A Major Sea-Level Fall at End of Frasnian (Upper Devonian) and Rapid Filling of the Jasper Basin, Sassenach Formation, Alberta Rocky Mountains
Eric Mountjoy and Sherry Becker
Near the end of the Frasnian, the westernmost Alberta Basin (exposed in the Rocky Mountains) was incompletely filled and some western reef complexes had considerable relief relative to the adjacent basin. This relationship is demonstrated by the westward thinning of the basin strata (Perdrix and Mount Hawk) around the southern margin of the Ancient Wall reef complex and along the western margin of the Fairholme reef complex southwest of Banff. The Sassenach Formation filled this depression (Jasper Basin and extensions to the south) in the early Famennian. The westward thickening, the presence of feldspar, and Nd isotopes in the Sassenach support a western Antler orogenic source for these clastic deposits.
In outcrop the Frasnian and Famennian paraconformity occurs between the Sassenach-Simla (or Ronde), and in the subsurface to the east between the Graminia-Blue Ridge. In the Jasper area, subaerial exposure of the Frasnian Simia Platform margin and adjacent slope is indicated by a thin, downlapping, lowstand sequence and absence of the upper part of the latest Frasnian linguiformis Zone. Following exposure, the entire 200m thick Sassenach Formation onlapped this slope and platform margin, which formed the northeast margin of the Jasper Basin, but did not cover the top of the Frasnian Simla Platform. Similar stratigraphic relationships occur along the southwestern margin of the Fairholme reef complex. A shallow, subtidal oncolite facies is developed at the base of the Sassenach In several sections this facies occurs sporadically in higher strata adjacent to the unconformity. At Mount Haultain these oncoids overlie reworked carbonate conglomerates (lowstand/ transgressive sequences?) derived from the underlying Simla, and in turn are overlain by columnar stromatolites. These facies represent a nearshore, agitated environment that shifted upwards stratigraphically along the northeast margin of Jasper Basin (previous Frasnian slope and platform margin) as sea-level rose. These relationships support a sea-level fall of about 150m (max) at the Frasnian-Famennian boundary and a corresponding rise during the Famennian Early/Middle triangularis Zone.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California