--> ABSTRACT: Fluvial Sedimentology of a Mesozoic Petrified Forest Assemblage, Shishu Formation, Junggar Foreland Basin, Xinjiang, China, by Cleavy L. McKnight, Oigao Gan, Alan R. Carroll, David Dilcher, Min Zhao, Yuan Hai Liang, and Stephan A. Graham; #91030 (2010)

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Fluvial Sedimentology of a Mesozoic Petrified Forest Assemblage, Shishu Formation, Junggar Foreland Basin, Xinjiang, China

Cleavy L. McKnight, Oigao Gan, Alan R. Carroll, David Dilcher, Min Zhao, Yuan Hai Liang, Stephan A. Graham

The Upper Jurassic(?) Shishu Formation of the eastern Junggar basin, Xinjiang, northwest China, is a fluvial sand unit containing an important assemblage of well-preserved, silicified tree trunks and rooted stumps. Numerous logs, up to 83 ft (25.5 m) long, occur at several levels within a 33.6-ft (10.3 m) stratigraphic section of fluvial sand, gravel, and mud and several paleosol horizons. The uppermost log-bearing layer includes a number of rooted tree stumps in growth position, with diameters of up to 8 ft (2.5 m). The maximum root length observed is 40 ft (12.3 m). The trees have been identified by Chinese paleontologists as Cupressinoxylon.

The petrified forest assemblage is preserved on the northeast margin of the Mesozoic Junggar foreland basin, a large continental basin subsiding under thrust loading from the south. Logs found within channel gravel units are oriented with their log axes parallel to the channel axis. Sedimentary structures, including epsilon and trough cross-stratification and imbricated channel gravels, indicate paleocurrent flow generally to the south, toward the basin center. The size of the logs suggests the presence of a major fluvial system. The epsilon cross-sets suggest a channel depth of 26 ft (8 m).

The oriented silicified logs and their enclosing clastic sediments provide important information on the depositional systems active on the northeastern margin of the Junggar basin in the Late Jurassic(?) time. Hopefully, further detailed study of the fossil trees, including the spacing of the rooted stumps, will provide new information on the paleoecology of Mesozoic forests and the climatic conditions prevailing in the region at the time of deposition.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.