NEW MAPPING OF LATE QUATERNARY UNCOMPAHGRE RIVER TERRACES OF WESTERN COLORADO

A new compilation of existing geologic and soil maps, along with elevation data, has yielded the first comprehensive map of late Quaternary terraces of the Uncompahgre River. The new map identifies 7 units including modern alluvium and 6 down-stepping strath terraces that rise up to ~115 m above the Uncompahgre River. Terrace river gravels are dominated by volcanic clasts derived from the San Juan Mountains that form the headwaters for the Uncompahgre River. Deposits are typically rounded, pebble-boulder gravel in a sand matrix, and the terraces are often veneered by loess with various stages of calcic soil development. The terraces extend discontinuously downstream from terminal moraines near Ridgway, Colorado to the confluence of the Uncompahgre and Gunnison rivers at Delta, Colorado. Near Delta, Colorado, the highest terrace level is associated with Lava Creek B tephra, which indicates that the terraces record the evolution of this valley over the past ca. 640 ka. Prior to this time, the ancestral Uncompahgre River migrated northeast down the dipslope of the Uncompahgre Plateau as the river followed the Dakota Sandstone-Mancos Shale contact. The new terrace map will serve as a geologic framework for subsequent dating of individual terrace levels, which will provide a more complete understanding of how glacial-interglacial cycles have influenced terrace development.