NEW EVIDENCE FOR ANCIENT LAKE ALAMOSA IN THE SAN LUIS BASIN OF COLORADO

In 1910, C.E. Siebenthal proposed the existence of a Pleistocene lake in the San Luis Basin of southern Colorado based on fossils and sediments exposed in Hansen Bluff, about 10 km southeast of Alamosa. Extensive studies of the bluff exposures in the 1980s revealed an early to middle Pleistocene section that has abundant plant and animal fossils, two volcanic ashes, and a paleomagnetic record that spans the Brunhes/Matayama boundary. The depositional environments at Hansen Bluff and from nearby cores include shallow to perennial lakes, playas, and meandering streams. However, in the century since Siebenthal’s first work, no one had found morphological expression of the Lake Alamosa or documented its maximum altitude or lateral extent.

During mapping of the Alamosa 1/2° x 1° sheet, I found several new exposures of shallow near-shore and lacustrine deposits. Beach gravels are preserved as bars, spits, and remnant deposits in saddles between bedrock-cored hills. Associated wave-eroded bedrock hills and knobs suggest that the ancient lake reached a maximum altitude of 2325-2330 m in the San Luis Basin. The most prominent features are well preserved at the basin’s southern end, along the northern margin of the San Luis Hills. Distinct spits at 2310-2330 m wrap around the southwestern side of Sierro del Ojita and Saddleback Mountain and a kilometer-long spit is well preserved on the eastern side of the Rio Grande, just north of the basin’s outlet. None of these or the dozen other larger constructional lake features in the area are well exposed, thus their relative age is presently unknown. Using sedimentation rates, Rogers et al. (Nat. Geog. Res., v. 1, 1985) suggested that the section at Hansen Bluff may be as young as 700 ka, an age that could be consistent with the stage III+ morphology calcic soil that has since developed. Overtopping of the lake and subsequent cutting through the San Luis Hills in middle Pleistocene time integrated the upper Rio Grande of the San Luis basin with the middle Rio Grande system to the south.

The age and extent of Lake Alamosa have practical implications in that the last deep-water cycle of the lake deposited clays in the center of the basin. These clays (blue clay of driller’s logs) form the highest aquitard between the upper unconfined aquifer and the lower confined (Alamosa Fm. and Santa Fe Gp.) aquifer in the basin, both of which are critical water resources in the basin.