RETURN PERIOD AND THE TREND OF EXTREME DISASTROUS RAINSTORM EVENTS IN ZHEJIANG PROVINCE

A provincial Disaster-causing Rainstorm Severity Index (DRaSI) is introduced to quantify the relationship between rainfall and its disastrous impacts on Zhejiang province of China, shortened as ZJ-DRaSI. ZJ-DRaSI is set up based on the DRaSI for single stations in combination with the coverage of rainstorms. The probability distribution function (PDF) of ZJ-DRaSI between 1971 and 2015 can be well fitted by the Wakeby Distribution with five parameters. It is found that decadal (e.g. 10yr, 20yr, and so on) return period values of ZJ-DRaSI related to typhoons are generally lower than that of non-typhoon events, implying that disastrous non-typhoon events have a higher frequency of occurrence. The extreme typhoon events have a significant cycle of 22.5 years, while the non-typhoon events have a significant cycle of 15 years. Both are currently at the high-value phase. The annual extreme value of ZJ-DRaSI exhibits an increasing trend of approximately 15% every 10 years.     

豫西济源地区中下三叠统遗迹网络分析

豫西济源地区下三叠统和尚沟组和中三叠统二马营组均为陆相河湖相沉积,本研究在和尚沟组和上覆二马营组中下段分别识别出遗迹化石5属6种和7属9种,这些化石材料成为探究大灭绝事件后陆地造迹生物复苏的良好材料。运用遗迹网络分析方法构建了和尚沟组和二马营组中下段遗迹网络,相关参数显示自早三叠世和尚沟组沉积期至中三叠世二马营组中下段沉积期,遗迹网络节点增加,平均度、平均加权度和图密度有所降低,网络直径、平均聚类系数和平均路径长度呈明显增加趋势,遗迹网络复杂化以及造迹生物对恶劣环境适应性的增强,这些可能是二叠纪—三叠纪之交灭绝事件(Permian-Triassic Mass Extinction, PTME)后生物复苏的潜在标志。推测PTME后华北地区生物复苏和生态恢复可能遵循着类似的模式,但后续研究仍需陆相实体化石证据的佐证与补充。     

琼东南盆地深水区WN-A井渐新世—中新世孢粉地层学及古气候

琼东南盆地深水区是21世纪南海北部大陆架地区天然气勘探的重点区带之一。为了划分该盆地WN-A井的地层时代及探明其沉积物所记录的古气候条件,笔者对该井分布于3 261.25~5 079.00 m井段共计108份岩屑样品进行了孢粉分析。研究结果显示,该井孢粉化石种类较丰富且分布连续,由下至上可划分为4个孢粉组合: Pinuspollenites-Polypodiaceoisporites-Juglanspollenites-Quercoidites E-Quercoidites D组合、Pinuspollenites-Juglanspollenites-Polypodiaceoisporites-Dicolpopollis kockelii组合、Quercoidites E-Polypodiaceoisporites-Pinuspollenites-Polypodiisporites组合和Pinuspollenites-Polypodiaceoisporites-Randiapollis reticulatus-Quercoidites E组合。其中：4 425.00~5 079.00 m井段孢粉组合的优势组分是Pinuspollenites、Juglanspollenites和Quercoidites D,指示热带、亚热带针叶、常绿、落叶阔叶混交林,气候较温凉;3 261.25~4 425.00 m井段孢粉组合中Quercoidites E占优势,指示热带、亚热带常绿、落叶阔叶混交林,气候温暖湿润。WN-A井孢粉组合记录了一次明显的由温凉转为温暖的气候变化,与全球渐新世—中新世的气候变化可以对比,其发生位置,即4 425.00 m可以作为WN-A井渐新统和中新统的界线。     

Petrogenesis and tectonic implications of early Palaeozoic igneous rocks of the western South Qilian Belt,central China

