Mapping lacustrine deposits and faults within the San Luis Valley, Rio Grande Rift, using airborne electromagnetic surveys

Abstract:

The San Luis Basin within the Rio Grande Rift is bound by the Sangre de Cristo Mountains to the east and the San Juan Mountains to the west. This structural basin is divided into sub-basins, with a deeper, eastern sub-basin partially underlying Great Sand Dunes National Park (GSDNP). This region has been the focus of geologic and geophysical studies by the USGS. In cooperation with the National Park Service, over 1,500 line-kilometers of airborne time-domain electromagnetic (TEM) data were collected within the eastern San Luis valley, including part of the GSDNP.

A primary target of the TEM survey is clay deposits identified in boreholes throughout the area. The clay forms a regional hydrologic barrier between upper unconfined and lower confined aquifers, and was deposited within Lake Alamosa, a Plio-Pleistocene lake that covered much of the valley. Resistivity models image the Lake Alamosa clay as a prominent conductive horizon throughout the survey area. This faulted and warped horizon serves as a marker in studying the interaction between sedimentation in Lake Alamosa and rift development. We present a probabilistic top-of-clay interface derived from stochastic inversion modeling of the TEM data.

Clay is imaged adjacent to the range front at GSDNP, and 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 zone suggested by gravity data. Based on the resistivity models, this zone is located 5-10 km west of the range front with greater offset 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 zone. Clay deposits abruptly thicken to the west at the fault, suggesting syn-fault deposition. Overlying the clay are west- and east-tapering sediment packages, which we interpret to be derived from the Sangre de Cristo and San Juan mountains, respectively. Mapping the extent and morphology of the clay has implications for both the evolution of Lake Alamosa and the style of rifting within the San Luis Valley.