Abstract
Field and petrographic descriptions of recent sabkha sediments at the eastern coast of Al-Kharrar lagoon, Red Sea coast of Saudi Arabia indicate the co-existence of gravel-sized, arrowhead, twinned gypsum crystals (< 23 cm long) and lenticular gypsum crystals (4–10 cm in size) of subaqueous and subaerial origin, respectively. Sand- and silt-sized, individual, lenticular and tabular gypsum crystals are widespread in the subsurface sediments of the sabkha. The twinned and lenticular gypsum crystals are highly enriched in sediment impurities, whereas tabular gypsum consists of clear crystals that are commonly associated with microbial lamination. The arrowhead, twinned gypsum crystals are formed subaqueously in a permanent supratidal pond that subsequently desiccated into a supratidal sabkha due to artificial construction of a barrier of concrete wall and gravelly dumps at the eastern margin of the lagoon. The lenticular and tabular gypsum crystals grow subaerially below the sediment surface of intertidal–supratidal sabkha by inclusive and displacive growth. The hindered supply of tidal floods to the former supratidal pond due to barrier construction leads to change in the depositional condition from upward, free growth of twinned gypsum crystals on the floor of the supratidal pond, to intrasediment growth of lenticular and tabular gypsum in a subaerial, supratidal sabkha setting. Deflation of fine sediments from the desiccated pond leads to exposure of a mixture of gravel-sized, arrowhead, twinned and lenticular gypsum crystals on the surface of the present day, supratidal sabkha setting. The results of this study indicate that the rapid change in the depositional condition due to human inference leads to desiccation of the supratidal pond, and changes in the morphology of the deposited gypsum from arrowhead, twinned crystals to lenticular crystals.