Sedimentology and provenance of Mesozoic nonmarine strata in western Mongolia: A record of intracontinental deformation

Western Mongolia is a structurally complicated and little-studied portion of the central Asia tectonic collage, yet contains well-exposed sequences of Mesozoic sedimentary strata that preserve an important record of ancient intraplate deformation. In order to document the record of Mesozoic sedimentary basin development and to provide a basis for interpreting the sedimentary record of intraplate deformation, we studied the sedimentology and provenance of Mesozoic strata at four locations in western Mongolia. Triassic sedimentary strata are missing across the study area, and Lower Jurassic strata unconformably overlie older rocks of various ages. Lower to Middle Jurassic basin fill in the northern portion of the study area is dominated by coarse, granite-bearing conglomerate and arkosic sandstone (Qm₃₅F ₄₉L₁₆) with a volcanic-dominated lithic fraction (Qp ₂₂Lvm₆₃Lsm₁₅). Paleocurrent indicators suggest southwest-directed sediment transport, reflecting uplift of an ancestral version of the Hanhöhiy Uul, a mountain range located north and east of the study area and composed mainly of Cambrian granite and associated volcanics. In the central portion of the study area, Lower to Middle Jurassic strata comprise an upward-fining kilometer-thick sequence from coarse matrix-supported fanglomerate to fine sandstone, shale, and coal suggestive of a mudload-dominated, meandering fluvial environment. Conglomerate clasts are dominantly basalt and granite, and sandstone is arkosic with higher QmFL%L and Qp-Lvm-Lsm%Lsm than equivalent sandstone to the north (Qm₄₁F₃₅L₂₄; Qp₂₄Lvm₄₁Lsm₃₅). Paleocurrent indicators suggest transport to the south-southwest. In the southernmost part of the study area, Lower through Upper Jurassic strata record a transition from mudload-dominated fluvial environments with abundant organic matter to fluvial flood-plain and/or mud flat with little organic matter and abundant calcisols. These strata are sharply overlain by Upper Jurassic-Lower Cretaceous angular polymictic conglomerate that in turn is overlain by Lower Cretaceous lacustrine mudstone and siltstone. Mesozoic sandstone at the southernmost field location is lithic rich and contains higherQpLvmLsm%Lsm than elsewhere in the study area (Qm₃₁F₃₃L₃₆; Qp₁₀Lvm ₅₀Lsm₄₀). Paleocurrent analysis suggests a fundamental reversal from south-directed paleoflow prior to deposition of coarse conglomerate to north-directed paleoflow during and after deposition of conglomerate. We interpret this reversal, the deposition of coarse conglomerate, and the lithic-rich sandstone compositions to reflect latest Jurassic uplift of the paleo-Altai Mountains to the south. Sm-Nd isotopic analysis performed on sandstone samples from the central part of the study area (Jargalant locality) yield depleted mantle derivation ages of ca. 1 Ga and ε_Nd values of -3.8 to -4.8, suggesting significant contributions from old continental crust. Sm-Nd isotopic analysis on local basement rocks, as well as granite and basalt cobbles within Jurassic strata, yield depleted mantle derivation ages and an isochron age of ca. 600 Ma, with initial ε_Nd values similar to that expected of depleted mantle at that time, suggesting an origin from juvenile crust. We interpret these results to indicate that a significant fraction of sand-sized detritus was derived from Archean granitic basement forming the southern margin of the Siberian craton, ∼50 km to the north. We favor a compressional tectonic setting for Mesozoic sedimentary basins of western Mongolia, although a strike-slip setting for these basins is also possible. This interpretation is based on several lines of evidence: (1) Mesozoic volcanics that might be expected in an extensional setting are lacking from the study area; (2) Jurassic coal contains low vitrinite reflectance values suggestive of a low heat flow more typical of a flexural basin setting than an extensional setting; and (3) adjacent sedimentary basins in western China contain well-documented evidence of Mesozoic contractile deformation and bear many of the same sedimentologic attributes as basins in western Mongolia.