MAPPING UNDER THE SAND: PROGRESS REPORT ON GEOPHYSICAL STUDIES IN GREAT SAND DUNES NATIONAL PARK, COLORADO

Great Sand Dunes National Park and Preserve is located at the eastern margin of the San Luis Valley at an embayment in the mountains. Although the Park is known for its majestic sand dunes, these features are fairly recent in geologic history. Much of the evidence of pre-dune history lies below the sand. The Park straddles the eastern, seismically active margin of a 5-6 km deep basin associated with the Late Cenozoic Rio Grande rift. Complex thrust packages in the mountains record earlier Laramide deformation. For ~3 my during the Plio-Pleistocene, a large lake occupied much of the San Luis Valley, leaving behind as much as several hundreds of meters of clay-rich deposits.<>

In cooperation with the National Park Service, the U.S. Geological Survey has been conducting geophysical surveys over the Park and vicinity, motivated by the need to characterize groundwater resources. The surveys include multiple types of airborne and ground-based methods. The methods each have unique strengths and limitations for imaging different aspects of the subsurface, which are most effective when integrated together and combined with independent information.

The geophysical studies in the Park are uncovering evidence of the influences of Laramide faulting on rift development and of rift faulting on lake deposition that are concealed under sand cover. Findings to date suggest the eastern rift margin is a series of steps down into the basin rather than one master fault. Shallow structural benches are evident adjacent to most of the range front (including one below the high dunes). These benches commonly have shallow dip to the west and show complex map patterns in detail, suggesting they may be Laramide thrust surfaces that were later cut by normal faults. The largest vertical displacement along the rift margin (the main rift fault) is located about 4-10 km west of the range front.

Lacustrine clay is imaged adjacent to the range front to the north and south of the dune field, and may extend some distance beneath the dune field itself. The top of the clay marks a faulted and warped surface. The surface is deepest to the north, where it is overlain by a thick sand body whose sharp eastern boundary is sub-parallel to the main rift fault. To the south, the clay is shallow, but thickens abruptly to the west of the main rift fault, suggesting syn-fault deposition.