Facies Successions in Peritidal Carbonate Sequences

Abstract

ABSTRACT High-frequency stratigraphic order in epicratonic sections is increasingly attributed to the widespread influence of Milankovitch-band climate forcing and attendant eustatic sealevel change on patterns of limestone and dolostone accumulation throughout much of the Proterozoic and Phanerozoic record. However, the actual existence of upward-shallowing lithofacies associations has rarely been explicitly demonstrated and, in contrast to such perceptions of periodic and global accumulation, many carbonate sequences can just as readily be interpreted as largely constituting unordered assemblages of various peritidal lithologies. Examination of published data from several long epicratonic sequences indicates that their division into shallowing cycles is a rather subjective exercise. Inferenc of repeated shoaling has commonly relied more heavily on the stratigraphic recurrence of particular units interpreted as representing extremely shallow to exposure conditions than on any documented tendency for groups of lithofacies to actually constitute upward-shallowing associations. Moreover, cycle definition via such picking of cycle tops results not only in a varied assemblage of overlying substitutable base and mid-cycle lithologies, but also leads to the designation of cycles that contain a relatively small number of stratal elements; most reported peritidal cycles contain only two stratal elements (mean = 2.25 lithofacies/shallowing cycle; n = 627) and thus are indistinguishable from sequences of randomly stacked peritidal units. Comparison of data on thicknesses and numbers of stratal elements in real-world cyclic and model chaotic sequences demonstrates that most Proterozoic and Phanerozoic sections exhibit little more meter-scale ordering of component units than would commonly be present in sequences of randomly associated peritidal lithofacies. On the basis of these considerations, we conclude that meter-scale cyclicity in many if not most epicratonic sequences is more apparent than real, that perceptions of repeated and eustatically driven platform flooding are largely incorrect, and that a substantial component of presumed meter. scale stratigraphic order in peritidal carbonates reflects little more than the random migration of various sedimentary subenvironments over specific platform localities durin long-term accumulation of peritidal carbonate.