Fine-grained debris flows in coarse-grained alluvial systems: Paleoenvironmental implications for the late Paleozoic Fountain and Cutler formations, Colorado, U.S.A.

The Fountain and Cutler formations are coarse-clastic alluvial wedges that mantled ancestral Rocky Mountain uplifts. Muddy granule sandstone (MGSF) is a volumetrically important facies to both systems. The facies is massive and unsorted with grain-size distributions that range from clay to granule. These deposits are best characterized as cohesive fine-grained debris flows. Yet, the MGSF rarely contains clasts > 10 mm, although other intercalated alluvial facies commonly contain cobbles and small boulders. Competence modeling was undertaken to assess the amount of water needed to account for the observed coarsest fraction of the MGSF. These results indicate that flows would need inflation with water by one-third to two-thirds, depending on clay mineralogy and the range of clay in the MGSF. This paper proposes that flows began as debris flows but underwent flow transformation through incorporation of water during flow in the paleohighlands. Dilation of the flow reduced competence, and each flow lost the coarser than 10 mm fraction. Flows would rheologically stiffen upon reaching the unconfined alluvial surface, ultimately behaving as cohesive fine-grained debris flows. The MGSF at both study areas does not fit into end-member facies models of alluvial fans. Depositional systems elsewhere that exhibit similar facies may want to consider flow transformation and the associated environmental implications when inferring the depositional setting.