Mesozoic Sedimentology, Stratigraphy, and Provenance of Western Mongolia as a Record of the Tectonic Amalgamation of Central Asia

Derek J. Sjostrom

Sedimentary deposits, paleocurrent analysis, and provenance studies from Mesozoic nonmarine basins of northwest China suggest that the area underwent deformation and unroofing in response to the Mesozoic tectonic amalgamation of central and southern Asia. Due to a lack of published English language investigations on the synchronous development of Mesozoic systems in adjacent western Mongolia, the extent to which Mongolian basins responded to Mesozoic tectonics remains poorly understood. Preliminary conclusions based on two years of field work suggest that the record of deformation preserved in Mongolian Mesozoic basins is similar to that of northwest Chinese basins.

Mesozoic basin fill at three locations in northwestern Mongolia consists of a series of Lower to Middle Jurassic alluvial deposits which fine upward from locally coarse matrix supported fanglomerates to fine sands, shales, and coals indicative of a meandering fluvial environment. Preliminary petrographic analysis of Lower and Middle Jurassic sandstones indicates a general increase upsection in percentages of potassium feldspar and plutonic quartz related to plagioclase, lithic volcanic grains, and volcanic quartz. Paleocurrent indicator directions range from southeast to southwest, suggesting Jurassic unroofing of an east-west trending Paleozoic volcanic sequence which lies unconformably below the Mesozoic sediments.

The complete Lower to Middle Jurassic section exposed at the Dariv location (N46° 20^prime; E94° degrees 60^prime) contains strata similar to that described above and is overlain by Middle to Upper Jurassic braided fluvial siltstones and sandstones, Upper Jurassic to Lower Cretaceous conglomerate with abundant extrusive volcanic clasts, and Lower Cretaceous lacustrine mudstones and siltstones. Paleocurrent analysis indicates northerly flow in the Lower Cretaceous conglomerates which, along with the depositional style, suggests uplift of the Altai Shan range to the southwest. The section exposed at Dariv is similar to time-equivalent strata of the Junggar Basin, and therefore may be a more distal record of the deformation associated with the Late Jurassic collision of the Lha a block with the southern continental margin of Asia.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California