LATE QUATERNARY LOESS AND SOILS ON UPLANDS IN THE CANYONLANDS AND MESA VERDE AREAS, UTAH AND COLORADO

Thin loess deposits have long been recognized on the uplands of the southeastern Colorado Plateau, along with dune sands derived largely through weathering and reworking of sandstone. The presence of loess and dust-affected soils in this region is important to native and modern farming cultures, and provides an opportunity to assess the link between alluvial and aeolian processes and their influence on the soil landscape. However, there have been few studies of soils formed on loess in this area, and the ages of loess deposits are poorly constrained. We sampled loess deposits and their soils from trench exposures on mesas in the Hatch Point area of Canyonlands as well as two outcrops in Mesa Verde National Park, and dated the deposits using optically stimulated luminescence and radiocarbon techniques. Three sandy loess and reworked loess units are present at Hatch Point. The oldest buried unit yielded two ages of 10,370 and 7,555 yr; it has a moderately developed soil with a cumulic argillic horizon and stage II-III calcic horizon. The middle unit also has a cumulic argillic horizon and stage I-II calcic horizon; 10 OSL ages and one radiocarbon age range from 6220 to 1385 yr. The overthickened argillic horizons indicate that pedogenesis was in part coeval with gradual loess deposition. The youngest unit is only locally present; it has a weak soil and a single age of 1740 yr. At Mesa Verde, two or three loess units are preserved in the two outcrops we examined; 6 OSL ages range from 51 to 17 ka. At least one buried soil is present between two loess units with ages of 51 and 34 ka. Based on older work, Holocene loess is likely present at Mesa Verde, but was not preserved at our sample sites. The ages of the loess units in both study areas correspond well with OSL-dated dune sands in Canyonlands National Park 20-30 km to the north and south of Hatch Point, and with dune sands on Black Mesa, Arizona, about 200 km to the south. The loess ages also are coeval with alluvial deposition in Canyonlands and in the Grand Canyon area, suggesting that loess deposition is linked with sediment supply from alluvial processes.