Northwestern Tarim Basin Research Articles

Aerodynamic properties of agricultural and natural surfaces in northwestern Tarim Basin

Friction velocity (u*) and aerodynamic roughness (z0) are atmospheric parameters that influence the flux of windblown dust. These parameters have not been quantified for different land use types in the Tarim Basin of China, one of the largest sources of atmospheric dust in the world. Wind speed profiles were measured and used to determine d, u* and z0 of a cotton (Gossypium hirsutum L.) field, red date (Ziziphus jujuba L.) orchard and native desert during the spring wind erosion season of 2012 and 2013. In addition, d, u* and z0 were estimated using Raupach's model. During periods of high winds sufficient to cause erosion in spring, d averaged 0.28, 0.067 and 0m; u* averaged 0.39, 0.51 and 0.35ms−1; and z0 averaged 7.0, 17.0 and 1.2mm for respectively the cotton field, red date orchard and desert. Estimates of d, u* and z0 according to Raupach's model during the same periods of high winds were 0.10, 0.16 and 0m; 0.56, 0.73 and 0.49ms−1; and 12.6, 21.9 and 1.8mm for respectively the cotton field, red date orchard and desert. The Raupach model, which depends on canopy height and area index, overestimated u* and z0 for the three land use types during most high wind events. The desert had the greatest potential for flux of windblown dust (due to the lowest z0) while the red date orchard has the lowest potential for dust flux (due to the highest z0).

Late Cenozoic transtensional fault belt discovered on the boundary of the Awati Sag in the northwestern Tarim Basin

Late Cenozoic transtensional fault belt was discovered on Shajingzi fault belt, NW boundary of the Awati Sag in the northwestern Tarim Basin. And numerous Quaternary normal faults were discovered on Aqia and Tumuxiuke fault belts, SW boundary of Awati. This discovery reveals Quaternary normal fault activity in the Tarim Basin for the first time. It is also a new discovery in the southern flank of Tianshan Mountains. Shajingzi transtensional fault belt is made up of numerous, small normal faults. Horizontally, the normal faults are arranged in right-step, en echelon patterns along the preexisting Shajingzi basement fault, forming a sinistral transtensional normal fault belt. In profile, they cut through the Paleozoic to the mid-Quaternary and combine to form negative flower structures. The Late Cenozoic normal faults on the SW boundary of Awati Sag were distributed mainly in the uplift side of the preexisting Aqia and Tumuxiuke basement-involved faults, and combined to form small horst and graben structures in profile. Based on the intensive seismic interpretation, careful fault mapping, and growth index analysis, we conclude that the normal fault activity of Shajingzi transtensional fault belt began from Late Pliocene and ceased in Late Pleistocene (mid-Quaternary). And the normal faulting on the SW boundary of Awati Sag began from the very beginning of Quaternary and ceased in Pleistocene. The normal faulting on Awati’s SW boundary began a little later than those on the NW boundary. The origin of Shajingzi transtensional normal fault belt was due to the left-lateral strike-slip occurred in the southern flank of Tianshan, and then, due to the eastward escape of the Awati block, a tensional stress developed the normal faults on its SW boundary.

Depositional facies and stratal cyclicity of dolomites in the Lower Qiulitag Group (Upper Cambrian) in northwestern Tarim Basin, NW China

