Abstract

The Permo-Pennsylvanian Cutler Formation near Gateway, Colorado, is the most complete (1,334 m) proximal section of alluvial-fan sediments deposited along the western flank of the Ancestral Uncompahgre uplift. Cutler facies can be correlated with depositional processes observed on modern “dry” alluvial fans. Proximal Cutler facies include matrix-supported bouldery debris-flow and channel-form streamflood conglomerates. Midfan sedimentation in the Cutler is represented by trough–cross-bedded, granular, braided-stream sandstones, laterally continuous streamflood conglomerates, and sheetflood deposits. Laterally continuous streamflood conglomerate was deposited at the mouth of large channels near the intersection point and consists of a cross-bed set as much as 2 m thick with a basal boulder-cobble lag. Rippled and laminated siltstone with gravel channels represents distal sheetflood sedimentation. Pedogenic features include rhizocretions and calcareous nodules.

Vertical changes in facies and maximum clast size delineate three megasequences on the scale of hundreds of metres thick. Each mega-sequence is composed of a coarsening-upward sequence of proximal facies overlain by a fining-upward sequence of more distal facies. Coarsening-upward sequences record periods of tectonic uplift and fan progradation, whereas fining-upward sequences result from tectonic quiescence and weathering-back of the mountain front. Small-scale cycles on the scale of ten metres occur within the larger megasequences and represent changes inherent to the alluvial-fan system (autocyclic).

Sedimentologic data on Cutler alluvial-fan sediments at Gateway support previous interpretations of semiarid or arid paleoclimate during Permian time along the western flank of Uncompahgria and may act as a standard of comparison for tests of the role of tectonism on sedimentation trends in the Paradox basin.

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