Primary Fluvial Oolites

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ABSTRACT An unusual occurrence of oolitic calcarenite is exposed in South Central Texas just south of the Balcones Fault Zone. Topographic location and fossil content suggest a late Pleistocene age. Thin section examination reveals a wide variety of oolites including normal, superficial, compound and disrupted types. There is convincing evidence that the oolites were deposited, and for the most part, actually formed in a fluvial environment. The oolite formation occurs in an irregularly shaped area which roughly parallels three contemporary streams for several miles. Strata composed essentially of oolites are cross-bedded, lenticular and irregularly intercalated with poorly sorted, pisolitic calcareous conglomerate, nonoolitic calcarenite and calcareous claystone. Cut-and-fill structures and ripple marks are distinct. Whole and fragmented shells of Goniobasis sp. and Polygyra sp., freshwater and terrestrial gastropods are common. X-ray analyses show that the oolites are composed entirely of calcite. The calcium carbonate from which the oolites were formed was probably dissolved from the Cretaceous Edwards fm., which underlies the area at a depth of 210 m, and then brought to the surface by artesian springs. The contemporary streams along which the oolite formation outcrops are still spring-fed from the Edwards artesian system and are supersaturated with respect to calcite. Oxygen and carbon isotope analyses corroborate the hypothesis of a freshwater depositional environment. Many fragments associated with the oolites, as well as may oolite nuclei consist of the calcareous remains of charophytes and blue green algae. It appears that both physico-chemical and biological processes were responsible for the formation of the oolites, not only in the immediate vicinity of the srpings but downstream as well.