Provenance of Conglomerate and Sandstone from Early Permian Shoushangou Formation in Xi Ujimqin, Inner Mongolia: Implications for Understanding Paleo-Asian Ocean Subduction

During the Late Carboniferous to Early Permian, a rift was formed by post-collisional extension after ocean closure or an island arc-related basin formed by Paleo-Asian Ocean (PAO) subduction in the Xi Ujimqin area. Nevertheless, the closure time of the PAO is still under debate. Thus, to identify the origin of the PAO, the geochemistry and U-Pb age of zircons were analyzed for the extra-large deep marine, polymict clastic boulders and sandstones in the Shoushangou Formation within the basin. The analyses revealed magmatic activity and tectonic evolution. The conglomerates include megaclasts of granite (298.8 ± 9.1?Ma) and granodiorite porphyry (297.1 ± 3.1?Ma), which were deposited by muddy debris flow. Results of this study demonstrated that the boulders of granitoids have the geochemistry of typical I-type granite, characterized by low Zr + Nb + Ce + Y and low Ga/Al values. The granitoid boulders were formed in island arc setting, indicating the presence of arc magmatism in the area that is composed of the Late Carboniferous to Early Permian subduction-related granitoid in southern Xi Ujimqin. Multiple diagrams for determining sedimentary provenance using major and trace elements indicate that Shoushangou sediments originated from continental island arc-related felsic rocks. Detrital zircon U-Pb age cluster of 330–280?Ma was obtained, indicating input from granite, ophiolite, Xilin Gol complex, and Carboniferous sources to the south. The basin was geographically developed behind the arc during the Early Permian period because the outcropped intrusive rocks in the Late Carboniferous to Early Permian form a volcanic arc. The comprehensive analyses of source areas suggest that Shoushangou sediments developed in a backarc basin in response to the northward subduction of the PAO. The backarc basin and intrusive rocks, in addition to previously published Late Carboniferous to Early Permian magmatic rocks of arc unit in Xilin Gol, confirm the presence of an Early Permian trench-arc-basin system in the region, represented by the Baolidao arc and Xi Ujimqin backarc basin. This study highlights the importance and potential of combined geochemical and geochronological studies of conglomerates and sandstone for reconstructing the geodynamic setting of a basin.     

珠江口盆地西江主洼油气差异分布机制

西江主洼是珠江口盆地一个低勘探程度洼陷，油气差异聚集特征明显，但其机理不清。本文在断陷盆地油气成藏理论指导下，利用研究区地质、地球物理和地球化学等资料，开展了烃源岩、断盖组合、储集体系和运聚模式等方面的研究。结果表明，裂陷期沉降、沉积中心有序迁移，造成主力烃源岩自东向西由文四段迁移至文三段、文一+二段，控制东、西部油气差异分布。恩平组区域性泥岩发育且晚期断裂缺乏，导致油气纵向上更易聚集于下构造层；区域泥岩减薄尖灭或晚期断裂切开盖层的区域，上构造层有一定油气分布。"源-汇"类型从宏观上控制储层优劣，影响下构造层油气富集程度；"仓储"运移是控制上构造层油气规模聚集的主要模式。下构造层古近系应作为重点部署方向，东部围绕文四段、西部围绕文三段、文一+二段烃源岩构成的含油气系统展开；上构造层勘探需关注珠海组，在隆起周边寻找具备"仓储"运移模式的有利区带。     

杨山晚古生代沉积盆地成因类型及其与桐柏-大别造山带关系的探讨

杨山晚古生代沉积盆地位于桐柏-大别山北麓,它具有明显的前陆盆地沉积特点,由早期(D₂?—C₁)的复理石建造到晚期(C₁—P?)的磨拉石建造;古生物地理分析表明其与华北、扬子陆块都有密不可分的联系,其间不可能有古洋盆的存在,因而它应当是桐柏一大别造山带碰撞造山过程中形成的前陆盆地。杨山晚古生代前陆盆地的形成说明,扬子陆块和华北陆块的陆-陆碰撞起始于晚泥盆世之前(S₃—D₂),而桐柏-大别造山带中生代的构造事件则可能代表一次大规模陆内逆冲-推覆作用。     

The role of sediments in the phosphorus cycle in Lake Lugano. I. Geochemical approach

The bottom sediment from three coring stations in Lake Lugano (Lago di Lugano) show major differences in their role in the P-cycles related to their geochemistry and characteristics of sedimentation. In the northern basin, the deepest sediment may be considered practically inactive, due to a permanently reduced condition at the sediment-water interface. In the southern basin, the sediments are active with respect to P-recycling with strong seasonal variations. One of the sites (Figino) behaves as a sink for P due to a high iron content and an important rate of detrital sedimentation.     

