Carbonate Eolianites in the Exumas — The Legacy of Vanishing Ebb Tidal Deltas During a Sea Level Rise

Abstract

The high-energy platform margin in the Exumas Islands portion of Great Bahama Bank is dominated by a tidal delta depositional motif, typified by islands (Pleistocene or Holocene) bounding open circulating tidal channels within which vigorous tidal flow leads to the formation of flood and ebb tidal lobes formed of ooid and other types of carbonate sands. As shown by the robust example of this motif associated with the tidal channel between Shroud and Hawksbill Cays, significant amounts of sediment form fully aggraded flood and ebb lobes and adjacent sand flats, and are reworked from the ebb tidal lobe by longshore currents and storms to nourish adjacent beaches and form back-beach sand ridges that quickly become cemented by meteoric cements. Isolated Early Holocene (~5,000 years old) eolianites on several islands in the Exumas, e.g. Gaulin Cay, Bitter Guana Cay, Cambridge Cay, O'Brien's Cay, and Warderick Wells Cay, are problematic, in that these significant relict deposits of ooid and peloid sands with large-scale foreset bedding clearly indicating sediment transport onto the platform are challenging to explain from the standpoint of the source of the carbonate sand. Age-related, coeval facies are lacking, and laterally adjacent shorelines generally lack appreciable Holocene sediment and commonly show signs of erosion and coastline retreat. We propose these eolianites are the result of ebb tidal lobe cannibalization by longshore currents and storms, and their apparent non-equilibrium state with the modern seaward coastline strongly suggests a major change in depositional conditions between time of deposition of the eolianites and present-day. It is likely that tidal delta lobe development occurred in these areas a few thousand years ago during a lower position of sea level when more vigorous tidal currents were concentrated in channels formed in deeper Pleistocene lows. Present sea level has resulted in less vigorous tidal currents in these areas and cannibalization through erosion (longshore currents and storms) of the ebb tidal delta lobe, beach sands, and the seaward portions of the eolianites themselves. The resultant, significant eolianite deposits at these localities are difficult to understand in the absence of their associated early Holocene facies belts, and would pose a very challenging dilemma to explain them in any ancient scenario. Associated facies are missing entirely or are severely truncated.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015