The Upper Cambrian Lower Qiulitag Group in the Tarim Basin, NW China, is overwhelmingly composed of cyclic dolomites. Based on extensive field investigations and facies analysis from four outcrop sections in the Bachu-Keping area, northwestern Tarim Basin, four main types of facies are recognized: open-marine subtidal, restricted shallow subtidal, intertidal, and supratidal facies, and these are further subdivided into ten lithofacies. In general, these facies are vertically arranged into shallowing-upward, metre-scale cycles. These cycles are commonly composed of a thin basal horizon reflecting abrupt deepening, and a thicker upper succession showing gradual shallowing upwards. Based on the vertical facies arrangements and changes across boundary surfaces, two types of cycle: peritidal and shallow subtidal cycle, are further identified. The peritidal cycles, predominating over the lower-middle Lower Qiulitag Group, commence with shallow subtidal to lower intertidal facies and are capped by inter-supratidal facies. In contrast, the shallow subtidal cycles, dominating the upper Lower Qiulitag Group, are capped by shallow-subtidal facies. Based on vertical lithofacies variations, cycle stacking patterns, and accommodation variations revealed by Fischer plots, six larger-scale third-order depositional sequences (Sq1–Sq6) are recognized. These sequences generally consist of a lower transgressive and an upper regressive systems tract. The transgressive tracts are dominated by thicker-than-average cycles, indicating an overall accommodation increase, whereas the regressive tracts are characterized by thinner-than-average peritidal cycles, indicating an overall accommodation decrease. The sequence boundaries are characterized by transitional zones of stacked thinner-than-average cycles, rather than by a single surface. These sequences can further be grouped into lower-order sequence sets: the lower and upper sequence sets. The lower sequence set, including Sq1–Sq3, is characterized by peritidal facies-dominated sequences and a progressive decrease in accommodation space, indicating a longer-term fall in sea level. In contrast, the upper sequence set (Sq4–Sq6) is characterized by subtidal facies-dominated sequences and a progressive increase in accommodation space, indicating a longer-term rise in sea level.

In situ geochemistry of Lower Paleozoic dolomites in the northwestern Tarim basin: Implications for the nature, origin, and evolution of diagenetic fluids

AbstractLower Paleozoic sedimentary rocks in the northwestern Tarim basin were strongly altered by complicated geofluids, which resulted in the occurrence of various diagenetic minerals (e.g., dolomite). Here, in situ major, trace, and rare earth element geochemistry of Lower Ordovician diagenetic dolomite grains as well as petrography were performed to unravel the geochemical features, the nature, and origin of the diagenetic fluids. The results indicate that different geochemical information can be detected within a single sample, even within a single dolomite grain. Five generations of diagenetic dolomite have been identified based on geochemical signatures, resulting from four distinct types of diagenetic fluids: (1) HREE enrichment (PAAS‐normalized), low ΣREE, no Eu anomaly, low Mn, Ba, moderate Fe, and high Sr contents are probably due to early burial dolomitizing fluids; (2) MREE enrichment, high ΣREE, high Mn, Fe, and low Sr content are likely to be associated with Devonian deep‐circulating crustal hydrothermal fluids; (3) flat or LREE enrichment pattern with obviously positive Eu anomaly is inferred to be linked to Permian magmatic hydrothermal fluids; and (4) flat REE pattern, moderate ΣREE, no Eu anomaly, low Mn, Ba, moderate Fe, and high Sr contents are probably due to late burial dolomitizing fluids. The significances of in situ method demonstrated in this study, compared with the whole rock analysis, include not only contamination‐free analysis but also unraveling the internal geochemical variation within a single sample or a mineral grain. Thus, for the geochemical study of complicated diagenetic process, in situ method should be preferentially considered.

Geology, geochemistry and genesis of the Cretaceous and Paleocene sandstone- and conglomerate-hosted Uragen Zn–Pb deposit, Xinjiang, China: A review

