Shoaling Cyclicity in Peritidal Carbonate Sequences
Bruce H. Wilkinson, Carl N. Drummond, and Nathaniel
Although prevalence of upward-shallowing cyclicity in epicratonic carbonates has been increasingly taken for granted in carbonate geology, division of peritidal sequences into shallowing-upward cycles is evidently a rather subjective exercise. Examination of data from several long sections indicates that inference of repeated shoaling often relies more heavily on stratigraphic recurrence of distinctive supratidal lithologies than on any real tendency for lithofacies to comprise upward shallowing associations. Cycle definition via picking of cycle "tops" not only results in varied assemblages of overlying "base" and mid-cycle lithologies, but also leads to the designation of cycles containing relatively few units; most reported upward shall owing peritidal cycles contain o ly two stratal elements.
In addition, most sections of platform carbonate can as readily be interpreted as comprising random assemblages of peritidal lithologies. Comparison of thicknesses and numbers of stratal elements in real-world sections of upward-shallowing "cycles" with the same characteristics of peritidal units in unordered sequences demonstrates that most Proterozoic and Phanerozoic sections exhibit little more meter-scale ordering than would result from stochastic accumulation.
On the basis of these considerations, we conclude that meter-scale cyclicity in many peritidal sequences is more apparent than real, that interpretations of repeated and eustatically driven platform flooding are based largely on the unsubstantiated premise that shallowing cyclicity should predominate in peritidal sequences, and that a considerable component of presumed meter-scale stratigraphic order in many epicratonic sections reflects little more than the random superposition of generally unrelated lithofacies elements during the long term accumulation of peritidal carbonate.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California