Early Palaeozoic granitoids in the South Qilian Belt, central China, record details of the tectonic evolution and crustal growth of the Qilian orogenic belt. Five representative granitoids from the western South Qilian Belt were sampled for zircon LA-ICPMS U–Pb dating, Lu–Hf isotopes, and whole-rock geochemical analyses. Zircon U–Pb dating of two porphyritic granodiorites and a porphyritic monzogranite yielded ages of 442.7 ± 3.5, 441.8 ± 4.3, and 435.4 ± 3.5 Ma, respectively. These granitoids exhibit a geochemical affinity to I-type granite, are metaluminous with a low aluminium saturation index (A/CNK = 0.75–1.15), have moderate Al₂O₃ and low MgO contents, high La/Yb and low Sr/Y ratios, and are depleted in Nb, Ta, P, and Ti, which suggests a subduction zone magmatic arc affinity, with mixing between a primary mantle-derived magma with lesser continental crustal material. The syenogranite and monzogranite from the South Qilian Belt, which yield U–Pb zircon ages of 440.4 ± 9.0 and 442.3 ± 1.2 Ma, respectively, have pronounced S-type geochemical affinities, are peraluminous with A/CNK values of 1.07–1.16, have relatively high SiO₂, Al₂O₃, K₂O, and Rb contents, low Y and Yb, low Sr/Y and La/Yb ratios, positive Th, U, and light Rare Earth Element (REE) anomalies, and depletions in Nb, Ta, Sr, and Ti. Their geochemical signature suggests derivation from partial melting of continental crust in a syn-collisional setting. The Hf isotopic data of zircons from the granitoids show a significant input of Paleoproterozoic crust in the crustal formation of the western South Qilian Belt in Palaeozoic. Compare the ε_Hf(t) value of S-type granite with that of I-type granite, the former may have a comparatively homogeneous source. Together with regional evidence, it is proposed that a collisional event occurred between the South Qilian Belt and the Central Qilian Belt at ca. 442–435 Ma.     

Variable character and diverse origin of hybrid event beds in a sandy submarine fan system,Pennsylvanian Ross Sandstone Formation,western Ireland

Hybrid event beds comprising both clean and mud‐rich sandstone are important components of many deep‐water systems and reflect the passage of turbulent sediment gravity flows with zones of clay‐damped or suppressed turbulence. ‘Behind‐outcrop’ cores from the Pennsylvanian deep‐water Ross Sandstone Formation reveal hybrid event beds with a wide range of expression in terms of relative abundance, character and inferred origin. Muddy hybrid event beds first appear in the underlying Clare Shale Formation where they are interpreted as the distal run‐out of the wakes to flows which deposited most of their sand up‐dip before transforming to fluid mud. These are overlain by unusually thick (up to 4·4 m), coarse sandy hybrid event beds (89% of the lowermost Ross Formation by thickness) that record deposition from outsized flows in which transformations were driven by both substrate entrainment in the body of the flow and clay fractionation in the wake. A switch to dominantly fine‐grained sand was accompanied initially by the arrest of turbulence‐damped, mud‐rich flows with evidence for transitional flow conditions and thick fluid mud caps. The mid and upper Ross Formation contain metre‐scale bed sets of hybrid event beds (21 to 14%, respectively) in (i) upward‐sandying bed set associations immediately beneath amalgamated sheet or channel elements; (ii) stacked thick‐bedded and thin‐bedded hybrid event bed‐dominated bed sets; (iii) associations of hybrid event bed‐dominated bed sets alternating with conventional turbidites; and (iv) rare outsized hybrid event beds. Hybrid event bed dominance in the lower Ross Formation may reflect significant initial disequilibrium, a bias towards large‐volume flows in distal sectors of the basin, extensive mud‐draped slopes and greater drop heights promoting erosion. Higher in the formation, hybrid event beds record local perturbations related to channel switching, lobe relocations and extension of channels across the fan surface. The Ross Sandstone Formation confirms that hybrid event beds can form in a variety of ways, even in the same system, and that different flow transformation mechanisms may operate even during the passage of a single flow.     

Chronostratigraphic framework and depositional environments in the organic‐rich,mudstone‐dominated Eagle Ford Group,Texas, USA

