FINE-GRAINED DEBRIS FLOWS IN COARSE-GRAINED ALLUVIAL SYSTEMS: PALEOENVIRONMENTAL IMPLICATIONS OF THE FOUNTAIN AND CUTLER FORMATIONS, COLORADO

Late Paleozoic structural basins in western equatorial Pangea typically contained an alluvial surface that graded towards a coeval shoreline. Despite the low-latitude setting of the region during this time, climatic interpretations that influenced these alluvial systems widely vary, including hot and arid, hot and humid, or periodically cool and humid. The early Permian Cutler Formation and the Pennsylvanian Fountain Formation are two records of these surfaces with varied climatic interpretations. Both of the units are coarse-grained containing cobbles and rarer boulders. Also common to both of these units is a massive muddy pebbly sandstone facies that is best characterized as a mass flow event. Grain-size analysis of these units span at least 15φ bins. Average clay weight percent for the facies is ~ 10% and 8% for the Fountain (n = 8) and Cutler (n = 5) samples, respectively.

Massive, poorly sorted deposits that exhibit abundant granule, sand and mud with this magnitude of clay should behave as cohesive debris flows during transport unless the flow was substantially inflated with water during each event to reduce the competency and cohesion of the flow. Conversely, if the flow was not inflated, then a cohesive debris flow should be capable of transporting the largest grain sizes observed in both systems. Yet, the coarse fraction (10 to 300 mm) common to both systems is conspicuously lacking in these facies. Theoretical modeling was undertaken to assess the amount of water needed to inflate the flow with water such that the matrix strength and buoyancy are capable of transporting only the observed grain size distribution. These results indicate that for a kaolinite-based slurry the matrix space would need to approximate 55% of the flow volume, which equates to a flow that is ~ 43 volume percent water. If the slurry was smectite-based, then the results are amplified and the flow would need to exhibit ~75 volume percent water. These fine-grained debris flows are common throughout the stratigraphic records of these two formations and suggest that water was readily available in both systems and support humid climatic interpretations. Whether flows inflated during the event through the incorporation of surface water or began with high-water content is not known at this time.