Upper Cambrian stromatolitic biostrome, Clinetop Member of the Dotsero Formation, western Colorado

Abstract

The Clinetop Member of the Dotsero Formation consists of carbonate pebble conglomerate and stromatolitic algal heads as much as 1.5 m thick. Isopach maps indicate that the Clinetop Member occupies an area 24 to 32 km wide and 64 to 80 km long (1,500 to 2,600 km 2 ) and trends approximately N60°W. The member consists of three stratigraphic units composed of 3 to 43 cm of basal carbonate pebble conglomerate, 0 to 91.4 cm of algal heads, and 0 to 2.1 cm of upper bioclastic and carbonate pebble conglomerate. The lower one-half to two-thirds of the algal heads are of the SH-V (cryptozoon) structural type, whereas the upper one-half to one-third are of the LLH-C (collenia) structural type. Channels filled with clastic carbonate as deep as 68 cm cut the algal-head unit. The orientation of the channels averages N46°E. Comparison of the algal heads to modern stromatolitic heads suggests that they formed in the intertidal environment of a tropical to subtropical sea. The SH-V algal structures formed in a high-energy, seaward, intertidal environment, whereas the LLH-C structures formed in a low-energy, landward, intertidal environment. Channels were formed by tidal runoff. A gradual regression of the sea across a stable platform is suggested to explain the distribution of the Clinetop Member.