STRATIGRAPHY AND STRUCTURE OF THE CAMBRIDGE FORMATION, BOSTON BASIN, MASSACHUSETTS: NEWS FROM UNDERGROUND

Argillaceous sedimentary rocks comprising the Cambridge Formation overlying the Roxbury Conglomerate in the Ediacaran Boston Bay Group have recently been documented in detail on the Boston Harbor islands and adjoining coastal outcrops of Hingham Bay. In northern portions the Boston Basin, however, these rocks are rarely exposed, and available descriptions come largely from subsurface borings and sampling during construction of decades-old water supply and sewerage tunnels. New data on whole rock geochemistry, mineral compositions and microstructures have been obtained from surviving materials from the Northern Metropolitan Relief Tunnel (NMRT), the Main Drainage Tunnel (MDT) and especially the City Tunnel Extension (CTE).

All the borings contain mainly K-deficient mica and Fe-rich chlorite (based on microprobing) with < 10% qtz + ab. Whole rock geochemistry shows that almost none of the analyzed CTE, MDT and NMRT borings overlap compositions of typical argillaceous rocks in a plot of SiO₂/Al₂0₃ versus Ca0+Na₂O/K₂O. Enrichment in Al₂O₃ and K₂0 pushes compositions of these samples towards potassium-rich bentonitic volcanic ash deposits. By contrast, Dorchester Tunnel samples from the south side of the Boston Basin, with compositions lower in Al₂O₃ and K₂0 and correspondingly richer in SiO_2, are more like typical shales. These distinctions are the basis for sub-dividing the Cambridge Formation into a lower Argillite Member (following usage of M.P. Billings) and an upper Transition Member that passes into trilobite-bearing Cambrian shales in the south reach of the Braintree Weymouth Tunnel. Analyses from this sequence also reveal shaly compositions.

Ages of the Roxbury Conglomerate (595-584 Ma) and Cambridge Fm (≤ 570 Ma) rule out the facies-relationship originally inferred from CTE mapping, but are consistent with a normal fault between Billings' Charles River syncline and a S-verging anticline of rollover origin to the north. At the same time, microstructures in CTE borings, including pressure solution and bedding locally transposed parallel to cleavage in the contact zone, reflect shortening more consistent with thrusting. U-Pb geochronology on zircon-bearing aplite intruding argillite in CTE Shaft 8 immediately north of the fault will provide a minimum Cambridge age bearing on this problem.