ABSTRACT

Structurally complex Pennsylvanian and Permian strata comprise the bulk of the Sangre de Cristo Range between La Veta Pass and Westcliffe in south-central Colorado. They are at least 14,000 feet thick near Crestone in the northern part of the area, and more than 9,000 feet thick at La Veta Pass on the south. The sediments are predominantly arkosic clastics, eroded from Precambrian crystalline rocks of the San Luis-Uncompahgre highland on the West, and rapidly deposited in a narrow, northwest-trending geosyncline. This sector of the Sangre de Cristos is nearly parallel with the geosyncline and situated near its western side. The terrigenous clastics become finer-grained and Desmoinesian limestones become abundant southeastward, reflecting southward decrease in uplift of the highland and a more basinward position of the southern exposures.

The Kerber formation (Morrowan ?), oldest Pennsylvanian unit in this area, apparently can be recognized only at Grayback Mountain, west of Huerfano Park; presumably it was partly removed by erosion prior to Atokan deposition.

The name Deer Creek formation is proposed for beds previously called Clastic member of the Sandia, from Huerfano River south to the New Mexico border. At its type locality west of Huerfano Park, this formation is late Atokan, 1,119.2 feet thick, and consists of variegated sandstones, siltstones, conglomerates, and shales chiefly of fluvial origin, with interbedded marine limestone and terrigenous clastics in the upper part. The Deer Creek formation is differentiated by its redbeds.

The Madera formation contains Fusulina rockymontana (Roth and Skinner) and is of Desmoinesian age. At La Veta Pass it is about 3,068 feet thick and is divided into the lower, Gray limestone member, 533 feet thick; the Arkosic limestone member, 2,369 feet thick; and the upper, Whiskey Creek Pass limestone member, 166 feet thick. The first two members accumulated under alternating marine and fluvial environments. The increasing predominance of continental sediments in the Arkosic limestone member reflects eastward migration of the shoreline. The Whiskey Creek Pass limestone records stability in both depositional and source areas.

The Madera grades northward into its stratigraphic equivalent, the Minturn formation. The Minturn is at least 5,000 feet thick and consists chiefly of grayish to greenish sandstones, conglomerates, and shales or siltstones. It is mainly fluvial, but marine sediments occur at several horizons. Near the top, a calcareous zone containing marine fossils is probably equivalent to the Whiskey Creek Pass limestone.

Overlying the Madera near La Veta Pass and Pass Creek Pass is a sandstone section about 1,000 feet thick, in fault contact with the overlying Sangre de Cristo formation. It is here named the Pass Creek sandstone.

The Sangre de Cristo formation, of late Pennsylvanian (?) and probably Permian (Wolfcamp) age, contains up to 8,000 feet or more of predominantly fluvial strata. It is differentiated from underlying units by its redbeds. Two members are recognized in the northern part of the area: the Lower member, characterized by piedmont cyclothems; and the Crestone conglomerate. The latter, a thick fanglomerate, is the only positive evidence of mountains in the San Luis-Uncompahgre highland, and is the last record of Paleozoic orogeny in this area.

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