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 THOMPSON, KOPERA, ROSS, BAILEY AND THOMPSON

Slate Island Slate

NANTASKET ROADS NANTASKET

Lovells Lovells Island HULL GUT

THE NARROWS Grape Island Grape Georges Georges Island Peddocks Island Peddocks Rainsford Island Rainsford Long Long Island Moon Island Castle Island Castle Logan AirportLogan Thompsons Island Thompsons THOMPSON, KOPERA, ROSS, BAILEY AND THOMPSON C1-3

KOPERA, THOMPSONBAILEY THOMPSON, ROSS,AND

Island Tunnel. Island

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Inter the along Harbor Boston g

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Island

Figure Cross2. Sectionofbedrock underlyin

Rainsford

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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 KOPERA, THOMPSONBAILEY THOMPSON, ROSS,AND 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 Boston Bay Group. This assumption and the resulting oversimplification of the stratigraphy led to incorrect structural interpretations along the south shore of the harbor, and elsewhere.

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Figure 5. Dorchester Bay Tunnel showing a slaty conglomerate layer dipping southeast, lower in the section than the diamictite layer at Squantum (from Clarke, 1885). (Vertical exaggeration steepens apparent dip.)

STRUCTURAL GEOLOGY

Boston and its harbor lie in a graben called the Boston Basin, which is part of the Avalon terrane, an exotic terrane that became attached to the rest of North America in Paleozoic time. The rocks in the basin were deposited during the Ediacaran Period when Avalon was located at mid latitudes south of the equator, off the western coast of Gondwana (Thompson et al., 2007). The northern part of the Hull Quadrangle is dominated by broad, open, upright folds. South of a major NE-trending fault zone the geology is more complex (Fig. 1).

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Crosby (1888) was the first to recognize the Brewster Syncline (Fig. 6), an open fold plunging about ten degrees northeast between Calf Island and the Brewster Islands. The fold deforms both the Cambridge Argillite and the intruding diabase sills, which demonstrates that the deformation happened after the sills intruded. Crosby speculated that the same synclinal fold extends southwest through Rainsford Island and North Quincy (“Wollaston Syncline” of Billings, 1976). This is exactly what we see in the Inter-Island Tunnel: a broad region of gently folded rocks, but overall synclinal in form, from about Long Island south to an important fault zone between Rainsford and Peddocks Island (Figs. 1 and 2).

Green Island N

SCALE

CZca – Cambridge Argillite Pzd - diabase breccia The Hypocrite Pzdm – magmatic pillow zone Pzdf - foliated diabase JPzd - diabase dike JPzd - thin diabase dike

Little Calf Island (numbers indicate locations of samples in tables 1-3)

Outer Brewster Island

Calf Island

Middle Brewster Is.

Great Brewster Island (eroded drumlin)

Shag Rocks

Little Brewster Island

Boston Lighthouse

Figure 6. Outer Boston Harbor Islands, showing the Brewster Syncline and sample locations. THOMPSON, KOPERA, ROSS, BAILEY AND THOMPSON C1-7

N North of Long Island, argillite and diabase sills observed in the Inter-Island Tunnel are arched by a broad anticline, which is the eastern extension of the Central

Anticline on the mainland (Billings, 1976). A stereonet Figure 7. Equal area plot KOPERA, THOMPSONBAILEY THOMPSON, ROSS,AND plot of bedding attitudes in the tunnel across the anticline of great circles that shows that the axis trends about N57˚E and plunges represent bedding across 22˚NE (Fig. 7). Open, upright minor folds at various the Central Anticline in scales are common both in the tunnel and on the harbor the Inter-Island Tunnel. islands.

Cleavage dips steeply to moderately northwest, roughly parallel to the axial planes of open folds. Boreholes along the Outfall Tunnel alignment ENE of Deer Island cross the broad nose of the Central Anticline and then follow the north limb of the Brewster Syncline. The Main Drainage Tunnel, which passes under Castle Island on its way to Deer Island, lies parallel to the north limb of the Central Anticline (Rahm, 1962). The corresponding syncline to the north, the Charles River Syncline, appears in the North Metropolitan Relief Tunnel at the very NW corner of the Hull 7 ½’ quadrangle (Billings, 1975). Morell et al. (2004) found that a stereonet plot of bedding defines a fold axis at N54˚E plunging 12˚NE, very similar to that for the Central Anticline. However, the axial plane of the syncline and associated cleavage dip steeply southeast. Morell et al. (2004) noted a second, north-dipping cleavage in a few borehole samples, which they suggested may be related to younger, south-verging folds and a reverse fault at the south margin of the basin. Cleavage in the Inter-Island Tunnel dips almost exclusively northwest. The exact age of the folding is unknown. Skehan and Murray (1980) believed it to be due to the late Paleozoic Alleghanian Orogeny. In their view, the earlier Taconian and Acadian Orogenies did not affect any of the Avalon terrane southeast of the Bloody Bluff fault. Alternatively, there may have been two phases of deformation: the overturned folds may be Alleghanian, similar to south-verging folds that deform Pennsylvanian rocks in southeastern Massachusetts and Rhode Island; the open, NE-plunging folds may be older (Thompson et al., 2007).

Although evidence for faulting was observed along the entire length of the Inter-Island Tunnel, many of the slickensided joints and even thin breccia zones do not show much displacement (Metcalf and Eddy, 1989). Slickensides on bedding planes indicate flexural-slip folding. All faults with reported zones of gouge and offset beds are shown on the map and in the tunnel cross-section. Even some of these, for example faults near Long

Island, do not disrupt the overall dip of the bedding. No fault seems to correlate to the Mt. Hope Fault (Billings, C1 -

1976; Kaye, 1980; Thompson, 1983), which is shown by Zen (1983) and Metcalf and Eddy (1989) as extending 7 northeast into the harbor. The lack of a major fault on strike with the Mt. Hope Fault supports the idea that the fault was active during early stages of rifting, but then was buried by the younger sediments (Thompson et al., 2014). We also see no evidence for major faults on either side of a Squantum-Moon-Long Island block, which Billings (1976) invoked largely because of his assumption of a unique diamictite horizon.

However, several major faults do cut the stratigraphy and folds south of Rainsford Island, where the Brewster Syncline is abruptly truncated. Some of the faults were predicted by “low-velocity zones” detected by seismic reflection surveys prior to the tunnel construction (Metcalf and Eddy, 1989, Appendix K, 1989). A major zone of altered, rotten rock some 400 feet thick was encountered in the tunnel and provided significant engineering challenges. The dominant faults strike NE and they may continue along the bathymetric lineament that extends NE between the Brewsters and Hull. Toward the SW, the fault zone may correspond to an unnamed fault south of the Wollaston syncline (Kaye, 1980). Kaye showed a fault in the same position on his unpublished map of the Hull Quadrangle, which he called the South Brewster Fault. We propose calling it the Cathedral Fault, after the huge concrete structure dubbed by tunnel workers as “the cathedral”, which was necessary to shore up the rotten zone in the tunnel. If our assumption is correct that gray argillites north of the fault are younger than reddish-brown and purple argillites south of the fault, then the north side moved relatively downwards. C1-8 THOMPSON, KOPERA, ROSS, BAILEY AND THOMPSON

South of the Cathedral Fault and approaching Nut Island in the Inter-Island Tunnel, beds steepen and become overturned. In Hingham an E-W reverse fault, perhaps an extension of the Blue Hills Fault (Billings, 1976), truncates a complicated, faulted pattern of conglomerates variously treated as belonging to the Boston Bay Group (Fig. 1 adapted from Crosby, 1893 and 1894) or to the older Lynn-Mattapan Volcanic Complex (Thompson et al., 2014). A similar fault was observed during construction of the Braintree-Weymouth sewage tunnel south of the shaft on Nut Island, west of Raccoon Island (Deere et al., 2004). The fault dips 73˚N (M.D.Thompson, pers. comm., 2011), with Cambridge Argillite on the north against a south-facing sequence of volcanics, conglomerate and argillite. Farther south in the Weymouth Quadrangle, the same tunnel intercepted a “healed thrust fault” between Cambridge Argillite and the Cambrian Weymouth Formation (Deere et al., 2004). These faults may be a manifestation of the same compression that formed the Central Anticline and Brewster Syncline. At the south margin of the basin, the more massive granite, conglomerate and volcanics may have folded less easily, with the result that the argillite was thrust southwards on top of them.

