MAPPING LACUSTRINE DEPOSITS AND FAULTS WITHIN THE EAST-CENTRAL SAN LUIS BASIN, RIO GRANDE RIFT, USING AIRBORNE ELECTROMAGNETIC SURVEYS

The northern San Luis Basin of the Rio Grande Rift is bounded by the San Juan Mountains to the west and the Sangre de Cristo range to the east. This structural basin is further divided into two sub-basins by the Alamosa horst, with the deeper, eastern sub-basin partially underlying Great Sand Dunes National Park (GSDNP). Since 2005, this region has been the focus of geologic and geophysical studies by the United States Geological Survey. In 2011, in cooperation with the National Park Service, over 1,500 line-kilometers of airborne time-domain electromagnetic (TEM) data were collected within the GSDNP and west toward the Alamosa horst.

A primary target of the TEM survey are electrically conductive clay deposits identified in boreholes and ground TEM measurements. The clay was deposited within Lake Alamosa, a Plio-Pleistocene lake that covered much of the valley and experienced multiple transgressions and regressions over its roughly three million year lifetime. Resistivity models derived from the TEM data reveal a pervasive conductive layer interpreted to be lacustrine clay of the Alamosa Formation. This clay horizon serves as an important marker in studying the interaction between sedimentation in Lake Alamosa and faulting of the rift. Clay is imaged adjacent to the range front to the north and south of the dune field at GSDNP, and there are indications that the clay may extend some distance beneath the dune field itself. The position of the clay relative to known faults records a history of syn- and post-depositional motion along these faults. The configuration of the clay surface is consistent with a north-trending, down-to-the-west fault suggested by gravity data and located 5-10 km west of the range front. Greater offset is indicated to the north, where the clay is overlain by a thick sand body whose sharp eastern boundary is sub-parallel to the inferred fault. The clay deposits abruptly thicken to the west at the fault, suggesting syn-fault deposition. Overlying the clay are east- and west-tapering sediment packages, with coarse-grained sediment thinning to the west and fine-grained sediment thinning to the east. Mapping the extent and morphology of the clay throughout the survey area has implications for both the evolution of Lake Alamosa and the style of rifting within the San Luis Basin.