Evaluating the Biogenicity of Fluvial-lacustrine Stromatolites from the Mesoproterozoic Copper Harbor Conglomerate, Upper Peninsula of Michigan, Usa

Abstract

The Mesoproterozoic (1.09 Ga) Copper Harbor Conglomerate represents alluvial fan, fluvial and lacustrine deposition in the Midcontinent Rift System. The formation outcrops in the Keweenaw Peninsula in the northwestern part of the Upper Peninsula of Michigan where it contains carbonate stromatolites preserved within both siltstone and conglomerate facies. The purpose of this study is to evaluate the biogenicity of these stromatolites, which lack direct microfossil evidence. The stromatolites were placed into their depositional context, their macro-scale features and thin section microfabrics were analyzed, and growth angles were measured of cobble-draping samples to determine if a phototrophic response existed. A methodology that uses magnetic susceptibility as a biosignature was also performed on these stromatolites. The results of these analyses reveal two distinct types of stromatolites. Stromatolites from the siltstone facies are interpreted as biogenic. They contain detrital laminae, hematite-rich micritic laminae, and fenestral fabrics. The stromatolites formed as microbial mats grew over a mudflat or sandflat with carbonate filled dessication cracks on an eroded topography. Stromatolites from the conglomerate facies are interpreted to have formed by a mix of chemical and biological processes. They are microdigitate and have abiogenic features such as isopachous laminae with radial fibrous calcite fans and botryoids. They also lack a phototrophic response, suggesting that growth was not controlled by cyanobacteria. These stromatolites also have some biogenic signatures such as conical wavy laminae that have been separated by gas build-ups. These stromatolites are interpreted as having formed in a flooded braidplain setting with restricted circulation. Magnetic susceptibility tests yielded inconclusive results in this case because the stromatolites in question contain secondary hematite. This study supports previous studies of these stromatolites, as well as microbial structures and organic-rich paleosols that have suggested freshwater microbial communities were abundant in the Midcontinent Rift during the Mesoproterozoic. It also highlights how variable environmental factors can influence stromatolite growth, even in similar depositional settings and with a consistent microbial presence.