DEPOSITIONAL MECHANISMS AND PALEOENVIRONMENTS OF LAMINATED FINE GRAINED FACIES IN THE EDIACARAN BOSTON BAY GROUP

Coarse clastic sedimentary strata, especially conglomeratic facies, comprise most of the outcrops of the Boston Bay Group but volumetrically, fine grained clastics of parts of the Roxbury Fm. and most of the Cambridge Fm. are more abundant. Within these strata intervals up to 10m thick are extremely deformed by slump folding but most of the sections consist of planar bedded mudstone and very fine sandstone/siltstone with numerous small scale sedimentary structures. Prior authors proposed, with very limited detailed sedimentological analysis, lake, tidal flat, shallow and deep shelf, slope, or deep basin paleoenvironments for parts or all of these fine grained facies. Recently completed restudy of the Squantum Member and of other diamictite and coarse clastic mass flow deposits mapped as the Roxbury Conglomerate established a slope and deeper basin environment of deposition. These coarser units are interbedded and intricately mixed with overlying and underlying laminated facies. Beneath the main diamictite bearing sequence at Squantum Head in Quincy, MA, thinly bedded and very thinly laminated gray mudstone and very fine sandstone have a rhythmicity consisting of mm to cm thick laminae. These laminae are comprised of couplets of silt/very fine sand overlain by extremely thin finer silt/clay laminae. The basal contacts of coarser laminae may truncate underlying laminae deformed by loading and micro-slumping. Where very fine sand comprises the basal portion of a couplet very faint normal grading is present and in some cases tiny intraclasts or clay chips have been reworked from underlying laminae. Traction structures such as ripples and channels occur but are extremely rare. There is an appearance of laminae bundling in some parts of the section; however, an analysis of laminae and couplet thickness distribution did not reveal any statistically relevant trends. Couplet thickness distribution appears random. Given the suite of sedimentary structures and bedding patterns the most likely mechanism of sediment transport for these thinly laminated strata was by very dilute low velocity turbidity current or hyperpycnal flow. The delicate nature of the laminae and their lateral continuity suggests deposition in a deep slope or basinal setting with low ambient current velocity.