Modeling carbonate microfacies in the context of high-frequency dynamic relative sea-level and environmental changes

Sequence stratigraphic models are used to interpret stratal architecture an d key stratal bounding surfaces in ancient carbonate platforms within the c ontext of changing accommodation space during third-order relative sea-leve l cycles. However, individual systems tracts are still described using stan dard microfacies that give a snapshot of limestone composition, but they do not take into account gradual changes in the marine environment resulting from variations in water depth during the cycle of relative sea-level chang e, Water depth is the single most significant collective control on a wide range of environmental gradients affecting carbonate sediment composition. During dynamic relative sea-level fluctuations stratigraphic changes in car bonate sediment composition are characterized by systematic shifts in the r elative importance of different limestone component grain types, forming te mporal continuums or relays. Relays are detected using computer-optimized J accard's similarity coefficient matrices to analyze presence/absence compos itional data. Individual relays between grain types may Link together sampl es that are generically unrelated to one another but are nevertheless genet ically related to deposition during the same unidirectional dynamic environ mental gradient. A stratigraphic relay identified within the basal beds of the mid-Cretaceou s Urgonian carbonate platform succession of SE France records unidirectiona l environmental gradients linked to changing water depth, characteristic of a transgressive systems tract. Periods of static water depth, such as the keep-up phase of a late highstand systems tract at the top of the Urgonian carbonate platform succession, are characterized by fixed compositional ass emblages. Stratigraphic breaks between individual compositional relays and assemblages occur at inflections in changing water depth, marking the bound aries between individual systems tracts and sequences, currently identified using stratal geometries and key stratal surfaces. The use of relays to model microfacies and identify individual system tract s and sequence boundaries has several advantages over existing methods, Thi s approach can be used where key stratal surfaces are difficult to distingu ish, such as in planar and concordantly bedded, inner platform settings and in arid depositional environments where physical evidence of subaerial pla tform exposure such as karstification is poorly developed. The technique ca n also be used to determine the genetic significance of unconformities pres erved in outcrop or core within a sequence stratigraphic context by differe ntiating unconformities formed at the extremes of relative sea-level cycles from those formed by other abrupt acyclical environmental changes.