Unfilled Accommodation Space - the Heel of Achilles of Cyclostratigraphy
Gregor P. Eberli and G. Michael Grammer
Carbonate platform cycles are assumed to be precise accommodation gauges and, consequently, systematic chances in the vertical stacking pattern of high-frequency cycles are often used to predict low-frequency sea level changes. Analyses of modern, Pleistocene and ancient carbonate cycles, however, indicate that filling of the accommodation space is facies-dependent and often incomplete. Thus, cycle stacking patterns might indicate inaccurate low-frequency sea level trends.
On modern Great Bahama Bank, filling of the Holocene accommodation space is controlled by facies and location. High-energy margin facies, like ooid shoals, have a high sedimentation rate and easily fill the available accommodation space, while the platform interior facies do not and, based on the last interglacial, probably never will fill all the accommodation space. Unfilled accommodation space can be calculated for the last interglacial. A core (Unda) located in the platform interior was taken in 7 m of water depth where the Holocene sediment thickness is 2.5 m. Sea level during the last interglacial is estimated to be about 7 m higher than at present. Thus, in the platform interior of GBB approximately 16.5 m of accommodation remained unfilled during the last interglacial. Cores of GBB document that this pattern persisted throughout the Late Pliocene and Pleistocene. Cores located at the platform margin show fewer but thicker cycles than cores from the platform interior for the same time interval, indicating that the platform interior cycles do not record maximum accommodation.
Cretaceous cycles of the Maiella platform (Italy) also show a relationship between location and cycles frequency with more cycles in the platform interior than at the margin. Pennsylvanian cycles of the Paradox basin display the dependency between cycles thickness and facies; Phylloid algal bioherms form mounds in otherwise thinner cycles, indicating different sedimentation rate and filling of accommodation space between bioherms and shelf carbonates.
In the rock record unfilled accommodation and, thus, amplitude of sea level is difficult to estimate. Consequently, extracting low-frequency sea level trends from vertical cycle stacking patterns can be difficult. For example, when facies belts shift during progradation or backstepping, the thickness of the cycles might reflect variable sedimentation rates rather than changes of relative sea level amplitudes.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California