MAPPING PLIO-PLEISTOCENE LAKE DEPOSITS UNDER GREAT SAND DUNES NATIONAL PARK, SAN LUIS VALLEY, COLORADO--INSIGHTS FROM RECENT DRILLING AND GEOPHYSICAL SURVEYS

Geophysical and drillhole data collectively suggest that deep-water Plio-Pleistocene clays were deposited farther east than previously documented in the San Luis Valley, southern Colorado, and subsequently subsided in response to rifting. Today the clays represent a regional confining layer that is fundamental to the hydrogeology of the valley. The extent and nature of these clays have been poorly known in Great Sand Dunes National Park and Preserve, which straddles the boundary fault of the Rio Grande rift on the eastern side of the valley. Recent drilling and geophysical surveys in the Park now provide a means to map massive clay layers to depths as great as 185 m and investigate relations between sediments and rift faulting. Sedimentation has been active in the rift since the Oligocene, with thicknesses reaching 6 km in the Baca graben under the Park. Over a period of 3 m.y. during Plio-Pleistocene time, a large lake occupied most of San Luis Valley. Thick (>500 m) clay accumulated in the central valley where deep water persisted, whereas interbedded sands and clays were deposited near the valley edges as the lake shoreline shifted location through time. Analysis of 77 transient electromagnetic (TEM) soundings acquired in and west of the Park (excluding the dune field and mountains) identifies an electrical conductor (<10 ohm-m) that corresponds well with massive clay in wells at depths of 70-200 m. Higher resistivities in sediments above the electrical conductor correlate with increasing sand content. In a recently drilled 100-m deep corehole near the southwest corner of the Park, the conductor corresponds to a clay-dominated sequence below 71 m that includes two thick clay layers of different color and an intervening layer dominated by sand. The electrical conductor was detected in all soundings, except those within 2-6 km of the range front. A contour map of elevations on the top of the conductor shows (1) a 120-m deep trough that generally corresponds to the deepest part of the Baca graben as indicated by 3D gravity modeling, and (2) two northeasterly striking, down-to-the-north normal faults. A concealed fault prominent in aeromagnetic data south of the Park corresponds to truncations of predominantly clay layers modeled in TEM profiles, suggesting that clay layers are fault bounded near the mountain front.