STROMATOLITES OF THE SHAKOPEE FORMATION (ORDOVICIAN, MINNESOTA): AN ANCIENT SHARK BAY OR RELICT OF A PRECAMBRIAN PAST?

Microbialites are organosedimentary structures, formed by accretion of carbonate through interaction with microorganisms, producing layered (stromatolite) or unlayered (thrombolite, microbialite) forms. Major mechanisms of growth include trapping and binding of detrital carbonate, precipitation of micrite within microbial mats, and precipitation of cement on and within structures. Trapped-and-bound stromatolites, typical of modern marine environments, have dominantly clastic textures. In contrast, Proterozoic marine stromatolites commonly have clotted micritic fabrics with varying amounts of isopachous cement. Because the dominant mechanisms of growth vary in time and space, stromatolite textural analyses provide useful information about ancient environments.

The Shakopee Formation, consisting principally of dolomite and sandy dolomite, was deposited near the shoreline of a shallow, epeiric sea that covered the interior of North America during the Ordovician. Stromatolites occur at several horizons, most notably in the upper portion, the Willow River Member, where stromatolite-bearing outcrops occur at several localities in southeastern Minnesota. Previous work suggested that modern marine stromatolites of Shark Bay, Western Australia are robust analogues for these stromatolites. This study examines Willow River stromatolite form and texture to better characterize the mechanism of formation.

Samples were collected from the Willow River Member at several localities in southeastern Minnesota. We examined stromatolite form and texture in hand sample, polished slab, and thin section and compared observations to examples of both modern and Proterozoic stromatolites. Willow River stromatolites are dominated by clotted textures, suggesting in situ precipitation of fine-grained carbonate during microbial mat growth, rather than trapping and binding of carbonate grains. In addition, The Willow River samples often also contain thin, isopachous laminae, suggesting a contribution by carbonate precipitation. Our analysis suggests that stromatolites from the Willow River Member differ substantially from modern marine forms, like those in Shark Bay, Australia, and that the precipitated stromatolites typical of the Proterozoic may be better analogues for these Ordovician forms.