STRATIGRAPHY AND STRUCTURE OF THE ROCKS UNDERLYING BOSTON HARBOR: NEW INSIGHTS ON THE CAMBRIDGE ARGILLITE AND ASSOCIATED DIAMICTITES AND DIABASE SILLS

William O. Crosby studied the islands of Boston Harbor in the late 1800s, producing excellent verbal descriptions but no maps. Many of his observations stand unchallenged. His detailed maps of the southern harbor shore are especially valuable as development has since obscured many outcrops. Later compilations (e.g. Billings, Kaye, Bell) imposed stratigraphy developed in Boston onto the islands and harbor perimeter.

Lithologic and structural data from new mapping of fifteen Boston Harbor Islands at 1:1000, integrated with data from sewage and outflow tunnels, shed new light on the Boston Bay Group and structures beneath the harbor. Ring fossils, identical to those previously reported in Hingham, are abundant much higher in the Cambridge Argillite on the outer harbor islands, and confirm a late Neoproterozoic age for the whole unit.

The Cambridge contains several debris-flow diamictites, including the so-called “Squantum Tillite”, at different stratigraphic levels, so that there is no reason to maintain member status for that layer nor to correlate all other diamictites with the Squantum. This more complex stratigraphy allows for a simpler interpretation of structures than in previous compilations. The Inter-Island Tunnel exposes continuations of the gently ENE-plunging Central Anticline of Boston and Brewster Syncline of the islands. These (Alleghanian?) folds are cut by numerous minor faults and truncated by a major NE-trending fault zone north of Peddocks Island. Soft-sediment slump folds are common throughout the Cambridge, but tectonically overturned beds are observed only near the south margin of the harbor, where the Cambridge Argillite was apparently thrust southwards by the Rock Island fault over a thin, previously deformed, north-facing sequence atop basement.

Subalkaline tholeiitic diabase sills in the outer islands intruded the Cambridge Argillite before deformation and lower greenschist regional metamorphism. Diabase/argillite contacts show tan-weathering, mafic chilled margins against gray, felsic, recrystallized melts with angular argillite clasts. A 30-m wide peperite-like breccia with similar gray fine-grained matrix is exposed on Green Island. A few steep, E-W dikes that cut the folded sills and argillite are more alkaline and resemble Paleozoic dikes on the mainland.