Abstract
The sedimentology and stratigraphy of New Providence Island, Bahamas, was analyzed using ground penetrating radar (GPR) and core correlation. The shallow Pleistocene carbonates that form New Providence Island were deposited during a series of sea level highstands. This study focuses on the internal architecture and distribution of the deposits from the last interglacial sea level highstand (MIS 5e). Ground penetrating radar (GPR) is used to image the geometries of the MIS 5e strata that are separated from the underlying MIS 9 strata by an unconformity. Outcrops in the western part of the island reveal a complex succession of the MIS 5e deposits, dividing it into six main lithofacies that range from eolianite to beach to subtidal deposits. Heights and geometries of different ridges in the study area are captured with a digital elevation model (DEM) of New Providence. High-precision 250 MHz GPR data, acquired in 2014 and 2015, images these lithofacies in the subsurface. The calibration of the GPR data with the outcrop facies and geometries facilitates the assessment of the distribution of the carbonate facies in the subsurface throughout the study area. This calibration reveals that some of the ridges that were mapped and interpreted beach ridges represent instead high energy subtidal bar settings. This finding prompts a revision of the notion of four down-stepping beach ridges with the onset of the glaciation at the end of MIS 5e. However, an exposure horizon within the MIS 5e subtidal deposits documents a fall of sea level of several meters during the last interglacial. This exposure horizon has been identified in outcrop, as a hardened caliche crust, and separates the early MIS 5e deposits of bioturbated subtidal grainstone facies from the late MIS 5e deposits with trough-cross bedding and foreset bedding that are typical for a subtidal bar. This horizon that is about 1m above present day sea level is identified in the GPR data and shows continuity throughout the GPR survey. The amount of sea level drop during this oscillation cannot be established from the exposure horizon but the subsequent rise of sea level was about 6 m higher than the first peak within MIS 5e based on the elevation of the highest point of the younger MIS 5e measured on New Providence. A 204-meter core from the northern part of the island provides the data for analyzing the deeper strata of New Providence Island. The entire core consists of mostly of high energy and shallow reef to fore-reef slope facies that is in concert with deposition in a platform margin setting. Only occasionally thin tidal flat/platform interior facies are observed. The core can be divided into three main successions of variable sediment compositions and textures. The basal succession consists of Miocene(?) dolomitized mixed skeletal lithofacies that show a variety of different facies from mudstones to various skeletal grainstones and floatstones. Above this basal facies, a thick succession of Pliocene dolomitized coral-bearing floatstone displays a shallowing upwards sequence from deep, algal-dominated facies into shallower, coralline reef facies punctuated by a series of exposure horizons. In the Pleistocene section above 38 meters, the lithofacies is dominated by skeletal peloidal grainstone and the mineralogy is calcite. This lithofacies also contains several exposure features that indicate high frequency changes of sea level. The change in mineralogy from limestone to dolomite across the Pleistocene-Pliocene boundary has been observed in other deep core borings on San Salvador and Great Bahama Bank.