HR: 17:00h
AN: H54B-05 INVITED    [Abstracts]
TI: Applying the Concept of Grade to Basin-Scale Stacking Patterns and Stratigraphic Architecture Along the Shelf to Basin Profile: An Outcrop Perspective
AU: * Pyles, D R
EM: dpyles@mines.edu
AF: Chevron Center of Research Excellence, Department of Geology and Geological Engineering, Colorado School of Mines, Golden, Co 80401, United States
AB: 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 and accumulate at the base-of-slope position. The morphology of 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 southern Wyoming, USA and the Upper Carboniferous Ross Sandstone of western 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. Although the two systems represent end members on the grade to out-of-grade continuum, they have several similarities: (1) both were deposited in epeiric seaways, (2) both were deposited in a time interval of ~1.2 My, (3) both were deposited during high-frequency eustatic changes in sea level, and (4) both have 500m of submarine fan accumulation. Despite these similarities, several differences are noted that appear to be related to grade. They are: (1) architectural and facies associations, (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. Stratigraphic and structural analysis of the Lewis Shale and Ross Sandstone support predictions made from stratigraphic modeling studies for first-order controls on grade. These controls are initial morphology of the slope, tectonically forced changes during deposition, and sediment supply.
DE: 3022 Marine sediments: processes and transport
DE: 3045 Seafloor morphology, geology, and geophysics
SC: Hydrology [H]
MN: 2007 Fall Meeting