SPONGES, CONCRETIONS AND CHERT IN THE PHOSPHORIA FORMATION, WYOMING

The Permian Phosphoria Formation in eastern Idaho and western Wyoming is a world-class economic phosphorite deposited in an epicontinental sea. Over a century of mining and scientific study of the basinal phosphorites have addressed the origins and concentration of phosphorus and organic carbon, but much less attention has documented the marginal facies and the presence of copious chert in the formation. Previous examination of chert beds in the Phosphoria have identified sponge spicules, but intact sponge bodies are obscure. This study measured sections of the Phosphoria along the northern margin of the basin between the Tetons and Gros Ventre ranges in western Wyoming, and in the process documented biogenic structures in chert that we interpret as meadows of sponges.

Within the Phosphoria Formation, chert occurs as dark beds, light-colored nodules and concretions, and toward the upper part (Tosi Chert Member) as a series of parallel, cylindrical concretions. Individual cylinders range in diameter between 5 and 10 cm, they are round to oval in cross section, and are up to a half-meter in length. Each cylinder in cross section has a central column with concentric dark and light rings of chert around it. Where exposed, the bottom few centimeters of the cylinder deforms the dolomitic sediment around it. In aggregate, the cylinders are closely packed and parallel to form up to 3 stacked beds, each with slightly differing orientations. We hypothesize these cylinders are individual sponges in life position, or taphonomically reoriented after death to form parallel masses. Similar preservation and orientation of “tubular cherts” described from Mississippian carbonates from Lost River Range in Idaho provide additional support for this interpretation. Discovery of thousands of in situ sponges in the Phosphoria cherts may provide new data to study chert formation from biogenic opal to final chert deposition.