@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}
}
@proceedings {316,
	title = {A structural framework for the Nashoba Terrane in eastern Massachusetts.},
	volume = {45},
	year = {2013},
	pages = {107},
	abstract = {The exhumation and tectonic significance of the migmatitic Cambro-Ordovician arc-complex of the Nashoba terrane, located between lower-grade rocks of the Avalon and Merrimack terranes in Massachusetts, has historically presented an enigma, in part, due to a lack of detailed analysis of internal structure. We propose a new terrane-scale structural model based on nearly a decade of detailed geologic mapping to provide a framework for future study. A subvertical NE-striking composite fabric (S (sub n/n-1) ) forms the dominant structural grain of the terrane. S (sub n) commonly deforms an older layer-parallel foliation (S (sub n-1) ) about meter- to kilometer-scale, upright to steeply inclined, NE- and SW-plunging, tight disharmonic folds (F (sub n) ). In the Nashoba Formation migmatites, S (sub n) commonly transposes a subhorizontal S (sub n-1) enveloping surface into spaced meter-scale subvertical shear bands that are absent in the dominantly metavolcanic Marlborough Formation. Fold axis-parallel mineral stretching and intersection lineations (L (sub n) ) are locally overprinted on S (sub n) by subhorizontal peak metamorphic to retrograde mineral lineations (L (sub n/n+1) ). Ambiguous D (sub n) kinematics in the NE transition SW along strike to top-to-NW normal fold vergence and drag along steep north-dipping S (sub n) axial planes and S (sub n) - S (sub n+1) shear bands. Later strain (S (sub n+1) - S (sub n+2) ) appears to be progressively partitioned at lower grade to pre-existing S (sub n) shear bands and discrete internal and terrane bounding fault zones which display early high-grade top-to-SE dextral or sinistral motion (S (sub n-1) - S (sub n) ) broadly overprinted by lower-grade top-NW movement (S (sub n+1) ). We propose a tentative tectonic history incorporating sparse existing geochronologic and petrologic studies: Top SW D (sub n-1) motion coeval with approximately 425 Ma sill-grade metamorphism and possible accretion. D (sub n) initiating syn approximately 395 Ma peak metamorphism with migmatite generation along S (sub n) and progressive bulk fabric development largely complete by the intrusion of the relatively undeformed approximately 349 Ma Indian Head Hill granite. Exhumation can be accommodated by well-documented syn-to-post D (sub n) regional sinistral motion combined with progressively lower grade top-NW extension along discrete structures continuing through deposition and deformation of presumed Carboniferous basin sediments along the terrane boundary.},
	keywords = {$\#$StaffPubs, Cambrian, eastern Massachusetts, exhumation, fabric, fault zones, faults, foliation, massachusetts, metamorphism, Nashoba terrane, Ordovician, Paleozoic, Structural geology, tectonics, United States},
	issn = {00167592},
	url = {https://gsa.confex.com/gsa/2013NE/webprogram/Paper215867.html},
	author = {Joseph P Kopera and Matthew A Massey}
}
