MICROBIALLY INDUCED SEDIMENTARY STRUCTURES AND PRESERVATION OF EDIACARAN-LIKE FOSSILS IN THE BOSTON BAY GROUP, MASSACHUSETTS

Over 200m of mudstone, very fine sandstone, and interbedded cobble to boulder diamictite comprise the clastic sedimentary section of the presumed Late Neoproterozoic Boston Bay Group at Hewitts Cove, Hingham, MA. A large portion of the section consists of very thinly laminated, very fine sandstone and siltstone, with numerous slumped and intraclastic horizons. Preservation of uniform planar graded laminae and ring-like fossils (positive epireliefs on bed tops) appears to be due in many instances to stabilization of bedding surfaces by microbial mats that developed on the bottom between low energy turbidity current events. No microbial filaments were observed in thin sections, but existence of biomats is strongly indicated by 0.1-0.6mm dark, pyrite-rich laminae. We interprete these laminae to be altered organic-rich, microbial biomats. Other inferred microbially induced sedimentary structures include rare irregularly wrinkled bed surface textures, peeled and rolled very thin pyritic laminae, abundant ragged-edged mat chips or intraclasts in thicker graded sand laminae, and irregular pyritic laminae that enclose and include isolated sand grains. Many slump folds have been truncated and overlain by graded sandstone beds suggesting significant erosion by turbidity currents on a distal fan-delta slope. Under such conditions microbial mats modified surfaces by trapping, binding, and stabilizing fine sediment thus preventing or inhibiting erosion and preserving detailed bed surface structures including those produced by an Ediacaran-like epifaunal benthic organism.