Abstract

Well exposed sequences of Belden Shale at Mosca and Fossil Ridge are 834.9 and 492.5 feet thick respectively and as measured. At both localities the lower portion of the formation is dominantly composed of black shale with very little limestone. These rocks record alluvial and marine deposition transgressing over an erosion surface, in part karstic, on top of the Leadville Limestone. At Fossil Ridge, red sandstones and shales rest on a well exposed solution surface at the base of the lower Belden Shale. At both localities the middle Belden Shale consists of alternating layers of black shale and black, fine-grained limestone. The upper part of the Belden Shale, not preserved at Fossil Ridge, consists predominantly of black shale with increasing amounts of siltstone and sandstone near the top, the base of the overlying Gothic Formation being placed at the point where sandstone becomes dominant. The upper member thus represents a regression caused by the incursion of alluvial sediments.

These two sections are of particular interest because of the general absence of structurally uncomplicated, unmetamorphosed outcrops of Belden Shale along the southwest margin of the Central Colorado Trough.