CHEMISTRY OF THE SILLS AND DIKES Martin E. Ross, Northeastern University

Major and trace element XRF analyses (Tables 1 and 2) were completed for 14 sills and 6 dikes from Calf Island, Middle Brewster Island, Outer Brewster Island, Grape Island, Green Island, Rainsford Island, and Slate Island. Two samples of felsic rheomorphic matrix associated with magma “pillows” developed within sills on Outer Brewster Island and Green Island were also analyzed.

Classification The major and trace element data are listed in Tables 1 and 2 respectively. The chemical types based on the scheme of Irvine and Baragar (1971) include 17 tholeiites (13 sills and 4 dikes), a picro-basalt sill, an andesite sill, an ankaramite sill, and a basaltic andesite dike. Two rheomorphic matrix samples (G-9B and OB-15B, Tables 1 and 2) are calc-alkaline. If plotted on LeBas et al.’s (1986) alkali-silica diagram (Fig. 8) the results are identical except for two sills: the andesite sill on Grape Island (PJT-1, Table 1) is a dacite (~rhyolite) and the ankaramite sill on

Figure 8. Classification of igneous rocks of Boston Harbor Islands using the total alkalies-silica diagram of LeBas et al. (1986). * = dikes; O = Calf Island sills; empty boxes = Middle Brewster Island sills; filled box = Outer Brewster sill; + = andesite and dacite “pillow” matrix samples (PJT-9B-G and PJT-15B-OB) and dacite/rhyolite sill on Grape Island.

Table 1. Major element compositions of the sills and dikes of the Boston Harbor Islands

Sample/ body* SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total**

1O-1-C dike 47.69 3.44 13.33 15.30 0.24 5.03 8.51 2.83 1.11 0.52 98.00 KOPERA, THOMPSONBAILEY THOMPSON, ROSS,AND 1O-2-C sill 46.96 1.03 17.09 9.74 0.19 10.28 10.56 1.76 1.15 0.08 98.84 1O-3-C sill 46.84 1.35 16.41 10.90 0.20 9.19 10.85 1.60 0.84 0.11 98.29 1O-4-C sill 46.07 1.17 14.70 11.27 0.20 11.17 10.66 2.91 0.08 0.09 98.32 1O-5-C sill 43.08 0.68 8.92 11.94 0.21 26.51 5.91 0.70 0.34 0.06 98.35 1O-6-C sill 43.01 0.71 8.55 12.15 0.21 27.35 5.76 0.23 0.18 0.05 98.20 1O-7-C dike 45.45 1.08 13.81 10.07 0.18 16.63 9.21 1.74 0.23 0.10 98.50 PJT-23-C sill 46.60 1.51 17.56 9.84 0.18 7.97 11.61 2.45 0.50 0.10 98.32 1O-15-MB dike 52.90 3.48 15.42 13.70 0.32 4.25 5.01 1.94 0.55 0.48 98.05 1O-16-MB sill 47.63 1.65 16.55 10.94 0.20 9.08 10.09 1.74 0.31 0.10 98.29 1O-18-MB sill 49.93 1.30 16.78 10.02 0.18 7.50 10.06 2.57 0.35 0.12 98.81 1O-21-MB dike 47.19 1.69 16.25 10.94 0.19 8.77 9.77 3.42 0.35 0.10 98.67 10-22-MB dike 46.70 1.97 16.02 11.54 0.21 8.25 10.99 2.23 0.43 0.15 98.49 PJT-20B-MB sill 46.86 1.17 17.04 10.29 0.19 10.68 10.24 1.57 0.62 0.09 98.75 PJT-31-MB sill 47.40 1.30 16.93 9.40 0.18 8.78 11.92 2.09 0.51 0.08 98.59 PJT-9B-G* 67.84 0.84 16.01 5.49 0.10 1.72 1.56 3.47 1.83 0.21 99.07 PJT-1-GR sill 72.67 0.65 14.37 4.84 0.15 0.99 1.77 1.14 2.30 0.09 98.97 PJT-11-OB sill 47.06 1.11 17.86 9.80 0.18 8.84 11.46 1.73 0.59 0.08 98.71 PJT-15B-OB* 62.04 0.96 16.72 7.54 0.15 3.57 2.38 4.38 1.07 0.14 98.95 PJT-7-R sill 41.61 3.82 19.14 10.80 0.94 5.04 12.56 0.36 3.60 0.59 98.46

PJT-14A-S sill 48.17 2.61 17.91 13.02 0.24 6.04 6.53 2.22 0.98 0.45 98.17

*C = Calf Is.; MB = Middle Brewster Is.; GR = Grape Is.; G = Green Is.; OB = Outer Brewster Is.; R = Rainsford Is.; S = Slate Is. PJT-9B-G is from a thick zone of felsic breccia on Green Island. PJT-15B-OB is from the felsic matrix in zone of mixing below a sill on Outer Brewster. C1

- **raw totals using Fe2O3 ranged from 99.42-100.10. FeO = Fe2O3 x 0.8998 9

Table 2. Trace element compositions of igneous rocks of Boston Harbor Islands. C1 - Sample* Nb Zr Y Sr U Rb Th Pb Ga Zn Ni Cr V Ce Ba La Ti Mg# 10

10-1-C 17.6 281 46.6 302 0 21.0 3 4 24 156 19 40 324 66 389 27 3.28 36.9

10-2-C 2.3 58 18.4 323 0 34.6 0 4 15 69 237 328 182 7 173 5 0.82 65.3 KOPERA, THOMPSONBAILEY THOMPSON, ROSS,AND 10-3-C 3.5 82 23.9 353 0 24.5 1 3 17 83 165 353 229 11 206 4 1.14 60.1 10-4-C 2.4 66 20.2 470 0 0.6 1 3 15 96 301 1476 235 10 100 4 0.95 63.8 10-5-C 1.3 35 11.6 18 0 15.9 1 2 9 65 1040 1763 129 4 32 2 0.61 79.9 10-6-C 1.3 33 11.0 19 0 8.0 1 1 9 64 1054 1738 116 6 24 3 0.58 80.1 10-7-C 2.9 71 21.7 179 0 3.7 1 3 14 36 557 946 180 12 71 4 1.01 74.7 PJT-23-C 2.8 69 19.5 534 0 11.5 0 3 16 70 112 209 206 8 204 4 1.15 59 10-15-MB 30.8 258 30.7 513 0 12.8 3 7 25 139 17 35 291 49 255 20 2.67 35.6 10-16-MB 4.6 99 22.6 284 0 10.2 1 3 16 80 163 329 215 12 94 7 1.26 59.7 10-18-MB 4.8 106 26.6 323 0 9.4 2 4 17 83 118 221 222 22 122 9 1.25 57.2 10-21-MB 4.2 96 22.6 563 0 13.2 1 5 17 84 169 330 222 9 157 4 1.38 58.8 10-22-MB 6.8 99 22.9 416 0 11.5 1 7 18 102 109 285 262 14 223 6 1.68 56 PJT-20B-MB 2.5 61 19.2 232 0 20.9 1 1 15 82 223 233 153 12 108 5 0.87 64.9 PJT MB-31 2.7 65 19.3 289 0 16.2 1 3 16 73 137 276 221 11 114 3 1.15 62.5 PJT-1-GR 8.9 184 39.6 126 1 76.4 4 10 17 121 8 30 71 52 534 24 0.61 26.7 PJT-9B-G 12.6 247 58.8 239 1 65.8 7 7 18 77 25 61 103 54 468 24 0.76 35.8 PJT-11-OB 2.0 57 18.2 334 0 19.7 0 1 15 75 175 263 194 10 94 5 1.02 61.7 PJT-15B-OB 13.3 244 35.6 239 1 32.1 8 134 21 87 30 68 113 53 323 24 0.80 45.8 PJT-7-R 33.4 265 30.3 382 0 129.2 2 4 23 96 49 16 231 52 552 23 2.65 45.4 PJT-14A-S 19.5 194 37.2 260 0 34.9 1 2 19 135 37 39 177 43 211 28 1.81 45.3

*C = Calf Is.; MB = Middle Brewster Is.; GR = Grape Is.; G = Green Is.; OB = Outer Brewster Is.; R = Rainsford Is.; S = Slate Is. PJT-9B-G is from a thick zone of felsic breccia on Green Island. PJT-15B-OB is from the felsic matrix in zone of mixing below a sill on Outer Brewster.

