Counting marmots…
Investigation of the Spread Eagle Peak area in the north-central Sangre de Cristo Range, Colorado, was undertaken to complete a series of geologic mapping studies in this region. The bulk of the range in th~s area is composed of a thick s equence of complexly folded and faulted Pennsylvanian and Permian s trata which may have an aggregate thrckness of as much as 14,000 feet. An area of approximately 70 square miles was mapped using aerial photographs with particular emphasis on the structural f eatures of the range. Precambrian rocks comprised of granite and metasediments occur in zones as much as 1-112 miles wide on both flanks of the range. Imbricate segments of lower P aleozoic strata c rop out in a fault zone between Precambrian,rock s and Permo-Pennsylvan ian clastics on the west flank of the range. These lower P aleozoic strata are probably remnants of Ordovician, Devonian and Missis- sippian rocks, chefly dolomites, limestones and sandstones. Strata of Perm-o-Pennsylva nian age may be divided into two general units: the lower unit, the Minturn Formation, is comprised of conglomerate, sandstone, and shale with thin l imestones interbedded near the top; the upper unit, the Sangre de Cristo Formation, is comprised of coarse arkosic sandstones and conglomerate. This thick sequence of sediments w as derived predominantly by erosion of Precambrian crystalline rocks of the San Luis-Uncompahgre highland to the west, and was deposited in a rapidly subsiding narrow trough, a zeugogeosyncline. No trace was found oi a large T ertiary stock which was pre- viously mapped along the Saguache-Custer .County line. Tertiary volcanic rocks covlering about 2-112 square miles occur in the northeastern part of the area. Dikes and sills also of Tertiary age occur separately and in clusters throughout the area. Pleistoccne valley glaciers were active throughout the hlgher parts of the range. In the area mappkd, two distinct stages of Laramide thrusting of opposing direction arc inferred from field data. In adjacent areas to the north and south most of the evidence reflects only a single stage of thrusting. A klippe, previously unreported in the Sangre de Cristo Range, was mapped near the crest of the range. . The structural f ramework of the Wet Mountain Valley is reinterpreted. Evidence is presented to show that this valley is probably genetically related to basin-and-range type structure rather than an intermontane synclinal depression. It is also proposed t hat the Wet Mountain massif be extended westward to include the West Mountain Valley and the Precambrian rocks which form the east flank of the range in the report area. A normal fault is inferred along the base of the range facing the Wet Mountain Valley. Displacement on this fault ie thought to be relatively small, but its genesis is probably similar to that of the high-angle fault which bounds the range on the west.
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