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Department of Geology, Faculty of Science, King Saud University, Riyadh, Saudi Arabia

ABSTRACT

The Tuwaiq Mountain Limestone is generally 200 m thick near the city of Riyadh (Lat.

2436N, Long. 4640E) in the centre of the Tuwaiq Mountains escarpment. It is basically subdivided into a lower section 110 m thick and an upper section 90 m thick. The latter is further subdivided into a lower 50 m of mainly coral and stromatoporoidal wackestones and packstones, and an upper 40 m of mainly dasycladacean wackestones and packstones. The identified dasycaldacean fossil algae include the genera Acicularia, Cylindroporella, Heteroporella, and Salpingoporella. Other associated algae are Cayeuxia and Thaumatoporella (Polygonella). The Tuwaiq Mountain Limestone is Callovian to Oxfordian in age. It has been cited as a source rock for some of the prolific oil production of eastern Saudi Arabia, besides being itself an oil-producer in some fields. The occurrence of dasycladacean fossil algae clearly indicates an upward regressive depositional cycle for the Tuwaiq Mountain Limestone in the area studied.