MODELING COMPETENCY THROUGH VOLUME EXPANSION OF FINE-GRAINED DEBRIS FLOWS HOUSED WITHIN THE FOUNTAIN FORMATION, COLORADO

The Pennsylvanian Fountain Formation comprises one of the coarse-clastic, wedge-shaped units that mantle the Precambrian cored uplifts of the ancestral Rocky Mountains. One of the most volumetrically significant facies within the lower 500 m of the Fountain Formation is a very poorly sorted, massive pebbly mudstone. Beds range up to 4 m but are commonly 1-2 thick. Grain size distributions are skewed to the fine fraction with a σ¹ of 3.6 Φ indicating very poor sorting. Clay and mud weight volumes range from 10 to 12% and 35 to 46%, respectively. Gravel fractions are < 10%. On average the finest 50 percent of the flows is < .065 mm, and the finest 10 percent is < .002 mm. To test for pedogenic accumulation of clays within this facies, we sampled beds that showed weak to strong pedogenesis. These samples had clay weight volumes of 17-26% depending on the degree of pedogensis, yet all of the pedogenic control samples had 70-116% more clay percent than unaltered deposits. Thus, a detrital origin for the clay is reasonable in deposits that show no pedogenesis.

The lack of sorting or stratification within the beds suggests a flow behavior similar to debris flows, however the lack of cobbles and boulders common to other facies stratigraphically adjacent suggest low competency uncharacteristic of 10-12% clay from debris flows elsewhere. Sweet and Soreghan (2010) proposed that the deposits could be the result of flow transformation from high-competency debris flow to low-competency, fine-grained debris flow. Here we explore an alternative hypothesis, that volumetric expansion of the flow from incorporated water significantly lowered competency. To test the expansion hypothesis, the flow was modeled to reflect 35%, 60% and70% volume matrix expansion where matrix is composed of water and clay. Assuming clay:water ratio as the sole competency driver, the model results suggest that flow competency varied from 57 mm, 3.5 mm and 1.7 mm, respectively, for each expansion percentage modeled. The coarsest 5% of these flows is typically >1.2 mm and ranged up to 4.5 mm suggesting that if the volume expansion model is the tenable mechanism, then the amount of matrix volume expansion of the flow was > 60% of which > 79% of that matrix space was water. This evidence suggests readily available and abundant water in the depositional system.