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The Pennsylvanian Minturn and Pennsylvanian-Permian Sangre de Cristo Formations of the northern Sangre de Cristo Mountains comprise a 3,800-m (12,500-ft) thick progradational sequence of coarse clastic sediments shed into a basin on the northeastern side of the late Paleozoic San Luis-Uncompahgre highland. From bottom to top, the mostly marine Minturn Formation contains probable deltaic (700 m, 2,300 ft), mixed fan-delta and prodelta (800 m, 2,600 ft), and fan-delta (600 m, 2,000 ft) deposits; the mostly continental Sangre de Cristo Formation contains distal alluvial fan (600 m, 2,000 ft) and proximal alluvial fan (1,100 m, 3,600 ft) deposits. This sequence of deposits coarsens and passes upward from mostly gray (reduced) nearshore marine strata in the Minturn to mostly r d (oxidized) continental strata in the Sangre de Cristo Formation. The sequence reflects the rise of the San Luis-Uncompahgre highland beginning in Middle Pennsylvanian and later time, as indicated by fusulinids identified in the Minturn Formation. At least three episodes of uplift are indicated by the distribution of unconformities, geometry of intertonguing facies, and abrupt vertical changes in facies.
The deltaic and mixed fan-delta and prodelta deposits of the lower and middle parts of the Minturn Formation consist of coarsening-upward cycles 30 to 300 m (100 to 1,000 ft) thick of shale, siltstone, sandstone, and conglomeratic sandstone. Some of the shales in the lower part of the Minturn are interpreted as having been deposited on the delta plain because they contain land plants in growth position. The mixed deposits in the middle part of the Minturn contain cycles of shale, proximal-turbidite sandstones, and conglomeratic sandstone; such cycles are interpreted as deposits of submarine fans overridden by fan deltas. Fan-delta deposits in the upper part of the Minturn consist of conglomeratic sandstone and thin limestone beds containing fossils of shallow-water marine invertebrate ; fan-delta sandstones locally contain large-scale cross-bedding interpreted as deltaic sedimentation units.
Continental deposits of the lower member of the Sangre de Cristo Formation consist of fining-upward cycles 2 to 37 m (6.5 to 121 ft) thick of cross-bedded conglomerate, sandstone, and siltstone, and siltstone deposited by braided streams on the distal parts of alluvial fans. The upper part of the Sangre de Cristo Formation, known as the Crestone Conglomerate Member, consists of proximal alluvial-fan deposits of conglomerate and coarse sandstone. Abundant poorly sorted conglomerates are interpreted as debris-flow and mud-flow deposits; sandstones containing horizontal stratification, low-angle cross-bedding, paleoplacers of black sand, and outsize clasts are interpreted as streamflow and sheetflow deposits.
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