Paper No. 5
Presentation Time: 2:25 PM
FOUR DISTINCT DEFORMATIONAL PHASES IN THE TACONIC FORELAND, CENTRAL AND SOUTHERN CHAMPLAIN VALLEY, VT & NY
Deformation of the Cambro-Ordovician shelf strata in the Champlain Valley can be divided into four distinct phases of which only the first fits the traditional Taconian timing. Three of these events (first, second, and fourth) were compressive, and have generally been confused with each other because later deformational episodes tended to reactivate thrusts developed during the earlier events. The third event was extensional and is recognized by map-scale structural, stratigraphic, and metamorphic discontinuities; its overprint by the last phase of compressive structures and the confusion about the relative timing of those structures has led to its interpretation as a pre-Taconic (or early Taconic) event. The first event is a minor emergent thrusting event that moved thrust sheets across and up submarine canyons after the development of the Chazy-Black River carbonates and before the infilling of the remnant canyons by the Trenton black shales; this is the Taconic deformation, sensu stricto. The second event is the major thrusting event in which the Taconics and Champlain thrust sheets were emplaced following the deposition of the late Ordovician flysch sequence; we interpet this as Caledonian deformation. The normal faults of the third event cut across the thrust structures developed during the second event, juxtaposing disparate stratigraphic sequences that represented separate thrust sheets; we interpret this as the back-arc opening event in earliest Devonian. The fourth event deforms these normal faulted boundaries and develops a consistent final compressive fabric; we interpret this as being Acadian deformation.