Marc S. Hendrix

Late Paleozoic tectonic amalgamation of northwestern China: Sedimentary record of the northern Tarim, northwestern Turpan, and southern Junggar Basins

Sedimentary rocks contained in basins adjacent to the Tian Shan provide a long and complex record of the late Paleozoic continental amalgamation of northwestern China, complementing that provided by rocks preserved within the range. This record, which comprises dramatic changes in sedimentary facies, sediment dispersal patterns, sandstone provenance, and basin subsidence rates, broadly supports previous interpretations of a two-part evolution of the Tian Shan: Late Devonian to Early Carboniferous collision of the Tarim continental block with the Central Tian Shan, followed by collision of this combined block with island arcs in the north Tian Shan and Bogda Shan in Late Carboniferous–Early Permian times. The first collision resulted in widespread angular unconformities within the Tarim basin. Continued convergence following the collision created a long-lived flexural foredeep along the northern margin of the Tarim block, which received at least 2000 m of Lower Carboniferous through Lower Permian fluvial and marine sediment derived from the interior of Tarim. Subsequent Early Permian continental extension of the northern Tarim basin resulted in the deposition of interbedded nonmarine siliciclastic sedimentary rocks and mafic to felsicvolcanic rocks. Sandstone within this interval was derived from the paleo–Tian Shan, and is composed predominantly of lithic volcanic grains similar to the rhyolite. In contrast to the Tarim basin, calc-alkaline volcanic rocks and volcanogenic sedimentary rocks dominated Carboniferous and Permian sedimentation in the northern Turpan and northwestern Junggar basins. Volcanic arcs remained active in the North Tian Shan and Bogda Shan through the early Late Carboniferous, depositing a kilometers-thick interval of deep marine sediment-gravity flows in the northwestern Junggar basin. Major arc magmatism ceased in the Late Carboniferous in response to closure of the oceanic basin between the combined Tarim/Central Tian Shan block and the North Tian Shan/Bogda Shan arcs. Upper Carboniferous through Lower Permian rocks in the northwestern Junggar basin compose the sedimentary fill of a bathymetric basin of oceanic depth (on the northern side of the volcanic arcs), culminating in a 1000-m-thick marine regressive sequence. Middle to Upper Permian sandstones were derived from the uplifted paleo–Tian Shan and bear the distinctive provenance imprint of granitic rocks presently exposed within the range. Late Permian subsidence of the Junggar basin accommodated >5 km of nonmarine sediments; however, the cause of this subsidence and its relationship to regional tectonic events remain controversial.

Late Oligocene-early Miocene unroofing in the Chinese Tian Shan: An early effect of the India-Asia collision

Apatite fission-track data indicate that Mesozoic strata exposed on the northern flank of the Chinese Tian Shan underwent ∼4-5 km of late Cenozoic unroofing, beginning at ∼24 Ma. This age apparently dates initial reactivation of the northern Tian Shan in response to the India-Asia collision, which continues to raise the mountain range today. Numerous studies of the Himalaya and Tibet suggest that a major shift from extrusion-dominated to crustal thickening-dominated tectonics occurred in latest Oligocene-early Miocene time, approximately coincident with the start of unroofing in the Tian Shan. This suggests that Tian Shan unroofing was a distant effect of that shift within the collision zone.

Sedimentary record and tectonic implications of Mesozoic rifting in southeast Mongolia

The East Gobi basin of Mongolia is a poorly described Late Jurassic–Early Cretaceous extensional province that holds great importance for reconstructions of Mesozoic tectonics and paleogeography of eastern Asia. Extension is especially well recorded in the structure and stratigraphy of the Unegt and Zuunbayan subbasins southwest of Saynshand, Mongolia, where outcrop and subsurface relationships permit recognition of prerift, synrift, and postrift Mesozoic stratigraphic megasequences. Within the synrift megasequence, three sequences developed in response to climatic and rift-related structural controls on sedimentation. Where best exposed along the northern margin of the Unegt subbasin, each of the synrift sequences is bounded by unconformities and generally fines upward from basal alluvial and fluvial conglomerate to fluvial and lacustrine sandstone and mudstone. Resedimented ashes and basalt flows punctuate the synrift megasequence. Rifting began in the Unegt subbasin prior to 155 Ma with coarse alluvial filling of local fault depressions. Subsidence generally outstripped sediment supply, and fresh to saline lacustrine environments, expanding southward with time, dominated the Unegt- Zuunbayan landscape for much of latest Jurassic–Early Cretaceous time. Episodic faulting and volcanism characterized the basin system for the balance of the Early Cretaceous. A brief period of compressional and/or transpressional basin inversion occurred at the end of the Early Cretaceous, prior to deposition of a widespread Upper Cretaceous overlap sequence. The driver(s) of Late Jurassic–Early Cretaceous extension remain uncertain because southeast Mongolia occupied an intraplate position by the beginning of the Cretaceous. Extension in the East Gobi basin was coeval with collapse and extension of early Mesozoic contractional orogenic belts along the northern and southern borders of Mongolia and probably was a linked phenomenon. Strike-slip faulting associated with collisions on the southern Asian and Mongol- Okhotsk margins likely also played a role in late Mesozoic deformation of the East Gobi region, perhaps partitioning the Gobi from apparently coeval large-magnitude contractional deformation in the Yinshan- Yanshan orogenic belt south of the study area in Inner Mongolia.

