NOYON-UUL SYNCLINE, SOUTHERN MONGOLIA - LOWER MESOZOIC SEDIMENTARY RECORD OF THE TECTONIC AMALGAMATION OF CENTRAL-ASIA

To explore the hypothesis that a regionally extensive early Mesozoic c ollisional foreland basin existed on strike to the east of documented Mesozoic foreland basins in western China, we conducted a reconnaissan ce study of a >5 km section of Upper Permian-Lower Jurassic(?) nonmari ne strata exposed in the Noyon Uul syncline in southern Mongolia. This paper documents the depositional settings of these poorly known strat a, their provenance, and their regional tectonic significance in the c ontext of the Mesozoic amalgamation of central Asia. We interpret Noyo n Uul strata to be composed of (1) a coarse-grained, braided fluvial f acies, (2) a meandering fluvial facies, and (3) a poorly oxygenated la custrine facies that represent three principal depositional environmen ts. Sandstone compositions are dominantly volcanic (mean compositions = Qm(20)F(23)Lt(57); Qp(13)Lv(82)Ls+Lm(5)), likely reflecting derivati on from Carboniferous and older volcanic-are sequences. Increased prop ortions of alkalic granite pebbles and cobbles above the first coarse- grained, braided fluvial unit and the first stratigraphic appearance o f 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. Overl ap of the Noyon Uul syncline by relatively undeformed Upper Jurassic a nd Lower Cretaceous strata, together with the presence of a regional u nconformity between Middle and Upper Jurassic rocks, requires a phase of Middle-Late Jurassic deformation. The erosional remnants of a Middl e-Upper Jurassic north-verging thrust belt have been documented just 1 00 km south of Noyon Uul, suggesting that the Noyon Uul deposits may b e foreland in origin. Specifically Triassic thrusting has not been rec ognized in southern Mongolia, but Permian-Jurassic contractile deforma tion associated with the collisional amalgamation of Asia is documente d or reasonably inferred for extensive areas of China to the south and west. Although sedimentary attributes of the Noyon strata are most co mpatible 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 comp atible with wide-spread Late Jurassic-Early Cretaceous rifting that oc curred across southeastern Mongolia.