A proposed integrated multi-signature model for peritidal cycles in carbonates

Abstract

A new integrated multi-signature model for meter-scale peritidal carbonate cycles presented here is relevant to the interpretation of these cycles in the stratigraphic record. Meter-scale shallowing-upward (micro)facies cycles are the most common type of peritidal parasequence. Peritidal parasequences are a fundamental component of carbonate sequence stratigraphic models. However, the existence of meter-scale cycles in the geological record has been questioned owing to the failure to prove statistically the existence of these cycles using data on (micro)facies and stacking pattern taken from previous studies. Nevertheless, the shallowing-upwards (micro)facies interpretive template remains widely used to identify these cycles and parasequences. We propose that the sedimentary signatures of meter-scale peritidal cycles vary systematically during the various phases of an underlying 3rd-order relative sea-level cycle. Although during specific phases of a 3rd-order relative sea-level cycle meter-scale peritidal cycles may be characterized by shallowing-upward units (e.g., early transgressive systems tract), during other periods the stratigraphic signal of cyclicity may be more cryptic and complex, causing them to be overlooked (e.g., late transgressive systems tract and highstand systems tract). We also propose that there is a half-cycle phase shift in periods of deposition and the formation of drowning surfaces, relative to 4th- to 5th-order relative sea-level cycles, over the course of a 3rd-order relative sea-level cycle. These points have ramifications for defining parasequences and using stacking patterns to determine peritidal cycle periodicity. Specifically, parasequences defined using flooding surfaces identify sedimentary units that inconsistently partition 4th- to 5th-order relative sea-level cycles at different stages of a 3rd-order relative sea-level cycle. We propose testable, new physical stratigraphic signatures for identifying meter-scale cycles that have consistent genetic significance, each cycle recording a relative sea-level cycle characterized by a 4th- to 5th-order relative sea-level rise and fall. Meter-scale peritidal cycles are recorded by an ordered pattern of systematically varying (micro)facies signatures related to their position within a 3rd-order relative sea-level cycle. The integrated multi-signature model of meter-scale peritidal carbonate cycles we propose here may assist in explaining the apparently contradictory evidence both for and against their existence in the geological record. As a result of the detailed consideration in this paper of the nature of parasequences and their variations with 3rd-order relative sea-level changes, a redefinition of parasequence is required and one is presented here.