Bedrock Influences on the Geomorphic and Alluvial History of a Semi-Arid Watershed, Northwest New Mexico – Simple or Complex Evolution?

The geomorphic history of alluvial systems in arid and semiarid settings typically reflects the complex interactions between driving and resisting forces, principally climate and geology, and the geomorphic processes and responses resulting from those interactions. It is not uncommon for the geomorphic response to regional climate change events to be similar over broad areas. However, in northwest New Mexico, the geomorphic evolution and Holocene alluvial record of the 1,200 km2 Kim-me-ni-oli Wash ephemeral discontinuous arroyo system have been influenced and complicated by the presence of contrasting bedrock types, such that nearby watersheds appear to have distinctive alluvial histories. Quaternary geologic and geomorphic data has helped to reconcile the complex, and apparent conflict in alluvial stratigraphy. This information includes: radiocarbon dated Holocene alluvial stratigraphy and correlation with dated stratigraphy in adjacent watersheds; abundance of archaeological sites in the region; weakly to moderately developed surface and buried soils having Btk and Bwk horizons; and geomorphic analysis of drainage basins that are underlain by contrasting bedrock types. The upper headwater region, underlain by resistant sandstones, shows consistent drainage morphometries; large volumes of Holocene age sandy alluvium several meters thick was temporarily stored in tributary and axial bedrock valleys. The middle and lower drainage basin, underlain by erodible mudstone, responded rapidly to base-level changes by cycles of cutting and filling. Small, local, resistant paleochannel bodies in the fluvio-deltaic mudstone bedrock locally impeded incision and caused non-uniform migration of base-level into some tributary basins. This has resulted in different degrees of basin development as reflected in variable morphometry and distribution of surface soils and their hydrologic characteristics. The seemingly simple geologic setting of this watershed underscores the necessity for carrying out thorough geologic and geomorphic characterization in order to understand the history as well as past, present, and future geomorphic responses to environmental change.