MICROBIALLY INDUCED SEDIMENTARY STRUCTURES AND LARGE SAND STROMATOLITES IN THE TERRESTRIAL AND MARINE/LACUSTRINE-TRANSITIONAL BAYFIELD GROUP (PROTEROZOIC), WISCONSIN

The Proterozoic Bayfield Group is the youngest of the post-rift sediments that infilled the ~1Ga Midcontinent Rift in northern Wisconsin, and its age is uncertain. It is comprised of the conformable Orienta, Devils Island, and Chequamegon sandstones. Here we report (1) new detrital zircon data that provides a maximum depositional age; (2) microbially induced sedimentary structures (MISS) in the eolian Devils Island Sandstone; and (3) 0.5 to >1 meter-scale sand stromatolites within a lacustrine or marine unit at the base of the Chequamegon Sandstone.

New detrital zircon data provides a maximum depositional age of 1012.5 +/-12 Ma based on 4 grains. The Devils Island Sandstone includes tangential cross-strata, sandy planar, trough cross-strata, and silty planar facies associations. These record eolian dune, interdune, fluvial, and playa depositional environments. MISS, including sand cracks, wrinkle structures, and mat fragments, are present in the sandy planar (interdune), silty planar (playa), and transitional tangential to sandy planar (dune toes to interdune) facies associations. A notable set of structures were documented at just one outcrop of the silty planar facies association. These are cm – to mm— scale, sinuous to concentric, branching, and locally pitted or scratched. They may represent trace fossils, or reflect physical modification of microbially bound sand and silt grains. The upper contact between the Devils Island and overlying Chequamegon sandstones is a regional flooding surface characterized by >1m incision overlain by three, meter-scale shoaling-upward cycles. Each cycle is comprised of trough cross-bedded sandstone that transition to large sand stromatolites, and is capped by thick sand cracks, microbial mat fragments, and a thin mud veneer modified by wrinkle structures.

Constraining the depositional age of the Bayfield Group remains enigmatic due to the difficulty of identifying small but important detrital zircon populations: These rocks may be rift-related or significantly younger than ~1012 Ma. Documentation of MISS in eolian facies adds to increasing evidence for significant microbial modification of Precambrian terrestrial settings. Meter-scale sand stromatolites are among the largest documented, likely significant for understanding Bayfield Group paleoenvironment.