Structural relationships of the Precambrian Carrizo Mountain Group, Carrizo Mountains, Trans-Pecos Texas
McAdoo, Richard Lee
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The Precambrian Carrizo Mountain Group in the Carrizo Mountains of Trans-Pecos Texas is composed of approximately 20,000 feet of low-grade metasedimentary and meta-igneous intrusive rocks that strike northeast and dip steeply southeast. It forms the upper sheet of the Streeruwitz Overthrust. Beneath the thrust are intensely folded, Precambrian rocks of the Hazel and Allamoore Formations that crop out in the Sierra Diablo immediately to the north. The primary mesoscopic structures of the Carrizo Mountain Group are a metamorphic foliation with a synoptic attitude of N50E-30SE, and a lineation in the plane of foliation whose general attitude is N28W-24SE. Thin section evidence shows the foliation and lineation to be of cataclastic origin. The metamorphic foliation and sedimentary bedding are parallel throughout the succession. In the Hazel and Allamoore Formations derived fold axes fall along a great circle. This circle represents a plane that strikes N25W and dips 22SW. The strike of this derived plane approximately parallels the trend of the lineation in the Carrizo Mountain Group. These structural relations suggest that the principal structures of the Carrizo Mountain Group are related to folds in the Hazel and Allamoore Formations. This evidence supports the theory of King and Flawn (1953) that emplacement of the Streeruwitz Overthrust caused both folding in the Hazel and Allamoore Formations and cataclastic development of cleavage and lineation in the Carrizo Mountain Group. Cleavage and bedding in the Carrizo Mountain Group show strong structural homogeneity, which lends support to Flawn's proposal (in King and Flawn, 1953) that the dominant structure of the succession is homoclinal. The orientation of the plane defined by the synoptic fold axes in the Hazel and Allamoore Formations may represent the general attitude of the Streeruwitz Overthrust in this region.