Noyon Uul syncline, southern Mongolia: Lower Mesozoic sedimentary record of the tectonic amalgamation of central Asia

Abstract

To explore the hypothesis that a regionally extensive early Mesozoic collisional foreland basin existed on strike to the east of documented Mesozoic foreland basins in western China, we conducted a reconnaissance study of a >5 km section of Upper Permian-Lower Jurassic(?) nonmarine strata exposed in the Noyon Uul syncline in southern Mongolia. This paper documents the depositional settings of these poorly known strata, their provenance, and their regional tectonic significance in the context of the Mesozoic amalgamation of central Asia. We interpret Noyon Uul strata to be composed of (1) a coarse-grained, braided fluvial facies, (2) a meandering fluvial facies, and (3) a poorly oxygenated lacustrine facies that represent three principal depositional environments. Sandstone compositions are dominantly volcanic (mean compositions = Qm 20 F 23 Lt 57 ; Qp 13 Lv 82 Ls+Lm 5 ), likely reflecting derivation from Carboniferous and older volcanic-arc sequences. Increased proportions of alkalic granite pebbles and cobbles above the first coarse-grained, braided fluvial unit and the first stratigraphic appearance of detrital K-feldspar in basal coarse-grained, braided fluvial strata suggest unroofing of Permian alkalic granites. The Noyon Uul deposits likely are syntectonic, as suggested by their thickness, environments of deposition, coarse-grained character, and accumulation rates. Overlap of the Noyon Uul syncline by relatively undeformed Upper Jurassic and Lower Cretaceous strata, together with the presence of a regional unconformity between Middle and Upper Jurassic rocks, requires a phase of Middle-Late Jurassic deformation. The erosional remnants of a Middle-Upper Jurassic north-verging thrust belt have been documented just 100 km south of Noyon Uul, suggesting that the Noyon Uul deposits may be foreland in origin. Specifically Triassic thrusting has not been recognized in southern Mongolia, but Permian-Jurassic contractile deformation associated with the collisional amalgamation of Asia is documented or reasonably inferred for extensive areas of China to the south and west. Although sedimentary attributes of the Noyon strata are most compatible with a foreland interpretation, it is possible that the Noyon deposits and their deformation may be related to strike-slip faulting. Neither the attributes of the Noyon deposits nor their ages are compatible with widespread Late Jurassic-Early Cretaceous rifting that occurred across southeastern Mongolia.