The UMM Al Binni structure in the Mesopotamian marshlands of Southern Iraq, as a postulated late holocene meteorite impact crater : geological setting and new LANDSAT ETM + and Aster satellite imagery

A c. 3.4 km diameter circular structure, discovered in southern Iraq on published satellite imagery by Master (2001), was interpreted to be a possible meteorite impact crater, based on its morphology (its approximately polygonal outline, an apparent raised rim, and a surrounding annulus), which differed greatly from the highly irregular outlines of surrounding lakes. The structure, which is situated in the Al ’Amarah marshes, near the confluence of the Tigris and the Euphrates Rivers (at 47°4’44.4” E, 31°8’58.2” N), was identified by Master (2002) as the Umm al Binni lake, based on a detailed map of the marshes published by Thesiger (1964). After the almost complete draining of the marshes since 1993, the lake has disappeared, and in recent Landsat TM, SPOT and ASTER satellite imagery, it appears as a light coloured area, due to surface salt encrustations. The alluvial plains of Iraq occupy a structural trough which is linked to active subductionrelated orogenic processes in the Zagros mountains of Iran. The bedrock in the region close to the Tigris-Euphrates confluence consists of marine clastics of the Dibdibba Formation (Miocene-Pleistocene). The overlying Holocene marine sediments of the Hammar Formation contain a Recent fauna consisting of gastropods, lamellibranchs, scaphopods, bryozoa, crab and echinoid fragments. The Hammar Formation, in turn, is overlain by Recent delta-plain and delta-front deposits of the Mesopotamian Plains, in which there were numerous marshes and permanent lakes until the recent destruction of the marshlands. 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. Because of the extremely young nature of the sediments in the marshlands of the TigrisEuphrates confluence area (<5000 years), it is difficult to find a geological explanation for the shape of the Umm al Binni structure. The postulate that the structure was formed by a Recent bolide impact can account for the simple bowl-shaped geometry with markedly polygonal outline, and the apparent rim and annulus around the structure in pre-1993 imagery. Master (2001, 2002) speculated on the possible consequences of this structure, if it was indeed of impact origin, for Bronze-Age Mesopotamia, and suggested that it might possibly be linked with an ~2350 BCE “ash” layer found at Tell Leilan (Syria) and in seasediment core off Oman, re-interpreted by Marie-Agnes Courty (1998) to be an impact fallout layer. Master (2001, 2002) also suggested that an impact-generated tsunami could have been responsible for the Babylonian and Sumerian “flood” legends of Atra-Hasis, Utnapishtim and Ziusudra, as recounted in the appendix to the Epic of Gilgamesh, and other accounts. Recent Landsat TM and high-resolution ASTER satellite imagery over the Al ’Amarah marshes shows a marked reduction in the amount of marshland vegetation in imagery from 2000, compared with imagery over the same area in 1976. The Umm al Binni lake now consists of a dry lake bed encrusted with white salt deposits. The high resolution ASTER imagery clearly shows a strikingly polygonal outline of the lake, which is in strong contrast to the highly irregular outlines of most of the other former marshland lakes within the region. The new images of the dry lake show a highly asymmetrical aspect to the lake: the southern half has smooth straight edges to the polygon sides, whereas in the northern half these edges are more irregular. The southeastern part of the crater is surrounded by a series of scalloped concentric zones, which are similar in appearance to ejecta blankets from young terrestrial and non-terrestrial impact structures. However, ejecta-type material is totally absent from the northern half of the Umm al Binni structure. If the feature is of impact origin, then it should have a symmetrial ejecta blanket surrounding it on all sides, unless it was the result of a very low-angle oblique impact, or if part of the ejecta blanket was eroded away. There is at least one example of a terrestrial impact structure (the Tsenkher structure in Mongolia) with only a partially preserved ejecta blanket, due to the removal by erosion of the rest of the ejecta around it. The high-resolution imagery shows the presence, in an area that was marshland just a decade ago, of a possible village or settlement about 4 km ENE of the Umm al Binni structure, from which paths radiate in all directions, possibly caused by domestic animal tracks. A road leads to this settlement from the northeast. This shows that the area is currently accessible overland. It is imperative that the structure now be studied on the ground in order to determine its origin, and that this be done before the proposed re-flooding of the marshes again makes the area inaccessible. However, all past and current attempts to study the structure on the ground have been frustrated by the extremely dangerous political and military situation that has prevailed in Iraq in the past 3 years. _______________oOo_______________ THE UMM AL BINNI STRUCTURE, IN THE MESOPOTAMIAN MARSHLANDS OF SOUTHERN IRAQ, AS A POSTULATED LATE HOLOCENE METEORITE IMPACT CRATER: GEOLOGICAL SETTING AND NEW LANDSAT ETM+ AND ASTER SATELLITE IMAGERY