Example of Stratigraphic Confusion: Type Locality of Butler Clay Member of Rockdale Formation (Wilcox Group), Bastrop County, Texas

Abstract

For 33 years, confusion regarding a type locality and a correct stratigraphic sequence has made impossible a definitive study of part of the Wilcox Group of southeastern Texas. Some confusion could have been avoided had procedures established by the American Commission on Stratigraphic Nomenclature been in effect and utilized. The clay pits at Butler in northern Bastrop County are the type locality of the Butler Clay Member of F. B. Plummer (1933, p. 587); the type locality is not in Freestone County as has long been thought. According to Plummer, the Butler Member is the lowest of the three members of the Rockdale Formation of the Wilcox Group (lower Eocene) and underlies the Simsboro Sand Member, the middle member of that formation. In the present study it is proved that the clay exposed in the pits overlies the Simsboro; hence it is the basal part of the Calvert Bluff Clay Member, which is the upper of the three members of Plummer. The members proposed by Plummer, except the Butler, have been raised to formation rank, and the Hooper Formation, proposed by H. B. Stenzel, has been adopted instead of the Butler Member. Clay beds in the Calvert Bluff and Hooper Formations weather to a clay soil which supports prairie grasses and mesquite bushes, whereas the sand beds of the Simsboro Formation weather to a pedalfer soil which supports a dense growth of blackjack oak, post oak, and cedar. Thus, by using vegetation criteria it is possible to map these formations with some reliability in areas where outcrops are poor.