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ABSTRACT

The upper part of the Riley Formation, Cambrian of central Texas, is primarily composed of a sequence of thoroughly trough cross-stratified deposits. The dominant lithologies range from fossiliferous glaucarenite to highly glauconitic bio-sparrudite. These cross-stratified deposits accumulated within a tidal inlet and associated lagoonal tributary and distributary channels. Tidal inlet-fill strata are underlain by shallow, open marine oosparites and biomicrites and are overlain by parallel bedded glaucarenites which accumulated as part of a barrier island complex. The parallel bedded deposits exhibit large scale, gently inclined strata, ripple cross-stratification, and a minor amount of vertical burrows.

Some glaucarenite units within the tidal inlet-fill have local concentrations of skeletal material, primarily trilobite carapaces. These concentrations are most abundant in the bottoms of troughs. Cementation by bladed to fibrous spar between the carapaces has resulted in the nodular appearance of these skeletal accumulations. Calcite clasts, with relict evaporite textures, occur within the carbonate nodules and surrounding glaucarenite. These clasts were eroded from the shallow subsurface of the barrier island as the tidal inlet migrated. The presence of the former evaporite clasts attest to an arid climate at the time of their formation.