THE CHIEMGAU CRATER STREWN FIELD: EVIDENCE OF A HOLOCENE LARGE IMPACT EVENT IN SOUTHEAST BAVARIA, GERMANY

 Kord Ernstson

Faculty of Geosciences, University of Würzburg, Germany. E-mail: kernstson@ernstson.de

 Michael A. Rappenglück

Institute for Interdisciplinary Studies, Gilching, Germany. E-mail: mr@infis.org

The Chiemgau strewn field in the Alpine Foreland was discovered in the early new millennium and comprises more than 80 craters in a roughly elliptically shaped area with axes  of about 60 km and 30 km. The crater diameters range between a few meters and a few hundred meters. Unless leveled by, e.g., farming activities, the craters exhibit more or less pronounced rim walls, and the diameter-to-depth ratio has been determined to be 7.5 on average. Geologically, the craters occur in Pleistocene moraine and fluvio-glacial sediments (gravel plains). The craters and surrounding areas so far investigated in more detail are featuring heavy deformations of the Quaternary cobbles and boulders, abundant fused rock material (impact melt rocks and various glasses), shock-metamorphic effects, and geophysical anomalies [1, 2, 3]. The hitherto established largest crater of the strewn field is Lake Tüttensee exhibiting an 8 m-height rim wall, a rim-to-rim diameter of about 500 m and a depth of roughly 30 m. Some 50 excavation pits around Lake Tüttensee have revealed a layer that reflects all aspects of a catastrophic event. The up to 1 m thick layer basically is a polymictic breccia of heavily shattered cobbles and boulders of Alpine lithologies mixed with largely undeformed however abundantly scratched and polished cobbles. The layer is rich in organic material like charcoal and splinters of wood. Animal bones and teeth, tufts of (possibly human) hair, and archeological Stone and Bronze Age artifacts like pottery shards and stone implements are intermixed. Abundant deeply corroded skeletal-like carbonate and silicate clasts contribute to the breccia. The corrosion is suggested to have originated from the action of high temperatures (decarbonization, melting) or/and the action of strong acid dissolution (nitric acid precipitation from the impact explosion cloud). Shock metamorphism in breccia clasts is indicated by PDFs in quartz and feldspar, diaplectic glass in feldspar, strong kinkbanding in mica, and intense microtwinning in calcite. The stratigraphic context of the breccia deposit, radiocarbon dating and the dating by Stone and Bronze Age artifacts exclude any relation with Alpine glaciations. Since any Holocene geological endogenetic processes can be eliminated, too, a meteorite impact to have formed Lake Tüttensee, the surrounding breccia layer as its ejecta blanket, and the whole crater field is most reasonable. Additional thermoluminescence [4] and radiocarbon dating [5] in the field of the other craters, and further dating by archeological finds confine the impact event to have happened in the first millennium B.C., most probably between 600 and 400 B.C [6]. The impact is substantiated by the abundant occurrence of metallic, glass and carbon spherules, a kind of accretionary lapilli and of strange matter in the form of iron silicides like gupeiite and xifengite, and various carbides. A gupeiite analysis from the Chiemgau strewn field closely resembles meteoritic suessite. Nanodiamonds and fullerene-like material have been reported to occur in the melt crust of cobbles from the strewn field [2]. The find context for the exotic matter is in most cases hardly compatible with an industrial production, and an origin from the impact process or/and as constituents of the impactor must be considered. The impactor is suggested to have been a low-density disintegrated, loosely bound asteroid or a disintegrated comet in order to account for the extensive strewn field.

References. - [1] Hoffmann, V. et al. (2004) Geophysical Research Abstracts, 6, 05041 [2] Hoffmann, V. et al. (2005) Abstract Int. Met. Soc. Conf., Gatlinburg [3] Ernstson, K. (2006) www.chiemgau-impact.com/gravimetrie/html [4] B. Raeymaekers, pers. com. [5] Fehr, K.T. (2005) Meteoritics Planet. Sci., 40, 187-194 [6] Rappenglück, B. & Rappenglück, M.A. (2006) Mediterranean Archaeology and Archaeometry, Vol. 6, No. 3, 101-109.