Triangulating from the cairns…
Northwestern Colorado was broken into a mosaic of fault blocks in the Late Paleozoic. Tectonic activity on these faults produced the sedimentary and structural basin of the northern Central Colorado trough, including its many subbasins, the two principal adjacent mountainous erosional areas, the ancestral Uncompahgre and Front Range uplifts, the basin-center ancestral Sawatch uplift, and many other uplifts within the basin. The Late Paleozoic faults in central Colorado have a dominant north-northwest to northwest trend; those in northwestern Colorado are east-west trending. The Pennsylvanian and Permian strata of northwestern Colorado record a history of active basin subsidence along numemus faults andcontemporaneous sedimentation of up to 5,000 m (17;OOO ft) of nonmarine to marine strata. Abrupt thickness variations and facies changes, onlap and overlap relationships, and, in a few localities, angular discordances occur adjacent to faults and uplift blocks. Faults of significant syndepositional offset throughout the Pennsylvanian and Permian include the Garmesa and Juniper-Axial-Indian Run fault zones, and the Gore, Boreas Pass, Elkhorn, South Park, Agate Creek, Pleasant Valley, Arkansas River; and Crested Butte faults. Many other intrabasinal faults were particularly active during the Desmoinesian, producing local horsts, grabens, and half-grabens. Movement on these structures significantly influenced development of thick evaporite sequences and abrupt facies and thickness changes. During the Early Pennsylvanian, deposition of marine shale, siltstone, and sandstone turbidites of the Belden Formation occurred in the basin-center; while marine carbonates of the Morgan Formation occurred in more basin-marginal positions. Uplift and erosion of portions of the ancestral front Range and Sawatch uplifts caused coarse-grained fluvial and fan-delta sediments to be shed to local basin-edge areas. The Desmoinesian strata record a history of vigorous tectonic activity and basinal subsidence which produced complex patterns of alluvial-plain sedimentation at the edge of high-relief, mountainous uplift blocks, particularly the ancestral Uncompahgre and Front Range uplifts, and basinal, coastal to marine and local salt-basin sedimentation. At least ten, shoalingupward depositional sequences (cycles) occur within Desmoinesian strata throughout the basin. In the northwestern portion of the basin, cycles of marine limestone and eolian sandstone occur in the Morgan Formation. Cycles of black marine shale and siltstone, carbonate rocks, well-laminated anhydrite, and, in some places, thick halite deposits occur in the Eagle Valley Evaporite in the center of the basin. Some cycles contain shoaling-upward sandstones, ranging from marine turbidites to shoreline, eolian, and fluvial deposits. The basin-marginal Gothic and Minturn Formations contain shoaling-upward cycles of marine limestone, shoreline shale, siltstone and sandstone, and fluvial sandstone and conglomerate. Algal bioherms occur conspicuously in several of the cycles. Much of the basin may have been alternately flooded by marine waters and subaerially emergent throughout the Desmoinesian. Nonmarine redbeds of the Maroon Formation and eolian sandstones of the Weber Sandstone dominate the Upper Pennsylvanian and Lower Permian strata of the basin. The topographic low of the basin was near its northeastern margin. Significant uplift and folding of the Wolfcampian strata (Maroon Formation) occurred prior to deposition of the fine-grained redbeds of the State Bridge Formation in the Guadalupian.
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