ABSTRACT

Small, conical sandstone mounds occur in some outcrops of the lower Wolfe City Formation (Upper Cretaceous) in Ellis and Navarro Counties, North-Central Texas. Biogenic mounds are common in modern shallow-marine environments (e.g., Callianassa mounds) but are rare in the geologic record. These biogenic mounds resemble the ichnogenus Chomatichnus sp. described originally from Carboniferous limestones of Lancashire, England (Donaldson and Simpson, 1962). This ichnogenus occurs as well-defined conical mounds having a central vertical burrow. Wolfe City Chomatichnus consists of calcite-cemented, glauconitic, bioclastic, argillaceous, fine-grained quartzarenite. The central vertical burrow is lined with shell debris; striae radiate from the central burrow. Thalassinoides, Gyrolithes, and Pseudobilobites occur in lower Wolfe City strata with Chomatichnus. The upper Wolfe City contains a profuse ichnoassemblage of Thalassinoides; Pseudobilobites; Gyrolithes; U- and star-shaped trace fossils; large-diameter, sinuous, horizontal burrows; and tripartite digitate burrows. The Wolfe City ichnoassemblage records the activities of infaunal crustaceans in a low-energy shallow-marine paleoenvironment. Chomatichnus has probably been preserved here because of early marine cementation and/or rapid sedimentation (storm) events.