GEOLOGIC MAP OF ALLUVIAL FANS ALONG THE ZAPATA AND BLANCA SECTIONS OF THE NORTHERN SANGRE DE CRISTO RANGE-FRONT FAULT SYSTEM, SOUTH-CENTRAL COLORADO

The Zapata and Blanca sections of the Northern Sangre de Cristo fault-system map area constitutes apex-to-toe widths of alluvial fans along the range front extending from the Blanca massif salient north to the southern edge of Great Sand Dunes National Park. Motivation for mapping was to create a surficial geologic framework and relative age context for fan development and modification. High-resolution topographic data was used to differentiate and assign preliminary relative ages to fan surfaces, mainly based on topographic position and surface roughness. Fan surfaces were described through detailed fieldwork cataloging sedimentological features and degree of modification undergone during periods of non-deposition. All fans in the mapping area are debris-flow dominated. Fan surfaces are split into nine units representing relative age by designation of old, intermediate, and young with sub-units of 1, 2, and 3—1 being oldest. Depositional fabrics evident of debris-flow are preserved most clearly on youngest fan surfaces that have undergone the least amount of modifying processes. Reworking and modification processes are expressed more obviously on older surfaces by increasing degradation of original depositional fabric, pedogenesis, weathering of clasts, gullying, input of eolian material, and faulting with age. Approximately seven coalesced fans in the northern portion of the map are comparatively smaller (<3 km radii), sourced from shorter, steeper, non-glaciated drainage basins, and tend to be relatively younger. Six larger fans (~5-6 km radii) are sourced from long, extensively glaciated drainage basins on the southern portion of the Blanca Massif. From north to south they are the North Zapata, South Zapata, Pioneer, Holbrook, Tobin, and Blanca fans, named by the dominant watercourse of the catchment from which they are sourced. Southern fan surfaces are older than those to the north and, except for Blanca, are capped by moraines. The South Zapata fan features the oldest mapped fan unit, and with a southern progression, the oldest unit of each fan decreases in age southward. With this surficial geological framework and relative age context, more detailed inferences can now be made about the range-front development of the western Sangre de Cristo Mountains that highlights alluvial fan development and modification.