Wartes, M. A., A. R. Carroll, T. J. Greene, Kerning Cheng, and Hu Ting, 2000, Permian lacustrine deposits of northwest China, in E. H. Gierlowski- Kordesch and K. R. Kelts, Eds., Lake basins through space and time: AAPG Studies in Geology 46, Chapter 8 p.123-132. Permian Lacustrine Deposits Of Northwest China Marwan A. Wartes Alan R. Carroll Department of Geology and Geophysics, University of Wisconsin-Madison Madison, Wisconsin, U.S.A. Todd J. Greene Department of Geological and Environmental Sciences, Stanford University Stanford, California, U.S.A. Kerning Cheng Research Institute of Petroleum Exploration and Development, China National Petroleum Corporation Beijing, People's Republic of China Hu Ting Research Institute of the Tu-Ha Petroleum Exploration and Development Bureau, China National Petroleum Corporation Hami, People's Republic of China INTRODUCTION Turpan-Hami basins (Figures 1, 2) (Hendrix et al., 1992; Greene et al., 1997, in press; Shao et al., 1999); therefore, Permian deposits of the Junggar and Turpan-Hami the Bogda Shan provide an oblique cross-section of Per- basins of the Xinjiang Uygur Autonomous Region of mian lake deposits of the unified Junggar-Turpan-Hami northwest China preserve some of the thickest and most basin. In addition to lacustrine sedimentation, the south- areally extensive lake strata on Earth. In the south Jung- ern margins of this basin periodically received coarse clas- gar depocenter, these nonmarine deposits are up to 5 km tic sediments shed from the ancestral Tian Shan (Figure 1) thick and organic-rich facies rank among the thickest (Carroll et al., 1990, 1995; Greene et al., 1997). Coarse clas- and richest petroleum source rocks in the world (Gra- tics were also shed southward into the basin from the ham et al., 1990; Lawrence, 1990; Demaison and Kelameili and west Junggar Shan, uplifts bounding the Huizinga, 1991; Carroll et al., 1992). In addition, Permian northeastern and northwestern edges of the basin respec- lacustrine deposits are estimated to span 1000 km along tively (Figure 1) (Lee, 1985; Carroll et al., 1990; Tang et al., strike, indicating that widespread lakes represented a 1997a; and Zhao and Tang, this volume). major paleogeographic feature of central Asia (Figure 1). Up to 5 km of total Permian subsidence is recorded Unfortunately, the remote location of these deposits has over a 40-50 m.y. span, yet the tectonic significance of this hindered detailed studies, and the western literature basin remains controversial. Hypotheses provided to contains only sparse reference to this important record explain Permian subsidence are numerous, often mutually of continental sedimentation. The purpose of this paper incompatible, and reflect the paucity of first-hand geo- is to briefly review the Permian nonmarine stratigraphy logic observations available to constrain the Permian and report on recent field-based studies documenting tectonic setting. Proposed hypotheses range from fore- the Permian lacustrine stratigraphy exposed along the land basin flexure (Watson et al., 1987; Carroll et al., north and south flanks of the Bogda Shan (Figure 2). 1995), to extension (Bally et al., 1986; Allen et al., 1991), to regional transtension (Allen et al., 1995). SETTING STRATIGRAPHY Paleocurrents and other data indicate that the devel- opment of Permian lakes pre-date the uplift of the The Junggar and Turpan-Hami basins portray a Bogda Shan, which now partitions the Junggar and complicated association of facies, particularly between 123 Figure 1—Simplified geologic map of the Junggar and Turpan-Hami basins illustrating the distribution of Permian outcrops (modified from Chen et al., 1985) and the known extent of Permian lacustrine facies. Box indicates the location of Figure 2. Permian Lacustrine Deposits of Northwest China 125 Figure 2—Location map of study area depicting Permian outcrops (modified from Chen et al., 1985). Strati- graphic details for named localities are shown in the regional correlation (Figure 3). Note paleocurrent data taken from clast imbrications and trough cross-beds indicating a north-northeasterly paleoflow (Carroll, 1991; Carroll et al., 1995; Greene et al., in press). These data are inconsistent with a Permian physiographic expres- sion of the Bogda Shan, supporting a hypothesis for a united Permian lake basin spanning both the Junggar and Turpan-Hami basins. lake marginal and basinal environments. In addition, Group are exposed at Zaobishan (Greene et al., 1997; precise understanding of the temporal framework for Wartes et al., 1999) (Figures 2, 3). The section consists these deposits is limited and regional correlations, of fine-grained sandstones, limestones, and dark such as that shown in Figures 3 and 4, await further gray calcareous