Bedrock geologic map of the Newton 7.5’ quadrangle, Middlesex, Norfolk and Suffolk counties, Massachusetts

The Newton quadrangle, located west of Boston, Massachusetts, is mainly underlain by clastic sedimentary and minor igneous rocks occupying the topographic Boston Basin (Crosby, 1880; Emerson, 1917; LaForge, 1932; Billings, 1976 and references therein). Fault blocks dominated by older plutonic and volcanic rocks truncate the Basin sequence on the west and south. Although fossiliferous Cambrian strata overlying plutonic basement had been reported some 30 miles away in North Attleboro, MA (Billings, 1929), the first quadrangle map including the Newton area (1:62,500 Plate I of LaForge, 1932) portrayed the plutonic units as “Early Paleozoic” and assigned all the others to the Devonian or Carboniferous periods. By the time the Newton quadrangle appeared at 1:24,000 scale (Kaye, 1980), emerging U-Pb zircon geochronology had revealed Neoproterozoic (Ediacaran in timescale of Gradstein et al., 2012) crystallization ages both for the Mattapan Volcanic Complex and the Dedham Granite on the south side of the map area (Kaye and Zartman, 1980; written communication of Zartman and Naylor, 1980 not published until 1984). The Boston Basin sequence in this map was inferred to be Neoproterozoic in the absence of a break with overlying Cambrian strata (Kaye and Zartman, 1980), an interpretation confirmed shortly thereafter by the discovery of Ediacaran microfossils in the top portion of the sedimentary section (“Vendian” in Lenk et al., 1982). In the map presented here, U-Pb zircon dates obtained by the author over twenty years with colleagues at MIT’s Radiogenic Isotope Lab constrain all map units except mafic dikes. During this interval, U-Pb analyses have become more precise and accurate, leading also to significant refinements to the dates used by Kaye in 1980, as discussed further in the section on Stratigraphy.