Table 3. Rare earth element (REE) compositions for selected dikes and sills from the Boston Harbor Islands and East Point Nahant.

Sample* La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Ba K Rb Ta 1O-1-C dike 29.3 66.74 8.8 42 9.5 2.9 10.5 1.5 9.9 1.7 5.6 0.6 5.1 0.6 379 1.11 22.6 1.5 THOMPSON, KOPERA, THOMPSONBAILEY THOMPSON, ROSS,AND

1O-2-C sill 3.4 8.4 1.3 6.1 2 0.7 2.5 0.4 3.5 0.6 2.1 0.2 1.8 0.3 161 0.87 27.9 0.2

1O-3-C sill 4.9 12.54 1.9 10 3 0.9 3.5 0.7 4.3 0.9 2.5 0.3 2.2 0.3 189 0.7 21.9 0.3

1O-4-C sill 3.8 9.95 1.6 7.9 2 0.9 2.8 0.5 3.8 0.7 2.1 0.2 2.1 0.3 109 0.06 0.6 0.3 1O-18MB sill 8.3 19.78 2.6 12.6 2.9 1 3.8 0.6 4.6 1 2.7 0.3 2.8 0.3 112 0.27 7.2 0.5 PJT-31MBsill 3.4 9.32 1.4 7.8 2.3 1 3 0.5 3.6 0.8 2 0.3 1.9 0.2 104 0.43 12.7 0.3

87-123a sill 7.7 16.89 2.1 10.9 2.1 1 2.8 0.3 2.2 0.3 0.8 0.05 0.8 0.05 605 2.07 51.3 0.8

92-72a sill 21.9 48.74 6.3 29.9 5.8 1.9 5.5 0.8 4.9 0.7 2.1 0.2 1.5 0.2 328 1.56 43.8 1.5 92-122a sill 37.2 86.46 10.8 47.6 9.3 2.8 8.7 1.2 6.6 1.3 2.8 0.3 2.2 0.3 557 1.8 37.4 2.4 92-130a sill 23.3 53.62 6.8 29.4 6 1.9 6.1 0.9 5.1 0.8 1.9 0.2 1.5 0.2 328 1.36 55.2 1.7 O6-2 dike 11.5 27.08 3.3 15 3.3 1.1 2.9 0.4 2.6 0.5 1.3 0.1 1 0.1 256 1.03 36.9 1.2

Sample * Nb U Th Sr

1O-1-C 21.51 0.6 2.6 311

1O-2-C 1.79 0.05 0.3 305

1O-3-C 3.21 0.05 0.4 330

1O-4-C 2.93 0.1 0.4 444

1O-18MB 4.53 0.4 1.6 285

PJT31MB 2.26 0.05 0.2 269

87-123a 11 0.1 0.7 533

92-72a 22.3 0.5 1.9 559

92-122a 36.93 0.8 3.6 572 C1

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92-130a 25.19 0.6 2.4 517 11

O6-2 16.17 0.3 1 217

*C = Ca lf Island; MB = Middle Brewster; 87 -123a, 92 -72a, 92-122a, 92 -130a and O6-2 are from East Point, Nahant (see Ross and Bailey, 2001). THOMPSON, KOPERA, ROSS, BAILEY AND THOMPSON C1-12

Rainsford Island (R-7, Table 1) plots as a tephrite. A dolerite sill on Calf Island (10-4), a dolerite dike on Middle C1 - Brewster Island (10-21), and an andesite sill on Outer Brewster Island (OB-15B) are borderline spillites on the 12

Hughes’ (1973) 100*K2O/(K2O+Na2O) vs alkalies diagram. In thin section the two dolerite samples show moderate to severe alteration of augite to fibrous uralite, pennine chlorite, and calcite with plagioclase altered to saussurite. KOPERA, THOMPSONBAILEY THOMPSON, ROSS,AND The composition of two dikes is distinctly different from those of the sills and other dikes in the islands, especially in trace elements. These two dikes, an E-W dike on Calf Island (10-1) and a NNW-striking dike on

Middle Brewster Island (10-15), have similar compositions (Tables 1 and 2). Noteworthy are their high TiO2

(>3.40%), P2O5 (>0.45%), Zn (>135 ppm) contents and low Ni (<20ppm) and Cr (<40 ppm) compared to the other island dolerites. It is unlikely that these two dikes served as source vents for any of the sills. Their compositions do resemble about half of the Nahant dikes and sills analyzed by Ross and Bailey (2001) and about a quarter of the Mesozoic CNE dikes analyzed by Ross (1992) as well as many of the Paleozoic dikes of the Avalon terrane in Massachusetts (Ross, 1990).

Major and trace elements: Boston Harbor vs Nahant sills and dikes The Boston Harbor Island dolerites and East Point Nahant dolerites overlap slightly in composition when plotted on an alkalies-silica diagram (Fig. 9) but more of the Nahant rocks fall in the alkaline field compared to only the ankaramite sill on Rainsford Island. The island dolerites show more overlap (Fig. 9) with the Mesozoic CNE dikes and especially the Paleozoic dikes of eastern Massachusetts analyzed by Ross (1990).

A marked difference between the Boston Harbor Island and Nahant dolerites is evident when plotted on the Nb/Y-Zr/Ti diagram of Pearce and Peate (1995) on which the Nahant dikes and sills plot within the alkaline field and the island rocks all plot in the subalkaline field (Fig. 10). The island dolerites do overlap with the field of the Paleozoic dolerites of Eastern Massachusetts. The island dikes and sills are lower in Nb and most are lower in Zr compared to Nahant dikes and sills and CNE dikes (Fig. 14). Most of the island dolerites are lower in TiO2 and Zr and Zr/Y than the Nahant dolerites of Ross and Bailey (2001).

Figure 9. Classification of igneous rocks of the Boston Harbor Figure 10. Classification of Boston Harbor Island igneous Island using the total alkalies-silica diagram of Irvine and rocks using the Nb/Y- Zr/Ti diagram of Pearce and Peate, Baragar, 1971. Also shown are fields for Paleozoic dolerites of 1995. Symbols as in Figure 8. Also shown are fields in coastal mainland Massachusetts, Paleozoic dikes at East Point, which Paleozoic dolerite dikes of coastal mainland Nahant, and Mesozoic dikes of eastern Massachusetts. Massachusetts and East Point, Nahant plot.

THOMPSON, KOPERA, ROSS, BAILEY AND THOMPSON C1-13

The major and trace element data clearly indicate that the dikes and sills of the Boston Harbor Islands represent a chemically distinct suite from the dikes and sill of East Point Nahant and CNE dolerite dikes, but overlap with many of the Paleozoic dolerites of Ross (1990 and unpublished data). As will be discussed in a later section, the island and Nahant dolerites plot within significantly different areas on tectonic discrimination diagrams as well.

Magma differentiation The magnesium number (Mg# or M’) is the ratio 100 x Mg/(Mg +Fe2+) and is very useful as a differentiation index. This is because these ferromagnesian components are enriched in minerals that crystallize at high temperatures (olivine and pyroxenes) and so will be enriched in rocks from magmas that have not undergone significant differentiation (i.e. more primitive magmas). As a result, rocks with high M’ are less differentiated than those with low M’. Plotting of M’ against various major and trace elements are used to test for differentiation by fractional crystallization or differences in degree of partial melting.

Plotting M’ against Zr (Fig. 11) suggests the range of compositions of the Boston Harbor rocks were produced by fractional crystallization of a gabbroic magma. Plotting M’ against CaO (Fig. 12) suggests plagioclase fractionation occurred. However, the absence of plagioclase phenocrysts in all but one dike and sill suggests plagioclase fractionation was not an important process.

Figure 11. Plot of Zr vs Mg# for Boston Harbor Island igneous Figure 12. Plot of Mg# vs CaO for Boston Harbor Island igneous rocks indicating fractional crystallization of a gabbroic magma more rocks indicating limited plagioclase fractionation. Symbols as in likely accounts for their variation than different degrees of partial Figure 8. melting. Symbols as in Figure 8.

