NEW DEVELOPMENTS IN THE SILURIAN-DEVONIAN BELT OF WESTERN CONNECTICUT

New geologic mapping and detrital zircon (DZ) analyses improve our understanding of the Silurian-Devonian (SD) interval in western CT. The Silurian Russell Mountain Formation (RM) and Devonian Straits Schist (SS) comprise a coherent, albeit sinuous, belt of lithologically consistent rocks that winds through the western CT uplands. The SS is an unlayered, coarse-grained mica-quartz schist whereas the RM consists of amphibolite, marble, calc-silicate, and substantive sections of rhythmically layered metasandstone that are commonly graded. In parts of the belt, DZ indicate the latter facies were incorrectly mapped as part of the pre-Silurian section, and a re-evaluation of rocks along strike demonstrates the likelihood of additional unrecognized SD section.

Detrital zircon (DZ) data from CT integrated with DZ data to the north provide the SD age constraints. Older Llandovery igneous rocks provide a lower age constraint in CT. DZ spectra from the Straits Schist display a mixed provenance that includes a strong Laurentian signal and prominent Ordovician Arc and Peri-Gondwanan (PG) peaks; however, the available RM spectra are dominated by the Laurentian ages. Recent DZ data from metamorphosed sandstones in the Woodbury quadrangle (central portion of belt) display SS-type spectra and contain grains as young as Early Silurian. These results present two implications: that Ordovician and PG grains may not have been deposited in the entire RM basin; and that the Woodbury rocks are best included with the Devonian SS rather than the Silurian RM metasandstones.

From north to south, the SD interval overlies PG rocks, metasedimentary rocks and migmatites, some likely no older than Cambrian, and Ordovician/ Silurian orthogneiss. Although the basal contact is interpreted everywhere as tectonized, available data suggest that the SD interval was originally deposited above a profound unconformity that was highly deformed in the Acadian. At the outcrop scale, original layering in the metapelitic SS has been replaced by a penetrative shear fabric. Both the RM and SS are locally mylonitic and boudinaged. The SS has been interpreted as a decollement in the northern gneiss dome belt. This is consistent with recent detailed mapping to the south that demonstrates previously unrecognized thrust imbrication of the SS and RM units.