Promises and Problems in Nonmarine Sequence Stratigraphic AnalysisÑExamples from Fluvial-Lacustrine Permian, Triassic, and Jurassic Deposits, Bogda Mountains, NW China

Wan Yang¹, Qiao Feng², Yiqun Liu³, Neil Tabor⁴, Qiang Guan⁵, Jihong Li³, Xiaohur Zhang³.  (1) Department of Geology, Wichita State University, Wichita, KS 67260, U.S.A., (2) College of Geoinformation Science and Engineering, Shangdong University of Science and Technology, Qingdao, China, (3) Department of Geology, Northwestern University, Xian, China, (4) Department of Geological Sciences, Southern Methodist University, Dallas, Texas, 75275, U.S.A., (5) East Junggar Division, Xinjiang Petroleum Bureau, PetroChina, Fukang, China  [email protected]

Applications of marine sequence stratigraphic models to nonmarine strata are difficult because of abundant erosional surfaces and rapid vertical and lateral facies changes in continental deposits. ÒCookie-cutterÓ applications of marine sequence templates and terminologies appear illogical because sedimentary processes, environmental conditions, and depositional mechanisms differ greatly between marine and nonmarine environments. Nevertheless, principles of time-stratigraphic analysis, that is, identification of multi-order time-surface-bound stratigraphic units, should be applicable to nonmarine strata.

About 2,000 meter-scale fluvial-lacustrine depositional cycles were delineated on two superbly exposed Lower PermianÐLower Triassic sections and one Lower-Middle Jurassic section in Bogda Mountains, NW China. Braided-stream, meandering-stream, lake-margin siliciclastic, lacustrine deltaic, and lacustrine mixed carbonate and siliciclastic cycles are identified. Lacustrine cycles are lake-expansion/transgressive-to-lake-contraction/regressive cycles termed high-order sequences as basic time-stratigraphic units. Intermediate-order sequences are interpreted on the basis of thickness proportions of transgressive-regressive lithofacies and cycle thickness and facies stacking patterns of high-order sequences and comprise lacustrine lowstand (LLST), transgressive (LTST), highstand (LHST), and regressive (LRST) systems tracts. LLST contains incising fluvial high-order sequences; LTST contains lacustrine cycles with thick transgressive lithofacies, showing upward-deepening trends; LHST contains cycles of deepest-water facies, showing upward-shallowing trends; and LRST contains lacustrine cycles with proportionally thick, well-developed paleosols. Intermediate-order sequences signify cycles of lake expansion/transgression, contraction/regression, and eventual termination. Low-order sequences are demarcated by major unconformities representing a major shift in tectonic and/or climatic conditions. Both intermediate- and low-order sequences are correlative in kilometers, albeit dramatic lateral changes in thickness and lithofacies, and are used to establish two-dimensional sequence stratigraphic frameworks.