Two sill samples (10-5, 10-6) and one dike (10-7) are primitive with M’ greater than 70 (Table 2) suggesting that olivine accumulation occurred in the magmas. Sill 10-5 and 10-6 are picro-basalts and dike 10-7 is a tholeiite. Olivine accumulation may also have occurred in the magmas of the nine samples with M’ from 70 to 58.

Samples from one dike and five sills were analyzed for REE’s (Table 3) and the data was plotted on a chondrite-normalized REE spider diagram (Fig. 13). Of the 5 sills and one dike analyzed for REE’s, two sills and the dike show faint negative europium anomalies, suggesting plagioclase fractionation occurred. Two of the sills show faint positive europium anomalies suggesting that either plagioclase accumulation or assimilation occurred. One sill (10-2) shows no europium anomaly. Note that the dike’s overall values are higher than those of the sills.

THOMPSON, KOPERA, ROSS, BAILEY AND THOMPSON C1-14 C1 - 14

THOMPSON, KOPERA, THOMPSONBAILEY THOMPSON, ROSS,AND

Figure 13. Chondrite-normalized REE spider diagram for selected samples of Boston Harbor Island dolerites. See text for discussion.

Tectonic environments indicated by sill and dike chemistries In general, tholeiites are formed from magmas derived from partial melting at relatively shallower depths typical of the outer (nearer trench) portions of island arcs or within grabens of continental rift zones. Alkaline basalts tend to form by partial melting at greater depths on the inner side of island arcs (farther from trench) or over horsts bounding continental rift zones (Lipman and Mehnert, 1975; Gill, 1970; Ragland, 1989). Owing to their more alkaline compositions in general, Ross (1992) concluded that Mesozoic dolerite dikes to the west along the coast of Massachusetts were in general more alkaline due to their being intruded into the horst adjacent to the Connecticut rift basin farther west.

The relatively flat REE patterns of Boston Harbor sills and dikes (Fig. 13) are similar to the typical island arc curve. The curve for dike 10-1 and sill 10-18, however, have negative slopes that resemble curves for alkali-olivine basalts and their weak negative europium anomalies (Fig. 13) are more typical of continental flood basalts. Also, sill MB-31 with a flat, concave downward curve resembles that of a MORB basalt more than an island arc basalt.

The Boston Harbor Island sills and dikes were plotted on numerous tectonic discriminant diagrams, two of which are presented here (Fig. 14 and 15). Most of the diagrams indicate that the Boston Harbor Island dikes and sills intruded into a plate margin, island arc, or back arc setting, while the Nahant dikes and sills and the Paleozoic and Mesozoic (CNE) dikes of mainland eastern Massachusetts intruded into a within-plate or continental arc setting. In general, the Boston Harbor Island dolerites have a more plate margin/oceanic signature than do the dolerites of Nahant and on the mainland, and appear to have been emplaced in an island arc setting (Fig. 14 and 15). Though highly speculative at this point, the trend from continental arc/within-plate magmatism at Nahant to a more oceanic/island arc type magmatism of the Boston Harbor Islands suggests a westward-dipping subduction zone during the Paleozoic when it is believed the island dolerites and at least some of the dikes were emplaced. A

THOMPSON, KOPERA, ROSS, BAILEY AND THOMPSON C1-15

Figure 14. Boston Harbor Island dolerites plotted Figure 15. Boston Harbor Island dolerites plotted on on the tectono-magmatic discrimination diagram of the tectono-magmatic discrimination diagram of Merchede, 1986. Nahant dolerites plot within the Wood, 1980. Nahant dolerites plot within the shaded shaded area. area.

Paleozoic age is consistent with the fact that the Boston Harbor sills are deformed by presumably Alleghanian folding. The northeast and east-west trending island dikes may be Mesozoic as has been determined for mainland dikes of similar trends (Ross, 2001). The northwest-trending island dikes may be Paleozoic like northwest dolerite dikes on the mainland. Little altered, east-west basaltic dikes in Hingham (Bailey and Bland, 2001; Crosby, 1894) are almost certainly Mesozoic.

DEPOSITIONAL PROCESSES AND PALEOENVIRONMENTS OF THE CAMBRIDGE ARGILLITE Richard H. Bailey, Northeastern University

At first glance outcrops of the Cambridge Argillite have the appearance of a rather monotonous sequence of dark, thinly laminated and bedded mudstone with a somewhat rhythmic alternation of finer and coarser beds. To better understand the environments and modes of deposition of the Cambridge, examination of textures, sedimentary structures, and bedding relationships at the microscopic or thin section, mesoscopic or bedding, and macroscopic or outcrop scales is necessary. Description of the Cambridge in the following sections is based primarily on the Boston Harbor Islands, and on nearby coastal outcrops of Dorchester and Hingham Bays; however, the authors have examined outcrop, tunnel, and borehole information throughout the Boston Basin and find generally similar features in more northerly portions of the Boston Basin where outcrops are less common.

Detailed description of Cambridge Argillite Microscopic Characteristics. Most Cambridge facies are very thinly or thinly laminated to thinly bedded successions of claystone or mudstone and very fine to fine sandstone. Claystone, now mostly altered to illite, chlorite, mica or other phyllosilicates, comprise darker black or gray-black laminae and range from 0.1 to 3 mm in thickness (Fig. 16, B). Thicker, homogeneous claystone or mudstone laminae 1 to 5 cm or more in thickness are more common in the more northerly and presumably more distal Cambridge facies. Dark gray to gray siltstone and very fine to fine sandstones alternate with mudstone laminae. These coarser laminae may exhibit normal grading (Fig. 16, A); however, the difference in grain size between the base and top of beds is very subtle and is often difficult to discern even in thin section.

THOMPSON, KOPERA, ROSS, BAILEY AND THOMPSON C1-16 C1 - 16

THOMPSON, KOPERA, THOMPSONBAILEY THOMPSON, ROSS,AND

A B

C D

E F

Figure 16. Sedimentary structures in Cambridge Argillite on Slate Island (A, B), Middle Brewster Island (C, D, F), Grape Island (E). A. Photomicrograph of laminated fine sandstone with graded beds (height of image is 1.5 cm); B. Photomicrograph of fine laminated mudstone with mud chips over scour surface (height of image is 1.0 cm); C. Laminated mudstone and sandstone; D. Fine sandstone and mudstone with scour surface and lenticular mudstone and sandstone intraclasts; E. Diamictite in black mudstone with deformed intraclasts and extrabasinal sand and pebbles; F. Bedding surface with Ediacaran ring fossils. THOMPSON, KOPERA, ROSS, BAILEY AND THOMPSON C1-17

Small-scale sedimentary structures visible in thin sections include lenticular silt and very fine sand stringers or lenses, small-scale slumps and mud curls, flattened and bent mud chips or intraclasts, and numerous scour structures. These latter structures are evident where slumped or deformed laminae are eroded and overlain by planar laminae (Fig. 16, B). Intraclasts or mud chips, where abundant, may give the mudstone a lenticular fabric or lamination. Other microscopic structures include load casts, injection structures, tiny sandstone dikes, and intrastratal microfaults. The mm or cm scale clastic dikes that transect laminae often have a sinuous form in cross section. This geometry results from dewatering and post-depositional compaction that wrinkles or corrugates the originally planar or gently curved dikes. These deformed injection structures have been misinterpreted as spiral burrows.

Mesoscopic Characteristics. The most common bed-scale structures observed in Cambridge facies are the nearly ubiquitous intervals of intrastratal folding and/or soft sediment deformation. Beds with abundant platy, angular, irregular, or rounded intraclasts, so called debrites, are also very common. The debrites may have a mudstone or a muddy sandstone matrix and they commonly rest on heavily scoured underlying beds (Fig. 16 D). In some instances rip up clasts have been dislodged but not transported from the underlying source bed. Many beds contain intricate interbeds of sandstone and mudstone indicating decelerating and accelerating turbidity current flow (Fig. 16, C). Sandstone intraclasts are also common where thicker sandstones and mudstone are interbedded. Lenticular bedding and starved ripples up to 1-2 cm in amplitude and 20 to 30 cm in wavelength are present in interbedded thin sandstone and mudstones. The ripple lenses exhibit small scale cross lamination and some thin planar sandstones are similarly cross laminated. Characteristic Bouma sequences are relatively rare in Cambridge successions. Several 10-15 cm thick, fine to medium sandstone beds (especially well displayed on Slate Island) exhibit Ta or Tb to Tc facies. Extrabasinal clast-bearing diamictites from 1-4 mm to 20-40 cm are common in mudstone sequences near or interbedded with coarse sandstone and conglomeratic facies (Fig. 16, E). Very thin diamictites resting on scoured surfaces may contain large intrabasinal and extrabasinal clasts (Fig. 16, E). Extrabasinal lonestones have the appearance of dropstones, in that they deform and penetrate adajacent laminae, but this deformation is primarily due to differential compaction of the original muddy debris flow and adjacent beds.

