PHYSICAL CHARACTERISTICS AND AGES OF LOESS DEPOSITS IN THE HATCH POINT-CANYON RIMS AREA, AND CORRELATION TO AEOLIAN AND FLUVIAL RECORDS IN SOUTHWESTERN UTAH

The Hatch Point-Canyon Rims area is an upland plateau located ~35 km south of Moab, Utah. Five trenches were excavated as part of a study examining geomorphic controls on the distribution of shrubland and grassland ecosystems. Trench logs, surficial deposit and soil descriptions, and physical property measurements were made to characterize exposed deposits that included loess, reworked loess, sheetwash, and fluvial-channel fill all of which contain weak to moderate textural B horizons and Stage I – III carbonate horizons. In addition, optically stimulated luminescence (OSL) ages and one ¹⁴C age were determined. The loess deposits consist of two previously unrecognized units, totaling about 2 meters thick. The lower loess unit was deposited between about 10,500 yr B.P. and 7,500 yr B.P. A short hiatus after the deposition of this unit is represented by soil development that included formation of abundant clay films and moderate structure. The upper unit was deposited between about 6,000 yr B.P. and <1,400 yr B.P. and consists of alternating beds of loess, re-deposited loess, and sheetwash deposits. The timing and stratigraphy of these two units in the Hatch Point-Canyon Rims area are consistent with OSL ages of Holocene dune mobilization and alluvial activity determined from previous work 30 km to the west in Canyonlands National Park. Deposition of the lower loess unit in the Canyon Rims-Hatch Point trenches corresponds to a time of dune activity in the Canyonlands area. Deposition of the upper loess, re-deposited loess, and sheetwash deposits corresponds to a time of alternating alluvial and dune activity in the Canyonlands area.