PLIOCENE TO MIDDLE PLEISTOCENE EVOLUTION OF THE UPPER RIO GRANDE, NORTHERN NEW MEXICO AND SOUTHERN COLORADO

In the middle Pleistocene, the upper Rio Grande had its headwaters in northern New Mexico. At that same time, the uppermost reach of the modern Rio Grande emptied into Lake Alamosa, which probably formed as a result of voluminous eruption of Servilleta flood basalt in the southern part of the San Luis Basin (SLB) between 3.7 and 4.8 Ma.

Servilleta basalt covers most of the Taos Plateau northward to the Colorado/New Mexico border. On the western side of the SLB, basalt is at the surface north to Antonito, Colorado. New USGS aeromagnetic data suggest the subsurface basalt flows continue at least as far north as La Jara at depths of 30-200 m. Servilleta basalt is also preserved on the eastern side of the SLB beneath the Costilla Plain, on San Pedro Mesa, and in the Culebra graben north to Fort Garland, Colorado. Drill-hole data show that basalt extends northward to Blanca in the shallow subsurface. Thus, by middle Pliocene time (ca. 3.5 Ma), Servilleta basalt probably blocked south-flowing drainages in Colorado: the resulting closed basin was occupied (episodically) by Lake Alamosa in which the sediments of the Alamosa Formation were deposited. This formation is well known for thick “blue clays” that form confining layers within fluvial aquifers of the SLB.

Alluvial and lacustrine sediment nearly filled the upper SLB prior to lake overflow, sometime around 450 ka as estimated from a preliminary 3He exposure date of ~439±6 ka on a boulder or reworked basalt and from strong calcic soils on shorelines, barrier bars, and spits at 7650-7670 ft elevation around the northern margin of the San Luis Hills. When the lake rose during a middle Pleistocene glacial cycle (perhaps marine OIS 12), it overtopped a hydrologic sill on Oligocene Tertiary rock of the Fairy Hills and cut a deep gorge. The integration of the upper SLB into the upper Rio Grande drainage led to downstream incision of the river, especially between the Red River and Culebra Creek, that later of which had previously been the Rio Grande's northernmost tributary.