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Putative domal microbial structures in fluvial siliciclastic facies of the Mesoproterozoic (1.09 Ga) Copper Harbor Conglomerate, Upper Peninsula of Michigan, USA



The Copper Harbor Conglomerate is a Mesoproterozoic (1.09 Ga) freshwater sedimentary sequence that outcrops in the Upper Peninsula of Michigan. The formation was deposited during infilling 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 formation. These domal structures range from millimeters to several centimeters in diameter and height, and are preserved in convex epirelief on fine-grained sandstone beds. 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 laboratory through inspection of cut slabs, petrographic thin sections, X-radiographs, and RAMAN spectroscopy. Results of these analyses reveal that the domal structures often contain weak, wavy horizontal bedding and laminae, and lack any vertical structures. These results support a biogenic origin of the domal structures instead of through abiogenic processes such as loading, sand volcanoes, or adhesion warts. These structures are akin to what were traditionally labeled as ‘sand stromatolites’, but are now known as ‘domal sand structures’. Along with previous descriptions of carbonate stromatolites, organic-rich paleosols, and microbial sand structures, our findings provide further evidence that mat-forming microbial communities thrived in the late Mesoproterozoic freshwater systems of the Midcontinent Rift.