ABSTRACT. U-Pb zircon data from the Squantum Member of the Roxbury Conglomerate and regionally overlying Cambridge Argillite constrain the age of terminal Neoproterozoic glaciomarine deposition in the Boston Basin, eastern Massachusetts.  The glacial sequence is younger than approx 595 Ma based on analyses of detrital zircons from sandstone underlying a prominent dropstone horizon in the Squantum type section, Quincy, Massachusetts.  A welded tuff clast from diamictite higher in the section (“tillite” of early workers) is comparable in age (595.5 + 2 Ma). Clasts of crystal-poor tuff (600 + 1 Ma) and porphyritic granophyre (610 + 2 Ma) were also dated.  Glaciomarine sedimentation was followed by deposition of Cambridge Argillite with a maximum depositional age of approx 570 Ma, based on the youngest zircon component in the Mystic River Quarry ash bed, Somerville, Massachusetts.   The exact stratigraphic position of the dated argillite in the northern part of the Boston Basin with respect to Squantum deposits farther south, however, is not clearly established.

            Dated clasts and detrital zircons from the Squantum Member match previously documented ages of volcanic and plutonic rocks produced during 625 to 590 Ma arc magmatism recorded in upland rocks surrounding the Boston Basin.  These age relationships imply that glaciation rapidly followed (and possibly overlapped) Avalonian tectono-magmatic activity in southeastern New England.  Older units were also exposed during glaciation based on comparable ages of detrital zircons from a quartzite clast in the Squantum diamictite and detrital zircons in the pre-magmatic Westboro Formation on the north side of the basin.  The presence of 1.3 to 1.0 Ga zircons in both detrital suites is consistent with sources in the West Amazonian craton or in Baltica.

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Created and maintained by Maria Raffi mraffi@wellesley.edu
dated created: September 15,2002