Stratigraphy of diagenesis: Limestone–dolomite cycles in a mid‐ramp setting, Miocene Arcadia Formation, South Florida, USA

The Arcadia Formation beneath Long Key, Florida Keys consists of a composite sequence with three third‐order sequences based on a regional correlation of gamma‐ray logs and cores. Deposition of the composite sequence was a result of a second‐order transgression on the underlying Suwannee Limestone, following an ~8 my hiatus. Combined, the three Arcadia third‐order sequences consist of a basal set of 10 fining‐up cycles (41 m thick), overlain by 22 coarsening‐up cycles (99 m thick). Both cycle types show mid‐ramp facies (~40–50 m depth) composed of molluscs, benthic foraminifera, bryozoan and red‐algal bioclastics. The mineralogy of the fining‐up cycles consists of alternations of a basal limestone and upper dolomite (75%–100%). The overlying, coarsening‐up cycles are predominantly limestone with only minor dolomite. The 10 basal cycles are pervasively dolomitised with enhanced permeability. The dolomitised intervals not only exhibit the highest permeability (mean = 3309 mD) but also contain an occasional, thin, low‐permeability (mean = 26 mD) cycle cap. Within the cycle, dolomite usually has an order‐of‐magnitude greater permeability than the limestone (mean = 214 mD). Dolomitisation of the upper part of the fining‐upward cycle produces a permeability inversion relative to the original finer‐grained (lower permeability) sediments. The intra‐cycle transition from limestone to dolomite suggests early dolomitisation, perhaps even syndepositional or prior to deposition of the subsequent cycle. The results of this study are consistent with the proposal by Goldhammer et al. that ‘stratigraphy of early diagenesis’ is operational, where marine diagenesis is favoured during deposition in the transgressive systems tract. The Miocene example described herein provides an example for the formation of limestone–dolomite cycles and may represent preferential dolomitisation during sea‐level transgression.