NEOGENE DIBDIBBA GRAVELS AND AN EARLY PLEISTOCENE AGE FOR THE ORIGIN OF WADI AL BATIN IN NORTHERN SAUDI ARABIA

Recent geologic mapping in northern Saudi Arabia provides the first evidence for a late Tertiary age of the classic Dibdibba gravels, and a new age and origin for Wadi Al Batin, a stream long-associated with these gravels. Since 1960 investigators have described the Dibdibba gravels as a mega alluvial fan of clasts shed off the Arabian Shield in response to Tertiary uplift and eastward tilt of the Arabian block and to Pleistocene climate fluctuations. The mid-Miocene Ajfar formation, dated by fossils, was found beneath the Dibdibba gravels, and abundant basalt clasts occur in the upper deposit that are not present in the lower deposit. Basalt clasts originated from Harrat Khaybar, K-Ar-dated at 5 Ma to 3 Ma, located in the upper drainage of Wadi Ar Rimah. We conclude that the Dibdibba gravels, capped by duricrust, accumulated during the late Miocene and Pliocene.

Wadi al Batin, a linear, ~250–km long valley that continues into Iraq and Kuwait, is incised >60m into the extensive Umm er Radhuma limestone plain that underlies the Ajfar formation. The incised valley must postdate Dibdibba gravel benches and the sinuous ridges of Dibdibba gravel in Kuwait. We propose that Wadi Al Batin originated by faulting along the Wadi Al Batin lineament in late Pliocene to early Pleistocene time after deposition of the Dibdibba gravels ceased, and the karst limestone plain was exposed by erosion. This Plio-Pleistocene faulting appears to be concurrent with that on the Central Arabian Graben System. Wadi Al Batin has been isolated from its headwaters since the expansion of the sand deserts in Saudi Arabia in Middle Pleistocene time. No apparent hydrologic source exists for a river to have eroded a deep, narrow valley into a near-flat limestone plain during the Pleistocene. Abundant karst landforms adjacent to the valley suggest that local runoff mainly infiltrates into the extensive limestone plain and is unavailable to support a large stream in Wadi Al Batin.