New Mexico Geological Society Annual Spring Meeting — Abstracts

Tributary alluvial fans in the lower Chama River Canyon, in northern New Mexico between Christ in the Desert Monastery and Big Eddy boat ramp north of Abiquiu Reservoir, affect the main channel form and slope of the Chama River in various ways. Some fans deposit bouldery debris flows into the river, creating rapids. Other fans deposit large volumes of sandy sediment, in some cases pushing the river to the opposite side of the valley. The purpose of this study is to determine the controls on alluvial fan size, morphology, and sediment texture that result in these differences.

Most tributaries have cut channels into their fans, creating exposures of the fan stratigraphy. Sample sites along the length of each tributary channel were used to determine particle size distribution, from which the median particle size, sorting, and other measures where derived. In the field, the fan boundaries and channel boundaries were mapped. Digital elevation models (DEMs) were used in ArcGIS to delineate the drainage basins for each fan, the area, relief, and slope angles of the drainage basins, and the area and slope of the fans. ArcGIS was also used to determine each geologic units' role on particle size and distribution on fan; in the study area the geologic units include Triassic Chinle Group through Cretaceous Dakota Formation sedimentary rocks.

Drainage basin area, stream length, and stream magnitude have a significant impact on particle size on fan. Larger area, longer length, and higher magnitude produce larger mean particle sizes on fans. The ratio between fan size and drainage basin size also appears important. Smaller fans tend to have less fining of clast size down fan, especially if their drainage basin is large in comparison with fan area. These fans produce bouldery debris flows into the river. Large fans, especially if their drainage basin is relatively small, have more down fan fining of particle size. In some cases these fans redirect the river channel with large volumes of sandy sediment.

Bedrock geology appears to have a strong influence on the fan's particle size, where a greater percent area of Dakota Fm produces abundant boulders and a larger mean particle size. Morrison Fm and Entrada sandstones tend to weather by granular disintegration, and along with Petrified Forest Fm mudstones, produce overall smaller particle sizes.