Upper Pleistocene Parabolic Ridges (I.E. 'Chevrons') From The Bahamas: Storm-Wave Sediments Or Aeolian Deposits? A Quantitative Approach

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.