Abstract
The Tuwaiq Mountain Limestone is one of five prominent rock units in the Upper Jurassic of central Saudi Arabia. The upper 90 meters of the formation, designated as the Upper Tuwaiq Mountain Limestone, consists of light gray, hard, ledge forming biomicrite with abundant algae, corals and stromatoporoids. The most common fossil algae are the oncoidal forms of the blue -green alga genus Cayeuxia . The coral genera Amphiastraea and Microsolena are among the commonest remains of the frame-builders of possibly small reef-like structures. The Upper Tuwaiq Mountain Limestone was deposited in open marine environment of moderate depth.