Abstract

The White Hills of Jeff Davis County, west Texas expose 1.5 km² of metasedimentary rocks, mostly marbles, surrounded by Tertiary volcanic and intrusive rocks. Previous workers concluded the metasediments to be Cretaceous fault blocks, but recent studies in the region have identified Eocene lacustrine carbonates interlayered with volcanic rocks suggesting another possible origin for the White Hills.

Dasycladacean algae in the metasedimentary rocks indicates a marine environment of deposition, which occurred in the region most recently in the Cretaceous. Comparison to Pennsylvanian through Cretaceous carbonate sequences in the surrounding area indicates that metasedimentary rocks are most similar to the Cretaceous Cox and Finlay Formations. Sedimentary structures, such as bedding, cross bedding, and ripple marks are preserved in the metaquartzite, but only bedding remains in the calcareous and dolomitic units.

This assemblage formed as a Cretaceous roof pendant of the Tertiary Sawtooth magma chamber. After thermal metamorphism, the entire block was uplifted to a point higher than the current elevation, then dropped into its present position after magma chamber subsidence following the emplacement of the Sawtooth Mountain pluton. The entire block exhibits the result of stress from multiple directions in the form of small faults, folds, joints and microshear.

Diopside, wollastonite, and hedenbergite occur in the metasediments near the igneous contact, and spurrite and paraspurrite occur away from the contact. Dolomite related to geothermal activity is present beyond the migration limits of silica and aluminum.