A palaeoecological analysis of the Whitestone Member of the Walnut Formation, Lower Cretaceous, Travis and Williamson Counties, Texas

Abstract

The Whitestone Member of the Walnut Formation is composed of a sequence of lime grainstones, and is surrounded vertically and laterally by units of monotonous lime mudstones. The interrelationships between the lime grainstones and the lime mudstones indicate that the Whitestone represents an isolated "pocket" of carbonate debris which formed on a middy bottom. The presence of a rudist dominated lime grainstone at the base of the grainstone sequence indicates that a small patch reef may have provided the nucleus for the resulting accumulation of carbonate debris. Breakdown of the skeletons of the organisms which inhabited this "nucleus" provided finer grained sandy material which eventually overwhelmed the patch reef and formed a locally restricted carbonate sand bank. This bank was subjected to the current activity of a shallow sea which resulted in the production of extensive cross bedding in the sands. Shallowing of the water resulted in the planation of the upper surface of the sand body, the planar surface becoming cemented in the submarine environment. This flat cemented surface was extensively bored by marine organisms, producing a remarkable geological horizon which is well exposed in many quarries in the area investigated. The carbonate sand bank was eventually covered by encroaching fine grained, muddy sediments. When compared with modern carbonate sand bodies, the Whitestone shows remarkable similarities in terms of petrological, sedimentological and paleontological characteristics. Although there are some discrepancies between the Cretaceous example and the modern analogues, these are minor and do not affect the overall comparison which indicates that the Whitestone formed in a shallow water marine environment in which slight fluctuations in water depth occurred. These fluctuations are represented by subtle changes which are reflected in the petrology and sedimentology of the unit.