PALEOENVIRONMENTAL ANALYSIS OF MESOPROTEROZOIC MICROBIAL MATS FROM THE COPPER HARBOR CONGLOMERATE, MICHIGAN: CONDITIONS THAT FAVORED THE DEVELOPMENT OF MICROBIAL COLONIZATION OF NON-MARINE ECOSYSTEMS

It is often assumed that life originated in the oceans and diversifying prior to colonizing land. Wellman and Strother (2015) argue that diversity of life in terrestrial settings 1.1 Ga was greater than in marine environments due to sulfidic ocean conditions, which would have hindered the evolution of eukaryotic organisms. Some of the earliest evidence for the development of terrestrial microbial life is found in the form of microbial mats in the alluvial fan and related deposits of the Copper Harbor Conglomerate at Union Bay, Michigan (Wilmeth, 2014). Despite the importance to the development of the terrestrial biota, little has been done to characterize the environmental setting of this deposit.

This study focuses on an outcrop at Union Bay that contains domal structures preserved in a fine-grained sandstone facies in the upper Copper Harbor. These structures are thought to form as sediment-trapping, mat-forming, microbial communities or MISS (Noffke et al, 2009; Wilmeth, 2014). The environments in which these mats grew in are loosely constrained. Facies and thin-section analysis of these strata identified 5 different facies which include: conglomeratic/scoured surfaces, fine-grained cross laminaed sands, fine-grained cross bedded sand, medium-grained cross-bedded sands, and planar-bedded sands with asymmetrical ripples. These are interpreted to represent the following environments respectively: fluvial, overland sheet floods, eolian, fluvial and sheet floods. MISS were found to occur in all of the environments, but were most abundant in planar beds and asymmetrical ripples (sheet floods). The occurrence of these suggest MISS development was strongly controlled by the water-table and sediment surface elevations and the availability and persistence of wet substrates controlled the location of the MISS. As suggested by Bottjer and Hagadorn, 2007, morphological changes of microbial mats are related to changes in depositional environments, suggesting that these Mesoproterozoic MISS were highly sensitive to the changing environmental conditions and therefore, changing sedimentary processes likely influenced the location and development of MISS.