Since the beginning of the century, several authors have hypothesized and documented the presence of bottom currents during the deposition of mudstones, including mudstones rich in organic matter, challenging the assumption that persistent low‐energy conditions are necessary prerequisites for deposition of such sediments. More processes responsible for transport and deposition of mudstones mean also more processes acting contemporaneously in different parts of a basin. Without a precise and robust chronostratigraphic framework, however, it is not possible to characterize these differences. The new data reported here provide a profoundly different understanding of the controls on sedimentation in distal continental shelf platforms. To enhance the understanding of the different coeval environments of deposition coexisting in a muddy system, the Upper Cretaceous Eagle Ford Group, deposited on the Comanche carbonate platform, has been investigated by integrating sedimentology, mineralogy, geochemistry and palaeoecology, and creating age models in different physiographic sectors using biostratigraphy and geochronology. Data from two cores and 41 outcrops were analysed with a telescopic approach, from grain scale to basin scale. Nine temporal stages over a ca 8 Myr interval (ca 98 to 90 Ma) were defined in an area that spans 75 000 km². Finally, the different environments of deposition recorded within each of the nine stages were interpreted. The construction of the chronostratigraphic framework also allowed: measuring the duration of a basin‐wide gradational increase in energy in the water column (ca 1 Myr) and a hiatus confined into the shallower water sector (ca 2 Myr); determining the mean eruption frequency of volcanoes (ca 9 kyr); and the time of inundation of the Western Interior Seaway (97·5 to 97·1 Ma). The context, the outcrops–cores–logs correlations, the large data set (Appendix  S1 ), the high‐precision and well‐calibrated constraints represent an unprecedented contribution for future regional facies models of organic‐rich units and for improvements of key aspects in the industry of unconventional resources.     

华北克拉通西部基底早前寒武纪地质事件、性质及其地质意义

综合华北克拉通西部陆块阴山地块、孔兹岩带和鄂尔多斯地块基底正、副片麻岩以及鄂尔多斯地块现代河流沙锆石U-Pb和Hf同位素资料,并根据最新获得的鄂尔多斯基底及盖层继承碎屑锆石U-Pb年龄及Hf同位素统计分析揭示,该陆块存在古、中太古代陆壳物质,自新太古代以来先后发生了新太古代~2.7Ga、2.55~2.45Ga以及古元古代2.2~2.0Ga和1.95~1.85Ga等多期构造热事件。由基底和盖层中各类岩石中获得的~2.5Ga锆石的εHf(t)多为正值,Hf陆壳模式年龄(t_(DM)~C)介于2.9～2.6Ga之间(峰值～2.75Ga),阴山地块存在～2.7Ga的岩石,鄂尔多斯地块基底有～2.7Ga继承锆石记录,证明新太古代存在一期重要的陆壳生长。2.55～2.45Ga的岩浆活动在西部陆块不同地质单元基底岩石中均有记录,出现大量壳源花岗岩和幔源岩浆侵入及麻粒岩相变质作用,它们的锆石εHf(t)值由负到正变化大,Hf陆壳模式年龄(t_(DM)~C)除少数接近岩浆活动年龄外,多数明显高于它们的形成年龄,指示了强烈的陆壳再造和一定陆壳生长,岩石组合及地球化学特征反映了汇聚挤压转为伸展环境,类似于华北陆块～2.5Ga广泛的岩浆变质事件及构造背景,揭示了不同陆块碰撞拼合而后转为伸展的构造演化过程。古元古代2.2～2.0Ga期间,沿鄂尔多斯地块北部及东部边缘出现大洋俯冲消减有关的陆缘弧花岗岩类,它们的锆石εHf(t)和t_(DM)~C值变化范围很大,表明在古元古代中晚期鄂尔多斯地块基底仍发生有陆壳增生和再造。此后在1.95～1.85Ga期间,沿鄂尔多斯地块北部的孔兹岩带和东部的中部构造带均发生顺时针P-T演化轨迹的变质作用,证明～1.95Ga鄂尔多斯地块相继与北部阴山地块和东部陆块碰撞拼合为一体,至～1.85Ga发生陆壳抬升与伸展、发生陆壳物质减压熔融的强烈混合岩化和大量S型花岗岩形成,其后发生镁铁质岩墙侵入,标志着华北陆块最终克拉通化完成。     