The Uragen Zn–Pb deposit in Wuqia County, Xinjiang, China, is hosted in sandstones and conglomerates of the Lower Cretaceous Kezilesu Group and argillaceous dolomite of the Paleocene Aertashi Formation in the Kashi sag, northwestern Tarim basin. The deposit is characterized mainly by low grade (Zn+Pb~3wt.%) and large tonnage (~3Mt metals in the southern ore zone, potentially 10Mt metals for the whole deposit), with an average Zn/Pb ratio of about 5.8. The orebodies are tabular and stratiform, mainly distributed in a syncline. The ores are characterized by disseminated sulfides with minor massive, veinlet, patchy structures, and fine-grained, locally colloform and framboidal, textures. The main primary metallic minerals are sphalerite, galena, pyrite and marcasite, and minor supergene minerals include smithsonite, hydrozincite, anglesite, calamine, jarosite, and limonite. The ore minerals mainly occur as replacement of cements, matrix and framework grains of the host rocks and as open space filling to a lesser extent. Based on the fact that orebodies straddle across a hiatus between the lower Cretaceous and Paleocene host rocks and on the replacement and open-space filling textures and structures, the mineralization is interpreted to be epigenetic. The tightly clustered Pb isotopic values of the main-stage ore minerals (206Pb/204Pb ranging from 18.608 to 18.663, 207Pb/204Pb from 15.619 to 15.669, 208Pb/204Pb from 38.677 to 38.839) suggest that a single source supplied the metals, whereas the wide range of S isotopes (δ34S from −27.9‰ to 14.6‰ VCDT) indicates multiple sources of reduced sulfur including those from thermal sulfate reduction (TSR) and bacterial sulfate reduction (BSR). The organic matter in the ores is characterized by relatively low concentrations of saturated hydrocarbons, high concentrations of aromatic hydrocarbons, asphaltenes and non-hydrocarbons, and low maturity compared to that in the host rocks and equivalent strata away from mineralization. Both hydrocarbons in the ores and host rocks show little biodegradation. These characteristics of organic matter suggest that the site of mineralization was buried to significant depths and heated to above 80°C before the charging of hydrocarbons (thus non-development of biodegradation), that some of the hydrocarbons were consumed in mineralization (thus decreased maturity), probably used as reducing agents for sulfate reduction in situ, and that bacteria-derived sulfur in the ores was not generated in situ. Mineralization probably resulted from mixing of a metal-rich fluid with multiple fluids carrying reduced sulfur from different sources (including some reduced sulfur from in situ TSR). However, it remains unclear whether the metal-carrying fluid was derived from a near-surface groundwater system and flowed downward to the site of mineralization at burial conditions, or was derived from deeper parts of the basin and flowed up to the site of mineralization as part of a hydrothermal system; the distinction of the two models requires a better understanding of the thermal profile across the deposit.

Major Controls on the Evolution of the Cambrian Dolomite Reservoirs in the Keping Area, Tarim Basin

The Cambrian dolomite reservoir is an important target in oil and gas exploration. The Penglaiba section in the Keping area is typically examined in studies dealing with the Cambrian dolomite reservoirs of northwestern Tarim Basin. Based on sedimentological, petrographic, and geochemical data, lithofacies and fluids are identified as the major factors that control the dolomite reservoir in the study area. Lithoacies are fundamental to reservoir evolution because they provide suitable channels for dolomitization and dissolution of fluids that, in turn, facilitate the formation of high quality reservoirs. The lithofacies which could form high-quality reservoirs in the study area are: slope slip (collapse) facies, gypsum related facies, and algae dolomite facies. The sources of fluids include seawater, meteoric freshwater, diagenetic/hydrocarbon fluid, and hydrothermal fluid. These fluids lead to dolomitization, penecontemporaneous meteoric dissolution, hypergene dissolution, organic acid dissolution and hydrothermal dissolution that result in secondary porosity, and as such, they have a significant contribution to reservoir evolution.

A Framework of Rock Mechanics in Northwestern Tarim Basin, China and its Geological Implications

A Framework of Rock Mechanics in Northwestern Tarim Basin, China and its Geological Implications

Tectono-thermal evolution of the northwestern edge of the Tarim Basin in China: Constraints from apatite (U–Th)/He thermochronology

The northwestern edge of the Tarim Basin, which contains the Kalpin Uplift and the north Bachu Uplift, lies between the South Tianshan and Tarim continental block and its tectonic evolution is closely correlated with the Tianshan. The pre-Mesozoic units are widely exposed in the northwestern edge of the Tarim Basin, which allow detailed geological observation and sample collection. In this paper, the apatite (U–Th)/He thermochronology was used to study the tectono-thermal evolution of the northwestern edge of the Tarim Basin. The sandstone samples from the Upper Neoproterozoic, Silurian, Devonian, Carboniferous and Permian units, etc. on the northwestern edge of the Tarim Basin were collected and used to determine apatite (U–Th)/He ages. The apatite He ages in the Kalpin Uplift reveal the tectonic uplift events during the Late Cretaceous (96–70Ma) and the Miocene (22–20Ma). These uplift events are linked to collisions of the Lhasa Block and the India plate with the southern Eurasian continental margin, respectively. However, apatite He ages in the north Bachu Uplift show only the Eocene event at ∼40Ma because the west Kunlun orogenic belt thrust over the southwestern edge of the Tarim Basin during the Eocene. The thermal modeling results reveal four cooling events in the Kalpin Uplift since the Late Neoproterozoic, corresponding to 460–445Ma, 350–320Ma, 220–210Ma and 20–10Ma but only one episode with 220–210Ma in the north Bachu Uplift since the Devonian. Based on the thermal modeling results, the north Bachu Uplift did not experience the cooling events at 460–445Ma, 350–320Ma and 20–10Ma shown in the Kalpin Uplift. This paper provides a new insight into the tectonic evolution of the Tianshan and the northwestern Tarim Basin.

