Publication Date

2017-08-09

Availability

Embargoed

Embargo Period

2019-08-09

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Marine Geology and Geophysics (Marine)

Date of Defense

2016-04-21

First Committee Member

Gregor P. Eberli

Second Committee Member

Donald F. McNeill

Third Committee Member

Paul M. (Mitch) Harris

Fourth Committee Member

James S. Klaus

Fifth Committee Member

R. Pamela Reid

Abstract

The Holocene and Pleistocene sedimentary strata in the Exuma Cays and New Providence Island, Bahamas, record sea-level oscillations during orbitally-driven sea-level highstands. Together, the orbital and sub-orbital sea-level changes primarily control shallow-water carbonate deposition, island accretion and preservation, resulting in extreme lateral and vertical heterogeneity of depositional facies. Field mapping and coring in the Exuma Cays document that exposed strata display a complicated array of stacked and laterally accreting marine and eolian deposits. Stratigraphic relationships, corroborated with age dating, are used to identify stratigraphic units and environments of deposition. Lateral accretion is a common motive in this shallow-water environment but it is an exception to the Law of Superposition that is widely used in physical stratigraphy. The lateral accretion is not the result of storm deposition but of forced lateral deposition when subsequent sea-level changes are below the amplitude of the previous fluctuation and its depositional topography. The Holocene flooding of New Providence Platform (NPP) and the Exumas windward margin portions of Great Bahama Bank records complex, shifting sediment patterns as the Holocene transgression caused a shift in the energy balance on the platform. Sedimentation started at the outermost platform margin around 6,700 ybp with the formation of beaches, back beach storm ridges, and eolian dunes. With rising sea level, this coastal system migrated towards the platform and in the process the early coastal system was largely eroded. Relict early Holocene dunes sitting directly on Pleistocene are the record of this early depositional system. Pleistocene antecedent topography acts as a barrier to the erosion caused by the Holocene transgression and also provides a template for complex Holocene carbonate sediment deposition and accretion. The changing energy balance also determines the preservation potential across the bank. Eastward of the islands, very little Holocene sediment is preserved; sediments on this outermost platform margin are either transported towards the platform or into Exuma Sound. In many places, the sloped outer margin is eroded down to the Pleistocene surface. Locally, the windward margins of the islands contain steep, eroding sea cliffs and small relicts of cemented (older) Holocene eolianites. On the leeward sides of the islands (facing the platform interior), the erosion is minimal and a more complete record of Holocene sedimentation is commonly present. The Pleistocene islands serve as the boundary between low and high sediment preservation. In addition, they focus the tidal currents, producing high-energy environments with increased sediment production. These sediments are deposited in ooid shoals or as accreting beaches on both the open-ocean and the platform interior of the Pleistocene islands. A multi-proxy approach of the sedimentary record of the strata deposited during the last Pleistocene Interglacial Highstand (MIS 5e), 129-116 kybp, provides strong evidence that eustatic sea level during MIS 5e was not a single rise and fall but oscillated as much as 10 m during MIS 5e dividing the sea-level highstand into early and late substages. The MIS 5e sea-level oscillation produced complex facies juxtapositions and lateral accretion patterns in reef, shoal, beach, and eolian deposits. In addition, at the end of late substage MIS 5e, sea level fell in downstepping pulses. The amplitude of the MIS 5e sea-level oscillation is constructed from the elevations and depths of three robust indicators of sea-level position: the beach-to-dune transition, in situ coral growth, and exposure horizons in outcrops and cores. In addition, regional 2D Ground Penetrating Radar (GPR) surveys document subsurface geometries and connectivity of subtidal units and prograding beach ridges on New Providence. These criteria document a 10+ m oscillation within the MIS 5e highstand that exposed the Great Bahama Bank platform, creating two separate depositional cycles within only ~13,000 years. The downstepping beach-to-dune transitions observed in cores from west to east in the Exumas are interpreted to represent pulsed ice buildup at the end of MIS 5e. Recognition of suborbital sea-level oscillations during interglacial highstands directly questions the commonly accepted premise that Milankovitch orbital frequencies controls carbonate cyclostratigraphy. A suborbital sea-level oscillation requires a suborbital forcing mechanism of much shorter duration than Milankovitch frequencies. This is important because it proves that multiple depositional cycles can be deposited during a single warm period thereby questioning orbital tuning methods commonly used in shallow-water carbonate cyclostratigraphy. In addition, the sea-level oscillations during interglacial times document rapid climate change during warm periods that includes short-term waning and waxing of ice sheets.

Keywords

Bahamas; Exuma Cays; New Providence; Sea Level; Sea-Level Oscillations; Pleistocene; Last Interglacial Highstand; MIS 5e; Holocene; Carbonate Windward Margin Stratigraphy; Shallow-Water Carbonates; Lateral Accretion; Sedimentary Record; Hurricane

Available for download on Friday, August 09, 2019

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