TUNNEL VISIONS BENEATH BOSTON HARBOR, MASSACHUSETTS: STRUCTURE AND STRATIGRAPHY OF THE EDIACARAN CAMBRIDGE FORMATION

"Argillite" of the Cambridge Formation dominates Boston Basin strata above Avalonian arc-related igneous rocks in SE New England, yet poor exposure of this sequence has long thwarted regional stratigraphic synthesis. Subsurface mapping records along with petrographic and geochemical data from borehole samples and tunnels leading to the Deer Island Sewage Treatment Plant in Boston Harbor are integrated here in a connected series of generally N-S cross sections spanning most of the Basin. Argillites fall into a "sandy" group with quartz and feldspar crystals studded though phyllosilicate-rich matrix and a crystal-poor "bentonitic" group characterized by illitic compositions consistent with altered volcanic ash and mudstone. Relationships summarized below in the North Metropolitan Relief Tunnel (NMRT), the InterIsland Tunnel (IIT) and Braintree Weymouth Tunnel (BWT) reveal a stratigraphic progression from sandy to bentonitic units.

Northern reaches of the NMRT north of Deer Island transect bentonitic argillite with microstructures like those in the hanging wall of the N-dipping, reactivated normal fault recognized to the west in the City Tunnel Extension. The hanging wall argillite contains volcanic ash that is > 30 Ma younger than 595-584 Ma Roxbury Conglomerate in the footwall. In the NMRT, the older footwall sequence south of the fault is sandy argillite. The base of the Cambridge section is transected in the IIT south of Deer Island in another upfaulted block. Here purple Roxbury-related siltstone is overlain without break by gray sandy argillite that continues southward into the BWT. The basal contact is repeated by faulting in the middle of the BWT, and the upper contact appears near its southern terminus in Weymouth, MA where bentonitic argillite passes upward into trilobite-bearing Cambrian strata.

A previous ²⁰⁷Pb/²⁰⁶Pb date from detrital zircon in the North Weymouth Shaft establishes a ~599 Ma maximum depositional age that is the sole U-Pb constraint on any of the Harbor section. Dual LA-ICPMS and CA-TIMS analyses in progress for quartzite from the shaft, sandy argillite farther north in the BWT and a possible tuff in the IIT can potentially tighten the maximum age and resolve emerging contrasts with customary correlatives among transitional arc-to-platform sequences in other West Avalonian terranes.