Stratal Order in Peritidal Carbonate Sequences

Abstract

ABSTRACT Speculation on the depositional origins and geological significance of meter-scale cycles in peritidal carbonates is becoming an increasingly prominent facet of sequence stratigraphic theory, the understanding of which bears directly on their appropriateness as chronostratigraphic entities as well as their usefulness as records of periodic extrabasinal forcing during sediment accumulation. In spite of the generally wide acceptance of the stratigraphic importance and interpretational significance of meter-scale parasequences, little has been done to quantitatively document the stratigraphic nature of regularly recurring lithologic associations or to verify the predominance of such cyclicity in shallow-water limestone/dolostone sequences. In order to determine the statistical extents and stratigraphic scales of stratal order in such sequences, we have examined several long sections of peritidal carbonate both with respect to the presence or absence of Markovian lithologic transitions and with respect to the upward-shallowing character of lithofacies associations. In contrast to common wisdom, these measures of stratal order suggest that lithologic manifestation of meter-scale cyclicity is relatively uncommon. All of the several sequences deemed cyclic via qualitative inspection in fact contain relatively few intervals of demonstrable lithologic order, and even fewer exhibit any tendency for contained units to shallow upsection. In reality, most parts of most shallow-water carbonate sequences exhibit little more str tal order than would be apparent in random sequences of peritidal lithologies. On the basis of these considerations, we suggest that discrimination of meter-scale cyclicity in epicratonic carbonates is perhaps more perceptional artifact than stratigraphic reality. Imminent and future efforts intended to fruitfully evaluate the importance of intrabasinal versus extrabasinal processes of sedimentation in shallow low-latitude settings should perhaps eschew more generic perceptions of periodic paleoclimatic forcing in favor of a less regimented view toward the importance of stochastic processes of carbonate accumulation.