The influence of carbonate platform morphology and sea level on fifth-order petrophysical cyclicity in slope and basin sediments adjacent to the Great Bahama Bank

Abstract

Despite several decades of study, details of the relationship between sea level and other factors controlling carbonate sedimentation require clarification. To address this issue, ODP Leg 166 drilled a slope to basin transect adjacent to the Great Bahama Bank to document Neogene and Quaternary sea level changes, and to precisely date the major unconformities within the slope and basinal succession. Here we describe the complex connection between the cyclicities evident in a range of sediment physical property parameters at Leg 166 sites and their relationship to sea level fluctuations, diagenesis, and the morphology of the adjacent carbonate bank margin. The characteristics of fifth-order petrophysical cycles in Miocene slope sediments of the Great Bahama Bank are fundamentally different from those in overlying late Pliocene and Pleistocene slope sediments. These intervals are separated by an early Pliocene sequence with characteristics that are transitional between the overlying and underlying sequences. We interpret the more symmetric character of the Miocene cycles as reflecting cyclic sedimentation controlled by the interaction between sea level fluctuations and the ‘ramp’ morphology of the adjacent platform, and the markedly asymmetric late Pliocene and Pleistocene cycles as representing the interaction between sea level and a ‘rimmed’ platform morphology. In the case of the Miocene cycles, diagenetic overprints cause values for petrophysical properties controlled by porosity to be inverted compared to cycles in essentially unaltered late Pliocene and Pleistocene sediments. A detailed understanding of the relationship between petrophysical cycles, diagenesis, sea level fluctuations, and platform margin morphology offers the potential to provide a proxy for inferring the controls on carbonate sedimentation in situations where the nature of the carbonate platform margin is unknown (e.g., where regional seismic data is poor or absent) and/or where sea level effects cannot be reliably determined by other means. These petrophysical proxies should be of greatest benefit where downhole logging data is used as the primary basis for sediment characterization.