SILURIAN STRATIGRAPHY OF WESTERN CONNECTICUT CONFIRMED BY U-PB SHRIMP ANALYSIS OF DETRITAL ZIRCONS IN THE STRAITS SCHIST

Silurian-Devonian metasediments of the Connecticut Valley – Gaspé trough are well-known in northern New England and Canada and are becoming better characterized by U-Pb dating of detrital zircons. The youngest zircons define the maximum age of deposition, and together with limited fossil evidence define depositional ages from later Silurian to Early Devonian. Reliable detrital zircon ages confirm the Silurian-Devonian age of the Waits River and Gile Mountain formations in Vermont (McWilliams et al., 2010) and the aluminous Goshen Formation in Massachusetts (Karabinos and Aleinikoff, 2011). These rocks have been further correlated in general with The Straits Schist of Rodgers (1985) into Connecticut. However, there is no quantitative evidence in Connecticut that this schist is younger than the early Paleozoic Collinsville and Trap Falls formations of central and western Connecticut. An unconformity above the latter two units is possible from the map pattern and provides qualitative evidence of the relative ages.

To test this correlation and the tectonic significance of a Silurian-Devonian unconformity west of the Waterbury dome we dated detrital zircons by U-Pb methods from a calcareous schist from the Collinsville Formation and from a quartzite from The Straits Schist from just below and above the unconformity near Stevenson (SV) on the Housatonic River. We also dated zircons from the Collinsville and Rowe formations farther northwest for context. Most of the zircon grains analyzed from below the unconformity are Mesoproterozoic but the SV sample also contains a substantial Paleoproterozoic population. In contrast, nine grains out of 52 from The Straits Schist are Paleozoic; the ages range from latest Cambrian to early Silurian. These ages show the sediments are early Silurian or younger, and define the first appearance of zircons with an eastern provenance. They also support mapping of an unconformity below The Straits Schist, consistent with a Salinic disturbance prior to the Acadian orogeny. An explanation of the amphibole cooling ages as old as ~408 Ma overlapping with deposition ages of some Connecticut Valley – Gaspé trough sediments remains a challenge.