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During the Late Devonian, sediments deposited in shallow marine, brackish, and tidal mud-flat environments accumulated in west-central Colorado. This sedimentary sequence comprises the Chaffee Group, which is divided into two formations; the lower third, the Parting Formation, consists predominantly of detrital rocks; the upper two thirds of the group, the Dyer Formation, is composed of carbonate rocks.
The Parting Formation is divided into three stratigraphic units, designated A, B, and C, in ascending order. Unit A is chiefly quartzose sandstone. Unit B is composed of four persistent beds which, in ascending order, are shale, thin dolomite, shale, and sandy dolomite or dolomitic sandstone that contains dolomite pebbles. Unit C consists of lenses and beds of quartzose sandstone, sandy shale, and sandy dolomite. On the east and northeast all three units change in facies to quartzose sandstone.
The Parting Formation is principally a transgressive sequence, but the middle dolomite bed and the top bed of unit B represent regressive phases. The source for the detrital sediments was a crystalline highland flanked by sedimentary rocks, located east and northeast of the study area. This highland persisted throughout Chaffee deposition, but yielded more detritus to the Parting Formation. The maximum advance of the Devonian sea occurred during upper Parting and lower Dyer deposition.
The Dyer Formation is a regressive carbonate sequence that can be divided into two carbonate members. The lower member consists of dark-gray, dolomitic, fossiliferous limestone, and herein is named the Broken Rib Member. The upper member consists of light-gray dense, stromatolitic dolomite, and herein is named the Coffee Pot Member. The Broken Rib Member apparently reflects deposition in a sublittoral environment. The stromatolites of the Coffee Pot Member are interpreted as having formed by the trapping and binding action of blue-green algae, which grew in the littoral zone. Intraformational breccias formed by the disruptive action of waves on desiccation polygons are associated with the stromatolites.
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