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An unusually complete section of several important Paleozoic(?) formations was exposed in the 12,806-m-long Wachusett-Marlborough Tunnel in the Clinton, Shrewsbury, and Marlboro quadrangles of east-central Massachusetts. The Marlboro Formation, the stratigraphically lowest formation cut by the tunnel, is at least 2,705-m-thick and has been divided into 31 members composed of distinctive sequences of eight lithologic types or facies referred to herein as lithologic units. Several of these units have recognizable subdivisions.

The overlying Nashoba Formation, at least 3,740 m thick, has been divided into 30 members composed of distinctive sequences of seven of the same eight lithologic units that form the members of the Marlboro Formation. Some of the lithologic unit subdivisions that are recognized in the Marlboro also occur in the Nashoba, but others do not. Additional lithologic unit subdivisions are found only in the Nashoba.

The layered rocks above the Nashoba Formation are divided into 15 unnamed formations or members composed of distinctive sequences of 12 lithologic units. Five of these lithologic units are recognized in the Nashoba and Marlboro Formations, but the other seven lithologic units are restricted to the unnamed formations or members.

Of the total measured stratigraphic thickness of 7,834 m, the Marlboro Formation makes up 34.5 percent; the Nashoba Formation, 47.7 percent; and the unnamed formations or members, 17.7 percent.

The northwestern part of the stratigraphic succession is in the chlorite and biotite zones of metamorphism, whereas the remainder of the succession ranges from the sillimanite-potassium feldspar to garnet zones or their mafic rock equivalents.

The originally sedimentary and volcanic section has been thickened by the intrusion of 2,521 m of igneous rocks. Three plutonic bodies (the Ayer Granodiorite, the Rattlesnake Hill muscovite granite, and the Andover Granite), having a total thickness of 1,422 m measured normal to the enclosing strata, account for 56 percent of the thickening.

The Marlboro Formation has been thickened 14.1 percent (444 m) by intrusive rocks; the Nashoba Formation, 17.9 percent (816 m); and the unnamed formations or members, 47.6 percent (1,260 m). The Andover Granite cut by the tunnel is intruded near the transitional boundary of the Nashoba with the Marlboro Formation and encloses rocks of Nashoba affinities. The unnamed formations have been thickened chiefly as a result of the intrusion of the Ayer Granodiorite and the Rattlesnake Hill muscovite granite plutons. Eighteen members of the unnamed formations or members compose a section 956 m thick which is devoid of intrusive rocks.

The protoliths of the layered rocks in the Wachusett-Marlborough Tunnel section were a succession of sedimentary and volcanic accumulations formed in environments that ranged from pelagic to shallow marine to subaerial. The protoliths of the Marlboro Formation were dominantly basaltic pyroclastic rocks and lava flows with interlayered quartz-rich and calcium carbonate-rich sedimentary rocks and limestone. The protoliths of the coticule (a garnet-quartz rock) and epicule (an epidote- and (or) hornblende-quartz rock) and quartzite, closely associated with calc-silicate rocks and amphibolite, were probably manganese-, iron-, and aluminum-rich sandstone or chert. These sedimentary rocks had well-developed beds that had been deposited as part of a volcanic sequence probably in a pelagic to shallow marine environment. The protoliths of the Nashoba Formation were probably predominantly sedimentary rocks whose clastic particles were derived from the weathering and erosion of volcanic rocks, including ultramafic and mafic to felsic flows, pyroclastic rocks, sandstone, limestone, and sandy lime mudstone. The Nashoba Formation is also made up in part of rocks whose protoliths were the same as those of the most common rocks of the Marlboro Formation.

The protoliths of the lower unnamed formations and members were probably fine-grained strandline deposits that graded upward into interbedded quartz-rich shale and high aluminum, sulfide-rich euxinic shale that formed offshore in an enclosed, noncirculating basin. These sedimentary rocks graded upward into a mixture of sandy lime mudstone, shaly sandstone, and volcanogenic sedimentary rocks, which were succeeded by more interbedded sandy lime mudstone and sandstone. These were overlain by a unique hornblende-actinolite ultramafic, pyroclastic volcanic rock, which was in turn succeeded by a thin, felsic, volcanogenic sequence.

The northwestward-dipping stratigraphic section is folded and cut by numerous faults. The axial planes of the folds commonly are overturned to the southeast; this suggests that the upper beds have been transported eastward relative to the strata below. Although premetamorphic faults are present, the dominant faults are postmetamorphic, northwestward-dipping thrust and reverse faults. Northeast-ward-striking, steeply dipping normal faults, mostly formed later than the thrust faults, are cut by northwestward-striking faults and by faults of other orientations as well; these are typically marked by quartz, calcite, and barite veins.

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