Implication of Unfilled Accommodation Space in Carbonate Depositional Systems for Cyclo-Stratigraphy

Gregor P. Eberli¹, Paul M. Harris², and G. Michael Grammer^3
1Comparative Sedimentology Laboratory, University of Miami, Miami, FL
²Chevron Energy Technology Company, San Ramon, CA
³Department of Geosciences, Western Michigan University, Kalamazoo, MI

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.

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