ABSTRACT: Redesignation of the Upper Lithofacies in the Hensel Formation on the Southwestern Flank of the Llano Uplift, Kimble County, Texas: A New Interpretation of the Depositional Environments

JONES, JAMES O. and AARON JOHN KULLMAN

The Hensel Formation is the terrigenous component of the last limestone-clastic couplet in the upper Trinity Group (Cretaceous). Most previous studies involving the Hensel have been in depocenters to the southeast of the Llano Uplift. Analysis of the Hensel to the southwest of the uplift has been cursory.

Exposures of the Hensel Formation along the upper Llano River drainage basin around Junction, Texas exhibit three distinct lithofacies. A basal conglomerate consisting of clast supported, high energy fluvial channel lag lenses exhibiting poor sorting and crude lateral accretionary crossbedding. The middle facies is extensively calcretized alluvial sandstones and mudstones with paleosol horizons and well developed rhizoconcretion zones. An upper marine facies is composed of fossiliferous calcareous siltstone intercalated with thin limestone beds. Clasts in the lower and middle facies, and the siltstone of the upper facies are derived from Pre-Mesozoic limestone and crystalline rocks from the Llano Uplift. The Hensel was deposited as a series of coalescing fan deltas that prograded from the positive Llano Uplift.

Fossil ostracodes and foraminifers in the upper facies indicate that it is Cretaceous in age and marine in origin and lithofacies analysis supports recognizing it a thin westward extension of the down dip laterally equivalent Glen Rose Limestone.

AAPG Search and Discovery Article #90941©1997 GCAGS 47th Annual Meeting, New Orleans, Louisiana