The Miocene/Pliocene boundary, as defined in the Mediterranean realm, is characterized by a distinctive lithostratigraphic change from evaporative continental to open marine sediments. This depositional sequence implies that the termination of the evaporative conditions associated with the "Messinian Salinity Crisis" was abrupt and would have required a major influx of marine water into the Mediterranean to have so dramatically altered the hydrologic regime. This rapid environmental change has been termed the "Terminal Messinian Flood" and its cause has been attributed to glacial eustatic and/or tectonic events.
A flooding event at the Miocene/Pliocene boundary has also been observed in the Bahamas. Shallow-water drilling on the platform top combined with seismic studies indicates that the loci of early Pliocene reef growth moved towards the platform interior, implying that a rapid rise in sea level caused the observed backstepping. Paleoceanographic investigations of sequences recently drilled at ODP Leg 166, Hole 1006A, located at the basinward end of a transect from the bank top and down the slopes of the Bahamas platform, provide new information to evaluate the global synchroneity of these two flooding events, as well as their possible causes and consequences.
Correlation of bio-, chemo-, litho- and sequence stratigraphic events recorded in pelagic sediments at ODP Site 1006 provides a scenario for the environmental changes occurring in the latest Miocene leading to the platform flooding in the earliest Pliocene. Tracing the isotope and lithologic events along the Bahamas transect using a sequence stratigraphic analysis indicates that, during this period, generally warmer sea-surface temperatures were associated with a high amplitude sea-level rise, probably characterized by relatively fast rates.
The onset of the major bank top flooding events at the Miocene/Pliocene boundary in the Bahamas appears to have been synchronous with events occurring during the "Terminal Messinian Flood" in the Mediterranean. Combined high resolution studies of continuous sections across the boundary in both areas imply a major overall eustatic sea level rise, which resulted in the eventual termination of evaporative conditions in the Mediterranean and forced the backstepping of the Bahamian platform margin.
Williams, T., and Pirmez, C., 1999. FMS images from carbonates of the Bahama Bank Slope, ODP Leg 166: Lithological identification and cyclo-stratigraphy. In Lovell, M.A., Williamson, G., and Harvey, P.K. (Eds.), Borehole Imaging: applications and case histories, Geol. Soc. Spec. Publ. London, 159:227-238.
Ocean Drilling Program (ODP) Leg 166 cored a transect of holes through the prograding carbonate sequences that form the western slope of the Great Bahama Bank, with the aim of detailing the relationship between the sequences and changes in sea-level over the last 25 Ma. A total of 1200 m of FMS resistivity images from Site 1003 (lower slope) and Site 1005 (mid-slope) were divided into three image facies types, with the aid of calibration against the recovered core. Type 1 was conductive (poorly cemented) sediment dominated by pelagic components, Type 2 was resistive (well cemented) sediment dominated by platform (neritic) components, and Type 3 was highly resistive (very well cemented) sediment, usually calci-turbities but occasionally hardgrounds. Much of the section is composed of metre-scale alternations between Type 1 and Type 2 sediment. We have used the cycle thicknesses in the Middle Miocene to obtain a sedimentation rate curve and to refine the biostratigraphy. The cyclicity is modulated by the precessional astronomical cycle. The FMS images were used to evaluate the lithostratigraphic position and significance of prominent isolated uranium peaks. The peaks tend to occur just below the tops of calci-turbidite-rich units, sometimes coincident with sequence boundaries and maximum flooding surfaces.
Reprinted with permission from The Geological Society of London.