Evolution of the Mesozoic Qamdo (Changdu) Basin, Eastern Tibet: Linkages between Sedimentation, Climate, and Regional Tectonics

Shang, Fei; Weislogel, Amy; Sun, Gaoyuan

The Qamdo basin, located in eastern Tibet, evolved from a passive margin basin developed on the southern margin of Qiangtang terrane into a retroarc foreland basin associated with crustal thickening caused by accretion of terranes and volcanic arcs during Mesozoic time. Sedimentological studies reveal that from Late Triassic to Early Cretaceous time, depositional systems shifted from, deep-marine/marginal-marine, to lacustrine, then to alluvial fan and braided river systems, indicating a continuous shallowing-upward trend over about 100 million years. Considering that Mesozoic global sea-level records show a continued sea-level rise since Late Triassic time, the deposystem change from marine to terrestrial systems of the Qamdo basin may well reflect Mesozoic regional tectonics.

The lower Upper Triassic Jiapila Formation comprises marine black shale with turbidite sandstone interbeds deposited in a clastic deep-marine setting. Vitrinite reflectance (%Ro) analysis of the Jiapila shale by other workers shows an average Ro value of 2.17%, indicating a maximum burial depth of 8000m based on geohistory modeling. The overlying bioclastic middle Upper Triassic Bolila Formation formed as carbonate platform deposits. The carbonate is characterized by well-developed secondary porosity system with an average porosity of 25%. The uppermost Triassic unit is the Duolagai Formation, comprises siliciclastic sediments deposited in a deltaic setting.

Overlying the Triassic marine-influenced deposits are Jurassic lacustrine deposits consisting of interbedded fine-grained sandstone, siltstone, red mudstone, and calcisols. The sandstone/mudstone cyclothems in Jurassic strata occurs at all scales from centimeter to regional-scale cycles of basin filling, which may reflect both regional climatic variations and tectonic-driven signals. Mud-crack structures are abundant and tend to increase upward in stratigraphic sections due to increased aridity, which may have resulted from regional surface uplift associated with tectonism.

The Lower Cretaceous Xiangdui Formation comprises abundant matrix-supported conglomerates, cross-stratified pebbly sandstone, red mudstone, and abundant calcisols, which suggest the deposition occurred proximal to the source areas under arid climatic settings, which may have also resulted from surface uplift and denudation of nascent mountain ranges related to accretion and convergent margin tectonism during the Early Cretaceous period.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013