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Original article (peer-reviewed)

Journal Terre & Environnement
Volume (Issue) 108
Page(s) 1 - 230
Title of proceedings Terre & Environnement

Abstract

This study includes geological mapping, sedimentological logging, and paleontological investigations on Mayaguana, a small island in the SE Bahamas. Samples were collected for petrographic and geochronological studies (amino-acid racemization, Sr-isotopes, U-series, and 14C-dating). Seven formations were identified on Mayaguana, four of which are new additions to the stratigraphy of the Bahamas. All consist of shallow-water carbonates (limestone and dolostone). They include the Rice Bay (Holocene), Whale Point (late Pleistocene), Grotto Beach (late Pleistocene), Owl's Hole (middle Pleistocene), Misery Point (early Pleistocene), Timber Bay (Pliocene), Little Bay (late Miocene), and Mayaguana (early Miocene) Formations. The four oldest stratigraphic units occur only along the north coast. They form a succession less than 12 m thick, spanning a time interval of 17 Ma, which translates to an accumulation rate of 0.6 m/Ma. The Plio-Pleistocene boundary was observed at +3 m apsl on the north coast exposures, but at depths between 6 and 10 m below sea level in cores drilled at more southerly locations. The youngest units are evenly distributed throughout the island. Relative sea level (rsl) during deposition of the described units can be estimated from (1) the occurrence of sedimentological indicators of paleo-depth, and (2) the elevation of these features apsl. The following rsl elevations can be estimated: <+10 m during the early Miocene; +2 m during the late Miocene; +3 m during the Pliocene; +5 m, >+8 m, and +9 m during the early Pleistocene; >+3 m during MIS 5e; and possibly +2 m during the Holocene. Interestingly, these marine deposits of various ages occur at about the same elevation, and were all deposited at a similar rsl (between +1 to +10 m apsl), which is surprising considering the difference in elevation between Mio-Pliocene and Quaternary sea-level stands. Derived from the combined thickness of the Mayaguana to the Misery Point Formations, the sediment accumulation rate on the Mayaguana bank for the corresponding period is much lower (0.6 m/Ma) than estimates calculated for carbonate platform tops of similar age (11 to 26 m/Ma). This particularity is best explained by a very slow subsidence that limited accommodation, and by the probable high elevation of the Mayaguana platform that permitted bank top flooding and associated carbonate deposition only during the highest sea-level stands of this time interval. During moderate sea-level highstands and lowstands, the bank top was emergent and subjected to karstification and pedogenesis. These two attributes (slow subsidence and raised position) could be linked to the ongoing transpression between the Bahamas and Hispaniola, or to the location of Mayaguana on a restraining bend of the Cauto fault system. Mayaguana is unique among Bahamian islands in that it comprises four stratigraphic units predating the middle Pleistocene, including Pliocene and upper Miocene dolostone, and foraminiferal limestone from the early Miocene. The subsidence history of this bank is thus radically different from that of other Bahamian platforms where Neogene rocks occur at depths of 10’s to 100’s of meters. This limited aggradation of the Mayaguana bank since the Miocene suggest that the platform may have remained stable for the past 20 Ma, implying that Neogene sea levels never exceeded modern datum by more than 10 m. Alternatively, due to its tectonic setting, it could have stood higher than other banks and would have been only flooded by the most important Neogene transgressions. Further drilling will resolve whether the asymmetry in the distribution of the ancient lithostratigraphic units is due to tectonic tilting or antecedent topography. Last but not least, the pristine and complete sediment successions from both this and the previous interglacial periods provide new and original data that complement, albeit contradict, earlier sea-level reconstructions for these time periods.
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