INSIGHT INTO THE COMPOSITE STRATIGRAPHY OF MIDDLE CAMBRIAN-AGE WEYMOUTH FORMATION, QUINCY AND WEYMOUTH, MASSACHUSETTS

Recently-completed deep rock test borings drilled for a bridge replacement project in Quincy and Weymouth, Massachusetts provides detailed insight into the stratigraphy of the Middle Cambrian-age Weymouth Formation. Planned to be constructed on drilled shafts bearing within this formation, design of the replacement bridge foundations depended on characterization of the bedrock including rock type, mineralogy, fracture characteristics, and rock strength. A Rock Mass Rating (RMR) system following AASHTO LRFD guidelines was used to initially quantify engineering characteristics of the bedrock and to facilitate design of the drilled shafts.

Both preliminary phase and recent test borings produced core from three sub-units (defined here) of the Weymouth Formation, including characteristic purplish-gray, greenish-gray, and red-maroon argillites and siliclastic mudstones, each with unique engineering properties and a range of unconfined compressive strengths. The rocks displayed relict sedimentary structures, ductile deformation, microfolding, faulting, brecciation, and prevalent quartz-healed fractures and rimmed calcite-filled tear/gash vein strain indicators. Here we present a newly-assembled composite stratigraphic section of the upper sub-units of the Weymouth Formation based on our recently obtained data.