In southwestern Connecticut and southeastern New York, a sequence of Precambrian rocks is unconformably overlain by a Cambrian-Ordovician quartzite-carbonate sequence. This, in turn, is unconformably overlain by a dark-gray fissile sillimanite schist that locally has calcite marble and calcite-rich mica schist at its base. The Paleozoic portion of this stratigraphic sequence correlates with the miogeosynclinal sequence of eastern New York and the western parts of Vermont, Massachusetts, and Connecticut.
A eugeosynclinal sequence is in contact with the fissile schist of the miogeosynclinal sequence. This eugeosynclinal sequence consists of the following rock units in order eastward from the fissile schist and apparently in order from oldest to youngest: (1) feldspathic garnet-muscovite-biotite schist commonly with sillimanite nodules and with some interbedded amphibolite; (2) garnet-muscovite-biotite schist with local sillimanite and (or) kyanite, amphibolite, and light-gray or white biotite-muscovite-quartz-feldspar gneiss locally containing garnet; (3) white biotite-muscovite-quartz-feldspar gneiss locally containing garnet; (4) interbedded garnet-muscovite-biotite-quartz-feldspar schist and muscovite-biotite-quartz-feldspar granulite; (5) dark-gray biotite- and (or) hornblende-quartz-feldspar gneiss with feldspar augen common. A suggested correlation of these units with formations in Connecticut and Massachusetts is as follows: (1) correlates with the Waramaug and Hoosac Formations; (2) and (3) with part of the Hartland Formation, Rowe Schist, and the Trap Falls Formation; (4) with parts of the Hartland, Taine Mountain, Moretown, and Fairfield Formations and the banded schist member of the Southington Mountain Formation; and (5) with parts of the Prospect, Collinsville, and Hawley Formations.
If the above correlation is correct, the eugeosynclinal sequence in southwestern Connecticut is the same age as the Cambrian-Ordovician miogeosynclinal sequence, and its structural position above the miogeosynclinal sequence can be explained by a major premetamorphic thrust fault similar to, if not actually, the Taconic thrust fault. The proposed correlation also suggests that Silurian rocks rest unconformably on overturned pre-Silurian rocks in the vicinity of the Westport, Bridgeport, and Long Hill quadrangles in southern Connecticut.