New Mexico Geological Society Annual Spring Meeting — Abstracts

Within extensional basins stratal thicknesses change across intrabasinal faults as well as at the basin margins. Superb outcrops at Kasha Katuwe Tent Rocks National Monument, in the northern Santo Domingo basin of the Rio Grande rift, provide an opportunity to examine the changing character of a stratigraphic section across an intrabasinal fault. Between the Camada and Pajarito faults a ~500-m thick section of west-tilted Miocene Peralta Tuff Member of the Bearhead Rhyolite and Cochiti Formation are truncated by an angular unconformity. Pliocene gravel of Lookout Park, generally less than 20 m thick, overlies the unconformity to form a high-level constructional surface. Lower Pleistocene Otowi Member of the Bandelier Tuff locally is present above soil developed in the gravel of Lookout Park and also fills valleys above gravel deposited in a channel incised below the gravel of Lookout Park. Basinward (east) of the Pajarito fault the Otowi Member is enclosed within Cochiti Formation with no evidence of the unconformities that are present at the base and top of the gravel of Lookout Park in the footwall of the fault. Greater subsidence permitted accumulation and preservation of an apparently continuous Miocene-to-Pleistocene section in the hangingwall of the Pajarito fault, whereas the equivalent section contains prominent lacuna marked by unconformities in the footwall. The footwall unconformities, therefore, do not have any regional significance with regard to episodes of deformation or incision history of the Rio Grande but simple reflect variations in accumulation and preservation across intrabasinal faults.