Abstract
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.
Abstract
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.