Davis Mountains And Barilla Mountains

The Davis Mountains together with the satellite Barilla Mountains constitute the largest contiguous segment of the Trans-Pecos magmatic province. They form an eroded volcanic plateau of approximately 115x75 km. The northern and northeastern margins are erosional while the southeast lavas are interstratified with volcanoclastic sedimentary rocks. The southwestern margin is an escarpment and probably related to the formation of the Valentine graben (Parker and McDowell, 1979, p. 1100). Many geologists have worked and published on the area (see Parker and McDowell, 1979, p. 1101 and Fig. 2) but there is no single, full account, although the Geologic Atlas of Texas, Marfa and Fort Stockton Sheets, cover the area on a scale of 1:250 000 (Barnes, 1979a and 1982). The volcanic rocks comprise a complex suite of lava flows, ash-flow sheets, bedded tuffs and interstratified sediments together with numerous intrusions. The lavas are predominantly sialic including trachyte, quartz trachyte and rhyolite, many of which are comenditic, and these are extensive but often form shallow shield-like features. Mafic lavas are subordinate and principally mugearite and hawaiite. There are two major ash-flow units in the area, the Gomez tuff and the Barrel Springs Formation. The Gomez tuff covers an area of 4300 km2 and is a pantelleritic ash-flow sheet, while the Barrel Springs Formation is a complex of welded and non-welded ash-flow sheets, tuffs and lava flows that covered some 3400 km2 with an initial volume of about 675 km3. The trachytes and rhyolites generally contain a sodic amphibole, usually arfvedsonite, and often aegirine and aegirine-augite. Analcime is a common constituent of the more mafic lavas.
Intrusive igneous rocks include stocks, plugs, laccoliths, sills and dykes together with agglomerate-filled vents. Rock types represented include trachyte, quartz trachyte, rhyolite, including peralkaline varieties, phonolite, trachyandesite, dacite, mugearite and hawaiite and their coarser grained equivalents. The most widespread coarse-grained rock is a syenite (McAnulty, 1955, p. 561) of alkali feldspar, aegirine-augite, olivine, apatite and usually a little analcime; (?)nosean, aenigmatite and arfvedsonite also sometimes occur. There appear to have been no detailed mineralogical studies of the area as a whole, but the Paisano volcano on the southern edge of the mountains has been mineralogically and petrochemically studied in detail by Parker (1983), while McDonough and Nelson (1984) have investigated geochemical constraints on magma processes. The general stratigraphic correlation for the whole area, based on K-Ar dating of many individual units, is presented by Parker and McDowell (1979, Fig. 3) who also list and indicate the areas covered by petrographic and field accounts.