Abstract

Two shoaling-upward or shallowing-upward successions of sediments, separated by a disconformity, developed in western Wyoming during late Middle and Late Cambrian time. The rock section, which is largely limestone, is over 130 m thick. The carbonate sediments accumulated in supratidal, intertidal, and shallow subtidal environments. The model consists of tidal flats, small and shallow shore lagoons where mud and/or sand shoals or barrier islands occur near the strandline, and larger, deeper epeiric (shelf) lagoons which are delineated by shoals. Wave and current energy is largely dissipated seaward of the epeiric lagoons except under storm conditions.

Excluding the limestone pebble conglomerates, the average carbonate rock contains small allochems scattered in a matrix of micrite and/or neomorphic calcite (microspar and pseudospar). Dolomite occurs as a replacement mineral in most beds. Trilobite, brachiopod, and echinoderm fragments, algal rods, ooids, peloids, micrite intraclasts, glauconite, and quartz grains form the allochem fraction. Shells, ooids, and glauconite were developed mainly in offshore areas and transported landward and onto the tidal flats.

The Middle Cambrian Park Shale Member (Gros Ventre Formation) contains edgewise micrite conglomerates, cryptalgal laminites, wavy bedding, and very thick micrites. Other features are wrinkle marks and small-scale ripple marks, rill marks, and desiccation structures. The sediments accumulated in shallow, shore lagoons and on intertidal and supratidal flats.

The limestone strata of the Upper Cambrian DuNoir Member (Gallatin Limestone) range in thickness from laminated to very thickly bedded. The upper DuNoir contains abundant ooids and zones of quartz sand. The sediments of the member were deposited under transgressive conditions on tidal flats, in shallow shore lagoons, and on mud-ooid and quartz sand shoals.

The beds of the Open Door Member (Gallatin Limestone) contain more fossil allochems than the underlying units. The sediments from which these rocks were formed were burrowed and highly bioturbated. The bedding, where preserved, varies from laminated to very thickly bedded. Most of the sediments of this prograding sequence accumulated in low energy epeiric lagoons delineated by lime mud shoals.

A disconformity separates the Upper Cambrian rock sequence from the overlying Bighorn Dolomite of Uppr Ordovician age.