ARCHITECTURE OF SCYTONEMA-LIKE MICROBIALITES FROM THE MORRISON FORMATION (UPPER JURASSIC), PARK CREEK RESERVOIR, COLORADO

Upper Jurassic microbialites have been reported from multiple localities within the carbonate wetland-lacustrine deposits of the Morrison Formation; however, analysis of microbial architecture has been limited. The Park Creek Reservoir microbialites occur within the undifferentiated “lower” Morrison Formation in north-central Colorado as a domed microbial lithostrome 20-40 cm thick and covering at least 3 km². Individual domes are 9-60 cm in diameter and display up to 20 cm of relief above the surrounding “interdomal” areas. Serial horizontal and vertical slab sections (0.5-1.5 cm thick) and petrographic analysis of individual domes point to a vertical zonation of the internal architecture as follows: 1) wavy to wrinkled to gently domical microbial laminates with minor oncoids; 5-10 cm thick; predominately micrite with minor chert; 2) minor oncoids and columnar stromatolites exhibiting parallel branching and coalescing columns which form secondary cauliflower to mammillary structures within individual domes; 15-20 cm thick; predominately chert with minor micrite; and 3) wavy to wrinkled stromatolitic domes (<1 cm of relief) with alternating (0.1-0.3 mm) light and dark micritic microbial lamina; 1-3 cm thick; abundant vertical and horizontal burrows (or borings?) infilled with detrital quartz and carbonate silt and sand. Other microbialite associated components include: ostracodes, charophyte gyrogonites, and bone fragments; ostracode and detrital quartz silt laminae above the microbialites; Magadi-type chert; minor to moderate intensity pseudomicrokarst; and minor desiccation cracks.

The internal structure of Morrison microbialites is similar to the internal architecture of Scytonema domes associated with the freshwater coastal marshes of Andros Island (Bahamas). The restriction of modern Scytonema mounds to the Andros freshwater coastal marshes provides a modern depositional analog for the Morrison microbialites. However, the association of Magadi-type chert with the Morrison microbialites highlights a key hydrogeochemical difference between the modern freshwater marshes of Andros and the ancient Morrison carbonate wetlands.