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Implication of Unfilled Accommodation Space in Carbonate Depositional Systems
for Cyclo-Stratigraphy*
By
Gregor P. Eberli1, Paul M. Harris2, and G. Michael Grammer3
Search and Discovery Article #50078 (2008)
Posted July 9, 2008
*Adapted from oral presentation at AAPG Annual Convention, San Antonio, Texas, April 20-23, 2008.
1Comparative Sedimentology Laboratory, University of Miami, Miami, FL, USA
2 Chevron Energy Technology Company, San Ramon, CA, USA ([email protected])
3 Department of Geosciences, Western Michigan University, Kalamazoo, MI, USA
Shallow-water carbonates are thought to fill the accommodation space in each high-frequency sea
level
change, but Holocene and late Pleistocene deposits show the geologic record to be much more complex, with examples exhibiting unfilled, irregularly filled, and overfilled accommodation space. Where filling occurs, it is often achieved with one facies, indicating that progradational patterns are likely the result of lateral stacking during subsequent
sea
level
changes.
As an example, the present-day topography on Great Bahama Bank from platform-margin dune ridges to the subtidal platform interior is approximately 12 m, and in a few cases up to 20 m. Both muddy and grainy tidal flat systems
locally fill accommodation space to mean
sea
level
. High-energy beach-dune ridges locally overfill accommodation space. Cores through tidal flats document that the provenance does not change significantly during the filling of the accommodation space; i.e., the high-energy grainy tidal
systems
remain grainy even in their uppermost portions. Pleistocene cores from the platform interior rarely display a shallowing-upward trend, but exposure surfaces rest directly on subtidal facies, indicating that in this environment accommodation remained unfilled until
sea
level
dropped. The lack of clear shallowing-upward trends in facies is common in the cores from the modern bank, indicating that facies boundaries move little during one high-frequency
sea
level
cycle
. Facies juxtaposition occurs more frequently in successive
sea
level
changes.
Based on the Pleistocene-Holocene succession of Great Bahama Bank, we speculate that many depositional cycles in the rock record, for which a change in the provenance is reported, might in fact be two cycles. In addition, the topographic relief in each cycle
might have been underestimated, which might have lead to miscorrelations of
cycle
tops.