GEOMORPHIC ANALYSIS OF TERRACE DEVELOPMENT ALONG THE ANIMAS RIVER

Due to the large number of gas well archaeological clearance surveys in the Animas River Valley of Northwest New Mexico, an evaluation of the terrace and floodplain surfaces is necessary for developing effective strategies for identifying and protecting cultural heritage sites. Despite a long history of geological and geophysical survey in the San Juan Basin, little documentation is present on the geomorphology associated with the basin’s Animas River. This poster presents an investigation of three proposed terraces, as well as the modern floodplain, that have been delineated for the Aztec and Flora Vista USGS Quadrangles using remote sensing and field observations. Soil and sediment characteristics of these surfaces were evaluated by auger coring and laboratory sampling of texture, carbonate and organic properties, where accessibility permitted.

The upper terrace is represented on both sides of the river as a Pleistocene strath terrace containing three to five meters of gravel with mixed aeolian sediment above the Cretaceous Nacimeinto Sandstone. The second terrace is represented inconsistently on either side of the Animas and was unavailable for testing due to property constraints. The lower terrace is represented on both sides of the river as a late Holocene fill terrace. The last phase of river migration on the lower terrace is obscured by concurrent fan deposition, which has promoted archaeological site preservation and prevented clear allounit delineation. The floodplain appears to have two clear allounits representing historic channel and bar migration, with areas of undifferentiated alluvium, due to fan and drainage channel mixing of the floodplain. Two migration phases were identified for the Animas representing an older phase with large meander loops and a more recent phase with straight braided channels.