Thompson, Kopera, Ross, Bailey and Thompson C1-1
THOMPSON, KOPERA, ROSS, BAILEY AND THOMPSON C1-1 BEDROCK GEOLOGY OF BOSTON HARBOR: CAMBRIDGE ARGILLITE AND ASSOCIATED DIABASE SILLS AND DEBRIS FLOWS by Peter J. Thompson, Dept. of Earth Sciences, University of New Hampshire, Durham, NH 03824 Joseph P. Kopera, Office of the State Geologist, University of Massachusetts, Amherst, MA 01003 Martin E. Ross, Dept. of Marine and Environmental Sciences, Northeastern University, Boston, MA 02115 Richard H. Bailey, Dept. of Marine and Environmental Sciences, Northeastern University, Boston, MA 02115 Margaret D. Thompson, Dept. of Geosciences, Wellesley College, Wellesley, MA 02482 INTRODUCTION The main goal of this field trip is to present the results of recent work in the Hull 7 ½’ Quadrangle, which is underlain almost entirely by the Cambridge Argillite. Sixteen of the islands that comprise the Boston Harbor Recreational Park were mapped at 1:1,000 during 2011 under contract with the USGS and National Park Service, through the office of the Massachusetts State Geologist (Thompson et al., 2011). The results of this mapping were integrated with data from sewage and outflow tunnels to produce a structural form-line map (Fig. 1) and a north- south cross section (Fig. 2). Joint data were collected from the islands for comparison to brittle data from the tunnels. Samples of the igneous rocks were collected for thin section study and geochemical analysis, which Marty Ross presents along with comparison to sills and dikes on the mainland. (Note that P.J. Thompson refers to the mafic dikes as “diabase”, whereas Ross prefers to call most of them “dolerite”.) Dick Bailey summarizes depositional mechanisms for the Cambridge Argillite, and in the final section, Meg Thompson reviews available U- Pb geochronological constraints on the Cambridge Argillite and other members of the Boston Bay Group.
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