12-4 Tectonostratigraphic Implications of Early Ediacaran Sedimentary Sequence in the Boston Basin, Eastern Massachusetts

Session: New advances in geological and geophysical research on the Appalachian orogen. (II)

Presenting Author:

Authors:

Abstract:

Recent high precision CA-ID-TIMS U-Pb zircon dates expand the previously documented 610-550 Ma interval of arc-related Ediacaran plutonism and volcanism in the southeastern New England Avalon zone and the geographic distribution of Ediacaran intrusives.  One of these dates—from felsite porphyry in a borehole drilled prior to construction of the MetroWest Water Supply Tunnel completed in 2003—further reveals a sedimentary sequence pre-dating volcanogenic deposits of the Boston Basin, eastern Massachusetts.  The sill intrudes laminated strata presumed at that time to be part of the Cambridge Formation that is overthrust by 609.52 ± 0.14 Ma Dedham Granite along the faulted western margin of the Basin.  However, the 618.66 ± 0.66 Ma weighted mean²⁰⁶Pb/²³⁸U date from the sill is significantly older than the 551.22 ± 0.20 Ma ash bed constraining the top of the Cambridge Formation in the former Mystic Quarry, Somerville, MA.  Dark gray argillite in the quarry (and from argillite sampled in the City Tunnel Extension running beneath it) also contrasts strongly with pale yellowish or olive gray shades of MetroWest strata.  Folding about a nearly EW-trending axis in the City Tunnel Extension results in bedding dips to the north or south, at odds with generally NW dipping beds mapped in the MetroWest Tunnel.  Surface exposures located north of the MetroWest alignment in Auburndale, MA are mapped as Cambridge Formation, but contain NW dipping bedding suggesting Pre-Dedham affinity.   

Recently, deep borings commissioned by the Massachusetts Water Resources Authority in preparation for a new water supply tunnel to branch NE from the MetroWest alignment show considerable lithological diversity in the "Pre-Dedham sequence".  Laminated units are typically fine sandstone and siltstone, in places containing graded beds.  Interbedded with these are coarser sandstones and conglomerates dominated by clasts of quartzite and felsite containing quartz fragments in a micaceous, possibly devitrified matrix.  These differ from < 595 Ma Roxbury-related deposits in which clast assemblages feature abundant purplish rhyolite  porphyries.

The ~619 Ma sill extends previously documented magmatic similarities between Avalonia in SE New England and terranes of mainland Nova Scotia into Early Ediacaran time.  The sill overlaps ages of volcanic tuffs and sub-volcanic porphyries in the Jeffers Group of the Cobequid Highlands, suggesting links between the intruded Pre-Dedham sequence and widespread Jeffers Group sandstones and turbidites.  

Geological Society of America Abstracts with Programs. Vol. 58, No. 2, 2026

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.