THE ORIGIN OF THE MORETOWN FORMATION, VERMONT – AN ALTERNATIVE PERSPECTIVE FROM THE SOUTHERN QUéBEC APPALACHIANS

In Vermont, the Moretown Fm is part of the Rowe-Hawley Belt, which consists of passive margin and supra-subduction zone metaigneous and metasedimentary rocks juxtaposed during the Taconian orogeny. The Moretown Fm is divided into Western and Eastern members and consists of quartz-rich metasedimentary rocks crosscut by mafic dykes of the Mount Norris Intrusive Suite (MNIS). Controversies regarding the origin of the Moretown have persisted for decades and its interpretation as an Ordovician supra-subduction zone basin remains equivocal. However, the new state map of Vermont and correlations of Cambrian-Ordovician rocks on both sides of the Vermont-Québec border bring new perspectives for the origin of the Moretown Fm.

The Rowe-Hawley Belt includes the Cram Hill Fm, which correlates with the Saint-Daniel Mélange of southern Québec. Both the Cram Hill and the Saint-Daniel mostly consist of black phyllites and sandstone/quartzite crosscut by mafic dykes and interlayered with basaltic flows that geochemically correlate with the MNIS. In the Western Moretown, the MNIS dykes cut a Taconian foliation, indicating that it was deformed prior to that magmatic event. In southern Québec, the Saint-Daniel is part of a syncollisional forearc basin that unconformably overlies an eroded basement of accreted ophiolites and Laurentian metamorphic rocks. Debris flows and conglomerates that mark this unconformity likely correlate with the Umbrella Hill Conglomerate. The latter is found at the base of the Cram Hill Fm and overlies continental margin metasedimentary rocks that contain tectonic slices of the Western Moretown. We believe that the Umbrella Hill marks the same unconformity as the one found at the base of the Saint-Daniel and that since it is interlayered with both the Eastern Moretown and the Cram Hill Fm, these two units may be synchronous and represent lateral facies variations. We suggest that the Western and Eastern members of the Moretown Fm likely formed in different settings at different times; the Western Moretown being part of the Laurentian passive margin and the Eastern Moretown belonging to an unconformably overlying forearc basin. Both the Eastern and Western Moretown were crosscut by late-collisional (Taconian) dykes of the MNIS, which may be attributed either to slab breakoff or subduction polarity flip.