High-Resolution Particle Size Analysis of Naturally Occurring Very Fine-Grained Sediment Through Laser Diffractometry

ABSTRACT In this paper, we present results from a large number of experiments aimed at quantifying method and instrument uncertainty associated with laser diffraction analysis. We analyzed the size distribution of fine-grained sediment ( 24 hours prior to analysis and using 60 seconds of ultrasonication during analysis. (2) Obscuration--a measure of the concentration of the suspension during analysis--produced the most reproducible results at about 20%. (3) Variations in refractive-index settings can significantly alter estimated grain-size distributions. (4) Assumed values for absorption (the degree to which sediment grains absorb the light) can have a profound effect on grain-size results. Absorption settings near 0 resulted in unexpected bimodal grain size distributions for sediments in the < 10 µm size fraction and significantly skewed the fine-grained tail of coarser samples, probably because of sub-optimal diffraction by particles with a diameter similar in size to the laser wavelength. Absorption settings closer to 1 produced very reproducible results and unimodal grain-size distributions over a wide range of refractive indexes. Our study has shown that laser diffraction can measure very fine-grained sediments (< 10 µm) quickly, with high precision ( 5% at 2 standard deviations), and without the need for extensive mineralogical determinations. These results make possible a new generation of studies in which high-resolution time-series data sets of sediment grain size can be used to infer subtle changes in paleohydrology.

Junggar basin, northwest China: trapped Late Paleozoic ocean

Abstract The Junggar basin originated during the late Paleozoic as either a remnant lower to mid-Paleozoic ocean basin or a mid-Carboniferous back-arc (intra- or inter-arc) basin bounded by emergent volcanic arcs south of the Siberian Craton. The retreating Junggar Sea left behind a regressive sedimentary section comprising at least 3–4 km of marine volcaniclastics in the southern Junggar area. Sandstones deposited in the Junggar basin since the Devonian are exclusively volcaniclastic, demonstrating that Precambrian basement rocks have never been exposed in the basin, and supporting the hypothesis that the Junggar is underlain by oceanic crustal materials. The mid-Carboniferous Junggar Ocean may have been a remnant ocean basin of early to mid Paleozoic age, or alternatively may have been an extending intra- or inter-arc basin formed behind an emergent volcanic arc in the northern Turpan region. Subsequent strike-slip deformation and the general sparsity of geologic and geophysical data from the Junggar area make it difficult to distinguish between these models. Late Early Permian through Triassic sediments of the Junggar basin are exclusively non-marine, deposited in a flexurally subsiding foreland basin during initial uplift of the ancestral Tian Shan mountains. The sedimentary section deposited during the Late Permian-Early Triassic appears to be inconsistent with a proposed rifting episode during this period.

