KIRBY, KENT CHARLES, and J. A. (TONI) SIMO, University of Wisconsin-Madison, Department of Geology and Geophysics, Madison, WI

ABSTRACT: Differences in Geometry and Stacking Patterns along a Carbonate Ramp Margin: Lower Carboniferous Pekisko Formation, West-Central Alberta

The Pekisko Formation of west-central Alberta demonstrates significant differences in stacking patterns and three-dimensional geometry along 80 km of a carbonate ramp margin. Subsurface control (approx. 400 wells, 15 core, in a 150 km x 75 km area) was used to document these differences and to develop an integrated regional view of carbonate ramp and basin deposition. This study highlights the necessity for stratigraphic interpretations on a regional three-dimensional scale, larger than most outcrop studies used to interpret basin and sea level history.

The Pekisko ramp (up to 60 meters thick) evolved from a lower transgressive stratal set into a relatively uniform prograding middle stage. However, the upper stage exhibits significant differences in geometry across the study area, including the growth of carbonate buildups (Waulsortian mounds) along the toe-of-slope. Carbonate ramp and mound deposition was terminated by an influx of shale that drapes the eastern area and thins westward to become indistinguishable from the overlying basin fill. The final basin fill consists of five prograding sigmoidal stratal sets.

Two stratigraphic sections, 80 kilometers apart, were constructed perpendicular to the ramp margin. The western section has a prograding upperramp set that contains evidence of synsedimentary fault control of the margin. The eastern section, with no evidence of contemporaneous faulting, has a retrogradational upper ramp, a wider margin (55 km versus 20 km), and contains a thick basin shale drape that is not present in the western section.

Both sections are analogous in size to large Outcrop belts (125 km x 20 km), yet because of local tectonic and environmental differences, interpretation of either section alone would yield an erroneous basin and sea-level history.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.