Abstract

The Acadian disturbance is a deformational event whose climax is of late, but not latest Devonian age and was an important episode in the geologic history of the Canadian Maritime provinces and the northern Appalachians. It is not generally appreciated that a contemporaneous tectonic episode affected a large part of the central and northern parts of the Rocky Mountains of the western United States. Strata as young as the Jefferson and Darby formations of late Devonian (Frasnian) age and down to pre-Cambrian basement have been truncated beneath a regional discontinuity at the base of latest Devonian (Famennian) Englewood, Exshaw and Parting or earliest Mississippian (Kinderhookian) Lodgepole and Madison formations.

The pre-Pennsylvanian rock record of the region is treated in the form of synthem stratigraphy in order to simplify nomenclature and to highlight episodes of hiatus and discontinuity in the section. In terms of synthem stratigraphy the Acadian discontinuity separates the older Piankasha from the younger Tamaroa Synthems (=Sequences) of Wheeler. Over a large part of central Wyoming the latest Devonian discontinuous basal thin sandy unit of the Tamaroa has been traditionally assigned to the Cambrian System, resulting in a misinterpretation of the geologic history of the area.