Terlingua-Big Bend

This is the most southerly part of the Trans-Pecos volcanic field in the United States, lying approximately between latitude 29°N and the Rio Grande. The southern half of the area lies in the Big Bend National Park and has been described in some detail by Maxwell et al. (1967) and illustrated with a geological map on a scale of 1:62 500. The area as a whole is also covered by a 1:250 000 geological map (Barnes, 1979b). Apart from the general account of the Big Bend area, other papers are for specific areas or intrusions such as the Terlingua-Solitario area (Lonsdale, 1940), the Christmas Mountains (Joesten, 1977) and the Rattlesnake Mountain sill (Carman et al., 1975). Geographically the area is complex and includes the Chisos Mountains, the Christmas Mountains, the Bofecillos Mountains, the Solitario Dome and numerous isolated hills, mountains, mesas etc. The area lies between two northwest-trending uplifted areas, the Mesa de Anguila to the west and the Santiago Mountains and Sierra Del Carmen to the east, with a central relatively low-lying area which may be a rift. Within the central low-lying area a considerable thickness of Tertiary volcanics is interstratified with sediments. Volcanics now extend over about one quarter of the area but originally coverage was probably complete. The greatest thickness of extrusive rocks occurs in the Chisos Mountains and in other mountainous centres such as the Christmas Mountains. Within the Big Bend National Park the two principal extrusive units are the South Rim Formation and below this the Chisos Formation. They include lava and pyroclastic flows and tuffs and are interstratified with conglomerates, sandstones and tuffaceous clays. The South Rim Formation consists principally of rhyolites, many with arfvedsonite, whereas the Chisos Formation includes numerous basaltic flows, which are probably hawaiites and mugearites, together with trachyandesites. The intrusive igneous rocks include plugs, laccoliths and various irregularly-shaped bodies which commonly arch the overlying rocks, together with numerous sills and dykes. Brief structural and petrographic details on over 300 intrusions within the Big Bend National Park are given by Maxwell et al. (1967, pp. 189-244). Rock types represented include basalt, hawaiite, mugearite, trachyte, quartz trachyte, rhyolite, phonolite, latite, trachyandesite and their coarser grained equivalents, although the majority are relatively fine grained. One distinctive suite of rocks is analcime-bearing and Maxwell et al. (1967) list 52 localities where they range from gabbroic varieties of andesine-labradorite, some alkali feldspar, augite and olivine to syenites in which the feldspar is essentially alkaline and the pyroxene aegirine. Notable localities include the Rattlesnake Mountain sill (Carman et al., 1975), which grades from an analcime monzonite to layers, stringers and ocelli of analcime syenite with similar rocks at Pena Mountain and Bone Spring (Maxwell et al. 1967, pp. 175-6), and the Terlingua-Solitario area with numerous intrusions (Lonsdale, 1940, Plate 2) ranging from analcime and analcime-nepheline syenite to analcime-bearing gabbros. Fayalite, arfvedsonite and aegirine granite, microgranite, quartz syenite and rhyolite form many of the larger intrusions including the arc of large intrusions on the west flank of the Chisos Mountains, the McKinney Hills in the east and the large intrusions of the Rosillos Mountains and Nine Point Mesa in the northeast and north. At the last three localities the dominant rock type is a hornblende-augite quartz syenite with fayalite and aegirine-augite zoned to a more sodic rim. A series of rocks from olivine syenodiorite through hornblende-augite syenite to quartz-hornblende syenite and granite occurs at Dominguez Mountain in the south, but this rock series is also represented throughout the area. An extensive dyke swarm radiates from Dominguez Mountain, but dykes are widespread and include a broad range of rock types.