BASALT GRAIN DISTRIBUTION IN THE CIMARRON RIVER AND ITS TRIBUTARIES, CIMARRON COUNTY, OK

Mesa de Maya, otherwise known as Black Mesa, is a plateau capped with Pliocene Basalt stretching for ~70 kilometers through parts of Colorado, New Mexico, and Oklahoma. The Cimarron River runs along the southern foot of the plateau for ~25 kilometers in New Mexico and Oklahoma. The objective of this study is to better understand the fate and breakdown of basalt grains during fluvial transport within the Cimarron River. Measuring the downstream behavior of these grains provides clues about the nature of fluvial sand provenance in semi- arid environments.

Bedload sand samples were collected at 14 sites from fluvial bedforms within the Cimarron River and its tributaries. Point counts were performed on approximately 200 medium-sized sand grains from each sample. Point count data was compared to transport distance. Point counts from a creek with headwaters on Black Mesa proper show basalt grain concentrations that decrease rapidly with distance. However, samples obtained from the Cimarron River indicate basalt grain concentrations remain constant through the 72 km of the study area. Basalt grains were also observed in tributaries of the Cimarron River that have no basalt outcrops in their drainage basins.

The presence of basalt grains in the tributary streams is attributed to the presence of basalt gravels in the Ogallala Formation. The Ogallala was deposited at approximately the same time as the basalt flows, thus basalt clasts were incorporated into its sand and gravel beds. Erosion of the Ogallala Formation from the Pleistocene through the present provides basalt grains to the Cimarron River and its tributaries. This source of basalt combined with the rapid decrease in basalt grain concentrations in the creek with headwaters on Black Mesa suggests that under modern semi-arid conditions few if any basalt grains from Black Mesa are being supplied to the Cimarron River. The Ogallala gravels must be the source of most of the basalt grains in the modern Cimarron River.