Stratigraphic Relationships and Detrital Composition of the Medial Ordovician Flysch of Western New England: Implications for the Tectonic Evolution of the Taconic Orogeny

We present new stratigraphic and petrographic data on the easterly-derived medial Ordovician flysch sequence of western New England and eastern New York State. A conformable relationship exists between the flysch and underlying early Paleozoic continental rise sediments within the western Taconic Allochthon. Farther east in the Allochthon we observe a possibly disconformable relationship. Contrary to previous interpretations, these facts suggest that at any place within this paleogeographic realm the sediments were undeformed and untransported at the initiation of flysch deposition. At least three sources contributed detritus to the flysch: (1) Taconic Sequence sediments; (2) deformed and metamorphosed pelitic, psammitic, and volcanic lithologies; and (3) igneous rocks of dominantly mafic-ultramafic affinities. These three sources are respectively interpretable as: continental rise, accretionary prism, and ophiolitic-volcanic arc assemblages, which are recognized in the regional geology of western New England. There is a progressive decrease in the age of the basal flysch from that within the allochthonous Taconic rocks to that deposited on top of the parautochthonous and autochthonous early Paleozoic carbonate shelf sequence. This indicates westerly progradation of the flysch synchronous with westward transport of allochthonous realms. Truncation of regional folds in Taconic rocks by shelf carbonate slivers on the basal thrust of the Allochthon shows that Taconic rocks were consolidated and strongly deformed well before the end of their westerly transport. The stratigraphic and petrographic data indicate progressive and diachronous east to west thrust stacking, deformation, and metamorphism of paleographic realms during the Taconic orogeny, a medial Ordovician continental margin-volcanic arc collision. They are evidence against emplacement of the Taconic Allochthon as one or several gravity slides detached from their origin.