TACONIC? WHAT TACONIC? IT IS MOSTLY CALEDONIAN!

Recent detailed mapping in western Vermont has revealed that the initial deformation of the Cambro-Ordovician shelf strata occurred in earliest Late Ordovician (early Caradoc), but pre-dated the major flysch deposition. This thrusting entailed relatively low displacement thrusts that migrated across and up submarine canyons which did not contain shale fill. Subsequent thrusting, including emplacement of the Taconic allochthons and the Champlain thrust sheet, post-dated the deposition of the shales which locally exceeded 2000m where they filled the submarine canyons. The age range and tremendous volume of shale involved in this later deformation argue against a Taconian age for this second episode of thrusting. A third episode of thrusting, mostly marked by reactivation of the earlier thrust surfaces, has also been discovered. Substantial beveling of structures occurred between the second and third episode of thrusting, indicating that the third episode is separated from the second episode by another significant time period. By analogy with nearby areas where post-Ordovician stratigraphy is preserved, we believe that an approximate time frame can be attached to these three episodes. We postulate that the initial minor episode of thrusting is the only expression of the Taconic Orogeny, sensu stricto. The second, and most important, episode is thought to be the result of the Caledonian Orogeny. The last of these events is expected to be the foreland portion of the Acadian Orogeny. Apparently, any potential Alleghanian signature is hidden within the more significant deformational signatures that dominate the area.