PETROGRAPHY AND GEOCHEMISTRY OF THE ALLAMOORE FORMATION, CULBERSON AND HUDSPETH COUNTIES, TEXAS (PRECAMBRIAN, TALC, ECONOMIC GEOLOGY)
Abstract
Metavolcanic, talc-bearing and carbonate rocks of the Allamoore Formation of Precambrian age (Y; 1,000-1,100 m.y.) crop out in an east-west-trending belt about 32 km long and 8 km wide (north-south) within southwestern Culberson and southeastern Hudspeth counties, Trans-Pecos, Texas. These three units, listed in ascending stratigraphic order, probably accumulated in a mature margin back-arc basin. Field and petrographic data suggest that extrusion of shallow marine pillow basalts and flows was accompanied by explosive venting of hyoloclastic breccias. The volcanic assemblage was covered by thinly-bedded, lenticular accumulations of limestone, dolostone, magnesite, chert, and possibly antigorite deposited within the intertidal and supratidal zones of barred shallow lagoons and/or associated ephemeral hypersaline lakes developed on sabkhas. Mat-building filamentous and coccoidal blue-green algae produced biohermal barriers, and biochemically created conditions favorable to the deposition of high-magnesium carbonate rocks. A thick sequence of intertidal to supratidal dolostones, characterized by silicified algal stromatolites (Stratifera, Tungussia and Conophyton) and containing interbeds of epiclastic tuffs, completes the stratigraphic sequence of the Allamoore Formation. The Hazel Formation of Precambrian age (Y; 975-1,000 m.y.) unconformably overlies the Allamoore Formation. Its basal conglomeratic facies was primarily derived from a folded and faulted southern highland composed of Allamoore Formation rocks. The conglomerate, along with arkosic sand and silt derived from exposures of Carrizo Mountain Group rocks, was deposited in the proximal, mid and distal facies, respectively, of a northward prograding, arid alluvial fan complex. The Allamoore and Hazel formations was complexly folded and faulted about 975-1,025 m.y.b.p., when rocks of the Carrizo Mountain Group (Precambrian Y; 1,200-1,300 m.y.) were thrust northward along the Streeruwitz fault, and were subjected to P-T conditions of greenschist facies metamorphism (350-450(DEGREES)C, 1-5 kb, 5-18 km burial depth). Metamorphism decreases northward away from the Streeruwitz fault and in the direction of thrusting. During metamorphism talc-bearing rocks were produced from the magnesium-rich precursor sedimentary rocks of the Allamoore Formation. Locally, the P-T conditions of metamorphism were sufficiently elevated by the emplacement of a thrust sheet of Carrizo Mountain Group rocks so as to cause the production of nearly pure talc bodies from the slightly earlier-formed talc-bearing rocks.
Subject Area
Geology
Recommended Citation
EDWARDS, GERALD, "PETROGRAPHY AND GEOCHEMISTRY OF THE ALLAMOORE FORMATION, CULBERSON AND HUDSPETH COUNTIES, TEXAS (PRECAMBRIAN, TALC, ECONOMIC GEOLOGY)" (1984). ETD Collection for University of Texas, El Paso. AAI8509024.
https://scholarworks.utep.edu/dissertations/AAI8509024