鄂尔多斯盆地西、南缘奥陶纪地质事件群耦合作用

北祁连造山带和北秦岭造山带在早古生代经历了相似的洋陆转化过程,于奥陶纪时发育了汇聚板块边缘的沟-弧-盆体系,分别形成了北西向展布的北祁连造山带走廊南山北缘早古生代岛弧及弧后盆地和东西向展布的北秦岭造山带早古生代岛弧及弧后盆地。期间,秦岭-祁连洋的俯冲造山作用和鄂尔多斯盆地西南缘沉积类型和内陆湖盆的发展演化之间存在有机的联系,构成了盆-山耦合体系,引发一系列构造事件、火山喷发事件和多种类型的事件沉积等。它们之间存在着一系列成因机制上的联系,有着共同的宏观背景。鄂尔多斯盆地西、南缘在几乎相同时期存在一次构造背景的转变,由被动大陆边缘转化为主动大陆边缘,并诱发了多期火山喷发事件,在盆地西南缘奥陶系形成多套斑脱岩夹层,这些斑脱岩可能为同时期或者稍后的钾盐矿(包括含钾卤水)的形成提供了重要物源。同时,鄂尔多斯盆地南缘由浅水碳酸盐台地陷落为深水斜坡,在盆地西、南缘奥陶系有规律的集中发育重力流沉积(海底扇、浊积岩等)、滑塌沉积和震积岩等事件沉积。从形成机制上,华南板块向北俯冲触发了火山活动和地震,火山喷发在奥陶系集中沉积了多套凝灰岩夹层,地震活动导致同时期大套重力流沉积,并触发相对深水区沉积物向深水区移动,使得重力流沉积转化为浊流沉积,形成了具有良好储层的浊积岩。统计表明,上述事件发育的时间与秦岭地区构造活动相对最活跃的时期基本一致。因此这些分布稳定的凝灰岩薄层和中奥陶世集中有规律分布的重力流沉积砂体为华南板块向华北本快俯冲背景下形成的,它们之间存在耦合关系。     

Origin and variability of a terminal Proterozoic primary silica precipitate,Athel Silicilyte,South Oman Salt Basin,Sultanate of Oman

The Precambrian–Cambrian Athel Silicilyte is a 400 m thick, salt‐encased siliceous succession in the South Oman Salt Basin. It is a self‐sourcing hydrocarbon reservoir and comprises up to 95% microcrystalline quartz and exhibits wavy discontinuous lamination, comprising thin, alternating organic‐rich and silica‐rich layers. Textures and geochemical fingerprinting indicate that it is a primary precipitate formed by microbially mediated precipitation of silica from sea water, within the water column at the sulphidic/oxic interface. The unique occurrence of the Athel Silicilyte in the terminal Proterozoic implies that optimal conditions for this style of silica precipitation occurred only briefly. Basin anoxia, coupled with the growth of microbial mats, low pH and high silica pore water saturations, created optimal chemical conditions for silica precipitation. Volumes of microcrystalline quartz are highest within the transgressive and early highstand systems tract and towards the centre of the Athel Basin. At the basin margins, and within the late highstand systems tract, volumes of microcrystalline quartz decreased as the volume of detrital sediment increased. Mass‐balance calculations indicate that silica‐enriched sea water would have been supplied to the basin by infrequent marine incursions that replenished ambient sea water in the upper part of the water column. In conclusion, precipitation of the Athel Silicilyte was driven by the coincidence of basin restriction, limited clastic input, availability of organic matter and water column anoxia. The observation that there are few documented examples of chert deposits younger than ca 700 Ma, prior to the Cambrian explosion, suggests that although silica budgets within marine basins probably remained high prior to the evolution of silica‐secreting organisms, direct precipitation from sea water was restricted. This is tentatively related to the gradual increase in alkalinity of sea water through the Palaeo‐Proterozoic and Meso‐Proterozoic, such that silica precipitation could only occur through the coincidence of basin anoxia and low siliciclastic input.     

藏南仲巴地区早白垩世日朗组物源分析及其构造意义

藏南仲巴地区早白垩世日朗组出露于特提斯喜马拉雅北亚带,整体为黄绿色火山岩屑砂岩,局部层位可见页岩与泥岩,分析为一套深海海底扇沉积组合。本文仔细分析了日朗组砂岩岩石学特征及鲍马序列和槽模沉积构造等沉积学特征,结果表明:日朗组砂岩成分成熟度和结构成熟度均不高,具有近源物源的特点;槽模构造古水流数据统计表明古流向由南向北,指示物质组分来源于南侧特提斯喜马拉雅和/或印度克拉通。砂岩碎屑组分统计结果表明日朗组的物源区构造背景属于克拉通内部及石英再旋回区。碎屑锆石U-Pb年龄频谱图对比进一步表明其物源区为印度稳定大陆边缘,外加一套早白垩世火山碎屑物质的输入。仲巴地区日朗组物源特征反映了印度大陆北缘早白垩世由深部断裂引起的一次强烈的火山事件,可能与印度大陆从澳大利亚-南极大陆裂解有关。