ABSTRACT

Outcrops of the lower part of the Cutler Formation near Gateway, Colorado, contain proximal alluvial-fan deposits of two principal lithofacies. The diamictite lithofacies (D) consists of a variety of sharply bounded, massive, muddy beds which are interpreted as deposits of subaerial debris flows. The gravelly sandstone lithofacies (S) consists of amalgamated beds of laminated and cross-bedded conglomeratic sandstones which are interpreted as water-deposited sediments. Clasts in S are rarely larger than 0.5 m in maximum diameter, but clasts in D are commonly much larger (up to 8 m). Both normal and inverse grading are visible in D, but grading is generally not well developed in either lithofacies. Imbrication of clasts, although more pronounced and more consistent in S, is fairly commo in both lithofacies.

Four subfacies of diamictite are distinguishable on the basis of clast packing and grading: massive matrix-supported (Dram), graded matrix-supported (Dmg), inversely graded clast-supported (Dci), and massive clast-supported (Dcm). Given the hydrodynamic and rheologic properties of debris flows as reported by previous workers, four debris-flow regimes are postulated on the basis of varying water content and clast interaction. Subfacies Dram and Drag typify deposits of flows having viscous clast interactions; subfacies Dci represents deposits of flows having inertial or colliding clast interactions. Subfacies Dram comprises deposits of flows with demonstrably high yield strengths. Subfacies Dcm represents deposits of flows in which yield strength was insufficient to support most clasts. This classification specifically recognizes turbulence, selective deposition, and bedload transport of clasts as essential characteristics of certain dilute debris flows. It also accommodates as debris-flow deposits some beds which have features usually associated with aqueous deposition, including clast frameworks, channelization, imbrication, and normal grading.