GSA Connects 2021 in Portland, Oregon

Paper No. 32-9
Presentation Time: 3:45 PM


GIANNINY, Gary, Fort Lewis College, 1000 Rim Drive, Durango, CO 81301

As the search for early life on Earth and possible evidence of life on Mars continues, understanding microbial carbonates on Earth, especially in stressed or hypersaline conditions can provide useful analogs. Microbial carbonates also host important oil and gas reservoirs. Here we present new outcrop and thin section data on microbial mat “laminites” and dendrolitic boundstones formed in part by the branching filamentous cyanobacterium, Ortonella, in the Carboniferous (Moscovian) carbonates of the Paradox Formation in southeastern Utah. A wide variety of microbial boundstone textures occur in late transgressive to latest highstand carbonates of the southwestern shelf of this basin, while lowstands were dominated by basin to shelf edge evaporites.

Microbial mat laminites are common in the carbonates of the lower Akah and Barker Creek oil and gas intervals in the lower Paradox Fm. and occur in 10-150 cm thick beds with crinkly laminae with peloids and micritic grain “bridging” crusts. In the lower-most Akah interval, distinctive grey and black microbial mat boundstones cap the subaerially exposed Sequence 2.9. At the base of this 20-30 cm thick bed are black dendrolitic “shrubs” or tufted mats composed of the acute-angle-branching micritized sheath molds (7-12 µ diameter ) of the cyanobacterium Ortonella. The outer wall of the shrubs is formed by 3-5 mm diameter “balls” of Ortonella, which stack to build shrubs ranging from 1-4 cm in height, 4-5 cm in diameter. Within the overlying microbial mat boundstones in the top 15 cm of the bed, 2-8 mm thick black to dark grey crinkly laminae are composed of multiple layers of erect 1-3 mm high Ortonella, an unidentified larger diameter branching microbial form, peloids, and micritized peloid clumps. Laminae contain distinct horizons of fibrous calcite cements, spar-filled fenestral porosity, mud cracks, soft sediment deformation and rip-up clasts.

Like the recently described modern dendrolitic microbial mat “shrubs” of Hamlin Pool, Australia, these Carboniferous dendrolites were formed by filamentous cyanobacteria in intertidal to seasonally supratidal environments. Unlike the Hamlin Pool dendrolites, the Ortonella-dominated microbialites were calcified, increasing the likelihood of preservation in more ancient strata on Earth, or similar environments on Mars.