OF SAND AND THROMBOLITES: EVIDENCE OF EPISODIC INUNDATION OF A CRETACEOUS SHORELINE

The Lower Cretaceous Bluff Dale Sand underlies the Glen Rose Formation throughout much of Central Texas. This unit is composed of a loosely consolidated fine grained silica sand, buff to grey in outcrop. The Bluff Dale Sand has been interpreted as a shoreline transitional member of the Twin Mountains Formation, a terrestrial conglomerate and sand deposit. While the Bluff Dale sand has been overlooked in the literature, this sand represents the turning point between the terrestrial and marine systems of the Lower Cretaceous, and contains many fossils indicative of the rising sea levels at this time. Most importantly, this unit contains thrombolites from two distinct flooding episodes within the sands. These thrombolite layers are locally continuous, and exhibit a bulbous habit both above and below.