Stratigraphy and environment of desposition of member 9 of the Rawls Formation, Presidio County, Texas



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Strata of member 9 cron out in the Santana, Redford and Presidio bolsons in the southern Bofecillos Mountains. Member 9 is correlative with the Tarantula gravels in the Rim Rock Country, the Delaho Formation in the Big Bend National Park, and with the basal parts of many of the bolson denosits which crop out along the Rio Grande. Annroximately 90 meters of coarse-grained sedimentary strata and intercalated volcanic flows comnose the member. Volcanic strata included in the member are the youngest known in this part of west Texas. The depositional environment for the sedimentary strata is interpreted as having been a series of coalescing alluvial fans that filled fault block basins which were formed along the trend of the Redford-Lajitas fault zone. The source for these strata was the older Tertiary volcanic flow rocks exposed around the flanks of the topographically high Bofecillos Volcano. A transgression of textural facies (distal over proximal) within the member indicates that that these strata were deposited during a period of tectonic quiescence. Renewed faulting in the area terminated the accumulation of member 9 strata. Strata deposited after this second period of faulting comnose the Redford-Presidio bolson-fills. Erosion since the introduction of a regional drainage system during the Pleistocene(?) has resulted in stripping away most of the originally denosited member 9 strata; only those strata contained within grabens formed during nost-member 9 tectonic activity have been preserved.



Geology, Texas, Presidio County.