Triassic synorogenic sedimentation in southern Mongolia: Early effects of intracontinental deformation

An important record of the earliest phases of intracontinental deformation in southern Mongolia is contained in widespread Upper Permian and Triassic sedimentary strata that crop out in the region. In this chapter, we provide documentation of stratigraphy, sedimentary style, sediment dispersal trends, and provenance relations from Upper Permian through lowermost Jurassic(?) strata at four separate localities in southern Mongolia: (1) Noyon Uul, a doubly-plunging syncline where we measured four separate sections through Permian, Triassic, and lowermost Jurassic(?) strata; (2) Tost Uul, a second doubly-plunging syncline where we measured one section in Permian, Triassic and lowermost Jurassic(?) strata; (3) Chonin Boom, a structurally disrupted series of Triassic strata located adjacent to the Tost fault; and (4) Toroyt, a well-exposed section of dominantly fine-grained strata in the western part of the study area. Strata of the study area are subdivided into five sedimentary facies on the basis of geometry of coarse clastic units, observed sedimentary structures, and comparison with modern and ancient analogs. These include: (1) gravelly, braided-fluvial facies; (2) alluvial and lake-plain facies; (3) meandering fluvial facies; (4) lake delta and open-lacustrine facies; and (5) profundal lake facies. Sandstone compositions throughout the study area are strongly dominated by lithic volcanic detritus (mean composition Qm₁₈F ₂₆Lt₅₆; Qp₆Lvm₈₈ Lsm₆), reflecting the erosion principally of local Paleozoic arc source terranes. Silicified volcanic and chert duraclast lithologies dominate conglomeratic facies across the study area; there are subordinate clasts of pink alkali granite, syenite, diorite, and vein quartz. Clast counts from the Noyon syncline are internally consistent within each synclinal limb but differ between the two limbs, suggesting that each limb received sediment from a different source terrane. Paleocurrent indicators from the north limb of the Noyon syncline record transport dominantly to the south and west, whereas indicators from the south limb record transport dominantly to the north and west, consistent with the interpretation of a broad depositional trough during Triassic time that funneled sediment westward down its axis. Paleocurrent indicators at Tost Uul, located west of Noyon Uul along strike, are dominantly west-directed, consistent with the interpretation of an axial drainage system. Restoration of 95-125 km of inferred post-depositional left-lateral strike-slip movement on the Tost fault, separating the Chonin Boom and Toroyt localities on the west from the Noyon Uul and Tost Uul localities on the east, brings the Chonin Boom section into subalignment with the Noyon Uul-Tost Uul synclinal pair and places the Toroyt locality farther to the northwest, obliquely down depositional dip. Combining our sedimentary environmental interpretations with this restored paleogeography, we infer that early intracontinental deformation in southern Mongolia produced a broad, east-west (present coordinates) depositional trough that funneled sediment through the Noyon Uul-Tost Uul-Chonin Boom localities to the Toroyt locality to the west. This trough was subsequently flooded in LateTriassic or earliest Jurassic(?) time by a regional lake. This history and style of sedimentation is broadly analogous to coeval Triassic-Jurassic sedimentation along tectonic strike in theTurpan-Hami basin of western China, where axial transport of coarse clastic sediments fed a similar regional lake system. Subsequent continued north-south shortening of southern Mongolia during Middle and Late Jurassic time folded and unroofed strata of the study area, as compression associated with thrusting in the Bei Shan intensified and propagated northward.