Petrography and genesis of Quaternary coastal gypcrete in North Kuwait, Arabian Gulf

Gypcrete crusts cover Late Quaternary playa deposits within the northern coastal flat of Kuwait Bay, Arabian Gulf. This study provides insight into the genesis of the gypcrete crust associated with the fluvial playa in an arid coastal area. The playa sediments are mostly muddy sand rich in diagenetic gypsum. The latter is concentrated at the top part of this playa where the gypcretic crust is developed. Field investigation, megascopic, microscopic and nanoscopic examination, and XRD analysis revealed the occurrence of several types of gypcretes, namely massive hard crust, hard clustered gypsum roses, gypseous rhyzocrete, and powdery crust. With the exception of the last type, all gypcretes are formed largely of gypsum with subordinate amounts of bassanite. Two genetic types of bassanite are recognised; these are replacive and precipitative. Replacive bassanite is the most common and is mainly represented by topotactic alteration of gypsum, whereas the precipitative mode is mostly restricted to the powdery crumbled crust and root cavity-fill. The micromorphology of both types of bassanite is discussed. It is suggested that the stability of bassanite is attributed to the elevated ambient temperatures in the presence of highly saline brine and organic acids.