Uplift, exhumation, and deformation in the Chinese Tian Shan

The terranes composing the basement of the Tian Shan were originally sutured together during two collisions in Late Devonian–Early Carboniferous and Late Carboniferous–Early Permian time. Since then, the range has repeatedly been uplifted and structurally reactivated, apparently as a result of the collision of island arcs and continental blocks with the southern margin of Asia far to the south of the range. Evidence for these deformational episodes is recorded in the sedimentary histories of the Junggar and Tarim foreland basins to the north and south of the range and by the cooling and exhumation histories of rocks in the interior of the range. Reconnaissance apatite fission-track cooling ages from the Chinese part of the range cluster in three general time periods, latest Paleozoic, late Mesozoic, and late Cenozoic. Latest Paleozoic cooling is recorded at Aksu (east of Kalpin) on the southern flank of the range, at two areas in the central Tian Shan block along the Dushanzi-Kuqa Highway, and by detrital apatites at Kuqa that retain fission-track ages of their sediment source areas. Available Ar/Ar cooling ages from the range also cluster within this time interval, with very few younger ages. These cooling ages may record exhumation and deformation caused by the second basement suturing collision between the Tarim–central Tian Shan composite block and the north Tian Shan. Apatite data from three areas record late Mesozoic cooling, at Kuqa on the southern flank of the range and at two areas in the central Tian Shan block. Sedimentary sections in the Junggar and Tarim foreland basins contain major unconformities, thick intervals of alluvial conglomerate, and increased subsidence rates between about 140 and 100 Ma. These data may reflect deformation and uplift induced by collision of the Lhasa block with the southern margin of Asia in latest Jurassic–Early Cretaceous time. Large Jurassic intermontane basins are preserved within the interior of the Tian Shan and in conjunction with the fission-track data suggest that the late Mesozoic Tian Shan was subdivided into a complex of generally east-west–trending, structurally controlled subranges and basins. Apatite data from five areas record major late Cenozoic cooling, at sites in the basin-vergent thrust belts on the northern and southern margins of the range, and along the north Tian Shan fault system in the interior of the range. The thrust belts *Now at ExxonMobile Exploration Company, P.O. Box 4778, Houston, Texas 77060, USA Dumitru, T.A., et al., 2001, Uplift, exhumation, and deformation in the Chinese Tian Shan, in Hendrix, M.S., and Davis, G.A., eds., Paleozoic and Mesozoic tectonic evolution of central Asia: From continental assembly to intracontinental deformation: Boulder, Colorado, Geological Society of America Memoir 194, p. 71–99. 72 T.A. Dumitru et al.

Occurrence, age, and implications of the Yagan–Onch Hayrhan metamorphic core complex, southern Mongolia

Mylonitic rocks associated with the south-dipping detachment fault of the Yagan–Onch Hayrhan metamorphic core complex in southernmost Mongolia indicate subhorizontal south-southeast–directed extension in the Early Cretaceous; synkinematic biotites give 40 Ar/ 39 Ar ages of 129 to 126 Ma. The Yagan–Onch Hayrhan core complex demonstrates that late Mesozoic localized high-strain extension, recently recognized in other parts of eastern Asia, also occurred in Mongolia. The presence of Mesozoic metamorphism at Onch Hayrhan, previously presumed to be Precambrian, brings into question the existence of the South Gobi microcontinent.

Evolution of Mesozoic Sandstone Compositions, Southern Junggar, Northern Tarim, and Western Turpan Basins, Northwest China: A Detrital Record of the Ancestral Tian Shan

ABSTRACT Sandstone compositional data can be a powerful tool in the interpretation of tectonic and climatologic influences on sedimentary basin fill, in addition to yielding important information about porosity in sandstone petroleum reservoirs. In order to explore these relationships, a modified Gazzi-Dickinson point-counting technique was used to analyze the composition of 143 Mesozoic sandstone samples from the southern Junggar, northern Tarim, and western Turpan basins of northwestern China. Results indicate that a Mesozoic, ancestral version of the Tian Shan physiographically separated the Junggar and Tarim basins and provided sand of very different composition to each basin. Mesozoic sandstone from the northern Tarim basin is diverse in composition and lithic-rich (Qm41F14L45; Qp42Lvm26Lsm32), is locally micaceous, and contains common radiolarian-chert grains and few dense accessory minerals. Inferred source rocks include upper Paleozoic alkali granite and metamorphic complexes, thick Silurian bedded-chert sequences, and lower Paleozoic strata of a passive continental margin. In contrast, sandstone from the southern Junggar and western Turpan basins is uniformly volcanic-rich (Qm21F21Lt58; Qp13Lvm68Lsm19), and contains abundant dense accessory minerals and only local radiolarian chert and mica. Inferred principal source rocks are Devonian-Carboniferous andesitic arc volcanics. The effect of sampling scale on sandstone composition outweighs that of plate-tectonic setting. Samples were derived mostly from medium- and coarse-grained fluvial systems that likely drained only portions of the ancestral Tian Shan and hence preserve local source-rock signatures, rather than an integrated compositional signal that can be directly compared to plate-tectonic petrofacies models. In addition, though Mesozoic basins of western China were most akin to broken foreland basins, Mesozoic sandstone is considerably more compositionally diverse and lithic-rich than that of modern or ancient broken foreland basins because of the variety of accreted terranes constituting the ancestral Tian Shan. Temporal changes in sandstone composition are consistent with episodes of Mesozoic deformation in the Tian Shan. Each deformational episode increased physiographic relief of the ancestral range, produced renewed downcutting and erosion of source rocks, and resulted in the deposition of compositionally very immature sandstone in adjacent basins. Although a regional early Mesozoic megamonsoon and an Early Cretaceous rain shadow cast across the northern Tarim basin are interpreted from regional facies and paleontologic data, neither paleoclimatic phenomenon appears to have significantly modified sandstone composition in the study area. Calculations of intergranular volume (% porosity + % cement) indicate that porosity in sandstone from the Tarim and Junggar basin depocenters was reduced principally by burial compaction and that the rate of porosity reduction was highest for lithic-rich samples.

