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Keywords:

  • Arabian Gulf;
  • carbonate hardground;
  • intertidal;
  • polygon;
  • tepee structures;
  • UAE

Abstract

This study provides the first quantitative description of modern giant polygons. These large-scale, up to 166 m diameter, sedimentological features are described here in detail from the intertidal zone of the Arabian Gulf near Abu Dhabi. These features form by the displacive carbonate cementation of carbonate sand below a 3 to 5 cm thick cover of unconsolidated sediment. The oxygen isotopic composition of cements is consistent with precipitation from evaporated sea water. Cementation proceeds via a poorly consolidated layer that grades downward into an 8 to 14 cm thick, well-cemented grainstone. The margins of this expanding hardground form the overlapping borders of the polygons. Tepee structures at the borders of polygons are only occasionally preserved, as supporting sediment is removed by strong tidal currents and high-energy events, such as storms, that erode the unsupported tepees. These observations have clear implications for the use of tepees in the interpretation of ancient polygons and their associated environments. When preserved in the sedimentary record, tepee structures are interpreted to indicate lower energy depositional environments than that observed in this study.