Macroscopic Characteristics. Large-scale slump or intrastratal folds with amplitudes ranging from 0.5 to 3 m are present on Rainsford and Slate Islands, Squantum, and in intervals in tunnels. These thick slump-folded intervals are occasionally interbedded with or laterally transitional into debrities with mega-intraclasts or olistoliths (large blocks or slabs of sediment that are transported but not significantly internally deformed). On Rainsford Island a slump folded interval at least 6 m thick also contains olistoliths over 2 m in size (Fig. 17). Lateral transition from deformed mudstone to a chaotic intraclastic debrite about 1 m thick can be traced laterally for about 10 m on Slate Island. The sliding surfaces on which the detached sediment slabs moved are also visible under the debrites or slumped intervals in several instances.

Inferred depositional mechanisms and environments Billings (1976) interpreted the Cambridge Argillite to be a deeper water deposit and in part to be a distal facies of an assortment of conglomerate, conglomeratic sandstone, and sandstone facies in the more proximal Roxbury Formation. Recent mapping and radiometric dating (see Thompson et al, 2014, and this field guide) indicate that the relationship of the Cambridge with portions of the Roxbury is problematic. Cambridge strata are interbedded with diamictites containing abundant extrabasinal clasts of basement lithologies and with fine to coarse lithic arenites. Therefore coarse proximal facies equivalent to thick sequences of strata mapped as the Cambridge Argillite were clearly nearby. A rapidly subsiding rift- or wrench-basin formed during the later history of a subduction/arc system on continental or microcontinental crust is consistent with most of the observed geological data (Bailey and Bland, 2001; Thompson et al., 2014). In such a basin high energy braided stream systems and their coastal equivalents can deliver sediment to the subsiding basin through fan deltas or gravelly tidal environments. Terrestrial debris flows or THOMPSON, KOPERA, ROSS, BAILEY AND THOMPSON C1-18 C1 - 18

THOMPSON, KOPERA, THOMPSONBAILEY THOMPSON, ROSS,AND

40 cm

Figure 17. Slump structures on Rainsford Island. Left: Large isoclinal soft sediment fold with about 2 m amplitude. Right: Outcrop to right of and below slump fold with large coherent but transported blocks. Most of the outcrop on the island consists of such slump-folded and disrupted Cambridge Argillite. even perhaps lahars can also contribute very coarse sediment directly to the basin. Facies representing these sorts of environments might be present in the lower portion of the Roxbury Formation or they might have been completely removed by erosion after basin filling.

Virtually all facies mapped as the Dorchester and Squantum Members of the Roxbury are best interpreted as turbidites and other sorts of subaqueous gravity mass flow deposits. Despite suggestions of a direct influence of glaciation on the deposition of the Boston Bay Group by many workers (for example see Socci and Smith, 1990; Passchier and Erukanure, 2010), there is little diagonostic evidence for glacial influence. Lonestones, often interpreted as glacial dropstones, almost always sit on scour surfaces with other pebbles, sand and granules, and with intrabasinal intraclasts indicative of lateral gravity transport (Bailey, 1984; Bailey and Bland 2001; Carto and Eyles, 2012). Undisputed examples of truly isolated dropstones in distal Cambridge facies have not been reported despite examination of thousands of meters of Cambridge Argillite in outcrops, boreholes, and tunnels.

Lenticular ripple horizons and sandstone with migrating ripple cross lamination are common along the southern edge of the Cambridge outcrop area and are especially well exposed on Slate and Grape Islands. This limited evidence of traction sand transport suggests higher energy flow on a deep shelf or upper slope possibly by storm- generated shelf currents, turbidity flow, or contour currents impinging on the slope. It is unlikely that these ripple- bearing strata represent shallower shelf facies since they are interbedded with diamictites, slump folds, and other gravity-generated structures discussed earlier. The thinly bedded character of much of the thick Cambridge succession requires innumerable, small pulses of sediment delivery, presumably by distal or low flow turbidity currents. As with most thickly filled turbidite basins, it is difficult to explain the precise mode of initiation of flow events. Hyperpychnal flow from rivers (dense clouds of sediment) can descend to the sea floor and flow down prodeltas into deeper water. This is a common mode of sediment delivery in modern basins fed by major rivers where hyperpychnal flows and subsequent tubidity currents are typically initiated by major rainfall events. In the tectonically active setting we reconstruct for the Boston Basin, earthquakes would likely trigger many flow and slump events, and small turbidity currents were probably sourced directly from the slope and not from the shelf edge. The pulsating delivery of coarser sediments into the Boston Basin would have interrupted the background deposition of silt and clay. Typical delivery mechanisms for background sedimentation in modern basins include very dilute sediment clouds or turbidity currents flowing over the shelf edge, settling of sediment directly from the water column, and hypopycnal flows into or near the top of the water column above the basin floor along water THOMPSON, KOPERA, ROSS, BAILEY AND THOMPSON C1-19

masses of different densities. Triggering events such as climate and rainfall cycles, oceanic storms, and even volcanic eruptions all introduce fine sediment into basins.

Thick, widespread diamictite strata such as those observed at Squantum, MA require a major depositional event such as failure and collapse of a significant portion of a gravelly shelf, a fan delta, and a portion of the slope. Such collapsed shelves and slopes have been observed in modern ocean basins (Bailey et al., 1989). Study of detailed stratigraphy within Squantum diamictite beds shows that they were produced by different types of gravity mass flow mechanisms that probably occurred over a significant time span; they were not instantaneous single event beds. Careful study of the lower contacts of thick diamictites also suggests that they were in part channelized, although only a few outcrops provide sufficient lateral exposure for detailed mapping of channels.

Abundant scour surfaces overlain by graded beds (Fig. 16 A, D) , the presence of flame structures and rip-up intraclasts in sandstone laminae (Fig. 16, C), and thicker sandstone beds with Tb to Tc Bouma units support turbidity current transport and deposition. The lack of obvious or distinct normal grading in many sandstone and siltstone laminae probably results from very low flow dilute turbidity currents transporting a homogenous silt and very fine sand load. The narrow range of grain sizes and low current velocity did not permit effective vertical sorting during deposition, hence graded beds are subtle and difficult to discern. Numerous tiny intraclasts in siltstones and mudstone indicates erosion of weakly cohesive mud chips or plates. Recent experimental work (Schieber et al., 2010) demonstrated the capability of such intraclasts to be transported distances of many kilometers. The cohesive nature of extremely thin claystone layers might in part be due to the presence of biomat firmgrounds (Fig. 16, B). Biomats might have helped to stabilize muddy laminae and to preserve circular Ediacaran fossils found on many Cambridge bedding surfaces (Fig. 16, F) (Bailey and Bland, 2001; Bailey and Fletcher, 2002). Biomats might also explain the delicate mud curls and wrinkled textures on some well preserved bedding surfaces.

