| Popis: |
Large-scale patterns of carbonate platform development are strongly influenced by the interplay between eustatic sea-level change and basinal subsidence or uplift. Improving understanding of the subsidence regime and drowning history of carbonate platforms is important for hydrocarbon exploration as drowning surfaces are known to form diagenetic seals on top of; or permeability barriers within platform reservoirs. In this context, taphonomic and diagenetic signatures may be useful as they have the potential to preserve evidence of the different stages of platform evolution, from platform growth through submergence to post- platform demise. The taphonomic and diagenetic processes operating within shallow reef/platform environments (< - 30 m) are relatively well understood in comparison to those that modify platform depositional fabrics in deeper water environments (> - 30 m). The aim of this study is to document and develop diagnostic models for the evolution of taphonomic alteration and marine diagenesis during progressive platform drowning. The potential of utilising the taphonomic and diagenetic criteria for the interpretation of platform drowning surfaces in the geological record is also discussed. The study is based upon petrographical, mineralogical and trace element analysis of 120 samples from a unique series of 7 successively drowned reefs offshore the Big Island of Hawaii. This exceptional dataset ranges across water depths of 150 – 1500 m and encompasses platform zones over 500 ka. New data from the palaeo-bathymetric interpretation of coralgal and encruster sequences, macro- and micro-boring analyses and analyses of marine diagenesis (cementation and dissolution), provide evidence to show that the platform drowning surfaces in Hawaii are characterised by a unique sequence of taphonomic and diagenetic changes with increased water depth. Six facies are defined and are interpreted as corresponding to four stages of platform growth, followed by a progressive drowning sequence and culminating in complete platform turn-off. The facies distribution and stages of taphonomic and diagenetic alteration with increased water depth during progressive platform drowning are illustrated in three schematic models. The characteristic taphonomic and diagenetic features of each stage of the models and their palaeo-environmental significance are the focus of this study. Although each sequence is not always observed in order or as complete, the schematic models presented provide a framework for documenting the taphonomic and diagenetic alteration of the platform drowning surface that occurs with increased water depth during progressive platform drowning. Many new interesting observations have been made from a much deeper drowned platform surface than has been described and analysed previously and a major advancement has been made in the understanding of the variability and distribution of taphonomic and diagenetic alteration of the platform drowning surface. |
