@proceedings {357, title = {Stratigraphy and structure of the rocks underlying Boston Harbor: new insights on the Cambridge argillite and associated diamictites and diabase sills}, volume = {44}, year = {2012}, month = {03/2012}, pages = {43}, edition = {2}, abstract = {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 {\textquotedblleft}Squantum Tillite{\textquotedblright}, 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. }, keywords = {$\#$StaffPub, $\#$StaffPubs, Boston Basin, Boston Bay Group, Boston Harbor, Cambridge Argillite, diabase, dolerite, harbor islands, sills}, author = {Thompson, Peter J. and Joseph P Kopera and Solway, Daniel R} } @Map {362, title = {Bedrock geologic map of the Newton 7.5{\textquoteright} quadrangle, Middlesex and Suffolk Counties, Massachusetts}, month = {10/2015}, edition = {15-03}, abstract = {

This map has been superseded by GM-17-01 at\ https://mgs.geo.umass.edu/newton

}, keywords = {Boston Basin, Cambridge Argillite{\textquoteright}, Eidiacarian, Lynn Volcanics, Lynn-Mattapan volcanics, Mattapan Volcanics, Precambrian, Roxbury Conglomerate}, author = {Thompson, Margaret D.} } @Map {371, title = {Bedrock geologic map of the Newton 7.5{\textquoteright} quadrangle, Middlesex, Norfolk and Suffolk counties, Massachusetts}, month = {05/2017}, publisher = {Massachusetts Geological Survey}, edition = {GM-17-01}, abstract = {

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 {\textquotedblleft}Early Paleozoic{\textquotedblright} 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 ({\textquotedblleft}Vendian{\textquotedblright} 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{\textquoteright}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.\ 

}, keywords = {$\#$BedrockMaps, $\#$MGSPub, argillite, Boston, Boston Basin, Cambridge, faults, Lynn, Mattapan, Middlesex Fells, Newton, Roxbury, volcanics}, author = {Thompson, Margaret D.} }