BEDROCK GEOLOGIC MAP OF GREATER BOSTON, MASSACHUSETTS: DIGITAL COMPILATION WITH NEW U-PB ZIRCON AGE CONSTRAINTS

Currently available geologic maps of Boston, Massachusetts and vicinity appeared before the widespread use of U-Pb geochronology and mainly reflect mapping through the 1970's, both at the surface and in water supply, sewerage and transportation tunnels. Starting around 1990, construction related to the Deer Island Sewage Treatment Plant in Boston Harbor and Boston's famous "Big Dig" (officially the Central Artery/Third Harbor Tunnel Project) has greatly expanded the subsurface database. This information has been digitally integrated with results of previous mapping, with aeromagnetic maps and with recently obtained U-Pb zircon dates to produce a 1:24,000 scale geologic map spanning (from E to W) the Boston South and Newton quadrangles.

The new map centers on the geological Boston Basin which contains conglomerate and basaltic to andesitic volcanics formed during extensional faulting late in the Avalonian tectonic cycle in southeastern New England. Overlying argillites record the transition to platform conditions. Detrital zircons from sandstone interbedded with Roxbury Conglomerate in the Newton Quadrangle were analyzed via LA-ICPMS to produce a probability density distribution showing the complete detrital age profile. Six of the youngest zircons were then re-analyzed using CA-TIMS to establish a precise maximum age of 598.87 ± 0.19 Ma (weighted mean ²⁰⁶Pb/²³⁸U date; MSWD = 1.3); the sixth grain is slightly older at ca. 603 Ma. The youngest ²⁰⁶Pb/²³⁸U date from zircon in similarly analyzed Squantum sandstone at the top of the Roxbury Conglomerate is 596.38 ± 0.40 Ma, and five other zircons yielded dates of ca. 598 Ma (weighted mean of 3 analyses), ca. 608 Ma and ca. 615 Ma. These results reflect the presence of detrital zircon derived from pre-Roxbury magmatic units uplifted by the Ediacaran Mount Hope Fault on the south side of the map area. Two CA-TIMS crystallization ages to be presented from Brighton volcanic interbeds in the Roxbury Conglomerate are approximately 10 Ma younger than the maximum ages for the sandstones.

This compilation originated as the first step in developing a depth-to-bedrock map for a seismic hazard analysis of the Boston area, but has evolved to round out age constraints needed for detailed comparison between southeastern New England and other northern Appalachian Avalonian terranes.