Oncolites in Cretaceous Yucca Formation, Presidio County, Texas: ABSTRACT

Abstract

Detailed stratigraphic work in the Yucca Formation, Presidio County, Texas, has revealed about 70 ft (21 m) of oncolite-bearing strata. The zone of oncolite occurrence is 180 ft (55 m) above the unconformity separating the Permian and Cretaceous rocks and 665 ft (203 m) below the initial occurrence of abundant Orbitolina. The zone varies both vertically and laterally from oncolite-supported conglomerate with micrite matrix to micrite with less than 10% oncolite content, to algal-encrusted, matrix-supported pebble conglomerate. Oncolites range from less than 0.5 in. (1.25 cm) to approximately 3 in. (7.5 cm) in diameter, and are commonly size sorted within the zone. The oncolites are most commonly nucleated about a fragmental piece of oncolite material and are classified as type-SS. In the pebble conglomerate facies, algal encrustations occur on very well-rounded, spheroidal clasts of siltstone, micrite, and chert. Very rare encrustations are present on clasts appreciably larger or smaller than the pebble size. Rarely is the algal material nucleated about organic remains such as pelecypod, gastropod, or brachiopod shells. The presence of this oncolite zone within an otherwise unfossiliferous section provides some control for interpreting depositional environments within the Yucca Formation. However, oncolites have been reported from several aquatic depositional environments both in the geologic record and as forming today. These environments include: lacustrine, fluvial; shallow, nearshore marine; and marginal marine brackish water systems. Local sedimentary features and broader stratigraphic relations effectively eliminate lacustrine and fluvial interpretation. End_of_Article - Last_Page 760------------