Giant polygonal Tepee structures discovered in the NE Red Sea - AL Wajh carbonate platform, KSA

<p>Carbonate tepee structures are believed to initiate through cement growth in shallow marine hardgrounds causing lateral expansion and leading to upward buckling of cemented layers commonly along polygonal boundaries.  They reportedly form in subtidal to supratidal marine settings and are stratigraphically important markers for exposure and cycle boundaries in ancient rock sequences.  Yet in modern carbonate settings only minor occurrences have been reported from the Arabian Gulf in Abu Dhabi and Qatar as well as in Australia.  We have discovered two spectacular fields of giant polygonal tepee structures on Sheybara Island, a part of the Al Wajh carbonate platform in the NE Red Sea, KSA. Satellite and drone data were used to measure the dimensions of polygons. Samples have been collected from three transects and two boreholes for age dating, petrographic and geochemical analysis. The tepee fields cover an area of 420,000 m<sup>2</sup> and 130,000 m<sup>2</sup>, respectively, in the supratidal to intertidal environment on the ocean facing side of the island.  Individual tepees are composed of chaotically superimposed rugged slabs reaching 3-10 cm in thickness.  Tepee ridges range in height from 10-50 cm.  Tepees are aligned along larger structures of well-defined polygonal shapes.  Their diameters range from 5m to 55m (n =100) with the majority having a diameter of 10-25 m (n=69).  Peculiar to many polygons is a central domal buckle with extensional fracture patterns. The tepees have formed in a well-cemented layer of shallow marine bioclastic sand to gravel-sized sediments composed predominantly of coral, red algae, benthic foraminfera, bivalve and gastropod debris that overlie a paleo-reef flat.  Grains are heavily micritized, cemented by clotted micrite and fibrous to acicular rim cements and occasionally covered by lace-like meshes of organic matter, likely indicating microbial activity.  SEM images from tepee samples show evidence for the presence of microbial activity - biofilms, morphologies that strongly resembles filamentous and coccoidal cyanobacteria, and mineralized cyanobacterial mats. Environmentally corrected C<sup>14</sup> age data indicate that polygons formed between 3000 to 1000 years before present (b.p.) correlating with a sealevel regression from a mid-Holocene sealevel highstand some 4000 to 5000 years b.p.  Dead and blackened finger corals commonly encrust tepees indicating that the elevated tepee crusts provide preferential seeding for coral colonialization upon re-submergence.</p>