VOLCANIC INFLUENCE OVER FLUVIAL SEDIMENTATION IN THE CRETACEOUS McDERMOTT MEMBER, ANIMAS FORMATION, SOUTHWESTERN COLORADO

There are spectacular examples of modification of modern fluvial systems due to volcanic processes during and after an eruption. These processes can include high influx rates of volcaniclastic sediment, drainage disruption, formation and failure of natural dams, changes in channel geometry and changes in channel pattern. Depending on the magnitude and frequency of disruptive events, the fluvial system might “recover” over a period of years or might change to some other morphology. The goal of this study is to examine a probable ancient analog of a fluvial-volcanic system, evaluate the preservation potential of volcanic features in the fluvial environment, and assess fluvial system recovery. The McDermott Member is the lower member of the Late Cretaceous Animas Formation in SW Colorado, and formed adjacent to the coeval La Plata Volcanic Center. Field studies were based on a proximal-distal transect of six measured sections near Durango, Colorado. In the field, 11 lithofacies have been identified including various types of sandstones, conglomerates, and mudrocks along with autobrecciated lava flows, lahars, and tuffs. Subsequent microfacies analysis suggests the lahar lithofacies might be subdivided into three types based on clast composition and matrix color, this might indicate different volcanic sources or sequential changes in the volcanic center. In addition, microfacies analysis of the tuffs suggests both ash-flow and ash-fall types are present. Two trends can be noted: (1) there is an overall fining-upward trend seen in a transition from lahars, lava flows, and fluvial conglomerates at the base to isolated sandstone-rich channels and extensive mudrock deposits near the top of the unit, and (2) there is a proximal-distal trend in the grain size and thickness of lahars, and the thickness and numbers of tuffs.