近50 a渭河流域洪水成因分析及防治对策

 通过对近50 a渭河流域洪水的年际变化、月际变化和潼关高程变化的综合分析得出，造成该流域洪水灾害的原因有降水量年内分配不均和年际变化大，滩面淤积加重，支流河口淤塞以及河势、流态的恶化等。针对这些原因，根据渭河流域洪水灾害的特点，提出了相应的防治措施，即降低下游高程，减少河道的淤积，增大河道泄洪能力；利用水库进行调水调沙，引进客水冲刷渭河下游；防洪工程要除险加固，提高防洪标准与抗洪能力；恢复林草植被，遏止水土流失，从源头上控制泥沙入河等。     

Weathering and climate change since l820 AD in Hoh Xil, Qinghai-Xizang Plateau, China——Evidence from element geochemistry of lake sediments of Gulug Co Lake

Element geochemistry of lake sediments has been widely used to detect climate change because element composition and ratios can reflect the weathering degree in the source area. Given the dement composition of lake sediments from Gulug Co Lake, Hoh Xil, Qing- hai-Xizang Plateau, chemical index of alteration （CIA）, index of composition variability （ICV） and other element ratios have been used to establish the weathering sequence of this area since 1820 AD. The weathering is so weak that the element composition change is more sensitive to climate change and autochthonous processes. From 1820 to 1984 AD, there were two drier periods with a wetter interval from 1870 to 1945 AD. After 1984 the weather showed a tendency of becoming wet.     

From source terrains of the Eastern Alps to the Molasse Basin: Detrital record of non-steady-state exhumation

Fission-track cooling ages of detrital apatite (AFT) in the East Alpine Molasse Basin display age groups corresponding to geodynamic events in the orogen since Jurassic times. These age groups are typical of certain thermotectonic units, which formed a patchwork in the Swiss and Eastern Alps. By a combination of petrographic and thermochronologic data, progressive erosion of source terrains is monitored in different catchments since the Oligocene. The AFT cooling ages show a decrease in lag time until when rapidly cooled debris derived from tectonically exhumed core complexes became exposed. After termination of tectonic exhumation, lag times of debris derived from the core complexes increased. Neither on the scale of the entire Eastern Alps, or on the scale of individual catchments, steady-state exhumation is observed, due to the highly dynamic changes of exhumation rates since Late Eocene collision.     

Paleoenvironmental records of water level and climatic changes from the middle to late Holocene at a Lake Erie coastal wetland, Ontario, Canada

Pollen and diatom assemblages, and peat stratigraphies, from a coastal wetland on the northern shore of Lake Erie were used to analyze water level and climatic changes since the middle Holocene and their effects on wetland plant communities. Peat deposition began 4700 cal yr B.P. during the Nipissing II transgression, which was driven by isostatic rebound. At that time, a diatom-rich wild rice marsh existed at the site. Water level dropped at the end of the Nipissing rise at least 2 m within 200 yr, leading to the development of shallower-water plant communities and an environment too dry for most diatoms to persist. The sharp decline in water level was probably driven primarily by outlet incision, but climate likely played some role. The paleoecological records provide evidence for post-Nipissing century-scale transgressions occurring around 2300, 1160, 700 and 450 cal yr B.P. The chronology for these transgressions correlates with other studies from the region and implies climatic forcing. Peat inception in shallow sloughs across part of the study area around 700 cal yr B.P. coincides with the Little Ice Age. These records, considered alongside others from the region, suggest that the Little Ice Age may have resulted in a wetter climate across the eastern Great Lakes region.     

Tracking recorded fires using charcoal morphology from the sedimentary sequence of Prosser Lake, British Columbia (Canada)

Quantitative analyses of variations in morphological features of charcoal were undertaken in a ²¹⁰Pb-dated sediment core from Prosser Lake (British Columbia, Canada). Seven morphological types of charcoal were defined by particle shape, major-minor axis ratio, apparent porosity and progradation to unburned material. The distribution of morphotypes and total charcoal abundances were assessed as a proxy for fire events recorded between 1919 and 2000 and to subsequent mechanisms of transportation-sedimentation to lake sediments. Charcoal morphotypes showed distinct relationships to recorded area burned by fires. Fragile charcoal fragments with highly irregular porosity (termed Type M) displayed the strongest correlation to burned area (r² = 0.51; P = 0.0001) and did not produce any false-positive signal for fires recorded within a radius of 20 km around the lake. We infer that high porosity and low density Type M fragments are aerially transported and directly deposited on the lake, and that the fragility of Type M charcoal prevents significant quantities from being secondarily transported and incorporated into the sedimentary record. We propose that charcoal morphology is an important but underutilized technique that can yield important insights into fire type, proximity and transportation-sedimentation processes.     

Stratigraphic evidence for multiple drainings of glacial Lake Missoula along the Clark Fork River, Montana, USA

Glacial Lake Missoula, a source of Channeled Scabland flood waters, inundated valleys of northwest Montana to altitudes of ∼ 1265 m and to depths of  >600 m, as evidenced by shorelines and silty lacustrine deposits. This study describes previously unrecognized catastrophic lake-drainage deposits that lie stratigraphically beneath the glacial-lake silts. The unconsolidated gravelly flood alluvium contains imbricated boulder-sized clasts, cross-stratified gravel with slip-face heights of 2-> 35 m, and 70- to 100-m-high gravel bars which all indicate a high-energy, high-volume alluvial environment. Gravel bars and high scablands were formed by catastrophic draining of one or possibly more early, high lake stands (1200-1265 m). Most glacial-lake silt, such as the Ninemile section, was deposited stratigraphically above the earlier deposits, represents a lower lake stand(s) (1050-1150 m), and was not deposited in lake(s) responsible for the highest discharge events. The glaciolacustrine silt-covered benches are incised by relict networks of valleys formed during the drainage of the last glacial lake. Significant erosion associated with the last lake draining was confined to the inner Clark Fork River canyon.