ABSTRACT: Sequence Analysis of the Jurassic Fernie Formation in Alberta--Condensed Deposition on a West-Facing Ramp

YOUNG, F. G. RICK, Home Oil Company Limited, Calgary, Alberta, Canada

The Fernie Formation comprises most of the Jurassic rock record in western Alberta and eastern British Columbia. It consists predominantly of medium gray to black shales, with minor siltstone, sandstone, limestone, dolomite, and chert. Surface sections in the Rocky Mountains are somewhat thicker than subsurface sections to the east, which are typically 70-110 m thick.

Both surface and subsurface expressions of the Fernie have been subdivided into informal members and beds, most of which exhibit abrupt or unconformable basal contacts. These represent hiatal gaps as much as an entire stage in duration, according to published biostratigraphic control. A comparison of the ages of nominally similar surface and subsurface members and their corresponding hiatuses reveals a poor match in most instances. This can be explained using sequence stratigraphic concepts, whereby units in the west (outcrop) represent mainly lowstand and transgressive systems tracts (LST/TSTs), whereas those to the east (subsurface) are highstand systems tracts (HSTs).

As a result of low clastic input from the craton to the east, depositional lenses alternated between eastern and western parts of the ramp as sea level fluctuated. Thus, hiatal gaps in the upper-ramp, stacked HST stratigraphic record correspond to depositional LST/TSTs in the lower ramp areas to the west, and vice versa. During several sea level lowstands, valleys were cut into underlying HSTs and subsequently filled during TSTs with estuarine, reservoir-quality sands such as the Rock Creek and Niton B sandstones. This analysis divides the Fernie into six condensed Type 1 sequences.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)