Thinly laminated, planar bedded mudstone in marine Phanerozoic basins is typically considered to be a deep water facies deposited under lower flow quiescent conditions. Post-Cambrian facies in many marine environments lack delicate lamination due to burrowing. In Proterozoic basins water depth must be interpreted cautiously because the lack of metazoan bioturbation permits occurrence of thinly laminated sedimentary rocks in a wider array of environments and facies. In the case of the Boston Basin, and the Cambridge Argillite in particular, the total suite of sedimentologic and stratigraphic evidence supports deposition in a deeper shelf-slope-basinal system. A sloping ramp or fan prodelta complex descending to a gentle slope and proximal deeper basin explains the majority of the preserved sedimentary structures. A steep slope is not necessarily required to deform and mobilize weak water- saturated muddy sediments, but a continual gradual slope permits fluidized debris flows to travel farther. The absolute depth of water in the deepest portion of the Boston Bay Group depositional basin is impossible to reconstruct but a water depth of at least several hundred meters would probably be necessary to provide the slope and gravitational impetus for the structures observed. The preserved Cambridge Argillite is estimated to be between 4 to 6 km thick and current dating suggests a maximum time span of basin history of perhaps 25 million years. These broad estimates provide a minimum rate of subsidence of 1km/5my with a realistic rate likely being much higher for Cambridge deposition alone. Accomodation for this rate of accumulation is similar to rates measured in some rift- and wrench-basin systems. Basin histories reconstructed by backstripping and geodynamic modeling suggest that rift/wrench basins go through a rift initiation phase with slower subsidence followed by a climax phase with rapid subsidence (Gupta et al., 1998). In the initiation phase depocenters are localized, whereas in the latter part of their history the basin will deepen and broaden significantly as regional subsidence accelerates and the depocenter expands. Such a sequence of crustal subsidence would explain the thickness pattern of the Cambridge and its overall retrogradational stratigraphy.

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GEOCHRONOLOGY AND STRATIGRAPHY OF THE BOSTON BAY GROUP Margaret D. Thompson, Wellesley College C1 - 20

LaForge combined Roxbury Conglomerate and Cambridge Argillite filling the Boston Basin into the Boston Bay Group beginning in 1932, but it took half a century more to find microfossils revealing the age of the argillite as latest Precambrian to earliest Cambrian (“Vendian” of Lenk et al., 1982). An ash bed near the base of the argillite KOPERA, THOMPSONBAILEY THOMPSON, ROSS,AND section in Somerville, MA subsequently yielded a slightly older U-Pb date of ≤ 570 Ma (Thompson and Bowring, 2000). The Neoproterozoic—specifically Ediacaran—age of unfossiliferous Roxbury deposits, however, has only lately been constrained via U-Pb zircon geochronology both for detrital zircons in Roxbury-related sandstones and for intrusive members of the associated Brighton Igneous Suite (Thompson et al., 2014). Figure 18 shows all currently available U-Pb zircon age constraints for the Boston Bay Group and immediately underlying basement units. Particulars of dates from the Boston Bay Group are discussed below.

STRATIGRAPHY OF GREATER BOSTON, MASSACHUSETTS

Braintree + Weymouth Fms

Cambrian 540 Ediacaran

550 ?? o o o o Aspidella? Figure 18. Simplified stratigraphy of the Boston Bay Group and underlying igneous basement, eastern 560 Cambridge Argillite Massachusetts. Age constraints are U-Pb zircon dates from the following sources: Dedham Granite North of Boston—Hepburn et al. (1993); Dedham Granite— 570 v v <570 v v v Ma v v Mystic Quarry ash bed Thompson et al. (2010); Lynn-Mattapan Volcanic Complex—Hepburn et al. (1993), Thompson et al. (2007, 2014); Roxbury Conglomerate and associated Brighton 580 igneous suite—Thompson et al. (2014); ash bed in S <596 Ma Cambridge Argillite—Thompson and Bowring (2000). 585 Ma Brighton Igneous Suite Group Boston Bay Lower Roxbury Conglomerate subdivided into FP— B <599 Ma Roxbury Conglomerate Franklin Park, B—Brookline and S—Squantum members 590 FP <595 Ma (Thompson et al., 2014). Aspidella? denotes ring fossils 597-593 Ma Lynn-Mattapan of Bailey and Fletcher (2002), Thompson et al. (2012a) in Volcanics the Cambridge Argillite. Complete list of dated basement 600 units in Table 1 of Trip B-2).

606 to

Complex 610 609.5 Ma Dedham Granite & Basement

Ma Dedham Granite North of Boston

The Roxbury Conglomerate is younger than 595.14 ± 0.90 Ma, the youngest detrital zircon obtained from three sandstones sampled in various parts of the Roxbury section (206Pb/238U-Pb chemical annealing-thermal ionization mass spectrometry [CA-TIMS] date of Thompson et al., 2014). This date comes from a sample low in the conglomerate section at Franklin Park, Dorchester, MA. The complete detrital age spectrum of this sample (as defined by laser ablation-intercoupled plasma mass spectrometry [LA-ICPMS] analyses of 91 zircons) shows a THOMPSON, KOPERA, ROSS, BAILEY AND THOMPSON C1-21

preponderance of pre-Ediacaran components that distinguish this sample from stratigraphically higher sandstones sampled in Newton, MA (Webster Conservation Area) and at Squantum Head, Quincy, MA. The Franklin Park sequence also differs from overlying conglomerate in terms of clast assemblages (Thompson et a., 2014) and therefore has been separated from the traditional Brookline Member (Crosby, 1880; Emerson, 1917; Billings, 1976) as the lowest member of the conglomerate. The Webster and Squantum samples, respectively representing the restricted Brookline and Squantum members of the Roxbury, yield slightly older maximum ages of 598.87 ± 0.71 and 596.39 ± 0.79 Ma, respectively (206Pb/238U CA-TIMS dates of Thompson et al., 2014). The younger Franklin Park date remains the best estimate of maximum age because this comes from the lowest sampled conglomerate. These and other Ediacaran dates from Roxbury detrital zircons are consistent with sources in the Lynn-Mattapan Volcanic Complex, Dedham and Dedham North granites shown in Figure 18, as well as other dated plutons surrounding the Boston Basin (Thompson et al., 1996 and 2010); pre-Ediacaran dates resemble those obtained from detrital zircons in the Westboro Quartzite (Thompson et al., 2012b).

A minimum age for the Roxbury Conglomerate comes from associated Brighton igneous rocks. These appear as the Brighton “Volcanics” in recent literature (Cardoza et al., 1990; Hepburn et al., 1993; Hepburn and Bailey, 1998; Thompson and Grunow, 2004), but the geochronology (details in Thompson et al., 2014) is consistent with some intrusive occurrences, as documented by various early workers (Benton, 1881; Burr, 1901; LaForge, 1932; Tierney et al., 1968). In particular, Brighton dacite near the base of the Roxbury and Brighton andesite some 1000 m higher in the section yield nearly identical weighted mean 206Pb/238U-Pb CA-TIMS dates of 584.19 ± 0.70 Ma and 585.37 ± 0.72 Ma, respectively. These results are best explained if the units are shallow sills, and in this case, the entire conglomerate section is older than approximately 585 Ma.

Volcanic ash from Cambridge Argillite in the Mystic River Quarry (Somerville, MA) was dated before the advent of single-zircon analyses and pre-treatment by chemical annealing to minimize Pb loss. Multi-grain zircon fractions from the sample yielded a variety of discordant 207Pb/206Pb dates, as well as a much younger concordant date of 570 Ma (TIMS dates of Thompson and Bowring, 2000). Because xenocrystic or detrital components are not uncommon in volcanic ashes (Landing et al., 1998, for example), we judged 570 Ma to be a good estimate of the age of the ash bed. Nevertheless, we reported it as ≤ 570 Ma because the possibility of still younger zircons could not be ruled out.

The Cambridge Argillite date, no matter whether it represents the true or maximum depositional age of strata low in the section, serves to demonstrate that the argillite is younger than the 595–585 Ma Roxbury Conglomerate as shown in Figure 18. Regional stratigraphic interpretations, beginning with Billings and Tierney (1964) and formalized by Billings (1976), that imply age equivalence between conglomerate in the southern half of the Boston Basin and Cambridge Argillite in the north cannot be reconciled with the collective geochronology.

ACKNOWLEDGEMENTS

This research was funded in part by Cooperative Agreement H4503090700 between the Department of the Interior, National Park Service, Northeast Region and the University of Massachusetts at Amherst. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Government, the Commonwealth of Massachusetts, the Massachusetts Geological Survey or the University of Massachusetts. Geochemical analyses were performed at the analytical laboratory of the Department of Geosciences at The University of Massachusetts, Amherst. In addition, one dike and five sills were selected for REE analyses performed by Acme Labs, Vancouver, B.C. We thank Wellesley's Cathy Summa for helping us get the necessary insurance waiver for the Calf Island trip.

