Facies stacking patterns in modern carbonate peritidal settings and their sequence-stratigraphic implications

The ‘shallowing-upward’ (parasequence) motif has long dominated the understanding of bed-scale carbonate cycles and, ultimately, sequence-stratigraphic models for platform-interior deposits of the rock record. To evaluate the viability of the shallowing-upward assumption, a review of mid- to late-Holocene coastal carbonate successions from many “classic” peritidal settings was undertaken. This assessment shows that the simple shallowing-upward assumption is largely unsupported by the aggregate of modern-systems observations. Tidal flats generally do not prograde directly into lagoons to create muddy shallowing-upward cycles, and lagoons are not observed to prograde over their own protective shoal barriers to create grainy shallowing-upward cycles. Instead, transgressive deposits comprise an important component of carbonate depositional successions, forming deepening-upward motifs, where cycle-base tidal-flat and lagoonal remnants are overlain by a ravinement surface, signifying the passing of a transgressing carbonate barrier, and then are overlain by open-platform deposits. Overlying this, regressive deposits, where present, are represented by lagoonal abandonment and grainy shoreface progradation capped by strandplains, with overlying accommodation only for thin, discontinuous, intertidal mud flats and their subtidal drainage channels in inter-ridge swales. These observations indicate that the construct of the parasequence is less useful than that of a high-order sequence containing transgressive and regressive components. The implications of these observations and interpretations are wide-ranging and have generated the need for updated sequence-stratigraphic models for coastal carbonates based on modern coastal systems, and with which the rock record can be compared. These models are meaningfully different from models currently in use with respect to predictions of large-scale subsurface connectivity.