Abstract

Late Pragian–early Emsian (∼ 405 Ma) microbial and metazoan reefs in the Al-Jawf area of northwestern Saudi Arabia occur in the 20-m-thick Qasr Limestone Member of the Jauf Formation, within a dominantly fine-grained marine siliciclastic sequence deposited on a broad shelf adjacent to the Arabian–Nubian Shield. Microbial reefs, composed of stromatolites, thrombolites, and to a lesser extent leiolites, are circular to elliptical in plan and have mounded-domical cross sections. Individual microbial reefs up to 6 m in height and 15 m across are commonly laterally and vertically juxtaposed in composite buildups up to 10 m high and 60 m across. Metazoan reefs, dominated by poorly preserved bryozoans, range from small subcircular mounds 10–60 m across, through ridges up to 300 m in length, to more complex structures and reach 4 m in height. The Qasr Member provides a rare example of well-exposed Early Devonian reefs in plan view. Satellite images show the spatial arrangement of Qasr mounds as hundreds of small patch reefs over a wide area. Regardless of type, the reefs and composite bioherms are generally NNE–SSW-oriented and many are grouped into parallel rows. This pattern, as in some modern patch-reef complexes, may reflect direct influence of paleotopography and water circulation patterns. The reefs are enclosed and separated by bedded deposits, marls, siltstones, and limestones: intraclastic floatstones, bioclastic floatstones–rudstones, and peloidal grainstones. The Qasr reef complex probably developed in generally low-energy environments between normal and storm wave base, in turbid to clear water. Early Devonian increase in microbial reefs, such as Qasr, challenges the view that post-Cambrian stromatolites were common only in normal marine subtidal settings in the wake of metazoan mass extinctions.