Evidence of Sea-Level Oscillations Within the Last Interglacial From the Miami Limestone and Bahamian Oolitic Shoals
Tidally influenced ooid shoals of the Pleistocene Miami Limestone and coeval deposits in the Bahamas were chosen to further document the record of sub-orbital sea-level oscillations within the last interglacial highstand (Marine Isotope Stage 5e or MIS 5e). Detailed sedimentological and stratigraphic investigations of cores and outcrops of the Miami Limestone are combined with a LIDAR-based digital terrain model (DTM) to relate the facies of the grainstone body to the geomorphologic structure and depositional geometries. The oolitic portion of the Miami Limestone, covering approximately 300 km2, comprises two distinct geometries: (1) a tidal bar and channel system which covers about 280 km2 and (2) a prograding barrier bar which covers 20 km2. Two main facies, mottled and cross-bedded, are identified within the oolitic portion. In several cores, characteristic features of subaerial exposure are found, such as dissolution, red staining, caliche crusts, and burrow-filling quartz. Two U/Th age dates, <155.2 ± 1.1 kyrs and 126.9 ± 0.9 kyrs, respectively, indicate that a key exposure horizon formed within MIS 5e and document a sea-level drop. It is also likely that the seaward accretion of the prograding barrier bar relative to the tidal bar and channel system is related to a sea-level oscillation. Thus, oscillating sea level during MIS 5e combined with syndepositional topography is responsible for sedimentological complexity and stratigraphic heterogeneity within the Miami Limestone. A comparison of the elevations of the shoal crests from the Miami Limestone with time-equivalent oolitic grainstone shoals in the Bahamas (Ocean Cay and New Providence Island) yields a difference of 17 m, provide an estimation of the amplitude of sea-level oscillation during MIS 5e. The implications of high-frequency sea-level oscillations during MIS 5e are twofold: 1) rapid climate changes and concomitant waxing and waning of ice sheets can occur within warm periods, and 2) sub-orbital oscillations within highstands generate additional cycles of deposition similar to the orbitally controlled depositional cycles themselves. The created stratigraphic complexity and heterogeneity have potential implications for reservoir characterization, modelling, and flow behavior of carbonate grainstone bodies in the ancient.
AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015