Upper Pleistocene parabolic ridges (i.e. ‘chevrons’) from the Bahamas: Storm‐wave sediments or aeolian deposits? A quantitative approach

Abstract

The Upper Pleistocene (Marine Isotope Stage 5e; ca 120 ka) stratigraphic record from the Bahamas comprises large, kilometre-long parabolic ridges of oolitic composition, that point landward, and have been up to now called 'chevrons'. A debate about their genesis has led previous researchers to consider two processes of deposition: (i) a catastrophic event involving giant storm-generated waves produced by specific climatic conditions at the end of Marine Isotope Stage 5e; and (ii) a more uniformitarian process which characterizes 'chevrons' as aeolian parabolic dunes because of their similar morphology. Since there are few unequivocal sedimentological criteria to discriminate aeolian from water-deposited sediments, only a quantitative, multi-method approach could provide enough evidence to produce a viable diagnosis on the genetic processes involved. Following this reasoning, the quantification of the morphological parameters of 'chevrons', a precise study of their sedimentary structures on previously and newly discovered sections, and several statistical grain-size analyses, all advocate for an aeolian origin. Moreover, when the aforementioned characteristics of 'chevrons' are compared with those of storm deposits (for example, washovers) and parabolic dunes occurring elsewhere on Earth, the dissimilarity with the former and the resemblance with the latter is evident. Amino-acid racemization dating, together with stratigraphic and petrographic investigations, constrain the age of the 'chevrons' to the late part of Marine Isotope Stage 5e. Their occurrence during this specific time interval can be explained by both strengthened easterly winds and drier climatic conditions associated with changing vegetation cover. Fixation of the arms by sparse vegetation, coupled with the loose 'chevron' nose sediment migrating farther inland, form the peculiar U-shaped morphology of these ridges.

Signatures of Pleistocene Marine Transgression Preserved in Lithified Coastal Dune Morphology of The Bahamas

“…The carbonate sediment production is limited to the shallow marine banks surrounding the islands, which forms a direct link between the source of sediment to construct aeolian dunes and sea level highstands during the Marine Isotope Stage 5, 7, 9, and 11 interglacial periods [6,[12][13][14]19,20]. The carbonate grains can experience rapid dissolution and diagenesis via early-stage vadose (i.e., meniscus, pendulous, and needle-fiber) cementation within tens to hundreds of years of deposition [9,21,22], increasing the preservation likelihood of the coastal dune topography. In The Bahamas and TCI, Pleistocene-aged duneforms compose most of the islands' high topography.…”

“…Fortunately, the recent availability of high-resolution DEMs has revealed the extent, shape, and variability of dune topography, as well as signatures of past and ongoing erosion across the region. These data allow for new analysis and interpretation of coastal dune morphologies at individual duneform to island to multi-island regional scales [9,15,23], as well as other components of island geomorphology [24,30].…”

“…Within these landscapes, the form and stratigraphy of coastal dunes provide particularly valuable environmental records that include sediment supply and wind climate [1], as well as storm frequency and intensity and properties of the land surface [2][3][4]. Though primarily composed of beach sand, coastal dunes commonly prograde inland, driven by onshore-directed winds [5][6][7][8][9]. The rates and patterns of this migration can be modified by pre-existing downwind topography, vegetation, and surface moisture to the point where dunes are stabilized [10].…”

“…Lithified wind-blown dunes of the Quaternary age are common across The Bahamas and Turks and Caicos Islands (TCI) [6,9,[12][13][14][15]. Unfortunately, connecting these dunes to contemporaneous shoreline positions remains a challenge because aeolian dunes can climb up and over pre-existing topography [16,17], potentially migrating kilometers inland without leaving a record of previous seaward positions.…”

Parabolic dune distribution, morphology and activity during the last 20 000 years: A global overview

“…(b) schematic cross‐section of a generic transverse dune and its internal sedimentary features showing the process of sand transport by wind (modified after Ringrose & Bentley, 2015). (c) Schematic cross‐section of a generic coastal carbonate parabolic dune and its internal structure (modified after Vimpere, Del Piero, et al, 2021). [Colour figure can be viewed at wileyonlinelibrary.com]…”