Glacial or Non-Glacial?  Formation of the Ediacaran Sedimentary Sequence of the Boston Basin


Our vote is for non-glacial. But ice may have been involved.


“Boston Basin originated as a faulted rift basin within the Avalon volcanic island chain.” Skehan, 2001, Roadside Geology of Massachusetts. This area of eastern Massachusetts was a separate entity back 580 million years ago (580 Mega-Annum, Ma). Around this time in the late Precambrian before shelled fossils existed, the Avalonia microcontinent on which the Boston Basin developed, was situated at about 60 degrees south latitude. In that setting, active volcanoes existed to what is now the south in the area of Mattapan, MA. These volcanic highlands and the weathered roots of granite now exposed in Dedham, MA acted as source rocks, that, through weathering and erosion, supplied the building blocks (sediment: large clasts and small sand-sized and silt and clay grains) that filled the Boston Basin that eventually held more than 5 km thickness of sediments.


As the overburden pressure increased near the base of the sedimentary pile, the sediments were turned to sedimentary rocks, and eventually the entire sequence became rocks, with sands being converted by compaction and cementation to sandstone and muds to mudstones and gravel to conglomerates. In a sedimentary basin of deposition, the coarser grain sized debris are found closest to the highlands, in the proximal position of the basin (i.e., close to the source area highlands) because they are too big and heavy to be moved any farther. The finest grained clastic or fragmental sediment such as silt and clay are found much farther from the source in what is called the distal area often in deep quiet water (distant from the source). In the case of the Boston Bay Group, the coarse proximal rocks are found south of the Charles River and these rocks are called the Roxbury Formation or Roxbury Puddingstone. The finest-grained rocks are now called the Cambridge Formation and are mostly found north of the Charles River. These two Formations make up the Boston Bay Group. Our depositional model will illustrate how the various sediments were moved through and settled in the basin over time.


The Roxbury Formation is further subdivided into three members, from the bottom up are: Brookline Member, Dorchester Member, and uppermost Squantum Member.


I hope to demonstrate that the features observed in the field at Squantum Head and associated units indicate the most of the Boston Bay Group were formed, not by ice but by subaqueous marine mass gravity flow processes.


Recent re-study and re-mapping of portions of the Boston Harbor Islands and surrounding coastal outcrops, including the type locality of the Squantum Member, called Squantum Head, Quincy, MA, by Prof. Richard Bailey (Northeastern University) and myself, allows a comparative analysis of the typical matrix-supported conglomerates (diamictites and other rock types). Many other outcrops expose a wide variety of coarse mass flow deposits and disturbed and slumped stratigraphic intervals. We focused on the conglomeratic facies within the Cambridge Formation and the underlying thinly laminated mudstone/sandstone facies that are interbedded with the diamictite or conglomerate facies (aka, Squantum and Dorchester Members).


These strata vary in bedding scale from centimeters to hundreds of meters and in lateral continuity from decimeters to probably kilometers. This talk will present our new observations that are based strictly on describing only field relationships and on grain size, shape, and fabric and whether conglomerates are clast-supported and therefore moved by turbulent flow processes or matrix-supported and therefore moved by laminar flow processes, which we interpret as an assortment of specific submarine mass gravity flow mechanisms. Following a presentation of our new data I will describe aspects of our new interpretation and model for the deposition of these intriguing rocks!