The Pennsylvanian and Permian sediments of a region in central Colorado are described in regard to their areal extent, lithologic characteristics, stratigraphic relations, and geologic age. Several stratigraphic units are mapped. Three of these are established as new members of the Maroon formation, namely, the Coffman conglomerate member of Pennsylvanian (?) age, and the Chubb siltstone and Pony Spring siltstone members of Permian age. The structure of the rocks in this area is discussed, a major contribution being the extension of the Trout Creek fault approximately 8 miles along its trace beyond its previously known location. An interpretation of the geologic history of the area is given, as determined from petrographic and stratigraphic evidence. It is concluded that the Weber (?) formation (Pennsylvanian), composed chiefly of shale, was deposited in a dominantly neritic, but at places littoral, environment; that the Coffman, consisting of arkosic conglomerate, sandstone, and shale, was formed in a delta and flood-plain environment; that the Chubb, composed of siltstone with arkosic sandstone in the upper part, originated under fluviatile conditions on coalescing flood-plains; and that the Pony Spring, consisting of siltstone with interstratified sandstone and shale, was also formed in a flood-plain environment.

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