Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
AAPG Annual Convention, Calgary, Alberta, Canada, June 19–22, 2005
Graded Versus Out-of-Grade Basin-Margin Morphologies and Their Relationship with Basin-Scale Stacking Patterns and Local Attributes of Submarine-Fan Strata: Lessons Learned from Outcrops of the Cretaceous Lewis Shale of Wyoming and the Carboniferous Ross Sandstone of Ireland
Regional shelf to basin studies on outcrops of Upper Cretaceous Lewis Shale of southern Wyoming, USA and the Upper Carboniferous Ross Sandstone of western Ireland demonstrate that: 1) both were deposited in epeiric seaways, 2) both were deposited in a time interval of ~1.2 my, 3) submarine-fan strata in both locally accumulate to 500m in thickness, and 4) both were deposited during high-frequency eustatic changes in sea level. Despite these similarities, the Lewis Shale contained a “graded” basin-margin morphology, whereas the Ross Sandstone contained an “out-of-grade” basin-margin morphology.
The distinct basin-margin morphologies of these formations are related to basin-scale stacking patterns and local attributes in submarine-fan strata. The “graded” basin-margin of the Lewis Shale is related to progradational basin-scale stacking patterns, whereas the “out-of-grade” basin-margin of the Ross Sandstone is related to aggradational basin-scale stacking patterns. The “graded” basin-margin of the Lewis Shale does not contain submarine canyons, submarine-fan strata are located basinward of the depocenter for their respective fourth-order stratigraphic cycle, depositional length of submarine-fan strata remains uniform, and submarine-fan strata contains a low proportion of slumps. The “out-of-grade” basin margin of the Ross Sandstone contains submarine canyons on the proximal slope, submarine-fan strata are located at the depocenter for their respective fourth-order stratigraphic cycle, depositional length of submarine-fan strata increases upwards, and submarine-fan strata contains a relatively high proportion of slumps.
First-order controls on basin-margin morphology
in the formations are: 1) initial morphology of the slope, 2) tectonically
forced changes during deposition, and 3) sediment supply.