BEDROCK GEOLOGIC MAP OF THE BOSTON SOUTH 7.5' QUADRANGLE, MIDDLESEX, NORFOLK AND SUFFOLK COUNTIES, MASSACHUSETTS

Until recently, available geologic quadrangle maps for greater Boston, Massachusetts, all dating from 1980 and earlier, appeared before U-Pb geochronology and geochemistry were routinely gathered on bedrock formations. Greatly expanded data in both categories are finally being translated into 1:24,000 scale maps, including the Newton 7.5 minute quadrangle published in 2017 by the Massachusetts Geological Survey and the adjoining Boston South quadrangle presented here. These two areas show generally the same stratigraphy, with Ediacaran granite and volcanic rocks overlain by conglomerate and argillite of the Boston Bay Group. Structural similarities include major ENE-trending folds cut by multiple generations of faults. Several significant advances over relations implied by older maps rest on Boston South observations highlighted below.

Stratigraphic refinements to stratified Boston-area rocks include re-assignment of the most southerly conglomerate in the quadrangle to the Lynn-Mattapan Volcanic Complex, here constrained by a weighted mean ²⁰⁶Pb/²³⁸U date of 593.19 ± 0.19 Ma on associated rhyolite. The dominance of pre-Ediacaran detrital zircons in sandstone interbedded with conglomerate in the fault block to the north warrants distinguishing this sequence from the traditional Brookline Member of the Roxbury Conglomerate. The new member is named for Franklin Park in the central part of the map, and the remaining Brookline Member is restricted to areas farther north and east in the quadrangle. Brookline sandstone contains an Ediacaran-dominated detrital suite at odds with previous interpretations linking all these conglomerates via proximal-distal transport across a simple sedimentary basin. The overlying Cambridge Formation has been subdivided using SiO₂/Al₂O₃ and CaO+Na₂O/K₂O ratios into a lower Argillite Member and an overlying Transition Member that passes upward into fossiliferous Cambrian strata in the neighboring Hull and Weymouth quadrangles.

Quincy Granite in the SE corner of the map has yielded a Lochovian CA-TIMS U-Pb date with uncertainty < 1 Ma, superseding an imprecise upper intercept date spanning the Ordovician-Silurian boundary. The contact with Cambridge transition strata to the north, shown by some as a thrust fault, is interpreted here as intrusive.