Abstract

Mineralogical study of five lower Eocene sands (Sequin sand, basal Wilcox; Simsboro sand, middle Wilcox; Sabinetown sand, upper Wilcox; Carrizo-Newby, basal Claiborne; and Queen City, middle Claiborne) have been carried on in Bastrop County, central Texas, by Master's degree students at the University of Texas. At present, it appears that there are three major sources for these sands: the Rocky Mountains, providing quartz-feldspar sands with epidote as a characteristic heavy mineral; the Ouachita foldbelt, providing low-rank metamorphic rock fragments and considerable garnet; and the southern Appalachians, yielding metamorphic rock fragments with a kyanitestaurolite heavy mineral suite. All these sands except the Simsboro are classed as subgraywackes, containing 5 to 30 per cent slate and phyllite fragments, about 5 per cent K feldspar, and 2-7 per cent chert. They are texturally sub-mature to mature, because they lack clay matrix and are moderately to well sorted. All the heavy mineral suites contain abundant opaques plus zircon, tourmaline and rutile, but since these minerals have no known specific source at present, they are disregarded in the discussion below which deals only with source-indicator heavy minerals.

The Seguin sand (S. M. Naseer Rizvi) contains a garnet-epidote-zoisite suite together with abundant slate-phyllite fragments. The source of this sand was apparently the Rocky Mountains plus the Ouachita foldbelt. The Simsboro sand (Jimmy B. Adams) is unique among these sands because of its orthoquartzite composition (over 98 per cent quartz and chert), and it is a beach sand in which significant rounding was going on during deposition. It is largely derived from older sedimentary rocks, probably Cretaceous and upper Paleozoic in northwest Texas. The Sabinetown (J. Richard Harris), is also a subgraywacke and contains garnet as its characteristic heavy mineral, although floods of kyanite and staurolite enter in the upper Sabinetown. It apparently has been derived chiefly from the Ouachita foldbelt with contributions from the southern Appalachians coming in increasing amounts higher in the section.

The Carrizo-Newby (Thomas W. Todd) contains abundant kyanite and staurolite with very little garnet or epidote, hence was derived largely from the Appalachians with possible minor contributions of metamorphic rock fragments from the Ouachitas. It probably represents the culmination of uplift in the southern Appalachians. The Queen City sand (Dean L. Callendar) also contains kyanite and staurolite, but in quantities considerably less than the Carrizo. Apparently it was coming chiefly from the Appalachians but was also diluted with contributions from sedimentary rocks in north Texas.

This study of heavy minerals and bulk petrography has shown a gradual eastward shift of the dominant source areas from the Rocky Mountains (Laramide orogeny) in lower Wilcox, to the Ouachita mountains in upper Wilcox and finally to the southern Appalachians (lower Claiborne). In addition to these major sources, all the sands received contributions from more local sources, including minor amounts of volcanic minerals.