BASIN HISTORY, DEPOSITIONAL MECHANICS, AND PALEOENVIRONMENTS OF MASS FLOW DEPOSITS IN THE EDIACARAN BOSTON BAY GROUP, MASSACHUSETTS

Recent restudy and remapping of portions of the Boston Harbor Islands and surrounding coastal outcrops permits a comparative analysis of the characteristic debrites, such as the well known Squantum Member, Roxbury Formation of the Boston Bay Group. In addition, there are well exposed outcrops containing a wide variety of coarse mass flow deposits and slumped and disturbed stratigraphic intervals. All of the conglomeratic facies considered in this analysis are within the Cambridge Argillite or are intimately associated with similar thinly laminated mudstone/sandstone facies within the Squantum Member. These deposits range in bedding scale from centimeters to hundreds of meters and in lateral continuity from decimeters to possibly kilometers. The thickest matrix-supported debrites contain an assortment of extrabasinal basement clasts up to a meter in diameter and intraclasts or olistoliths up to several meters in length in a fine-grained mudstone/sandstone matrix. These extrabasinal clast-bearing debrites, representing classic debris flows, vary in thickness, proportion of fine matrix, and in size and character of intraclasts. It is difficult to determine conclusively from sedimentary evidence alone if these debrites entered the basin from the mountainous, tectonically and volcanically active source area as terrestrial mass flows or if they were submarine mass flows resedimenting proximal coarse facies deposited on a narrow shelf or ramp. Given the nature of the matrix and the ubiquity of large basinal intraclasts we favor the latter interpretation. Thinner extrabasinal clast-bearing, clast-supported debrites, typically in the pebble to cobble size range, probably represent distal edges of shelf or slope derived flows that have outrun the more plastic, matrix-rich portion of the flow. Very common debrites, often overlooked in the Boston Bay Group, are those containing only or largely mudstone or fine sandstone intraclasts or olistoliths. In some outcrops it is possible to trace laterally an interval with intense soft sediment deformation and/or slump folding into an intraclastic or intraformational conglomerate. These intraclastic debrites are clearly sourced from a muddy and cohesive slope.