The Roaches and Ashover Grits: sequence stratigraphic interpretation of a ‘turbidite-fronted delta’ system

A single Upper Carboniferous fluvio-deltaic cycle (Namurian, R₂b5) in the south part of the Pennine Basin of northern England has been reinterpreted using a sequence stratigraphic approach. In upward succession the deposits comprise basinal mudstones, localized thick density current deposits, delta slope deposits and delta-top sandstones, followed abruptly by basinal mudstones. Earlier interpretations linked these elements with a single, mainly regressive, cycle, referred to as a turbidite-fronted delta. Recent evidence for strong glacio-eustatic sea-level fluctuations in the Namurian suggests that it is unlikely that the previous simple model can explain the spatial arrangement of all the lithological units. This paper attempts to identify elements of the Exxon sequence stratigraphic model, in which changes of sea level are an essential part. The basinal mudstones represent deep-water deposition and imply a highstand of sea level. Thick density current deposits are now known to be localized close to the basinward limit of delta progradation, so cannot be seen as a ubiquitous component of the depositional system. They may form a detached fan, or a fan located at the foot of the delta slope. Slope siltstones include turbidite-like sandstones, but these are interbedded with tractional sandstones and Pelecypodichnus trace fossils, giving no clear indication of water depth. The delta-top sandstones, some coarse and pebbly, can in places be shown to consist of two parts separated by a significant erosion surface now regarded as a type 1 sequence boundary. The erosion surface is locally incised about 80 m into the delta deposits, forming a major palaeovalley. Giant cross-beds (foresets 20 m thick) forming part of the palaeovalley fill are restricted to the basinward end of this feature. A curve of relative sea level inferred for the R₂b5 interval suggests a fourth-order cycle in which two sharp rises are separated by a gradual fall. Possible minor (fifth-order) rises and falls may be superimposed. © 1997 John Wiley & Sons, Ltd.     

Are carbonate barrier islands mobile? Insights from a mid to late-Holocene system,Al Ruwais,northern Qatar

Barrier islands are important landforms in many coastal systems around the globe. Studies of modern barrier island systems are mostly limited to those of siliciclastic realms, where the islands are recognized as mobile features that form on transgressive coastlines and migrate landward as sea-level rises. Barrier islands of the ‘Great Pearl Bank’ along the United Arab Emirates coast are the best-known carbonate examples. These Holocene islands, however, are interpreted to be anchored by older deposits and immobile. The mid-Holocene to late-Holocene depositional system at Al Ruwais, northern Qatar, provides an example of a mobile carbonate barrier island system, perhaps more similar to siliciclastic equivalents. Sedimentological and petrographic analyses, as well as ¹⁴C-dating of shells and biogenic remains from vibracored sediments and surface deposits, show that after 7000 years ago a barrier system with a narrow back-barrier lagoon formed along what is now an exposed coastal zone, while, contemporaneously, a laterally-extensive coral reef was forming immediately offshore. After 1400 years ago the barrier system was forced to step ca 3 km seaward in response to a sea-level fall of less than 2 m, where it re-established itself directly on the mid-Holocene reef. Since that time, the barrier has retreated landward as much as 1000 m to its current position, exposing previously-deposited back-barrier lagoonal sediment at the open-coast shoreline. In modern neritic warm-water carbonate settings mobile barrier island systems are rare. Their construction and migration may be inhibited by reef formation, early cementation, and the relative inefficiency of sourcing beach sediments from open carbonate shelves. Carbonate barrier island systems likely formed more commonly during geological periods when ramps and unrimmed shelves predominated and in calcite seas, when meteoric cementation was minimized as a result of initial calcitic allochem mineralogy. As with their siliciclastic analogues, however, recognition of the influence of these transient landforms in the rock record is challenging.     

