Stratigraphy and petrology of the upper part of the riley formation, upper cambrian of Central Texas



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The strata of the upper part of the Riley Formation (Upper Cambrian) in central Texas display an intimate interfingering of two lithosomes, one terrigenous and the other carbonate, extending from the northwest and southeast respectively. These strata accumulated in a shifting complex of nearshore depositional environments as determined by analysis of the six major lithofacies. These lithofacies include: 1) a finely laminated and flaser-bedded calcareous very fine sandstone (tidal flat), 2) a burrowed or lenticular-bedded calcareous very fine sandstone (shallow subtidal), 3) a trough cross-stratified, hematitic and calcareous poorly sorted medium to fine sandstone or terrigenous pelmatozoan biosparite (migrating tidal channel deposits, cutting previous two facies), 4) a trough cross-stratified glaucarenite (tidal inlet accumulations of offlap deposits), 5) a thick-bedded, rippled or burrowed terrigenous to pure biomicrite and biosparite (shallow subtidal to shallow shelf) which includes oosparite accumulations (wash-over carpets from shoals), and 6) a trough cross-stratified trilobite-brachiopod glauconitic biosparudite (tidal inlet and channel accumulations of offlap deposits). The history of these strata from Middle Cedarina-Cedaria time to Aphelaspis time is one of episodic progradation in an overall transgressive sequence. Accumulation began with a tidal flat and tidal channel depositional system similar to the German North Sea coast. As terrigenous input waned shallow subtidal to shallow shelf carbonates were deposited across much of the area and oolite shoals developed on the platform margin. Eventually a tidal inlet and lagoon system prograded across the area before a major regression occurred in post-Aphelaspis time, resulting in a disconformity over part of the outcrop area. Diagenesis in the carbonate lithofacies encompasses: cementation, dolomitization, dedolomitization, glauconitization, neomorphism, pyritization, and silicification.