Applying the Concept of Grade to Basin-Scale Stacking Patterns and Stratigraphic Architecture Along the Shelf to Basin Profile: From Outcrop Perspective

David R. Pyles¹, James P. Syvitski², and Roger Slatt^3
1Department of Geology, Chevron Center of Research Excellence, Golden, CO
²Institute of Arctic and Alpine Research, Department of Geological Sciences, University of Colorado, Boulder, CO
³Department of Geology, University of Oklahoma, Norman, OK

The concept of grade is used to describe the net effect of numerous sedimentary processes that, over time, define the morphology of the upper bounding surface of sediment deposition. Two end members characterize the continuum of basin margin morphology: graded and out-of-grade. Recent studies have related basin-margin morphology to specific regional stratigraphic stacking patterns. Graded basin margins contain graded (equilibrium) depositional profiles that advance or prograde into the basin. The morphology of these profiles remains unchanged through time and space. Out-of-grade basin margins contain over-steepened profiles, and as a result, sediments bypass the shelf edge and the proximal slope accumulating at the base-of-slope position. The morphology of slope profiles in out-of-grade basin margins changes through time and space. While regional studies have related large-scale stratigraphic stacking patterns to grade, there are no published results documenting the effect of grade on local stratigraphic architecture along shelf to basin profiles.

The shelf to basin floor stratigraphic architecture of the Upper Cretaceous Lewis Shale of Wyoming, USA and the Upper Carboniferous Ross Sandstone, Ireland represent graded and out-of-grade depositional systems, respectively. The graded Lewis Shale contains progradational basin-scale stacking patterns, whereas the out-of-grade Ross Sandstone contains aggradational basin-scale stacking patterns. Several parameters of local stratigraphic architecture appear to be related to grade: (1) distribution of architectural elements and facies, (2) presence/absence of incised valleys, (3) presence/absence of submarine canyons, (4) lengths of submarine fans, (5) partitioning of sandstone across the physiographic profile, (6) position of submarine fans through time, and (7) position of submarine fans relative to depocenters.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas