GEOMORPHIC EVOLUTION OF THE LA BOTICA ARCHAEOLOGICAL SITE AND LA JARA CANYON: RELATIONSHIPS TO MIDDLE PLEISTOCENE LAKE ALAMOSA AND THE RIO GRANDE GORGE, SAN LUIS VALLEY, SOUTH-CENTRAL COLORADO, USA

The La Botica archaeological site lies along La Jara Creek, a tributary to the Rio Grande along the southeastern flank of the San Juan Mountains. The site is approximately 20 km upstream from the mountain-piedmont junction, where the Pliocene Los Mogotes composite shield is east-tilted along the western margin of the San Luis basin. The site lies nested in La Jara Canyon approximately 100 meters below the canyon rim and 100 meters above La Jara Creek on a fluvial tread cut on a landslide block. The adjacent canyon walls expose the Pliocene (4-5 Ma) lava flows overlying the Los Pinos Formation and Miocene (20-21 Ma) lava flows of the Hinsdale Formation. The stratigraphic positioning of Pliocene lava flows over weakly lithified deposits of the Los Pinos Formation resulted in toreva-block landslides, as La Jara Creek incised. Cooling fractures present in the lava flows readily produce abundant colluvial and talus deposits. The terrace tread underlying the site is composed of thin, discontinuous, “lag” fluvial gravels mantling eroded landslide debris, which is overlain by two substantial intervals of loess deposition. The earlier loess deposit is capped by a very well-developed soil likely dating to the middle Pleistocene. Deposition of the second loess deposit ceased about 800 B.P. Human occupation of the site began in the early Holocene (Atlantic chronozone) and continued into the nineteenth century. Local residents continued gathering medicinal plants at the site into the 1980s. We target the lower loess unit for Optically Stimulated Luminescence (OSL) and upper for bulk organic carbon radiocarbon (AMS) dating techniques to provide a minimum age for abandonment of the fluvial surface and the termination of Pleistocene loess deposition. We tentatively place the geomorphic evolution and associated landforms into a middle Pleistocene incision sequence, associated with base level control by Lake Alamosa and Rio Grande Gorge incision. Herein, abandonment of broad alluvial deposition and capture of La Jara Creek into its canyon occurred with initial draining of Lake Alamosa ~400-385 ka. Rapid incision and undercutting of Pliocene basalts created the landslide block by ~250 ka. By 200 ka, the landslide obstruction had been removed and incision continued along La Jara and Rio Grande Canyons at ~0.5 mm/yr.