Depositional history, tectonics, and detrital zircon geochronology of Ordovician and Devonian strata in southwestern Mongolia T.M. Gibson 1 , P.M. Myrow 1,† , F.A. Macdonald 2 , C. Minjin 3 , and G.E. Gehrels 4 1 Department of Geology, Colorado College, 14 E. Cache La Poudre Street, Colorado Springs, Colorado 80903, USA 2 Department of Earth and Planetary Sciences, Harvard University, Oxford Street, Cambridge, Massachusetts 02138, USA 3 Research Center for Stratigraphy and Paleontology, School of Geology and Petroleum Engineering, Mongolian University of Science and Technology, Ulaanbaatar-46, Mongolia 4 Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA ABSTRACT Lower Paleozoic successions of the Gobi- Altai zone of southern Mongolia record an abrupt facies transition from deposition of predominantly fine-grained uppermost Ordovician through lowermost Devonian carbonate and marl facies to deposition of coarse clastic strata of the Lower Devonian Tsakhir Formation. The Tsakhir generally fines upward from alluvial-fan cobble and pebble conglomerate to interbedded coarse- and fine-grained marine siliciclastic and car- bonate strata, which were deposited within a tectonically active basin. The marine strata, deposited in a storm-influenced proximal to distal fan delta, include unusual event beds that grade from pebble conglomerate to hummocky cross-stratified grainstone and sandstone. These beds represent sediment emplaced by gravity flows during flood events and reworked by large gravity waves associ- ated with storm events. The interpreted link between flood deposition and storm wave re- working supports a hyperpycnal flow inter- pretation for these deposits. The sudden facies transition at the base of the formation represents the sedimentological and stratigraphic signature of Early Devo- nian tectonism in the Gobi-Altai zone. The general upward-fining pattern of the Tsakhir is interpreted as a response to the creation of accommodation space at a greater rate than progradation of the fan delta, in large part due to tectonic subsidence, although some compo- nent of eustasy may have been involved. The production of steep relief and deposition of associated volcanics suggest a transition from relatively passive deposition to active tectonics in this region during the Lochkovian to Pra- gian stages of the Early Devonian. We herein introduce the term “Tsakhir event” for this important tectonic transition. Range-bound- ing faults for this event are not preserved, but alluvial-fan deposition, the development of unconformities, renewed subsidence, and magmatism throughout the Gobi-Altai zone all suggest syndepositional tectonism. Detrital zircon spectra from both Ordovi- cian and Devonian strata contain Archean to Paleozoic ages. Minor differences between Ordovician and Devonian samples suggest changes in source regions and/or trans- port paths prior to, and after, the Tsakhir event. The paleoenvironmental setting of the Tsakhir Formation requires short transport distances, and thus the age spectrum of a sample from this formation represents proxi- mal basement rocks of the Shine Jinst region of the Gobi-Altai zone. Basement rocks are not exposed in the Shine Jinst region, but the wide variety of ages in all of the detrital spec- tra suggest a nearby continental source. Our detrital age spectra contain peaks that coincide with basement ages and magmatic events on the adjacent Mongolian microcon- tinent and also have strong similarities with recently published spectra of nearby land- masses in Neoproterozoic to Paleozoic paleo- geographic reconstructions, namely, Siberia, North China, eastern Gondwana, and Tarim. These similarities extend to spectra of late Neo- proterozoic to middle Paleozoic rocks through- out Gondwanaland and also Siberia, illustrat- ing the somewhat limited utility of detrital spectra for determining the tectonic affinities of crustal blocks at this time in Earth history. INTRODUCTION Southern Mongolia is centrally located within the Central Asian orogenic belt, a Neoprotero- zoic to early Mesozoic accretionary zone that juxtaposed microcontinents and island arcs be- tween Baltica, Siberia, Tarim, and North China (Şengör et al., 1993; Zorin et al., 1993; Lamb and Badarch, 1997; Heubeck, 2001; Badarch et al., 2002; Wang et al., 2005; Briggs et al., 2007; Windley et al., 2007; Kelty et al., 2008; Kröner et al., 2010). The Central Asian orogenic belt is the largest area of Phanerozoic crustal growth on Earth (Şengör et al., 1993), but the timing and nature of the orogenic events remain poorly constrained. This is due in part to the paucity of studies on temporally constrained syntectonic strata in southern Mongolia, and also due to post-Paleozoic tectonic complications. Detailed sedimentological data combined with chrono- stratigraphically constrained geochronological data are needed in order to reconstruct the tec- tonic history of this region and decipher regional relationships with adjacent tectonic blocks. In this paper, we provide the first detailed sedimentological and stratigraphic study of the Lower Devonian Tsakhir Formation in the Shine Jinst region of southern Mongolia (Fig. 1). This unit records a stratigraphic tran- sition from underlying Upper Ordovician to lowermost Devonian quiet water carbonate de- posits to coarse conglomerate, sandstone, and siltstone of the Tsakhir, which has been inter- preted to record uplift and erosion of these older carbonate units (Lamb and Badarch, 1997). Our sedimentological analyses, in combination with detrital zircon geochronology data for a suite of chronostratigraphically constrained samples that span this tectonic transition, provide impor- tant insights into the depositional history and the tectonic evolution of the Gobi-Altai zone, and more generally, the Central Asian orogenic belt. GEOLOGICAL SETTING AND STUDY LOCATION Shine Jinst is located in the Gobi-Altai zone of southern Mongolia, directly south of the Main Mongolian Lineament (Fig. 1), which separates For permission to copy, contact editing@geosociety.org © 2013 Geological Society of America 877 GSA Bulletin; May/June 2013; v. 125; no. 5/6; p. 877–893; doi: 10.1130/B30746.1; 12 figures; Data Repository item 2013145. † E-mail: pmyrow@ColoradoCollege.edu