ORIGIN OF THE LATE QUATERNARY DUNE FIELDS OF NORTHEASTERN COLORADO

Stabilized eolian deposits, mostly parabolic dunes and sand sheets, co ver much of the landscape of northeastern Colorado and adjacent parts of southwestern Nebraska in four geographically distinct dune fields. Stratigraphic and soil-geomorphic relations and accelerator radiocarbo n dating indicate that at least three episodes of eolian sand movement occurred between 27 ka and 11 ka, possibly between 11 ka and 3 ka, an d within the past 1.5 ka. Thus, eolian sand deposition took place unde r both glacial and interglacial climatic conditions. In the youngest e pisodes of eolian sand movement, Holocene parabolic dunes partially bu ried Pleistocene sand sheet deposits. Late Holocene sands in the Fort Morgan and Wray dune fields, to the south of the South Platte River, h ave trace element ratios that are indistinguishable from modern South Platte River sands, but different from Ogallala Formation bedrock, whi ch has previously been cited as the main source of dune sand on the Gr eat Plains. Sands in the Greeley dune field, to the north of the South Platte River, have trace element concentrations that indicate a proba ble Laramie Formation source. Measurements of parabolic dunes indicate paleowinds from the northwest in all dune fields, in good agreement w ith resultant drift directions calculated for nearby weather stations. Thus, paleowinds were probably not significantly different from prese nt-day winds, and are consistent with a South Platte River source for the Fort Morgan and Wray dune fields, and a Laramie Formation source f or the Greeley dune field. Sand accumulated downwind of the South Plat te River to form the Fort Morgan dune field. In addition, sand was als o transported farther downwind over the upland formed by the calcrete caprock of the Ogallala Formation, and deposited in the lee of the upl and on the southeast side. Because of high wind energy, the upland its elf served as a zone of sand transport, but little or no sand accumula tion took place on this surface. These studies, which demonstrate the importance of fluvial-source sediments for dune fields in Colorado, ma y be applicable to other dune fields in North America. Because modern drift potentials in northeastern Colorado are among the highest in the world, the present stability of dunes in the region may be in part a function of the dunes being supply-limited rather than solely transpor t-limited. Extensive (similar to 7700 km(2)) late Holocene dunes docum ent that eolian sand in northeastern Colorado is very sensitive to sma ll changes in climate or fluvial source conditions.