Phylogeography and conservation genetics of the relic Gymnocarpos przewalskii (Caryophyllaceae) restricted to northwestern China

Gymnocarpos przewalskii is restricted mainly to the arid desert areas of northwestern China, and is listed as a rare and endangered species of second conservation priority in the China Red Data Book. A phylogeographic study was conducted using two chloroplast regions (psbA–trnH and ycf6–psbM), together with various population genetic analyses, to examine genetic variation, population structure, and evolutionary history of the species. In all, 25 haplotypes were detected, 14 of which were found in the northwestern Tarim Basin of southern Xinjiang Autonomous Region. G. przewalskii showed high levels of total genetic diversity (h T = 0.849) and average gene diversity within populations (h S = 0.350). A higher N ST than G ST (P < 0.001) indicated significant phylogeographic structure across the species range. Nested clade phylogeographic analysis (NCPA) suggested that genetic structure in G. przewalskii has been heavily affected by past fragmentation, which likely resulted from aridity and the expansion of desert in northwestern China during the Quaternary. Mismatch analyses were consistent with NCPA in additionally suggesting contiguous range expansion for populations in the northern Tarim Basin and the Yumen region of Gansu Province, which may have occurred during a less arid interval. Human disturbance was identified as the greatest current threat to the species.

Biostratigraphic correlation of the Ordovician black shales in Tarim Basin and its peripheral regions

Ordovician black shales are widely distributed in the Tarim Basin and its peripheral regions, and some of them may serve as potential hydrocarbon source rocks. The present study of the Ordovician graptolite fauna from these shales, together with the yielded conodonts and chitinozoans etc., permits a refined correlation of the rocks. Based mainly on a new collecting of the graptolites and a study of the faunas in Kalpin and Kuruktag regions, and the successful identification of the new graptolite material from a few drill cores within the basin, we are able to update our knowledge of the Ordovician in the regions, and draw some conclusions: (1) The most widespread distribution of the black shales in the Tarim Basin and its peripheral regions, which correspond to the Nemagraptus gracilis Zone, may be related to a global sea-level rise during this time interval. (2) Black shales of Ordovician occur most frequently and extensively in the Kuruktag (also spelled as Quruq Tagh) region in eastern Tianshan Mountains, spanning Tremadocian to mid-Katian (D. spiniferus Zone) temporally and extending southeastwards into Manjiaer Depression. The black shales of the D. spiniferus Zone may even extend into the central Tarim Basin. (3) Three different bio- and litho-facies belts (Bachu: carbonate platform and reef belt; Kalpin-Aksu: marginal platform and upper slope belt; Wushi: slope belt) are recognized in the northwestern Tarim Basin. (4) The internationally well-correlated Saergan black shale, which has been considered to possess high potential for hydrocarbon source rock, may possess a restricted distribution in the Kalpin and Aksu areas.