Noyon Uul syncline, southern Mongolia: Lower Mesozoic sedimentary record of the tectonic amalgamation of central Asia

To explore the hypothesis that a regionally extensive early Mesozoic collisional foreland basin existed on strike to the east of documented Mesozoic foreland basins in western China, we conducted a reconnaissance study of a >5 km section of Upper Permian-Lower Jurassic(?) nonmarine strata exposed in the Noyon Uul syncline in southern Mongolia. This paper documents the depositional settings of these poorly known strata, their provenance, and their regional tectonic significance in the context of the Mesozoic amalgamation of central Asia. We interpret Noyon Uul strata to be composed of (1) a coarse-grained, braided fluvial facies, (2) a meandering fluvial facies, and (3) a poorly oxygenated lacustrine facies that represent three principal depositional environments. Sandstone compositions are dominantly volcanic (mean compositions = Qm 20 F 23 Lt 57 ; Qp 13 Lv 82 Ls+Lm 5 ), likely reflecting derivation from Carboniferous and older volcanic-arc sequences. Increased proportions of alkalic granite pebbles and cobbles above the first coarse-grained, braided fluvial unit and the first stratigraphic appearance of detrital K-feldspar in basal coarse-grained, braided fluvial strata suggest unroofing of Permian alkalic granites. The Noyon Uul deposits likely are syntectonic, as suggested by their thickness, environments of deposition, coarse-grained character, and accumulation rates. Overlap of the Noyon Uul syncline by relatively undeformed Upper Jurassic and Lower Cretaceous strata, together with the presence of a regional unconformity between Middle and Upper Jurassic rocks, requires a phase of Middle-Late Jurassic deformation. The erosional remnants of a Middle-Upper Jurassic north-verging thrust belt have been documented just 100 km south of Noyon Uul, suggesting that the Noyon Uul deposits may be foreland in origin. Specifically Triassic thrusting has not been recognized in southern Mongolia, but Permian-Jurassic contractile deformation associated with the collisional amalgamation of Asia is documented or reasonably inferred for extensive areas of China to the south and west. Although sedimentary attributes of the Noyon strata are most compatible with a foreland interpretation, it is possible that the Noyon deposits and their deformation may be related to strike-slip faulting. Neither the attributes of the Noyon deposits nor their ages are compatible with widespread Late Jurassic-Early Cretaceous rifting that occurred across southeastern Mongolia.

Characteristics of Selected Petroleum Source Rocks, Xianjiang Uygur Autonomous Region, Northwest China (1)

The sedimentary basins of Xinjiang Uygur Autonomous Region, China, are moderately to poorly explored for petroleum. Volumetric adequacy of petroleum source rocks is a critical exploration risk in these basins, particularly because source rock data are limited. This study provides new source rock data and speculatively assesses the source rock potential of Xinjiang basins. The Junggar (Zhungaer) basin, the best explored of the Xinjiang basins and containing a giant oil field, is underlain in many areas by an Upper Permian lacustrine oil-shale sequence remarkable for its organic richness and oil source quality. Depending on its position in the basin, the Permian section ranges from immature to overmature and is inferred to be the principal source of oil in the basin. Upper Triassic-Middle Jurassic coal measures, including lacustrine rocks, constitute a secondary source rock sequence in the basin. The smaller, intermontane Turpan (Tulufan) basin contains a very similar Upper Triassic-Middle Jurassic sequence, which, where sufficiently buried, probably comprises the only significant oil source sequence in the basin. The vast Tarim (Talimu) basin offers the greatest variety of potential source rocks of all Xinjiang basins but remains the least well documented. From limited but geologically planned and focused sampling, Cambrian, Carboniferous, and Permian strata are not considered major oil contributors in the dominantly shallow marine Paleozoic section of the northern Tarim basin. Only Ordovician black shales appear to have significant potential. The Upper Triassic-Middle Jurassic sequence of the northern Tarim basin is similar to that of the Junggar and Turpan basins--a section rich in coal and lacustrine shale that constitutes another potentially significant oil source. Due to the size, stratigraphic packaging, and structural relief of the northern Tarim basin, Paleozoic and Mesozoic potential il source beds range from immature to overmature.