白垩系膏岩层序对铅锌矿床形成的贡献——以云南兰坪金顶超大型铅锌矿床云龙组为例

通过野外地质观察, 结合勘探资料和前人研究, 本次用K-Ar法对兰坪盆地金顶超大型铅锌矿床云龙组中的伊利石开展测年研究, 并分析典型勘探线剖面的主要围岩、矿体及矿石结构构造特点, 论述了矿区膏岩层序可能的时代及其在成矿过程中的作用和方式, 形成如下初步认识: 金顶铅锌矿区云龙组中黏土矿物伊利石的K-Ar法年龄为(105.7±1.4) Ma, 与前人测得地层中黄铁矿Re-Os等时线年龄(114±13) Ma相近, 表明该套膏岩层序的地层时代为白垩纪阿尔布期(Albian); 膏盐岩呈似层状、透镜状、角砾状赋存于云龙组中上部, 经后期成矿作用改造而形成呈似层状、角砾状、不规则状产出的铅锌矿体, 并常与膏盐岩共存; 铅锌矿体的就位空间既包括原膏岩层序的空间位置, 也包括高孔隙度的景星组砂岩; 巨量的云龙组膏盐岩在成矿过程中通过硫酸盐还原提供丰富的硫, 使铅锌矿床达到超大型规模。因此, 巨量膏岩层的消耗是金顶矿床巨量金属得以富集成矿的关键因素之一, 这对特提斯造山带含膏岩层序内寻找类似矿床具有普遍意义。同时, 该类膏岩层序的对比研究, 对于“金顶式”的MVT型铅锌矿床的区域找矿潜力评判也具有重要的指示价值。     

深时全球海平面变化重建方法的回顾与展望

全球海平面变化对古地理、古生物、古气候演化以及能源矿产分布具有重要的控制作用。然而,目前学界对深时海平面变化的驱动机制尚不清楚,部分归因于缺乏高精度全球海平面变化的恢复。文章回顾了全球海平面变化研究的起始与发展过程,归纳出五种类型的深时海平面变化重建方法和技术,即地层学、沉积学、洋盆动力学、同位素地球化学方法和大数据技术。并总结了上述研究方法的原理、优势和不足,并以白垩纪全球海平面重建为实例,讨论了当前深时全球海平面重建的难点和争议点,并对未来深时全球海平面变化重建进行了展望。     

2017年四川九寨沟MS7.0地震及余震定位研究

采用特定震源台站校正定位方法对九寨沟M_S7.0地震及其余震进行了精定位研究,结果显示九寨沟地震主震深度为19km左右,余震主要分布于5~15km深度范围;与单事件定位方法相比,其余震展布和断层的线性特征更吻合,更具有平面特征;与双差定位法相比,精定位事件覆盖范围更广。精定位结果显示,断层整体为虎牙断裂西北向的延伸,根据余震分布和特定震源区的校正项分布,认为此发震断层应分南、北两段;余震区长度为40km左右,西北段宽度为10km左右,东南段宽度为6.5km左右。     

沧东断裂第四纪活动性探测结果

根据人工地震探测结果,分别在天津东丽区山岭子村北、大港区中塘村和宁河县北淮淀村布设了三条钻孔剖面,对沧东断裂开展了钻孔勘探工作。经微体古生物鉴定和地层年代测试,确定了各钻孔第一、第二、第三和第四海相层的位置。通过以海相层为标志层的地层对比并结合中塘村BZ1孔古地磁测试结果,对沧东断裂的活动性进行了分析研究。结果表明,沧东断裂晚更新世以来没有发现活动的迹象,其最新活动年代可能在中更新世。如果以海河断裂为界,从位于沧东断裂北部的北淮淀钻孔剖面和位于南部的中塘钻孔剖面的地层对比发现,沧东断裂北段、南段活动性差异不大。同时,已有的一些研究表明,海相层的观测可能对地震信息反映较好。除沧东断裂探测外,天津南、北断裂、海河断裂对21口钻孔进行了海相层鉴定,目前开展的蓟运河断裂探测也将对海相层位置进行确定,这些结果,对利用咸水层观测地震信息的层位选择有很大帮助。     

