LAHARS OR NOT LAHARS? AN EVALUATION OF THE VOLCANIC CONNECTION FOR THE LATE CRETACEOUS MCDERMOTT FORMATION NEAR DURANGO, COLORADO

Migration of the Late Cretaceous transcontinental seaway from southwestern Colorado was marked by deposition of the McDermott Formation. This distinct stratigraphic unit marks the uplift of the Laramide San Juan dome with concomitant development of the San Juan basin, and the onset of terrestrial environments that continues today. Understanding the origin of the McDermott Formation is essential to gaining insight into the geologic and tectonic processes at this important strand of time.

Documentation of stratigraphic facies within the McDermott Formation in the Animas River valley south of Durango reveals six major units and several intraformational erosion surfaces. Sedimentary facies in the McDermott Formation reflect alluvial processes that range from debris flows to hyperconcentrated and dilute flows. Conglomeratic units contain abundant clasts of fine- to coarse-grained igneous rocks with phenocrysts of augite and hornblende. The textures and mineral compositions of these clasts are similar to shallow intrusive rocks in the La Plata Mountains to the northwest.

The origin of the McDermott Formation is controversial. A widely accepted hypothesis is that it formed by deposition of lahars, which carried volcanic debris produced by eruptions that were centered over the La Plata Mountains. No conclusive evidence for this proposed volcanic event, however, has been found and documented. We offer a competing model in which the southern margin of the La Plata Mountains laccolithic complex was oversteepened and collapsed. Intrusive material released by this flank collapse was then transported and deposited by debris and hyper-concentrated flows. This latter model requires little or no volcanic material, which is consistent with the dominant igneous material in the McDermott Formation.