ABSTRACT

Sequence stratigraphic models are used to interpret stratal architecture and key stratal bounding surfaces in ancient carbonate platforms within the context of changing accommodation space during third-order relative sea-level cycles. However, individual systems tracts are still described using standard 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 change. 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 carbonate sediment composition are characterized by systematic shifts in the relative importance of different limestone component grain types, forming temporal continuums or relays. Relays are detected using computer-optimized Jaccard's similarity coefficient matrices to analyze presence/absence compositional data. Individual relays between grain types may link together samples that are generically unrelated to one another but are nevertheless genetically related to deposition during the same unidirectional dynamic environmental gradient.

A stratigraphic relay identified within the basal beds of the mid-Cretaceous Urgonian carbonate platform succession of SE France records unidirectional 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 assemblages. Stratigraphic breaks between individual compositional relays and assemblages occur at inflections in changing water depth, marking the boundaries 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 tracts and sequence boundaries has several advantages over existing methods. This approach can be used where key stratal surfaces are difficult to distinguish, such as in planar and concordantly bedded, inner platform settings and in arid depositional environments where physical evidence of subaerial platform exposure such as karstification is poorly developed. The technique can also be used to determine the genetic significance of unconformities preserved in outcrop or core within a sequence stratigraphic context by differentiating unconformities formed at the extremes of relative sea-level cycles from those formed by other abrupt acyclical environmental changes.