2020年5月6日新疆乌恰5.0级和5月9日柯坪5.2级地震总结

2020年5月6日、5月9日,新疆地区南天山西段先后发生乌恰5.0级和柯坪5.2级地震,系统总结2次地震发生前出现的地震活动和地球物理观测异常,其中：①地震活动：震前存在调制地震集中、地震窗、5级以上地震成组等中短期异常;②地球物理观测：2次地震震中附近震前出现形变、电磁和流体观测异常,其中形变异常3项、电磁异常4项、流体异常1项,主要分布在柯坪5.2级地震震中附近。通过对2次地震序列进行跟踪,发现：乌恰5.0级地震余震较少,震后60天内共记录M_L 3.0以上余震4次,最大震级为M_L 4.5;柯坪5.2级地震后余震较丰富,震后60天内共记录M_L 3.0以上余震10次,最大震级为M_L 4.7,计算得到序列h值为1.6,b值为0.73。综合分析认为,2020年5月新疆地区2次5级以上地震前存在的地震活动异常较少,但区域地震活动水平较强,主要存在具有中短期指示意义的地球物理观测异常。     

2020年2月3日四川青白江5.1级地震总结

针对2020年2月3日四川青白江5.1级地震发生前的地震活动和地球物理观测异常、构造背景、震源物理参数,以及地震序列特征等进行系统总结。结果表明,青白江5.1级地震发生前存在小震调制比高值、地磁低点位移、重力固体潮高噪声等中短期异常;地震序列活动特征显示,此次地震为一次孤立型地震序列,序列参数计算结果与当前余震活动水平基本相符。另外,青白江地震发生在龙泉山断裂带上,震源机制计算结果显示为一次逆冲型破裂事件,与龙泉山断裂构造性质一致。同时,此次地震发生在四川盆地的少震弱震区,对于川滇及川滇藏交界地区后续7级以上地震的发生具有一定预测意义。     

2020年1月25日西藏丁青5.1级地震总结

2020年1月25日,藏东昌都丁青地区发生5.1级地震,尽管此次地震发生在监测能力较低地区,但震前仍监测到小震调制比高值、低b值等地震活动中短期异常。文中系统总结了地震构造背景、震源物理参数、序列特征以及震前出现的地震活动和地球物理观测等异常,结果发现：震源机制解显示为拉张型破裂,最近断裂为巴青—类乌齐断裂;序列活动特征、序列h值和b值计算结果显示,此次地震为主余型地震序列。目前,藏东地区仍存在一些地震活动和地球物理观测异常,表明该地区存在发生6级以上地震的强震背景,丁青5.1级地震的发生未能缓解该地区强震危险性。     

利用断裂带首波分析甘孜—玉树断裂带拉张盆地结构特征

对断裂带及附近区域地层精细结构的描述是理解地震孕育和发生过程的基础.不同板块或块体边界在构造或区域应力作用下,常常会形成速度间断面和低速的断裂带,间断面和低速带的存在会产生特殊的断裂带地震波,比如断裂带首波和围陷波,并会影响地震的物理过程、破裂速度和破裂方向等.本文主要利用2010年4月14日M_W6.9玉树地震后布设的流动地震台站,对沿着甘孜—玉树断裂带传播的断裂带首波进行识别和分析.分析结果表明,在甘孜—玉树断裂带的不同区域均观测到了断裂带首波,在西段的结隆拉张盆地附近的3个台站沿断裂带界面的平均纵波速度差异值在5%～8%,而其他区域的平均速度差异为1%～3%.通过台站位置分布和断裂带首波特征关系,初步分析了断裂带拉张盆地的结构,结果显示结隆盆地的长度为～40km,宽度为5.35～5.97km,深度不超过5km,在地表浅层形成了一个低速区,分别与巴颜喀拉块体(NE)和羌塘块体(SW)产生了两个物性差异界面,但没有延伸到主震和余震区震源深度.我们的结果表明结合密集台阵资料,通过断裂带首波特征分析可以为断裂带精细结构及几何特征提供一种新的技术方法和途径.