SOUTH SHORE STRATIGRAPHY OF THE BOSTON BASIN: INSIGHTS FROM THE BRAINTREE-WEYMOUTH TUNNEL

Sparse surface outcrops on the south side of the Boston Basin include gray argillite assigned to the terminal Neoproterozoic Cambridge Argillite and fossiliferous red and green mudstones and slates, respectively assigned to the Lower Cambrian Weymouth Formation and Middle Cambrian Braintree Slate. Also present are various unfossiliferous argillaceous units and the micaceous "Milton" quartzite for which age remains unclear. The Braintree-Weymouth Tunnel, currently under construction between the Weymouth Fore River on the south and Houghs Neck on the north, offers an opportunity to document continuous subsurface relationships needed to clarify regional stratigraphy and structure.

Preliminary work has aimed at comparing red beds encountered in the North Weymouth Shaft of the tunnel with surface samples of the Weymouth Formation in nearby Mill Cove. Red beds in the tunnel are brownish gray siltstones composed of quartz, white mica and hematite based on x-ray diffraction. Electron microprobe analysis reveals accessory zircon, xenotime, monazite, ilmenite, rutile and apatite. Neither method uncovered chlorite or albite. In comparison, the representative Weymouth samples are grayish red siltstones containing quartz, albite, white mica, chlorite and hematite with accessory zircon, sphene, epidote, and apatite. White mica in the Weymouth Formation is more deficient in interlayer cations than micas from the shaft samples (K+Na=0.63 vs. 0.82-0.91). Neither mica is true muscovite (K+Na=1). These results show that the red beds in the Weymouth Formation and the North Weymouth Shaft have distinct mineralogies and mineral compositions.

The sequence in the shaft also contains minor olive gray and dark greenish gray interbeds and significant amounts of micaceous, quartz rich lithic wacke with a mineral assemblage similar to the "Milton" quartzite. Available 207Pb/206Pb dates from detrital zircons in an olive gray siltstone interbed in the shaft establish a maximum depositional age of 599 Ma for this sequence. This date allows the possibility that red beds and associated rocks in the shaft are significantly older than the Lower Cambrian Weymouth Formation. Possible correlatives in the Newfoundland Avalon Zone include the Rencontre, Chapel Island or Random formations.