mudstones organized in a series of detailed work. Nonetheless, a synthesis of existing and parasequences 1-3 m thick. These cycles generally new data are summarized in a basin-oblique west to grade upward from stromatolitic limestone, to lami- east chronostratigraphic cross-section of the Permian nated mudstones, and are capped by a coarsening- (Figure 3). The stratigraphy and physical characteris- upward succession of siltstone to fine-grained tics illustrated by the correlation allow for a simplified sandstone. This cyclicity is interpreted to reflect tripartite division of these deposits into phases of transgressive-regressive fluctuations in lake-level lacustrine development (Figure 3). (Wartes et al., 1998), perhaps in response to varying climatic aridity. Several 20-30 m intervals of Lower LOWER PERMIAN Permian lacustrine facies have also been docu- mented in the eastern end of the Turpan-Hami basin The first phase of lacustrine deposition spans the at Tian Shan Xiang (Figures 2, 3, 5). They are Lower Permian and is the least well documented, having expressed as laminated, dark gray mudstone con- been noted in just two localities along the Bogda Shan taining fish scales, algal lamination, and localized (phase I on Figure 3). These deposits follow an overall soft-sediment deformation (Figure 5). marine regression marked by a diachronous retreat of The Early Permian age designation for these marine waters from the region. Marine deposition deposits is based largely on stratigraphic position; continued in deeper portions of the basin, particularly however, interbedded volcanic rocks occur at both near the south Junggar depocenter, well into the Early localities (Figure 3). Future radiogenic age determina- Permian (Carroll et al., 1990) (Figure 3). tions of these igneous rocks will provide a critical con- An 80 m thick succession of lacustrine rocks of the trol on the chronostratigraphy of Lower Permian Lower Permian Yierxitu Formation of the Aqibulake lacustrine deposits. Figure 3—Permian chronostratigraphic correlation along a basin-oblique transect of the Bogda Shan. Localities are from the southern Junggar (northern Bogda Shan), western Turpan-Hami (northern Tian Shan), and northern Turpan-Hami (southern Bogda Shan) areas. Lacustrine development can be divided into three phases, which are shown at right as I, II, and III, and correspond to the organization of the text description. The phases are based on age relationships, sedimentary facies, lake type, and other diagnostic characteristics. Note the indicated stratigraphic location of detailed sections displayed in Figure 5. Distances between localities are not shown; refer to Figure 2 for true lateral separation. Formation subdivisions and regional nomenclature are shown in Figure 4. Permian Lacustrine Deposits of Northwest China 127 Figure 4—Subdivision and correlation of Permian units in the Junggar and Turpan-Hami basins (mod- ified from Zhao, 1982; Chen et al., 1985; and Liao et al., 1987). LOWER-UPPER PERMIAN 1992; Clayton et al., 1997; Carroll, 1998). Another poten- tial source for these oils is the slightly older Pingdiquan The next phase of lacustrine development (labeled II Formation of the northeastern Junggar basin (Tang et al., on Figure 3) is early-Late Permian in age and has 1997a) (Figure 4). The Jingjingzigou Formation is slightly received the most attention due to the extremely rich more fossiliferous than the Wulapo Formation with source rocks occurring in this interval along the north- reports of numerous ostracode genera including Tomiella, west flank of the Bogda Shan (Graham et al., 1990; Car- Darwinula, and Darwinuloides, as well as palynomorphs roll et al., 1992, 1998). Recent studies have begun to such as Cordaites sp. (Zhang, 1981; XPGEG, 1991). correlate these organic-rich facies with widespread, The Jingjingzigou Formation grades upward into coeval lacustrine mudstones preserved south of the the thick oil shales characteristic of the Lucaogou For- Bogda Shan in the Turpan-Hami basin (Greene et al., mation (Figure 5). The Lucaogou Formation consists of 1997; Wartes et al., 1998). The following description of more than 800 m of finely laminated, organic-rich this phase of lacustrine deposition will begin with rocks mudstone. Dolomite also occurs as occasional nodular of the Upper Jijicao Group in the southern Junggar beds and as cement in very fine to fine-grained sand- basin, followed by equivalent strata of the Taodonggou stones. Total organic carbon (TOC) commonly exceeds Group in the Turpan-Hami basin. This separation is 20% in the richest facies and averages 4% over one 800 m warranted due to the differing formation names and, to interval (Carroll et al., 1992). Coeval organic-rich a lesser degree, lithologic character (Figures