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. The field trip starts out in Squantum in order to discuss the stratigraphic position and depositional setting of diamictite layers in the section. Then we travel by boat to Calf Island to observe the upper part of the Cambridge Argillite intruded by sills and dikes. Our boat route will circle around the Brewster Islands, affording an overview of the Brewster Syncline. Other features will be pointed out along the way. PREVIOUS WORK William O. Crosby (1880, 1888) published vivid verbal descriptions of the geology of the Boston Harbor islands, but no maps. His excellent maps of the southern coastline from Quincy to Nantasket (1880, 1889, 1893, 1894) show details of the geology, including outcrop locations. We relied heavily on these maps for portions of the Hull Quadrangle on the mainland, largely because many of his outcrops have become inaccessible due to extensive development in the intervening years. We also consulted Crosby’s unpublished field notebooks at the MIT Libraries Archives. Large-scale maps of Worlds End and Hewit Cove (Bailey and Bland, 2001) and of Houghs Neck (Thompson and Bailey, 1998) were also incorporated into our map. Five islands (Green, Calf, Middle Brewster, Outer Brewster and Rainsford) had been mapped by personnel of Hager and Richter at 1:2500 as part of a study before construction of the Inter-Island and Outfall Tunnels (Metcalf and Eddy, 1989, Appendix I). Other maps that include the Hull Quadrangle (Zen, 1983; Billings, unpub.; Kaye, unpub.) imposed a stratigraphy developed to the west in Boston (Emerson, 1917; LaForge, 1932; Billings, 1976) onto the islands and harbor perimeter. Faults were also extended from the mainland into the harbor. The interpretations we present differ from Zen (1983) in several important ways. C1 - 2 Logan Airport THOMPSON, KOPERA, ROSS, BAILEY ANDTHOMPSON BAILEY THOMPSON, ROSS, KOPERA, Castle Island Lovells Island THE NARROWS Long Georges Island Island Thompsons Island Rainsford Island NANTASKET ROADS HULL GUT Moon Island Peddocks Island Grape Island Slate Island THOMPSON, KOPERA, ROSS, BAILEY AND THOMPSON C1-3 THOMPSON, KOPERA, ROSS, AND ROSS, THOMPSON, BAILEY THOMPSON KOPERA, Island Tunnel. Island - g Boston Harbor along the Inter the along Harbor Boston g C1 - 3 Island Figure Cross2. Sectionofbedrock underlyin Rainsford C1-4 THOMPSON, KOPERA, ROSS, BAILEY AND THOMPSON CAMBRIDGE ARGILLITE The bulk of the sedimentary rocks in the Boston Harbor islands and northern part of the Inter-Island Tunnel are fine-grained, laminated, gray to pale green to purplish-gray or black strata, all treated here as Cambridge Argillite. The thin bedding and other sedimentary features in these rocks are typical of turbidites, deposited when fine sediment slowly settles out onto the sea floor from turbidity currents. Shaler (1869) named the presumably youngest unit in the Boston Bay Group, the Cambridge Slates. Billings (1929), noting the massive, non-fissile character, referred to the same rocks as the Cambridge Argillite. True slaty cleavage is well developed in a few places, such as the northwest arm of Rainsford Island (Fig. 3) and Slate Island, where slate was quarried starting in colonial times (Snow, 1971). Ring fossils that are identical to structures found in England and Wales have been found in numerous localities on the islands and on shore (Bailey and Bland, 2001; Thompson et al., 2012). Fossil ages in the British deposits are constrained by Ediacaran dates between 559.3 ± 2.0 Ma and 604.7 ± 1.6 Ma (inferred from SHRIMP [Sensitive High-resolution Ion Microprobe] U-Pb analyses of zircons from associated volcanic rocks by Compston et al., 2002). Although Ediacaran microfossils were found during subway construction near Harvard Square (Lenk et al., 1982), the only available U-Pb date for the Cambridge Argillite is ≤ 570 Ma from a mainland locality in Somerville, Massachusetts (details in section on geochronology). Rocks in the central and northern parts of the harbor are dominated by tan-weathering, greenish gray argillite. Numerous sedimentary structures such as cross-beds, dewatering structures and slump folds provide original topping information; no beds are overturned except in isoclinal, intrastratal slump folds, best exposed on Rainsford Island, where soft-sediment isoclines up to two meters in amplitude are refolded by tectonic folds (Fig. 4). North of the Figure 3. Cambridge Argillite on Rainsford Island, Figure 4. Refolded slump fold cut by steeply dipping viewed toward the southwest parallel to cleavage. cleavage (dashed line), Rainsford Island. Cathedral Fault Zone, the tectonic folds are typically open and upright, making it valid to estimate the thickness of the Cambridge Argillite along the crest of the Central Anticline (Fig. 1). Using an average plunge of 20° and assuming a continuously upright section, the Cambridge is at least 4542 meters thick between the Roxbury Conglomerate in Boston and the Brewster Islands. An unknown additional thickness lies northeast of the Brewsters. In the Inter-Island Tunnel south of the Cathedral Fault Zone and at Squantum, argillite tends to be purplish or reddish-brown. Crosby (1880, p. 203) wrote, “…no sharp distinction is possible between the brownish and grayish slates. The most that can be said is, that the slate immediately overlying…conglomerate is very likely to be of brownish or purplish tints, and that these colors are rare in slates not occupying this stratigraphic position.” This observation suggests that the more colorful argillites were deposited in a more proximal position than the gray. And THOMPSON, KOPERA, ROSS, BAILEY AND THOMPSON C1-5 it follows that we might expect gray argillites higher in the section as the deposition progressed in a deepening basin. Matrix-supported diamictite is exposed in the Hull Quadrangle at Moon Island, which is on strike with identical rocks along the south shore of Squantum, all dipping south (Fig. 1). Another layer of south-dipping diamictite holds AND ROSS, THOMPSON, BAILEY THOMPSON KOPERA, up the ridge on the north shore of Squantum (Squaw Rock), which is generally taken to be the type locality of the “Squantum Tillite” (Sayles, 1914). Billings (1976) interpreted the two diamictite layers as one and the same, repeated by a fault. The argillite above and below the northern layer appears virtually identical: purplish-gray, with sandy interbeds, yet according to Billings the argillite below the diamictite is the Dorchester Member of the Roxbury Conglomerate and that above it is the Cambridge Argillite. Kaye (1980) incorrectly showed the southern layer dipping north, with a syncline between the two. We prefer an alternative hypothesis (Socci and Smith, 1990), that the two diamictites represent two separate debris flows. It is now evident that the Cambridge Argillite contains multiple diamictite layers, consistent with Bailey and Bland’s (2001) depositional model for the Boston Bay Group, whereby diamictites represent debris flows that swept out from time to time into a deepening basin. Clarke (1885) reported a conglomerate layer dipping south in the sewage tunnel between Squantum and Dorchester (Fig. 5). He did not note whether it is clast-supported or matrix- supported, but perhaps it is a yet a third diamictite layer. Crosby (unpub. notebook no. 2, 1886) inspected the material from the tunnel, which included mostly argillite, with “a little slaty conglomerate”, possibly diamictite. Geologists who logged boreholes north of Long Island (Metcalf & Eddy, 1989) reported several thin layers of “conglomeratic argillite”, which we interpret also as diamictite. Billings ascribed great importance to the diamictite at Squantum, assuming it to be a unique horizon within the