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TRIP LOG C1 - The meeting point is the American Legion parking lot on the north side of Dorchester Street in the Squantum 22 section of Quincy, beyond Nickerson Beach and before the bridge to Moon Island [UTM: 42.302247° N, 71.011052° ]. We will not visit the famous diamictite at Squaw Rock (also called Squantum Head or Chapel Rocks), often considered the “type section” of the Squantum Member of the Roxbury Formation, which lies north of KOPERA, THOMPSONBAILEY THOMPSON, ROSS,AND the meeting point. Field guides are available for that locality (Bailey, 1987; Bailey and Bland, 2001). We will car pool to the Town of Quincy’s Orchard Beach, where parking is limited along the seawall on Bayside Road.

Mileage 0.0 Retrace your route back toward Quincy (turn right from the parking lot). 0.2 Turn left on Bellevue Road. 0.6 Turn left on Brunswick Street and follow it to the seawall. 0.8 Turn right and park along the beach. Walk to the north end of the beach, where large granite blocks are stacked, and onto the small outcrop.

STOP 1: Orchard Beach, Squantum The pennisula of Squantum (Fig. 1) is underlain by approximately 1.1 km of strata striking generally to the northeast and dipping about 30 to 50 degrees to the southeast. The lower 120 m or so of the section are comprised of the diamictite at Squaw Rock. This lower diamictite has an interbedded transition with underlying sandstone and mudstone, mapped as the Dorchester Member of the Roxbury Formation. The upward transition into overlying sandstone and mudstone, mapped as the Cambridge Argillite, occurs as a series of diamictite and pebbly mudstone beds that give way to a purplish mudstone and siltstone. About 690 m of Cambridge mudstone underlie much of the rest of Squantum pennisula although it is very poorly exposed. Outcrops of a younger diamictite occur intermittently along the east shore of the penninsula and in sparse outcrops in the neighborhoods on the south and southeast facing slopes. The lower and upper contacts of this diamictite are not exposed, however outcrop width indicates that it is at least 300 m thick.

STOP 1A: North Orchard Beach Debris Flow and Interbedded Mudstone This small outcrop exposes a debris flow deposit or debrite about 2 to 4 m thick. The exact thickness is difficult to determine because underlying and overlying mudstone is intimately contorted into the debrite. The entire debris flow is deformed by slump folds and the orientations of several of the larger folds are shown in Fig. 19. The matrix of the debrite is a grayish mudstone with patches of fine sandstone and siltstone. Intraclasts are abundant at all scales and range from small mudstone or sandstone blebs or clots to a purplish sandy mudstone olistolith several meters on a side. About 5 m from the beginning of the outcrop there is a 1.5 m long bedded sandstone/mudstone block entirely enclosed within the matrix (Fig 20, E). Black sandstone intraclasts with very ragged and deformed margins are very distinctive.

One intraclast (Fig. 20, F) has been identified in the literature as a stromatolite head (Passchier and Erukanure, 2010), but it clearly is a deformed mudstone intraclast. Extrabasinal clasts are primarily felsic volcanics, with rare granitic and quartzite pebbles or cobbles. As you walk to the northeast corner of the outcrop near the large concrete vent, the outcrop is a distinctive purple, different from the gray matrix of the debrite. The margins of the purple block are faulted or tectonized but it seems to be a large olistolith entrained by the debris flow. Northwest of the area in Figure 19 along the shoreline you traverse about 40 m of mudstone. Beyond the last outcrop of deformed mudstone the section is covered until you get near the top of the lower diamictite on the north side of the causeway, north of the meeting point. Please do not try to cross the causeway from the beach, as permission is required from the Boston Police Department to be on the causeway to Moon Island. Walk back south along the beach.

STOP 1B: South Orchard Beach Island Diamictite. To visit the small offshore island at the south end of Orchard Beach you should begin at a falling tide or near low tide as the island becomes inacessible at mid-tide. The island exposes about 35 m of typical diamictite. Diamictite textures range from fine matrix supported pebble and THOMPSON, KOPERA, ROSS, BAILEY AND THOMPSON C1-23

Surface rubble

Figure 19. Stop 1A. Sketch map of debrite at north end of Orchard Beach, Squantum, MA; mapped by R.H. Bailey. THOMPSON, KOPERA, ROSS, BAILEY AND THOMPSON C1-24 C1 - 24

THOMPSON, KOPERA, THOMPSONBAILEY THOMPSON, ROSS,AND

A B

C D

E F

Figure 20. Outcrop images from field stops: Orchard Beach island diamictite (A-D) and north Orchard Beach debrite (E-F); ruler is 15 cm. A. Sandstone interbed in diamictite with graded sandstone laminae, note clast from overlying diamictite into sandstone bed; B. Closeup of clast deflecting and penetrating underlying laminae; C. Pebbly diamictite with muddy sandstone matrix; D. Cobble-boulder diamictite with sandy mudstone matrix; E. 1.5 m long laminated sandtone intraclast in debrite; F. Thinly laminated mudstone intraclast. Note that this intraclast was misidentified as a “stromatolite hemispheroid” (Passchier and Erukanure, 2010).

THOMPSON, KOPERA, ROSS, BAILEY AND THOMPSON C1-25

small cobble diamictite (Fig. 20, C) to clast-supported coarse cobble to boulder diamictite (Fig. 20, D). As at most other Squantum-type diamictite localities felsic volcanic clasts predominate with fewer but larger granite and quartzite clasts. The largest clast in the outermost outcrop is a granite boulder about 70 cm long. It is very difficult to see bedding clearly in the homogeneous diamictite and bed orientation must determined by looking for finer sandstone or conglomerate interbeds. One scour channel indicates a topping direction toward the southeast. Cleavage dips steeply northwest.

Return to the vehicles and retrieve any vehicles left at the Meeting Point. Reset the odometer.

0.0 Turn right on Dorchester Street, which turns into East Squantum Street. 0.8 Turn left on Quincy Shore Drive. 3.4 Right on Sea Street. 3.8 Left on Southern Artery, MA Rt.3A. 4.8 Left on Washington Street, still MA Rt.3A. 5.3 Stay on 3A through the traffic circle. 8.1 Turn left on Shipyard Drive and look for the Hingham Shipyard parking lot.

We will travel by boat to Calf Island and the leaders will point out various features along the way. Most of the larger islands in the harbor are drumlins with no exposed bedrock. From the tunnels we know that most of the rocks underlying Boston Harbor are Cambridge Argillite. Many of the smaller islands would not exist were it not for the presence of more resistant diabase sills.

The boat will follow the Weymouth Back River past Hewitts Cove, and then veer north west past Slate and Grape Islands, where bedding and foliation both dip steeply south (Fig. 1). Off to the west lies Houghs Neck, with Nut Island at the tip, which is the southern terminus of the Inter-Island Tunnel. Kaye (unpub., 1989) reported that red sandstone was found in a boring north of Nut Island. A boring on the west end of nearby Peddocks Island showed graywacke with thin beds of red shale. In both areas the red beds appear to be of limited extent and to grade laterally east and west into similar non-red sediments. Peddocks Island features several drumlins connected by tombolos. We’ll travel north past Peddocks and then through Hull Gut into the dredged Nantasket Roads.

The Cathedral Fault lies between Hull and the islands beyond. Off to the northwest is the Narrows, which was the main approach to Boston (hence the fort on Georges Island) until deeper channels were dredged. The sewage treatment facility on Deer Island is the northern terminus of the Inter-Island Tunnel. The boat will pass Lovell Island and head northeast toward Calf and the Brewsters.

One needs permission from the National Park Service to land a boat on Calf Island, and a permit to conduct scientific investigations. A landing craft can be arranged for a fee through UMass/Boston.

STOP 2 CALF ISLAND. Calf Island is the largest of the outer harbor islands (Fig. 21). The bedrock consists of five or six diabase sills and minor argillite between some of them. Beds and the sills dip gently east-northeast and are folded by open, northeast-trending folds. So far, meaningful U-Pb dating of the sills has been stymied by the paucity of zircons.

STOP 2A. LANDING COVE AND ARGILLITE We land at a small bay on the west shore of Calf Island, with a view to the right of Cambridge Argillite dipping about 24 degrees away from us, capped by a diabase sill. Why is this bay here? Why is the argillite exposed along this stretch, but not to the north or south? Apparently the rocks across the middle of the island have been lifted up in a horst block, so that the contact between argillite and diabase has shifted relatively toward the east. Clamber up the shingle berm to look east across a salt marsh to another small bay on the eastern shore. A NE-striking fault is poorly exposed along the north side of this bay. As sea level continues to rise, the island will eventually be split into two smaller islands.

