ABSTRACT. The Cambridge Argillite (maximum age ca. 570 Ma) is the most extensive bedrock formation underlying the Boston Basin of eastern Massachusetts. Because this unit seldom crops out, structural relationships reported here are based on subsurface data from tunnels converging at the Deer Island Sewage Treatment Plant in Boston Harbor. The North Metropolitan Relief Tunnel (NMRT) and the Main Drainage Tunnel (MDT), first mapped by the late Marland Billings, transect the argillite along alignments oriented S 53^o 05’ 27’’ E and N 69^o 43’47’’ E, respectively. Bedding measurements from these tunnels and from the Massachusetts Water Resources Authority's more recent Deer Island Shaft, combined with fabrics in available core borings reveal a more complete sequence of structural events than previously documented.

A stereogram of bedding poles in the NMRT and the Deer Island Shaft shows a p girdle defining a fold axis oriented 54/12 NE. Microstructures in core samples from the NMRT confirm Billings’ report of penetrative, south-dipping cleavage. This foliation is found on both limbs of and lies parallel to the axial plane of his Charles River syncline. Two samples also contain a north-dipping S2 foliation not observed by Billings on the north limb of the syncline. This foliation parallels axial-planar cleavage in a south verging anticline in the City Tunnel Extension (CTE) farther north in the basin. NNE trending, steeply dipping joints from the Deer Island Shaft represent a regional joint system also found in the CTE.

The Charles River Syncline in the NMRT is consistent with folding in other areas of Boston Harbor. Strata at Calf and Middle Brewster Islands, in the MDT and in the Inter-Island Tunnel are all folded about ENE trending, gently plunging axes. S2 cleavage in the NMRT suggests that south verging folds in the CTE postdate the Charles River syncline. Joints in the Deer Island Shaft may relate to NNE faults cutting Pennsylvanian Norfolk Basin fill to the south.