Limestone-Sandstone Clinoforms in Middle-Permian Lacustrine Deposits, Bogda Mountains, NW China - Implications for Progradational Infilling of Intermontane Lake Basins

Wan Yang¹, Wei Guan¹, Yong Wang², Qiao Feng², Yiqun Liu³, and Neil Tabor^4
1Department of Geology, Wichita State University, Wichita, KS
²College of Geoinformation Science and Engineering, Shangdong University of Science and Technology, Qingdao, China
³Department of Geology, Northwest University, Xian, China
⁴Department of Geological Sciences, Southern Methodist University, Dallas, TX

Stratigraphic architecture and lithofacies of two clinoform packages in Roadian Lucaogou Fm., Bogda Mountains, NW China, provide insight into factors controlling sediment infilling of intermontane lake basins. Seven dip and strike sections were measured, covering 4x6 sq. km. The lower clinoform package was deposited after a transgression on an alluvial-fan complex. The progradational platform is ~5 m thick, extending ~200 m down dip and ~4 km laterally. Alternating packstone/grainstone and lithic sandstone form clinoform cycles 20-50 cm thick, with a maximum 15-degree dip to NE-NW. Sigmoidal clinoforms steepen progressively. Blackened peloids, ooids, skeletal and lithic grains, and cross beddings in limestones indicate high-energy littoral conditions. Sandstones contain some carbonate grains and cross beddings, indicating similar conditions. Clinoform cycles thin to 3-15 cm ~100 m downdip and have more silt; they amalgamate into a 1.5-m thick package ~4 km updip.

The lower package is truncated and overlain by a 30-cm-thick sublittoral shale, where the upper clinoform package developed. It consists of thick lithic-quartzose sandstone clinoforms capped by thin (10s cm) peloidal sandstone to packstone/grainstone. In the north, it is 4-8 m thick and overlain by a conglomeratic transgressive lag; clinoform cycles are 40-100 cm thick, dipping to NNW. In the south, the package thickens to 17 m; clinoform cycles are 1-6 m thick, but dip to SW.

High-order clinoform cycles comprise intermediate-order clinoform-package cycles bound by transgressive surfaces. The cyclicity suggests two-order variations in net precipitation, resulting in changes in sediment type and lake-level of a balance-filled lake. Moreover, shoreline morphology and depositional topography probably controlled depositional loci and progradational direction.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas