THE ORIGIN AND HISTORY OF CENOZOIC MEGALAKE SUDD, ALONG THE RIVER NILE IN SUDAN AND SOUTH SUDAN

We describe an African paleolake that may be relevant to the topic of this theme session. Between Juba and Khartoum, the River Nile flows through the vast Sudan Basin at the center of which is the great swamp of the Sudd, at about 400 m elevation. Beginning in 1864, evidence accumulated that there was once an enormous lake filling the Sudan Basin. But that hypothesis foundered about 1970, apparently falsified by the absence of paleoshorelines and of any site for a natural dam, and today there is rarely any mention of it. The hypothesis is now being reconsidered after discovery of evidence from diatoms for a lacustrine paleoshore line at about 515 m (El Shafie et al., 2011, J. Afr. Earth Sci.), and a spillway at the same elevation leading toward Paleolake Chad, as well as evidence from Mediterranean sapropels that a lake-draining flood may have occurred at about 3.2 Ma (Álvarez, 2023, JAES).

The present paper asks when the lake first came to be. Initially it looked like this could not be answered because the evidence needed would be buried beneath the sediments of the Pliocene lake. But extensive exploration for hydrocarbons has been carried out in the Sudan Basin, first by Chevron Overseas Petroleum and, more recently by other companies, especially the Chinese, and some information on the results has been published. There have evidently been three extensional episodes — in the Early Cretaceous, in the Late Cretaceous, and in the Paleogene. Each episode began with deep, narrow rift valleys and ended with a broad topographic sag. Although lakes have existed at times during this evolution, there has probably not been a continuous lacustrine history. The history leading to Megalake Sudd in the strict sense appears to have begun in Neogene time, and perhaps in the Paleogene.

The Megalake Sudd hypothesis has implications worth considering. Breaking of a natural dam and diversion of the Nile northward to the Mediterranean would have profoundly changed the drainage of the African Continent. In addition, the presence of a lake 1.5 times the area of the Caspian Sea, today’s largest lake, in the middle of North Africa should have had major climate and geographic consequences during a critical time in the evolution of Australopithecus and early Homo.