Drowning history of a Miocene carbonate platform (Zhujiang Formation, South China Sea)

The identification and interpretation of drowning events in the geologic record can aid significantly to the reconstruction of the depositional, tectonic and eustatic history of a study area and often improve reservoir and seal prediction in carbonate rocks. The differentiation between drowned platforms showing a record of continuous deepening and those with a record of exposure followed by rapid deepening remains, however, problematic. The Zhujiang carbonate platform (Liuhua 11-1 field, South China Sea) study shown here provides an example of an integrated approach combining high-resolution geochemistry, microfacies analyses and foraminiferal biostratigraphy in order to improve the reconstruction of environmental conditions prior, during and after platform demise and drowning. The Zhujiang carbonate platform displays the following vertical succession of four facies types i) skeletal grain facies with a miogypsinid/lepidocyclinid-dominated fauna deposited in a moderately deep (< 50 m), oligotrophic back-reef setting; ii) in situ corals in patch-reef facies in an oligotrophic lagoon (< 10 m); iii) rhodoid facies with in situ red algal crusts, dominated by Heterostegina sp. and spiroclypeids, possibly capped by a subaerial exposure surface. Well-rounded rhodoids representing a mesotrophic lagoon dominate the upper portions of the rhodoid facies; iv) pelagic marine shales of the Hanjiang Formation burying the carbonate platform after drowning. This facies succession, in combination with geochemical evidence suggests a deepening-upward trend. This trend might have been interrupted by transient subaerial exposure but no evidence for meteoric diagenesis was found at the drowning unconformity topping the carbonate platform. Instead, microfacies analyses suggest that platform demise may be related to progressive changes in environmental conditions, including increasing nutrient-levels and/or decreasing temperature up-core towards the drowning unconformity. These findings are of significance for those concerned with Miocene carbonate factories and, more specifically, the demise of carbonate platforms in general.