Distribution Characteristics and Significance of Hydrocarbon Shows to Petroleum Geology in Kalpin Area, NW Tarim Basin

Abundant hydrocarbon shows are distributed in five different regions in the Kalpin area, northwestern Tarim Basin: Aksu hydrocarbon shows region, Subashi hydrocarbon shows region, Sanchakou hydrocarbon shows region, Piqiang hydrocarbon shows region, and Sickl hydrocarbon shows region, totaling 27 hydrocarbon show points. From the statistical analysis of all hydrocarbon shows in the study area, the paper summarizes the distribution characteristics. Combined with field investigation, previous documents of Kalpin area and oil-source correlation of hydrocarbon shows, the authors indicate that large multilayered oil-bearing paleo-reservoirs existed in the geological process in the Kalpin area, such as Aksu paleo-reservoirs and Piqiang paleo-reservoirs. Despite destruction by several periods of tectonic events after the formation of paleo-reservoirs, many reservoirs may still be left undamaged. Giant gas fields below the Middle Cambrian gypsum-salt bed could be preserved in the Kalpin area. In addition, the Middle-Lower Permian Balikelike formation, with good oil generating condition is an important exploration target in the Kalpin area.

Pyrite trace element geochemistry of mafic granulite xenoliths from Xikeer: implications for the source of Cu in the sediment-hosted mineralization in the northwestern Tarim Basin (Northwest China)

The Xikeer mafic granulite xenoliths are hosted by a Cenozoic basanite sill (∼20 Ma) in the northwest Tarim Basin, northwest China. Sulfides, identified in these xenoliths consist mainly of pyrite. Two groups of pyrite (types A and B) can be distinguished based on petrography and trace element geochemistry, as determined by laser-ablation ICP-MS analysis. Type A pyrite is subhedral and in general has lower trace element abundance than type B pyrite. Type B pyrite is fractured with ragged anhedral morphology and has extremely high Cu, Tl, Ni, and Co abundance. The low Co and Ni contents combined with a positive correlation between Cu, Cr, and Pb all indicate that the type A pyrite may have formed from a high-temperature magmatic hydrothermal fluid. In contrast, the Cu-, Tl-, Ni-, and Co-rich type B pyrite was probably formed in a relatively low-temperature basinal environment. The Cu mineralization at Xikeer can be explained when Cu, Tl, Ni, and Co were leached from the intruded basanites and subsequently enriched in circulating basin brines. Such enrichment of Cu in basin brines could provide an important Cu source for the Cu mineralization in the Xikeer district and other Cenozoic sub-basins in the Tarim Basin.

Carbon and strontium isotopes and global correlation of Cambrian Series 2-Series 3 carbonate rocks in the Keping area of the northwestern Tarim Basin, NW China

One hundred and ten carbon and nineteen strontium isotopic values of outcropping Cambrian Series 2 and Series 3 carbonate rocks in the Penglaiba section of the Keping area were analyzed. Effective isotopic data with little influence of diagenesis were used to address the global correlation. The δ 13C values exhibit two major positive excursions (peaking at 3.1‰ and 3.3‰) and three major negative excursions (peaking at −3.0‰, −4.2‰ and −3.2‰). The carbon isotope excursions (peaking at −3.0‰ and −4.2‰) across the Cambrian Series 2-Series 3 boundary show good correlations with similar variations reported in Siberia, Laurentia, North China and South China. In contrast, the other three carbon isotope excursions (peaking at 3.1‰, 3.3‰ and −3.2‰) do not have a fairly good global correlation because of the lack of biostratigraphic data. With respect to the 87Sr/ 86Sr ratios, they show good correlation with those reported for Laurentia, and further support the global δ 13C comparison. On the basis of these new data, it is showed that the combination of δ 13C curves and 87Sr/ 86Sr variations serves as a powerful tool for correlation and subdivision of Cambrian strata in the Tarim Basin of northwest China, and provides new data for global correlation.

Paleozoic structural deformation of Bachu uplift, Tarim basin of Northwest China: Implications for plate drifting

Based on high-resolution 2D seismic profiles, the Paleozoic structural deformation characteristics of Bachu (巴楚) uplift of northwestern Tarim basin, NW China, are exhibited in this article. The deformation happened during three main geological periods: the end of Middle-Late Ordovician (O2–3), the end of Early-Middle Devonian (D1–2), and the end of Late Permian (P2). In the Bachu uplift, there developed a series of NW-trending thrust faults and imbricate structures due to the effect of the NW-SE compression stress towards the end of Middle-Late Ordovician (O2–3) (middle Caledonian movement), and there developed some NNE-trending thrust faults and fault blocks under the control of the NEE-SWW compression stress at the end of Early-Middle Devonian (D1–2) (early Hercynian movement). However, at the end of Late Permian (P2) (late Hercynian movement), some NE-trending thrust faults and associated folds developed as a result of the NE-SW compression stress. The first-stage (O2–3) deformation is obviously more violent than those of the latter two stages (D1–2 and P2), which implies that the Tarim plate drifted quickly to the north at around the same time basin.

