ABSTRACT

The Middle Pennsylvanian Minturn Formation and the Pennsylvanian and Permian Sangre de Cristo Formation of the northern Sangre de Cristo Range form a 4,000-m (13,000-ft) thick, progradational sequence of fan-delta and alluvial-fan deposits. This sequence was deposited along the western margin of the central Colorado trough during faulting and uplift of the late Paleozoic Uncompahgre highland of the Ancestral Rocky Mountains.

The Minturn Formation is composed mostly of sandstone and shale deposited by fan deltas that prograded into the central Colorado trough. The Minturn of the northern Sangre de Cristo Range is divisible into (1) a turbidite-bearing facies, (2) a limestone-bearing facies, and (3) a red-bed facies. The turbidite-bearing facies is interpreted as deposits of fan deltas that prograded onto the sea bottom below wave base. The limestone-bearing facies is interpreted as deposits of fan deltas that prograded onto a shallow sea bottom above wave base. Turbidites and shallow-marine limestones, although they make up only a minor part of the Minturn Formation, are mutually exclusive deposits that serve to distinguish the two facies. In the limestone-bearing facies, sandstones containing deltaic foresets overlie thin shallow-marine limestones and are considered diagnostic of that facies. Both facies contain thick intervals of sandstone and shale interpreted as deltaic and alluvial deposits. Where it onlaps the Uncompahgre highland, the lower part of the Minturn Formation contains quartzose and arkosic red beds of probable alluvial origin.

The continental Sangre de Cristo Formation conformably overlies the Minturn Formation basinward, but it unconformably overlies Minturn Formation and Precambrian basement near the Uncompahgre highland. The Sangre de Cristo Formation contains (1) a sandstone facies deposited on the distal surfaces of alluvial fans, and (2) a conglomerate facies (Crestone Conglomerate Member) deposited on the proximal surfaces of alluvial fans. The sandstone facies consists of fining-upward cycles of red conglomeratic sandstone and siltstone interpreted as braided-stream deposits. The conglomerate facies consists of poorly sorted conglomerates interpreted as debris-flow and mudflow deposits, and sorted conglomerate and sandstone interpreted as streamflow and sheetflow deposits.

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