Structural geology in the Crested Butte Quadrangle is more complex than previously reported. Outcrop relationships previously explained as unconformities are the result of large faults at Hunter's Hill and in Slate River valley.

The following lithologic assemblages are recognized in the Absarokan rocks of Colorado: black shale and limestone, gypsum, fine buff clastic rocks, coarse buff reworked clastic rocks, coarse red reworked clastic rocks, coarse red arkosic clastic rocks, and red siltstone. All of these except the gypsum and red siltstone assemblages are present in the Crested Butte Quadrangle. The Belden shale is the black shale and limestone assemblage, and was deposited unconformably upon the Leadville limestone during the beginning of the invasion of the Absarokan seas. A new name, the Gothic formation, is introduced for the overlying coarse and fine buff clastic rock assemblages. The type section is at Copper Creek. This formation was deposited under predominantly marine conditions as the Ancestral ockies and the Central Colorado trough became sharply differentiated. The Maroon formation is restricted to the coarse red arkosic and reworked clastic rock assemblages above the Gothic. It was deposited as tectonic differentiation continued, but after the seas had withdrawn from the area. Red siltstone, not present in the Crested Butte district, was deposited during the final reduction of the Ancestral Rockies after tectonic differentiation ceased. To complete the cycle the Jurassic Entrada sandstone was deposited unconformably upon slightly deformed Absarokan rocks. The Gothic thins toward the northeast with concurrent thickening of the Maroon. Also the Maroon is absent in the southwest part of the area. These facts indicate a progressive northeastward shift of the site of deposition i this area during the Absarokan.

Absarokan fossils from this area, separated on the basis of the type of enclosing rock, fall into more or less distinct groups. Although the fossils are mostly of Des Moinesian age, there are a few Morrowan species in the Belden and many post-Des Moinesian species in both the Belden and Gothic. It is suggested that the Morrowan species here may be restricted to black shale and limestone producing

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environments, and the post-Des Moinesian species here to environments characteristic of nearly filled or dried up seas. Their presence at Crested Butte may then be explained by the existence of such environments in this area during Des Moinesian time.