Volanic Geology of the Davis Mountains Trans-Pecos Texas: First Year Report

This report describes the results of the first-year study of the volcanic rocks of the Davis Mountains, Trans-Pecos Texas. Oligocene volcanic rocks in the Davis Mountains constitute the major part of the eastern, alkalic belt of the Trans-Pecos volcanic field. Yet, because of their volcanic and stratigraphic complexity, the Davis Mountains remain the most poorly mapped and least understood part of the field. The geology of the Davis Mountains as shown on the Geologic Atlas of Texas is based on regional extrapolation of formations established in a few detailed studies. Unfortunately, even these detailed studies commonly grouped a variety of lithologic types of doubtful genetic relationship. Extrapolation of these composite units into areas studied only from aerial photographs has further confused true relations. Very few source areas, calderas or otherwise, have been even tentatively identified. Only one stratovolcano and one caldera have been relatively thoroughly studied.

Nevertheless, the Davis Mountains are worthy of detailed study for several reasons. They are a distinctive, large-scale example of alkalic continental volcanism, comparable in areal extent and volume to major calcalkalic fields such as the San Juan Mountains of Colorado. All igneous rocks are alkalic; several are peralkaline. The volume of individual volcanic units is much greater than in most peralkaline volcanic fields. Many of the ash-flow sheets are rheomorphic; the degree of secondary flow ranges up to extreme examples in which a pyroclastic origin is largely obscured. Additionally, several large-volume silicic units have the outcrop and textural scale features of lavas but the areal dimensions of ash-flow tuffs. The origin of these units, whether extremely rheomorphic tuffs or unusually large-volume and extensive silicic lavas, is actively debated.