Umm al Binni lake, a possible Holocene impact structure in the marshes of southern Iraq: Geological evidence for its age, and implications for Bronze-age Mesopotamia.
by
Sharad Master
Impact Cratering Research Group, Economic Geology Research Institute, School of Geosciences, University of the Witwatersrand, South Africa

A near circular, vaguely polygonal, ~3.4 km-diameter postulated impact structure [1] in the Al ‘Amarah marshes (at 47°4’44.4” E, 31°8’58.2” N), in southern Iraq, has now been identified on a map [2] as the Umm al Binni lake. After the partial draining of the marshes in 1993 [3-6], the lake has shrunk, and in recent Landsat TM and SPOT satellite imagery [7, 8], it appears as a light coloured area, due to surface salt encrustations.

Geological setting and age: The alluvial plains of Iraq occupy a structural trough which is related to active orogenic processes in the Zagros mountains of Iran [9]. The bedrock in the region close to the Tigris-Euphrates confluence consists of marine clastics of the Dibdibba Formation (Miocene-Pleistocene) [9, 10]. The overlying Holocene marine sediments of the Hammar Formation contain a Recent fauna consisting of gastropods, lamellibranchs, scaphopods, bryozoa, crab and echinoid fragments [11]. The Hammar Formation in turn is overlain by Recent delta plain and delta front deposits of the Mesopotamian Plains, in which there are numerous marshes and permanent lakes [9, 12]. It is estimated that the Recent sediments of the Tigris-Euphrates plains were deposited in the last 5000 years, during which 130-150 km of seaward progradation has taken place [9].

Because of the extremely young nature of the sediments in the marshlands of the Tigris-Euphrates confluence area (<5000 years), it is difficult to find a geological explanation for the strikingly circular shape of the Umm al Binni structure, which differs markedly from the highly irregular shapes of the surrounding marsh lakes. The postulate that the structure was formed by a Recent bolide impact can account for the simple bowl-shaped geometry with slightly polygonal outline, and the apparent rim and annulus around the structure in pre-1993 imagery [1].

Speculations and implications for Bronze-Age Mesopotamia, if Umm al Binni is of impact origin: The impact, with the energy of hundreds of Hiroshima-sized nuclear bombs, would have had a devastating effect on the regional environment. Since there are no accounts in the writings of Herodotus [13] and Nearchus [14] or later historians, the event must have taken place in the Bronze Age at the dawn of recorded history (between ~3000 and ~1000 BC), and may have inspired the flood legends of Ziusudra and Utnapishtim [15]. If the postulated impact site was under water, the water column would have absorbed some of the energy, resulting in a smaller crater than if the impact had been on dry land [16]. Hence estimates of the bolide diameter (~150 m), based on the crater diameter (~3.4 km), are only a minimum, and the bolide could have been larger and more energetic. A wet impact would have generated huge tsunamis, which would have lashed all the port cities of Mesopotamia, such as Ur, Uruk, Shurrupak, etc, within a radius of a few hundred km. The 2.6 m-thick “Flood” layer at Ur [17] could be a tsunamite, and the ~2350 BC “ash” layer found at Tell Leilan (Syria) [18] and in sea-sediment core off Oman [19] could be a fallout layer [20] from a dust cloud generated by the impact. Umm al Binni may possibly be linked with other young postulated impact structures in western Iraq (Al Umchaimin [21]), Estonia [22] and Argentina [23]; and with the iron meteorite from a post-“Flood” burial site at Ur [24].

Only a thorough field investigation coupled with geophysical studies and dating, together with macroscopic and microscopic evidence for shock metamorphism, will be able to prove an impact origin for the Umm al Binni structure. Until then the implications raised in this discussion will remain only as speculations.

