BEDROCK TUNNELS OF MASSACHUSETTS: WINDOWS ON GEOLOGICAL EVOLUTION OF THE REGION

A system of bedrock tunnels and aqueducts has been constructed for water supply for Greater Boston by the Metropolitan Water Resources Authority (MWRA) and its predecessor commissions beginning after the mid-19th century and just now being completed. The main artery of this system of tunnels transports water from Quabbin Reservoir in the Bronson Hill anticlinorium of western Massachusetts to and within the Boston Basin. The segment from Quabbin Reservoir to Wachusett Reservoir is called the Wachusett-Coldbrook Tunnel; that from Wachusett Reservoir to Southborough, the Cosgrove Tunnel; that from Marlborough to Newton, the MetroWest Tunnel; and the City Tunnel, which is the segment that transports water through the western Boston Basin to a network of tunnels which distributes it throughout Boston. Additionally there are tunnels driven for other purposes within the Boston Basin and beyond. The E-W trend of this MWRA tunnel system for water supply is approximately normal to the structural trends within four tectonic terranes, as well as the terrane-bounding faults that separate these geologically distinct blocks from one another. From W to E these once independent tectonic units are referred to as the Bronson Hill, the Merrimack, the Nashoba, and the Avalon terranes. In a plate tectonic interpretation, the Bronson Hill is an Ordovician volcanic arc formed near the edge of the Laurentian Supercontinent. The easternmost of these three terranes, the Avalon, consists of Proterozoic continental shelf sedimentary rocks which upon its breakup gave rise to 620 to 600 Ma arc granitic batholiths and volcanics along the margin of the resulting Gondwanan Supercontinent. The Merrimack and Nashoba terranes contain granitic batholiths of Silurian, Devonian and Silurian, Ordovician and Cambrian ages respectively, that suggest that they formed by a series of collisions between and among these Gondwanan microcontinents during transit from the South Pole to Laurentia. The final collision about 400 mya in Silurian time culminated in a more tightly welded collision boundary in the Devonian Acadian orogeny.