TSK 11 Göttingen 2006 Geerdts et al. Evolution of the Tamtsag Basin / NE-Mongolia — part I: basin fill Poster Peter Geerdts1 Marc Vogler1 Buyan Davaa1 Andreas Henk1 Introduction The Tamtsag Basin in NE Mongo- lia is part of a widespread basin sys- tem which formed during Late Juras- sic and Cretaceous times (Graham et al. 2001, Qing-Ren et al. 2003). It is filled with continental sediments and volcanics which can reach up to 4 km in thickness. Rifting and subsequent basin inversion led to a complex basin geometry characterized by several horst and graben structures. The geody- namic causes for regional basin forma- tion are discussed controversially and several hypothesis ranging from oro- genic collaps via subduction rollback to collision-induced rifting have been put forward. Scientific research on the Mesozoic basins in Mongolia has so far concentrated on the East Gobi Basin to the south (Graham et al 2001, Prost 2004, Johnson 2004) and some work has also been published on the Hailar Basin (Qing-Ren et al. 2003), the northeast- ward continuation of the Tamtsag Basin into China. Fundamental data on the fill and tectonics of the Tamtsag Basin in between is still missing. This is partly due to poor exposure as most of the basin fill is covered by Cenozoic sedi- ments and only locally, near the border- ing faults, rocks are accessible for sur- face investigations. However, recent dis- coveries of oil in the Tamtsag and Hailar Basins have resulted in intense explo- ration activity and a strong interest in 1 Geologisches Institut, Universität Freiburg Albertstrasse 23b, D-79104 Freiburg, Germany the area. This contribution describes the results of a field campaign in fall 2005 focusing on the basin fill while a companion paper (Davaa et al. this vol- ume) deals with the basin structure and hydrocarbon potential of the Tamtsag Basin. Basin fill So far little has been published on the stratigraphy of the Tamtsag Basin. A comparative synopsis of the Mesozoic stratigraphy in the adjacent basins is given by Qing-Reng et al. (2003). Basement rocks consist of metamor- phics and intrusives of Permian to De- vonian age. Triassic strata is miss- ing. Basin subsidence commenced in the Lower-Middle Jurassic with sedi- mentation of alluvial/fluvial conglomer- ates, sandstones and intercalating coal- beds. Voluminous Upper Jurassic vol- canics with sedimentary interbeds are known from the Tamtsag Basin and also found in the Hailar and Erlian Basins. Using mainly borehole data, Neves (2000) identified a Lower Creta- ceous, clastic, continental rift-fill with fluvio-deltaic conglomerates and sand- stones. Volcanics occur. A general fin- ing upward-trend leads into deep wa- ter, lacustrine mudstones and shales overlain by Upper Cretaceous fluvial- lacustrine mudstone-sandstone facies. Clastic Cenozoic deposits of conglomer- ates, sandstones, and mudstones cover most parts of the basin, concealing the Mesozoic units. At the western margin the filling of the Tamtsag Basin is accessible at surface and two areas have been mapped in de- tail to get an insight view of the litholo- gies and depositional environments of the basin fill. In one of the areas mapped the pre-rift 1 Geerdts et al. TSK 11 Göttingen 2006 Figure 1: Location of the Tamtsag Basin and adjacent Mesozoic basins (Graham et al. 2001) basement rocks of the Tamtsag Basin are exposed. They consist of low- to high-grade contact-metamorphic schists and turbidites with basaltic interlay- ers of presumably Devonian to Permian age. Contact metamorphism was caused by voluminous granitic intrusions. The syn-rift sequence in the area can be di- vided into a volcano-sedimentary and an effusive volcanic unit. The former consists of large scale, proximal ign- imbrite deposits which are overlain by terrestrial, fluvial and lacustrine sedi- ments (conglomerates, breccias, sand- / silt-/ mudstones), frequently interfin- gering with volcanic rocks (tuffs, ign- imbrites). There is a strong lateral lithologic change in this unit across the mapping area. It is conformably over- lain by lavas of basaltic-basaltic to an- desitic composition, presumably indi- cating a time of enhanced basin ex- tension. K/Ar-dating of this unit is in preparation to provide a radiomet- ric age and exact stratigraphic posi- tion, respectively. The effusive volcanics are concordantly followed by units of terrestrial-sedimentary origin, with con- glomerates, sand-, silt-, and mudstones rich in plant fossils and containing a unit of black shale that could serve as a po- tential source rock. Furthermore, vol- canics (rhyolithe, tuffites) are found in this unit. The second mapping area covers more than 70 square kilometers and is located southeast of the one described above, closer to one of the main basin-bounding faults. Most of the area is covered by Cenozoic sediments. The central part is an uplifted structure providing good outcrops. It comprises no basement rocks, but a thick volcanic sequence of rhyolite, andesite and two basalt gen- erations, possibly of Late Jurassic to Early Cretaceous age. Field observa- tions show that the rhyolitic volcanics are overlain by andesite. Basalts cross- 2 TSK 11 Göttingen 2006 Geerdts et al. Figure 2: Preliminary stratigraphic column of mapping area 1. cut the rhyolite and andesite and, hence, are of younger age. The center of the ridge is formed by a dome-like structure of rhyolitic composition. The magmatic units are overlain by a well rounded, possibly basal conglomerate. The sec- ond, younger basalt overlies the con- glomerate. This indicates two separate stages of volcanic activity. A lens of ma- rine carbonate sediment with crinoidal fossils was found intercalating with the basalt. The stratigraphic position of this unit remains unclear and requires further investigations as so far no ma- rine sediments have been described from the Tamtsag Basin. Acknowledgements Financial sup- port by the Deutsche Forschungsge- meinschaft is gratefully acknowledged. ‘Mineral and Petroleum Authorities of Mongolia’ kindly supported our work in the Tamtsag Basin. References Qing-Ren M et al. (2003) Tectonics of the late Mesozoic wide extensional basin system in the China–Mongolia border region. In: Basin Research 15, 397–415 Graham SA et al. (2001) Sedimentary record and tectonic implications of mesozoic rifting in southeast Mongolia. In: Geological Soci- ety of America Bulletin, 113/12, 1560–1579 Prost GL (2004) Tectonics and hydrocarbon systems of the East Gobi basin, Mongolia. In: The American Association of Petroleum Geologists Bulletin 88(4) 483–513 Johnson CL (2004) Polyphase evolution of the East Gobi basin: Sedimentary and structural records of Mesozoic-Cenozoic intraplate de- formation in Mongolia. In: Basin Research 16, 79–99 Neves R et al. (2000) Mongolia, Tamtsag Basin, evidence for widespread, high qual- ity, mature Lower Cretaceous Source Rock. Abstract, AAPG International Conference & Exhibition. 3