Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Local and Regional Sources for Upper Permian-Lowermost Triassic Fluvial-Lacustrine Fills in the Tarlong-Taodonggou Half Graben, Southern Bogda Mountains, NW China
(1) Geology, Wichita State University, Wichita, KS.
(2) Geological Sciences and Engineering, Missouri University of Science and Technology, Rolla, MO.
(3) Geology, Northwest University, Xi'an, China.
(4) College of Geo-information Science and Engineering, Shandong University of Science and Technology, Qingdao, China.
The Tian Shan suture zones were identified as the source of Permian fluvial-lacustrine fills in the Tarlong-Taodonggou half graben of the greater Turpan-Junggar basin. Thin-section petrographic study of 23 fine to very coarse lithic arenites from Upper Permian-Lowermost Triassic Wutonggou low-order cycle suggests both distant suture zones and local rift shoulders are sources. Samples are at a 33-m average spacing, covering the 735-m-thick Wutonggou. Three hundred points are counted in each sample to identify the composition and texture of framework grains. The combination of monocrystalline quartz (Qm), mudrock (Sm), and basaltic lithic (Vb) grains is the most diagnostic: stable Qm represents non-sedimentary and recycled sedimentary sources; physically unstable Sm and chemically unstable Vb represent local sources. Two petrographic facies, A and B, are evident on ternary plots. Facies A has the mean composition of Qm7Vb38Sm55, and is common in Lower Wutonggou; Facies B having the mean of Qm71Vb6Sm23 is common in Upper Wutonggou. Facies A contains clean euhedral volcanic quartz and abundant lithics, whereas Facies B has abundant anhedral quartz with common vacuole lines and some mica and metamorphic grains.
Paucity of Qm and abundance of Sm and Vb in Facies A suggest local sources, because Sm and Vb are unstable and likely experienced short transport. Sm grains may have been derived from the exposed early-rift Lower-Permian fluvial and lacustrine mudrocks. Vb may have been derived from local and, possibly, from regional sources: the thick pre-rift Upper-Carboniferous basaltic lava flows and obducted oceanic basalts in the suture zones, respectively. Dominance of Qm in Facies B indicates a source rich in monocrystalline quartz. The persistent humid climate during Late Permian suggests increased transport distance in an enlarged catchment, instead of increased chemical weathering, in late Wutonggou time as a reasonable explanation for increased quartz content. The quartz-rich source rocks may be an unroofed granitic core, unroofed magmatic arcs, the metamorphic basement, and/or Carboniferous marine sandstones in the suture zone. Future geochemical and heavy mineral studies may ascertain the source lithology for the Facies B. The above interpretation is supported by 135 paleocurrent measurements, which indicate a dominantly north-flowing drainage system originating from the high-relief suture zones.