Temporal constraints and pulsed Late Cenozoic deformation during the structural disruption of the active Kashi foreland, northwest China

In response to the ongoing Indo‐Asian collision, structural deformation has encroached into the Tian Shan foreland in western China since the early Miocene. In order to reconstruct a detailed history of foreland deformation along the southern margin of the Tian Shan, we have synthesized extensive mapping, analysis of seismic sections, magnetostratigraphy of the foreland fill and associated growth strata, apatite fission track dating, changes in sediment accumulation rates, and geodetic surveys of deformed fluvial terraces. Across an area exceeding 6500 km2 that spans the Kashi foreland in the northwestern Tarim Basin, we use these data to place temporal constraints on individual structures that delineate at least 4 distinct stages of deformation. Stage 1 involved initial uplift of hinterland structures beginning at 20–25 Ma. Stage 2 began at ∼16.3 Ma when the basement‐involved deformation front stepped south to the Kashi Basin Thrust that bounds the foredeep strata. Stage 3 occurred from 13.5 to 4 Ma, as the deformation front migrated south in episodic steps above thin‐skinned detachment and ramp‐flat folding. Finally, stage 4 began after 4 Ma with growth of the Keketamu Anticline followed by rapid southward migration of the deformation front to the Atushi and then Kashi detachment anticlines. On the basis of the deformed Tertiary strata and available seismic data, calculations of the minimum magnitude of shortening yield totals ranging from 10 to 32 km since the early Miocene, of which 7–12 km occurred since ∼4 Ma. Although average shortening rates since 16.3 Ma were 0.8–1.3 mm/a, rates were faster from 16.3 to 13.5 Ma (1.1–3.2 mm/a), after which they decreased to 0.5 mm/a between 13.5 and 4 Ma before increasing to 2.2–2.7 mm/a after 4 Ma. Similarly, the rate of southward propagation of the deformation front was initially 3–7 mm/a for the first 10 Ma of basin development, slowed to 1.5 mm/a between 13.5 and 4 Ma, and increased to more than 10 mm/a since 4 Ma. These changes in shortening and structural migration rates over time suggest that the loci of orogenic deformation shifted in and out of the foreland during the Neogene. Surveyed stream terraces suggest that, although most current deformation is located along the southern front of the foreland, modest deformation and shortening continue in the hinterland. Structural style within the foreland has also changed over time from imbricate thrust faults with hanging wall folds along the northern margin, to ramp‐flat faults and detachment folds along the southern margin. Where the basement rocks are previously deformed, such as in the hinterland of the Kashi foreland, thick‐skinned fault ramps and related fault propagation folds dominate the structural style. As syntectonic sedimentation within the foreland produced thick, subhorizontal stratigraphy, the structural style evolved to detachment and fault bend folding above these footwall flats within the Neogene strata.

Dual influence of the rejuvenation of Southern Tianshan and Western Kunlun orogen on the Cenozoic structure deformation of Tarim Basin, northwestern China: A superposition deformation model from Bachu Uplift

