PUTATIVE MICROBIAL STRUCTURES IN FLUVIAL SILICICLASTIC FACIES OF THE MESOPROTEROZOIC (1.09 GA) COPPER HARBOR FORMATION, UPPER PENINSULA OF MICHIGAN, USA

The Copper Harbor Formation is a Mesoproterozoic (1.09 Ga) freshwater sedimentary sequence that outcrops in the Upper Peninsula of Michigan and northern Wisconsin. The formation was deposited during erosion of the failed Midcontinent Rift and contains fluvial, lacustrine, and alluvial fan facies. This study describes and analyzes the formation of small domal structures preserved in fluvial sandstone facies within the lower portion of the Copper Harbor. These domal structures range from millimeters to several centimeters in diameter and are preserved in convex epirelief on beds of fine-grained sandstone, some of which also include ripples and desiccation cracks. The structures have a pustulose texture and a patchy distribution on bedding planes. Slabs containing the structures were collected in the field and analyzed in the lab through inspection of cut slabs, thin sections, and x-radiographs. Results of these analyses reveal that the domal structures often contain weak horizontal bedding and laminae, and lack any vertical structures. These results do not support non-biogenic formation of these structures through loading, sand volcanoes, or adhesion warts, but do support the hypothesis that these domal structures were microbially mediated. These Copper Harbor structures are akin to what were traditionally labeled as “sand stromatolites,” but are now known as “domal sand structures.” Carbonate stromatolites are also well known from lacustrine deposits within the conglomerate facies higher in the Copper Harbor. These domal sand structures suggest that mat-forming microbial communities thrived in the late Mesoproterozoic freshwater systems of the Midcontinent Rift. Analogous environments are known to have existed near the Noachian-Hesperian boundary (~3.7 Ga) on Mars, where alluvial fan and fluvial structures were deposited on the eroding flanks of impact craters. Copper Harbor microbial structures such as those described and analyzed in this study may therefore provide useful search images in the pursuit of evidence for ancient life on Mars.