THOMPSON, KOPERA, ROSS, BAILEY AND THOMPSON C1-26

C1 - 26

THOMPSON, KOPERA, THOMPSONBAILEY THOMPSON, ROSS,AND

Figure 21. Bedrock geology of Calf Island, showing sample locations in tables 1-3. THOMPSON, KOPERA, ROSS, BAILEY AND THOMPSON C1-27

The argillite is thinly bedded and weathers to a pale grayish-green color. Depending on the tide, up to about 10 meters is exposed below the sill. In all the argillite exposures encountered, keep your eye out for primary structures such as cross-bedding, slump structures, and ring fossils. The presence of ring fossils here in the upper part of the Cambridge Argillite (Thompson et al., 2011) as well as lower in the section (Hewitts Cove and Slate Island, Bailey and Bland, 2001) shows that the processes which formed them persisted for a long time. Please, no collecting of ring fossils except from loose material.

STOP 2B. DIABASE SILLS. After taking a close look at the argillite, we will follow a path counter-clockwise around to the SE corner of Calf. We will cross three diabase sills, including those where samples 10-4-C, 10-5-C, and 10-6-C were collected (Tables 1 & 2). Note thin dark mafic-rich layers a few centimeters thick, which locally merge (Fig. 22, A). Two sills can be seen in contact: the lower one is altered to a greenish color, in sharp contact with the overlying reddish-weathering sill. Look for chilled margins to conclude age relations. Note apparent columnar jointing.

Argillite reappears on the next promontory south, a spot much favored by nesting sea birds (don’t try coming here in July!), where the argillite is arched beneath the dolerite sills in an open, NE-plunging anticline. Cleavage in the argillite strikes NE, parallel to the fold’s axial plane, and dips 75° NW. All these rocks are still within the horst block.

STOP 2C. ARGILLITE-DIABASE CONTACTS. Depending on the tides, we will explore as much of the section in the cliff below an old chimney as possible. (The ruins were actress Julia Arthur’s 1891 “cottage”.) This section was studied by Wellesley College student Janet Sarson (1998). Sarson concluded that the thin layers of argillite preserved between the sills seem to have partially melted in contact with the hot intruding magma. The felsic melt in turn was injected into cracks in the cooling diabase sills, both upward (Fig. 22, B) and downward. The felsite and argillite are chemically identical (Sarson, 1998).

STOP 2D. FOLIATED DIABASE. At the south end of the cliff, all these layers are cut by a steep fault, beyond which much of the dolerite is greenish-weathering and foliated. The foliation wraps around some curious, roundish masses of unfoliated dolerite a few meters across, best exposed at the southern promontory. We can discuss the origin of these blocks. Are they boudins? Crosby also noted these masses and on his third visit to Calf (unpub. notebook no.4, 1901) wrote that they were “still a mystery”! The foliation is steep and strikes NE, reappearing along the east shore 50 meters north of yet another exposure of argillite and dolerite pillows with felsic matrix. It would seem that the foliated zone must be a shear zone, because the argillite strikes northwest toward the interior of the island, at a high angle to the foliated rock.

STOP 2E. MAFIC PILLOWS IN FELSIC MATRIX. Excellent exposures of argillite and diabase pillows in a felsic matrix (Fig. 22, C) will be visited at the SE tip of the island. Crosby (unpub. notebook no. 1, 1901) described such pillow-like shapes as a “marble paper pattern”. He wrote in notebook no. 4 (1901?) “[there is] much evidence that the slate was not thoroughly lithified when the [dolerite] was intruded.” One of the present authors agrees, and by extension, concludes that the sills might also be Neoproterozoic. Discussion welcome! From here we will retrace our route to the landing cove.

STOP 2F. CROSS-CUTTING DIABASE DIKE AND SILL/ARGILLITE CONTACTS. If time allows, we will walk north along the west shore to see diabase/argillite contacts cut by an E-striking, vertical, 20-meter wide diabase dike (10-1-C), whose chemistry is quite different from that of the sills (Tables 1 & 2). It may be significantly younger than the sills. The contact between one of the sills and the argillite is especially interesting (Fig. 22, D).

THOMPSON, KOPERA, ROSS, BAILEY AND THOMPSON C1-28 C1 - 28

THOMPSON, KOPERA, THOMPSONBAILEY THOMPSON, ROSS,AND

A B

C D

Figure 22. Features in diabase sills on Calf Island. A. Dark chloritic horizons in layered sill at Stop 2B. B. Felsic dikes cutting upward into diabase dill in the cliff at Stop 2C. C. Cross section view of mafic pillows in a rheomorphic felsic matrix at the southeast corner of Calf (Stop 2E). D. Pods of diabase at the base of a sill, west shore of Calf Island (Stop 2F). One gets the sense that the mafic material intruded toward the right.

As we leave Calf Island, the boat will circle around the Brewster Islands in a clockwise direction (See Fig. 6). Like Calf Island, Little Calf Island is also composed of east-dipping diabase on the west limb of the Brewster Syncline. We will pass through “The Hypocrite”, a very deep channel with unpredictable currents between Little Calf and Green Island. Green Island is entirely diabase, cut by a 30-meter wide zone of rheomorphic felsite breccia. We will cross the Brewster Syncline, passing through “The Flying Place” between Outer and Middle Brewster Islands, where we get a close look at the sills dipping northwest. On our way back to Hingham we’ll pass by Shag Rocks and Little Brewster Island, which is held up by a lower, thinner diabase sill. The Boston Lighthouse, built in 1783, is the second oldest working lighthouse in the United States.

THOMPSON, KOPERA, ROSS, BAILEY AND THOMPSON C1-29

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Thompson, M.D., A.M. Grunow and J. Ramezani, 2007, Late Neoproterozoic paleogeography of the Southeastern New England Avalon Zone: Insights from U-Pb geochronology and paleomagnetism, Geological Society of C1 - America Bulletin, v. 119, no. 5/6, p. 681-696. 32

Thompson, M. D., Ramezani, J., Barr, S. M., and Hermes, O. D., 2010, High-precision U-Pb zircon dates for Ediacaran granitoid rocks in SE New England: Revised magmatic chronology and correlation with other Avalonian terranes, in Tollo, R. P., Bartholomew, M. J., Hibbard, J. P., and Karabinos, P. M., editors, From KOPERA, THOMPSONBAILEY THOMPSON, ROSS,AND Rodinia to Pangea: The lithotectonic record of the Appalachian Region: Geological Society of America Memoir 206, p. 231-250. Thompson, M. D., Barr, S. M., and Grunow, A. M., 2012b, Avalonian perspectives on Neoproterozoic paleogeography: Evidence from Sm-Nd isotope geochemistry and detrital zircon geochronology in SE New England, USA: Geological Society of America Bulletin, v. 124, p. 517-531. Thompson, M.D., Ramezani, J., and J.L. Crowley, 2014, U-Pb zircon geochronology of Roxbury Conglomerate, Boston Basin, Massachusetts: Tectono-stratigraphic implications for Avalonia in and beyond SE New England, American Journal of Science, v. 314, in press. Thompson, P. J., Kopera, J. P. and D. R. Solway, 2011, A report on the bedrock geology of Boston Harbor to the National Park Service, 2011: Submitted to the U.S. National Park Service Geological Resources Inventory Program. 31 p., 15 plates, and GIS database files. Thompson, P. J., Kopera, J. P., and Solway, D. R., 2012, Stratigraphy and structure of the rocks underlying Boston Harbor: New insights on the Cambridge Argillite and associated diamictites and diabase sills: Geological Society of America Abstracts with Programs, v. 44, no. 2, p. 43. Tierney, F. L., Billings, M. P., and Cassidy, M. M., 1968, Geology of the City Tunnel, greater Boston, Massachusetts: Journal of the Boston Society of Civil Engineers, v. 55, p. 60-96. Wood, D. A., 1980, The application of a Th-Hf-Ta diagram to problems of tectonomagmatic classification and to establishing the nature of crustal contamination of basaltic lavas of the British Tertiary Volcanic Province: Earth and Planetary Science Letters, v. 50, p. 11-30. Zen, E-an, ed., 1983, Bedrock geologic map of Massachusetts, United States Geological Survey, 1:250,000.