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

ABSTRACT The Hensel Formation is the terrigenous component of the last limestone-clastic couplet of 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: 1) a basal terrestrial facies of conglomerate consisting of clast supported, high energy fluvial channel lag lenses exhibiting poor sorting and crude lateral accretionary crossbedding; 2) a middle terrestrial facies of alluvial sandstones and mudstones with paleosol horizons, calcrete and well-developed rhizoconcretion zones; and 3) an upper facies that is composed of fossiliferous calcareous siltstone intercalated with thin fossiliferous limestone beds at the top and barren calcareous siltstone beds in the lower portion. Clasts in the lower and middle facies, and the mudstone 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 alluvial fans/fan deltas that prograded from the positive Llano Uplift. Fossil ostracodes including Bairdia sp., various agglutinated foraminifers, and small bivalves in the upper facies indicate that it is Cretaceous in age and marine in origin. This, and lithofacies analysis, supports recognizing it as a thin westward extension of the down dip, laterally equivalent Glen Rose Limestone.