Stratigraphy and sedimentology of Quaternary units around and within Dean's Blue Hole, Long Island, Bahamas
The Bahamas are known for their numerous carbonate platforms that have recorded glacio- eustatic variations since the Cenozoic. The northern part of the archipelago has been heavily studied by geologists since the 60's and has provided key information on surface geology, regional tectonics and the dynamic of ice sheets through time. Long Island is a narrow and elongated landmass located in the SE corner of Great Bahama Bank (central part of the Bahamas archipelago). Previous research (Hearty, 2010) showed that it mostly consists of eolianites of Middle Pleistocene to Late Holocene age. Marine facies (e.g. fossil reefs and beaches) have only been observed at two localities (Curran et al., 2004; Kindler and Godefroid, 2015). The depth to the base of the Lucayan limestone (i.e. the Plio-Pleistocene boundary) indicates a regional subsidence rate of 1.35 m/105 years (Pierson, 1982). Our study area is located in the southern part of Long Island where both the lagoon- and the ocean- facing shorelines take an E to W trend. The main geological feature of this zone is Dean's Blue Hole (DBH), which is the second deepest marine sink hole in the world (202 m). We investigated and mapped the stratigraphic units exposed in the vicinity of DBH, and also scuba dived down to a depth of 60 m into the latter. The sedimentological characteristics of the identified stratigraphic units were examined in the field. Samples were collected by hand and with a jack hammer at irregular intervals between 10 m above and 30 m below the sea level for petrographic analyses, image analysis, amino-acid racemization dating and U-Th dating. Long Island's surface geology is composed by six different units that have been correlated with three formations (Carew and Mylroie, 1995a) going from the MIS 9 to Holocene: The Owl's Hole Formation, the Grotto Beach Formation and the Rice Bay Formation. The section logged at DBH (coord.: N 23°06'23”, W 75°00'31”) comprises three vertically stacked shallowing-upward successions, called hereafter Sequences I to III, in descending order. About 28 m thick (+10 to -18 m), Sequence I can be correlated with Marine Isotope Stage (MIS) 5a (i.e. the 80 ka BP sea-level highstand). The underlying Sequence II is ca. 8 m in thickness, and is logically assigned to MIS 5e (i.e. the 120 ka BP sea-level highstand) due to its position, oolitic-peloidal composition (Kindler and Hearty, 1996), and relatively low diagenetic grade. Occurring between 27 and 30 m below sea level, Sequence III is made of much altered bioclastic boundstones and grainstones, likely deposited in reefal (base) and intertidal (top) environments. It is tentatively correlated with a Middle Pleistocene interglacial (MIS 9 or 11) due to its high grade of diagenetic alteration. If our correlation is correct, the difference in elevation, between the Middle-Upper Pleistocene boundary in the blue hole and its equivalent inland, could be related to (1) a topographic effect, (2) differential subsidence of Great Bahama Bank, and (3) down-warping of the DBH area along a hidden fault plane. The latter hypothesis, which is supported by the abrupt change in the orientation of the Long Island shorelines in this region, could further provide a clue on the anomalous depth of DBH. We also observed fenestrae porosity at +25 m above the sea level, only in oolitic eolianites. These features were also observed on other islands in the Bahamas up to 40 m above the sea level (Bain & Kindler, 1994; Hearty et al. ,1998; Kindler & Strasser, 2000). In order to bring new information on the subject, we performed image analysis on more than 200'000 particles to determine the differences between beach and eolian fenestrae with the aim to characterize the formation process. It appears that there is a binary distribution of the size of these fenestrae with greater fenestrae formed by beach process, but also a significant difference in the orientation distribution depending on the genesis process. iii Finally, because of the observed differences between our A/I values calculated from the DL- Val measured by Reverse Chromatography, and older values measured by Ion Exchange Chromatography, we decided to perform several corrections for this dating method. Therefore, the correlation was based on petrographic and stratigraphic observations and we applied the APK model to transform the measured DL-values following the formula defined by Whitacre et al. (2016), in order to give A/I values and age ranges for sediments on Long Island. It was especially successful for MIS 7 and MIS 9 units that are diagenetically altered. This three axis study initially aimed to provide a primary geological knowledge for the Dean's Blue Hole area on Long Island, which has not been studied yet. Further studies will be able to use the geological map generated as a basis of their future works. The originality of this work was to use the Dean's Blue Hole as a natural outcrop to study older units than the ones normally expected on the Bahamian islands. Unfortunately, this was not the case and the oldest unit found was correlated with the Middle Pleistocene. However, regarding the changing of the axis of the island at this place, this could give some leads to explain the abnormal depth of the Dean's Blue Hole and help to better understand these karstic features. The possible discovery of a criterion to discriminate between beach and eolian fenestrae whether they are induced by storms or swash, can yield to corrections in some studies that have wrongly interpreted paleo sea-levels. It is also a good start to observe differences of porosity and permeability in these type of deposits in order to improve the accuracy of reservoir geology models. And finally, this work provides a preliminary database for AAR dating on Long Island and the possible corrections to apply on DL-values measured by RP in order to have comparable data with A/I values measured by IE.
Citation (ISO format)
VIMPERE, Lucas. Stratigraphy and sedimentology of Quaternary units around and within Dean’s Blue Hole, Long Island, Bahamas. 2017.
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