IMPACT ORIGIN OF TABUN KHARA OBO CRATER, MONGOLIA, CONFIRMED BY DRILL CORE STUDIES

Tabun Khara Obo (TKO) is a 1.3 km diameter crater structure centered at 44°17’50”N, 109°39’20”E in the Gobi Desert, SE Mongolia. Morphologically the TKO structure is well outlined. The crater rim rises above the bottom of the depression by 20-30 m. The crater is located on an uplifted Proterozoic intrusive metamorphic rock block emerging from East Gobi Basin. The basement rock lithologies comprise schist, gneiss, and amphibolite. Earlier work suggested that the structure is of its impact origin, but no convincing impact characteristics had been documented. Surface mapping and sampling carried out in 2007 did not reveal structural, mineralogical and geochemical evidence of an impact origin. In 2008, we drilled into the center of the crater and reached the maximum depth of 206 m, with 135 m of core samples recovered. From the top, the following lithologies were intersected or: 0-3 m aeolian sand; 3-140 m lake deposits - mud, evaporates; 140-174 m lake deposits-gypsum with carbonate and mud; 174-185 m polymict breccia: greenschist, gneiss; 185-204 m monomict breccia, brecciated quartz-feldspar-mica schist. The breccias start at 174 m as polymict breccias with angular clasts of different lithologies and gradually change downwards to breccias with a dominating lithology until grading into monomict breccia. At the bottom of the hole, we see strongly in-situ brecciated quartz-feldspar crystalline schist. The breccia cement is also changing from gypsum and carbonate cement to the fine-grained rock matrix. In some cases the cement is recrystallized. Some quartz grains in the breccia (depth intervals 192, 194.2, 196.4, 199.3, 201.6 and 204 m) display planar deformation features with impact-characteristic orientations as measured by the universal stage. The discovery of unambiguous shock features in drill core samples confirms the impact origin of the Tabun Khara Obo crater in Mongolia. The age of the structure is not yet known.