HYDROGEOLOGIC CONTROLS BY DEPOSITIONAL AND LITHOFACIES TRANSITIONS - GREAT SAND DUNES NATIONAL PARK AND PRESERVE, COLORADO

The Great Sand Dunes National Park and Preserve encompasses several depositional environments preserved in sediments of Miocene-Pliocene Santa Fe Formation, Pliocene-Pleistocene Alamosa Formation, and in late Pleistocene – Holocene deposits: all important hydrostratigraphic units. Lithofacies transitions control recharge, movement, discharge and interaquifer leakage of ground water through the National Park.

The 2000 “Great Sand Dunes National Park and Preserve Act” directed the Interior Department to obtain water rights to fulfill the purposes of the Park. Accordingly the National Park Service obtained an in-place ground water right in 2008, the first of its kind in Colorado. As a decree condition, ten piezometers were constructed north-south along the western boundary of the Park in 2009. The piezometers generated new data on the uppermost 300+ feet of sediments that preserve evidence of several geologic environments and lithofacies transitions.

The dominant lithofacies in the eastern half of the Park is an alluvial fan or bajada facies characterized by cobble, gravel, and coarse to medium sand, with lenses of silty clay: high-permeability materials from granite and arkosic conglomerates of the adjacent Sangre de Cristo range. A similar lithofacies, with varying degrees of induration, is preserved from the surface to several thousand feet depth, reflecting rapid uplift of the range as the San Luis Basin developed. Deep continuity of the high-permeability facies allows ground water recharge along the mountain front.

Progressing westward, the major sediment source transitions to volcanic rocks from the San Juan Range. This lithofacies is transitional: Sangre-sourced fan deposits to the east, and Rio Grande- sourced alluvial, aeolian, and lacustrine deposits to the west. Further west, the basin floor is dominated by finer-grained aeolian, lacustrine, and alluvial deposits. Aeolian deposits have formed the visually arresting sand dunes that are the chief feature of the Park. Each lithofacies is associated with a characteristic range of hydraulic conductivity and varying degrees of stratification and induration which control aquifer characteristics and interaquifer leakage. Ground water modeling of the Park reflects the transitions, but at a manageable level of hydrostratigraphic representation.