallow marine paleoenvironment. Chomatichnus has probably been preserved because of early marine cem ntation and/or rapid sedimentation (storm) events.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994