STRATIGRAPHY AND DEPOSITIONAL SETTING OF NEOPROTEROZOIC STRATA ON SOUTHERN BOSTON HARBOR ISLANDS, BOSTON BAY GROUP, MASSACHUSETTS

Outcrops on several Boston Harbor Islands represent the most extensive surface exposures of fine grained facies in the southern Boston Basin. About 290 m of Neoproterozoic strata are exposed on Slate Island in Hingham Bay. Beds are nearly vertical to slightly overturned and strike 72 to 80 degrees. Less extensively exposed and partly correlative facies are present on nearby Grape and Racoon Islands. Strata are comprised of the following facies in order of abundance: 1. thinly laminated and bedded dark mudstone and fine sandstone, 2. very thinly laminated gray-black mudstone, 3. laminated mudstone/sandstone with cross-laminated sandstone lenses, 4. graded sandstone with platy mudstone intraclasts overlying scour surfaces or with mudstone flames, 5. 0.2 to 3m intervals of slump folded and intraclastic mudstone/sandstone, 6. 2 to 20 cm thick diamictites with pebbles, granules, and highly deformed sandstone/mudstone intraclasts in sandstone/mudstone matrix. Alternating sandstone and mudstone beds are typical of episodic delivery of coarser sediments during gravity flow events. Slump folds, common scour truncations of slump folds, diamictites (debrites), and sandstones with Tb-c intervals suggest considerable paleoslope and deposition on a distal pro-fan-delta slope. Thin section analysis reveals mm scale structures similar to those described above. Sudden depositional events buried microbial biomats (now thin pyritized laminae) and circular to elliptical Ediacarian-like fossils. Lack of isolated outsized clasts (potential dropstones) in any of these facies argues against involvement of floating ice in deposition of these Neoproterozoic strata.