WAS THE STRAITS SCHIST OF WESTERN CONNECTICUT DEPOSITED IN A RETROARC FORELAND BASIN?

The Straits Schist (SS) is a pelitic schist with varying amounts of quartzite that outcrops in a sinuous N-S band across western Connecticut. The SS overlies metasedimentary or metaigneous rocks and is thought to be unconformable. Except in the northern Bristol Dome, all pre-SS rocks sampled for detrital zircons (dZ) have only Laurentian and older signatures. In contrast, SS samples also contain dZ indicative of both an eastern Gondwanan (~600Ma) and Arc (Ordovician) sources. Ord.-Sil. igneous rocks in the Bronson Hill Arc (454-442Ma) and in the Waterbury Dome (434-437 Ma, Dietsch et al., 2010) are compatible with the younger detrital zircon populations in the SS (~450-414 Ma). The protolith for the SS is interpreted to be a thick succession of mudstones suggestive of quiet, deep water deposition with periodic input of coarser sediment. The deposition of such a thick, widespread mudstone succession indicates a foundering of the SS basin, and probably represents a period of rapid subsidence.

The geologic expression of the onset of rapid subsidence in the SS can be demonstrated in the Long Hill area. At this locality marbles and amphibolites underlie the SS. The marbles are well laminated, suggestive of a shallow water algal laminite protolith. The marble/SS contact is sharp, overlain by 10 cm of mixed quartz + calcite, and is unmodified by ductile deformation. The contact between the marble and the SS represents an abrupt change from shallow water carbonate to deeper water mudstone deposition (i.e., a deepening event) without an intervening subaerial unconformity. Such abrupt deepening events are observed in better-preserved foreland basins and can be tied to loading by thrust sheets and a concomitant rapid subsidence response. This tectonic scenario, coupled with the change in dZ provenance leads us to suggest the SS was deposited in response to a tectonic (Salinic?) thrust loading event in a backarc basin receiving both Laurentian and Gander/ Arc sediment; i.e., a retroarc foreland basin.