STRUCTURAL GEOLOGIC FEATURES OF THE LOWER CANYONS, RIO GRANDE WILD AND SCENIC RIVER, TEXAS

The Lower Canyons reach of the Rio Grande is an 83 mile section that extends from Heath Canyon near La Linda to south of Dryden, Texas. The geology of the area includes the Lower Cretaceous Edwards and Trinity limestones, including Glen Rose (Trinity), and Maxon and Del Carmen Formations (Edwards). Structurally, the area is located west of the Devil's River uplift, southeast of the Marathon Basin, and east of the Laramide structures in the Persimmon Gap area of Big Bend National Park. The area is being studied to evaluate the hydrodynamics of numerous springs that emerge from trans-boundary aquifers, the Edwards-Trinity Plateau aquifer that extends from eastern Brewster County north to Midland and east to the Texas Hill Country and two aquifers in the Mexican State of Coahuila. The structural features of the area include several large scale NW trending folds. The Bullis fold is a broad anticline with a fold axis bearing of 318 degrees. The fold is broken approximately along the axial plane by a reverse fault. In the same area minor folds exist perpendicular to this structure. The Lower Canyons area is pervasively fractured by a system of joints trending roughly NNW. We have divided these joints into three groups based upon their magnitude of fracture and their spatial repeat distance. These include a “fine” set of joints with a repeat interval of 20 meters and an average trend between 334 and 355 degrees, a “medium” set with a spatial repeat interval of 130 meters and an average trend of 310 to 320 degrees and a “coarse” set with a spatial repeat pattern of 2.49 km and a trend of 310 to 320 degrees. Additionally a thrust fault has been mapped on the Mexican side of the river downstream from the Bullis Fold. The Rio Grande in the Lower Canyons follows a structurally controlled “rectangular” pattern. The relatively linear reaches have trends of approximately 285 degrees and 15 degrees (90 degree angular difference). The axial trends of the Bullis and other smaller folds in the area are consistent with a Laramide tectonic association, even though this area is east of the major Laramide related structures. The structures downstream from the Bullis fold hint at a transpressional origin with joint trends indicating right lateral stress. This is consistent with structures to the west in the Persimmon gap area.