PRELIMINARY MAGNETIC ANALYSIS OF THE USGS BP3 CORE, GREAT SAND DUNES NATIONAL PARK, COLORADO

Rock magnetic analysis of a 99.4-m(326-ft)-deep core within the San Luis Valley along the southwestern boundary of Great Sand Dunes National Park, south-central Colorado, provides important information on the age of the recovered sediments. Within the San Luis Valley, subsurface clay deposits are ubiquitous and are thought to be deposits of the laterally extensive, ancient Lake Alamosa. Lithology, magnetic susceptibility, paleomagnetism, and anisotropic magnetic susceptibility were studied in the approximately 22.8 m (75 ft) of recovered core. We identified three broad lithologic units in the drillhole: an uppermost sand unit with the presence of interbedded clays increasing with depth (0 m–71.9 m; 0 ft–236 ft depth), alternating beds of thick blue clay and sand (71.9 m–89.9 m; 236 ft–295 ft), and a lower gray clay with more abundant sand content (89.9 m–99.4 m; 295 ft–326 ft). Magnetic susceptibility values correlate well with these general unit classifications; higher susceptibility values were associated with the more sandy horizons. Forty-seven paleomagnetic samples collected from the core were subjected to alternating field demagnetization, and the results of best-line-fit analysis of the paleomagnetic data demonstrate a change from normal to reversed polarity with depth, just above the blue clay. This reversal most likely represents the 0.78-Ma Brunhes-Matuyama boundary and thus provides a unique age constraint at depth. The Brunhes-Matuyama boundary also was observed within a core collected from Hansen Bluff, 38 km to the southwest. Measurement of anisotropic magnetic susceptibility for these samples demonstrated that minimum susceptibility axes were steeply inclined, as expected if the sediments had been little affected by drilling deformation. Coupled with aerial electromagnetic studies across the valley we believe this magnetic analysis will lead to a better understanding of the distribution and history of the clays of the region.