Late Jurassic-Early Cretaceous Synrift Sedimentation in the Tsagaan Suvarga Basin, Gobi-Altai Region of SW Mongolia
Surface geological studies integrated with new seismic reflection data from southwestern Mongolia reveal a pattern of Late Jurassic-Early Cretaceous synextensional basin evolution prior to transpression, inversion, and uplift of isolated ranges during the Cenozoic India-Asia collision. The Tsagaan Suvarga basin contains one of the best-exposed, most complete successions of Mesozoic synrift sedimentation, and serves as a potential analogue for other synrift subbasins that evolved into larger integrated basin systems during protracted extension. New sedimentologic and provenance investigations conducted as a part of a regional exploration program incorporate analyses of depositional systems, detrital zircon U-Pb geochronology, and sandstone compositions. Depositional systems of the ~2500 m-thick Tsagaan Suvarga succession include: (a) restricted coarse-grained talus cone, alluvial fan, and fan-delta systems indicative of syndepositional slip on the west-dipping fault system bounding the eastern basin margin; (b) lake-margin deltaic systems containing viable sandstone reservoir facies; (c) basin-center (profundal) lacustrine deposits with abundant oil-shale source-rock facies; and (d) principally basaltic volcanic rocks and associated mass-flow assemblages (up to 200 m thick) recording local slope instability. U-Pb ages of detrital zircon grains show a relatively restricted range of age signatures, commonly revealing unimodal populations with age peaks focused at ~250 and ~300 Ma, consistent with derivation from Carboniferous-Triassic magmatic-arc rocks. An absence of syndepositional zircons indicates either limited contributions or zircon-poor compositions for known magmatic rocks of Late Jurassic-Early Cretaceous age. Although representative sandstones for the broader region show arkosic to feldspathic litharenitic modal compositions, mature quartzose sandstones are important locally. Regionally similar provenance patterns and stratigraphic units, particularly the lacustrine oil-shale mudrock facies, characterize other Mesozoic synrift basins of northern China and Mongolia, suggesting a similar mode of basin evolution and/or regional integration of formerly isolated basins during Late Jurassic-Early Cretaceous extension.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California