References: [1] Master, S. (2001). A possible Holocene impact structure in the Al ‘Amarah Marshes, near the Tigris-Euphrates confluence, southern Iraq. Meteoritics and Planetary Science, 36(9), Suppl., p. A124. [2] Thesiger, W. (1964). The Marsh Arabs. Longmans, Green. Reprinted, Penguin Books, Harmondsworth, 1967, 233 pp. [3] North, A. (1993a). New evidence shows marshlands draining away. The Middle East, London, No. 227, Oct. 1993, 22-23. [4] North, A. (1993b). Saddam’s water war. Geographical Magazine, July 1993, 10-14. [5] Pearce, F. (1993). Draining Life from Iraq’s marshes. New Scientist, No. 1869, 11-12. [6] Pearce, F. (2001). Iraqi wetlands face total destruction. New Scientist, No. 2291, 4-5. [7] Munro, D. C. & Touron, H. (1997). The estimation of marshland degradation in southern Iraq using multitemporal Landsat TM images. Int. J. of Remote Sensing, 18(7), 1597-1606. [8] Partow, H. (2001). The Mesopotamian Marshlands: Demise of an Ecosystem. Early Warning and Assessment Technical Report, UNEP/DEWA/TR.01.2.Rev.1. Division of Early Warning and Assessment, United Nations Environment Programme (UNEP), Nairobi, Kenya. www.grid.unep.ch/activities/sustainable/tigris/marshland. Also: http:// gridz.cr.usgs.gov/publications/meso.pdf. [9] Larsen, C. E. and Evans, G. (1978). The Holocene geological history of the Tigris-Euphrates-Karun delta. In: Brice, W. C. (Ed.), The Environmental History of the Near and Middle East Since the Last Ice Age. Academic Press, London, 227-244. [10] Macfayden, W. A. (1938). Water supplies in Iraq. Iraq Geol. Dept., Publ. No. 1, Baghdad, 206 pp. [11] Hudson, R. G. S., Eames, F. E. & Wilkins, G. L. (1957). The fauna of some Recent marine deposits near Basra, Iraq. Geol. Mag., 94(5), 393-401. [12] Lees, G. M. and Falcon, N. L. (1952). The geographical history of the Mesopotamian Plains. Geogr. J., 118, 24-39. [13] Herodotus (1972). The Histories. Revised, with an Introduction by A. R. Burn. Penguin Books, Harmondsworth, 653 pp. [14] De Morgan, J. (1900). Délégation en Perse, Mémoires, Tome I, Paris, 4-48. [15] Sandars, N. K. (1960). The Epic of Gilgamesh. Penguin Books, Harmondsworth, 128 pp. [16] Örmo, J., Shuvalov, V. & Lindström, M. (2001). A model for target water depth estimation at marine impact craters. Meteoritics and Planetary Science, 36(9), Suppl., p. A154. [17] Woolley, C.L. (1954). Excavations at Ur: A record of Twelve Years’ Work. Ernest Benn, London, 262 pp. [18] Weiss, H., Courty, M.-A., Wetterstrom, W., Guichard, F., Senior, L., Meadow, R. & Curnow, A. (1993). The genesis and collapse of Third Millenium North Mesopotamian civilization. Science, 261, 995-1004. [19] Kerr, R.A. (1998). Sea-floor dust shows drought felled Akkadian Empire. Science, 279, 325-326. [20] Courty, M.-A. (1998) Causes and effects of the 2350 BC Middle East anomaly evidenced by micro-debris fallout, surface combustion and soil explosion. In: Peiser, B.J., Palmer, T. & Bailey, M.E. (Eds.), Natural Catastrophes During Bronze Age Civilisations: Archaeological, Geological, Astronomical and Cultural Perspectives. British Archaeological Reports -S728, Archaeopress, Oxford, 252 pp. [21] Merriam, R. and Holwerda, J. G. (1957). Al Umchaimin, a crater of possible meteoritic origin in western Iraq. Geogr. J., 123, 231-233. [22] Raukas, A., Tiirmaa, R., Kaup, E. & Kimmel, K. (2001). The age of the Ilumetsa meteorite craters in southeast Estonia. Meteoritics and Planetary Science, Vol. 36(11), 1507-1514. [23] Schultz, P.H. & Lianza, R.E. (1992). Recent grazing impacts on the Earth recorded in the Rio Cuarto crater field, Argentina Nature, 355, 234-237. [24] Graham, A.L., Bevan, A.W.R. & Hutchison, R. (1985). Catalogue of Meteorites. Fourth Edition (Revised and Enlarged). British Museum (Natural History), p. 357.

Date received: February 26, 2002