Based on new high-resolution seismic profiles and existing structural and sedimentary results, a superposition deformation model for Cenozoic Bachu Uplift of northwestern Tarim Basin, northwestern China is proposed. The model presents the idea that the Bachu Uplift suffered structure superposition deformation under the dual influences of the Cenozoic uplifting of Southern Tianshan and Western Kunlun orogen, northwestern China. In the end of the Eocene (early Himalayan movement), Bachu Uplift started to be formed with the uplifting of Western Kunlun, and extended NNW into the interior of Kalpin Uplift. In the end of the Miocene (middle Himalayan movement), Bachu Uplift suffered not only the NNW structure deformation caused by the Western Kunlun uplifting, but also the NE structure deformation caused by the Southern Tianshan uplifting, and the thrust front fault of Kalpin thrust system related to the Southern Tianshan orogen intrudes southeastward into the hinterland of Bachu Uplift and extends NNE from well Pi1 to Xiaohaizi reservoir and Gudongshan mountain, which resulted in the strata folded and denuded strongly. In the end of the Pliocene (late Himalayan movement), the impact of Southern Tianshan orogen decreased because of the stress released with the breakthrough upward of Kalpin fault extending NE, and Bachu Uplift suffered mainly the structure deformation extending NW-NNW caused by the uplifting of Western Kunlun orogen.

Late Neoproterozoic paleomagnetic results from the Sugetbrak Formation of the Aksu area, Tarim basin (NW China) and their implications to paleogeographic reconstructions and the snowball Earth hypothesis

In order to better constrain the Neoproterozoic paleogeographic reconstruction of continents and to improve the understanding of the snowball Earth hypothesis, paleomagnetic investigations were carried out in the Aksu area of the northwestern Tarim basin. Forty-eight sites of samples were collected from the Sugetbrak and Chigebrak Formations. Twenty-four sites of sandstone and volcanic rock from the Sugetbrak Formation revealed stable characteristic remanent components (ChRm) isolated between 500 and 680 °C. The computed magnetic directions from these components are consistent and significantly distinguished from those of younger ages. Both normal and reverse polarities have been observed, and a positive fold test is revealed after bedding corrections at 95% confidence level. Anisotropy of magnetic susceptibility measurements shows primary sedimentary fabrics with weak anisotropy degrees. A paleomagnetic pole is calculated at 19.1°N, 149.7°E, k = 11.2, A 95 = 9.3° with n = 24, yielding a paleolatitude of ∼27°N for the sampling area. The chemo-stratigraphic correlation of this section with reference ones reveals an average age of ∼595 Ma. A new paleogeographic reconstruction has been attempted showing general low paleolatitude positions for the Australia, South China and Tarim blocks at about 600 Ma. This observation provides new evidences for the snowball Earth hypothesis.

Internal structure and paleoecology of the lower Permian Uzunbulak reef complex of the Tarim Basin, Northwest China

The internal construction and biotic communities of the Uzunbulak reef of the northwestern Tarim Basin are studied for the first time. The reef was built during the Sakmarian, while the reef substrate and capping beds are of latest Asselian and earliest Artinskian ages, respectively. The reef substrate beds are composed of skeletal and oncoid grainstone. Those fusulinid-dominated skeletal shoals and oncoid banks indicate a high-energy environment and produced local topographic highs on which the reef grew. Reef framework consists mainly of calcisponge bafflestone, calcisponge-Thartharella framestone, and Tubiphytes, Archaeolithoporella and Girvanella boundstones. Calcisponges were the primary frameconstructors that baffled high-energy currents. Archaeolithoporella, Tubiphytes, Girvanella and possibly microbes acted as the primary binders for the boundstone framework. Fusulinids and brachiopods were common reef dwellers. The interreef facies sediments are composed of skeletal-crinoid wackestone-packstone. Most of bioclasts have thick, micritized envelopes. The back-reef facies deposits consist of alternating skeletal packstone to wackestone and black shale. Sea-level fluctuations were probably accountable for the reef growth and demise.

Surveying of the deformed terraces and crust shortening rate in the northwestern Tarim Basin: Comment

Surveying of the deformed terraces and crust shortening rate in the northwestern Tarim Basin: Comment

Surveying of the deformed terraces and crust shortening rate in the northwestern Tarim Basin

Relief surveying and chronology study were carried out on the deformed river terraces across the Artushi anticline belt in the northwestern Tarim Basin. The crust shortening rate of this anticline belt since late Pleistocene was calculated to be up to 5-6 mm/a. The total crust shortening rate from the northwestern Tarim Basin to southwestern Tianshan since late Pleistocene was estimated to be over 10 mm/a.