Depositional history, tectonics, and detrital zircon geochronology of Ordovician and Devonian strata in southwestern Mongolia
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 carbonate 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 associated with storm events. The interpreted link between flood deposition and storm wave reworking supports a hyperpycnal flow interpretation for these deposits. The sudden facies transition at the base of the formation represents the sedimentological and stratigraphic signature of Early Devonian 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 component 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 Pragian stages of the Early Devonian. We herein introduce the term “Tsakhir event” for this important tectonic transition. Range-bounding 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 Ordovician and Devonian strata contain Archean to Paleozoic ages. Minor differences between Ordovician and Devonian samples suggest changes in source regions and/or transport 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 proximal 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 spectra suggest a nearby continental source. Our detrital age spectra contain peaks that coincide with basement ages and magmatic events on the adjacent Mongolian microcontinent and also have strong similarities with recently published spectra of nearby landmasses in Neoproterozoic to Paleozoic paleogeographic reconstructions, namely, Siberia, North China, eastern Gondwana, and Tarim. These similarities extend to spectra of late Neoproterozoic to middle Paleozoic rocks throughout Gondwanaland and also Siberia, illustrating the somewhat limited utility of detrital spectra for determining the tectonic affinities of crustal blocks at this time in Earth history.