ABSTRACT

The Lower Cretaceous Hensel Formation in central and north-central Texas consists of predominantly terrestrial deposits. Study of Hensel outcrops in Kimble, Gillespie, and Blanco Counties, south of the Llano Uplift, reveals a facies evolution that is associated with a major marine transgression. This evolution is expressed in both a north-south facies tract and in the overall stratigraphic succession. Four major depositional systems are recognized. (1) Basal, valley-fill deposits are limited to localities proximal to the source area. (2) Low-sinuosity, bedload channel-facies overlie the valley-fill and are widely distributed. (3) Low-sinuosity channel-facies evolved into more distal, somewhat finer grained, coastal-plain fluvial systems. (4) Both the fluvial and coastal-plain facies are characterized by extensive flood-basin muds and small, ephemeral arroyos. The pervasive development of calcrete, or caliche, within the overbank deposits, and the overall depositional style of the unit imply a semi-arid, seasonal climate. As the marine trangression progressed, the sediment supply decreased, and channel gradients lowered. Deposition of the shallow marine or lagoonal carbonates that overlie the Hensel resulted from the final inundation of the source area.