PRELIMINARY CYCLOSTRATIGRAPHIC CORRELATION OF MID-PERMIAN FLUVIAL-LACUSTRINE QZJ LOW-ORDER CYCLE IN TURPAN-JUNGGAR RIFT BASIN, NW CHINA – PROBLEMS AND PERSPECTIVES

Common erosional surfaces and lateral thickness and lithologic changes in nonmarine rift deposits hamper accurate time-stratigraphic correlation. We test strategies to establish a cyclostratigraphic framework and correlation of mid-Permian Quanzijie low-order cycle (QZJ LC) among six sections in the 80-km² Tarlong-Taodonggou half-graben in the southern Bogda Mountains, NW China and the Dalongkou section 80 km to the north. QZJ LC contains conglomerate, sandstone, siltstone, shale, and minor tuff and paleosols. Sedimentary texture and structure, and stratal geometry suggest meandering stream and minor marginal and littoral lacustrine environments. Systematic environmental changes define two types of high-order cycles (HC): fluvial HCs of thin channel-fill conglomerate and sandstone and thick, red, and structureless mudrocks; lacustrine HCs of lake margin Gleysols and sublittoral shales. Many overbank mudrocks are enigmatic with a possible loess origin.

The QZJ LC is bounded by a basal unconformity separating fluvial deposits from underlying lacustrine limestone and shale, and a top conformity separating Gleysols from overlying lacustrine deltaic deposits. The base signifies a regional tectonic uplift causing lake withdrawal and fluvial incision and peneplanation; the top a major climatic shift to humid conditions causing lake return. An overall upward-fining trend as reflected by upsection decreasing conglomerate and increasing mudrock corresponds to a fluvial-lake-margin environmental change. It is enhanced by basal Calcisols indicating arid climate and top Gleysols with abundant Fe-Mn pisoids indicating humid climate. The shift may correlate to the mid-Permian global icehouse-greenhouse transition. QZJ LC in the north is thicker and dominated by coarse fluvial deposits than that in the south. It is 72-178 m thick and contains 12-55 HCs. These variations make intra- and inter-graben stratigraphic correlation at the HC scale difficult. The uncertain correlation suggests that other attributes (e.g., CaCO₃, TOC, and silt content, and magnetic susceptibility) are needed to reveal systematic climatic and environmental changes to aid in accurate time-stratigraphic correlation.