Driving processes and recognition criteria for complex subaerial unconformities in a non-marine succession: Implications for basin margin shaping during forced regressions

A regional sedimentological and sequence stratigraphic analysis is presented in this paper to provide new insights into the morphological complexity and sedimentary mechanisms of unconformities giving shape to basin margins. Although the development of unconformities entails significant sediment delivery to be considered when analysing ‘source to sink’ systems, as well as large palaeogeographical changes related to basin reconfiguration episodes, the complexity that unconformities can reveal towards basin margins has been scarcely explored. A sedimentological and architectural analysis combining outcrop and subsurface datasets shows five palaeoenvironmental evolutionary stages and four key stratigraphic surfaces in a constrained ca 2 Ma Valanginian interval on the southern margin of the Neuquén back-arc basin (Argentina). A complex-type subaerial unconformity was identified, composed in turn of two subaerial unconformities exhibiting both single and composite motifs, different morphologies and facies shifts representative of large palaeogeographical changes. In the studied stratigraphic interval, two erosional stages occur linked to combinations of exclusively non-marine-driven processes involved in unconformities developing. The two subaerial unconformities differ in nature and distribution, representing a novel case of complex unconformities and stratigraphic architectures in non-marine lowstand wedges. The Valanginian complex subaerial unconformity entails a high diachroneity along strike and depositional dip, implying that the hiatus created in landward settings occurred during relative sea-level fall and rise stages during a period longer than in basinward areas. Disagreeing with classical sequential models, two third-order sequence components of a complex lowstand wedge are preserved in proximal settings. Subsidence-controlled accommodation and interplay between second-order and third-order cycles were combined, increasing the prospects of sediment storage and preservation potential of composite sequences towards landward areas. This work improves current comprehension about complex subaerial unconformities formation and related lowstand architectures in proximal settings, providing criteria to understand and revaluate lowstand wedge geometries, particularly for more complex examples, such as the case reported in this contribution.     

Event chronostratigraphy: A high-resolution tool for dating the recent past

The most recent part of the geological timescale presents us with some of the greatest challenges for dating. With the exception of ²³⁰Th/²³⁴U methods, whose use is restricted to rather specific depositional environments, there is no established geochronometric tool capable of dating more than a fraction of the recent past at a resolution adequate to tackle the environmental issues of this period. Event stratigraphy, the investigation of comparatively rare and abrupt occurrences that leave some trace in the stratigraphic record, has been widely employed as a means of correlation and dating of older geological strata. Yet this approach has frequently been overlooked in efforts to establish chronologies of the recent past. It is ironic, therefore, that because of the acceleration of human activity, stratigraphic events have almost certainly occurred with greater frequency over the last few centuries than at any preceding time in Earth history. Because the history of human-induced events is usually well-established, the markers of such events have immense chronostratigraphic value. They may be employed in circumstances in which radiometric techniques may not be suitable, and may offer higher-resolution dates than those associated with conventional dating methods. Dated event horizons may also provide the essential means by which to validate geochronometric analyses of the recent past.Event markers may be divided into those that produce discontinuities in the rock record and those (of much greater value in the terrestrial deposits that are the focus of most investigations of the recent past) that leave some tangible signal in the rocks. These signals may be the result of either natural factors or human-induced processes, and may occur in a range of temporal contexts. They may mark the instant of occurrence of a short-lived phenomenon, or they may represent the abrupt disappearance or sudden appearance of some feature. This paper reviews each of these markers, focussing specifically on their application to the chronology of the recent past and the global environmental transformation that has taken place during this time.     

Dating the recent past

The dramatic environmental changes of the last 500 years are likely to continue into the future and to have an increasing impact on both the Earth and human society. Any understanding of future environmental change is thus critically dependent on our capacity to reconstruct the environmental changes of the past. Fundamental to this is an ability to place the environmental responses of the last half millennium within a reliable chronological framework. Unfortunately, this most recent part of the geological timescale presents us with some of the greatest challenges for dating. With the exception of ²³⁰Th/²³⁴U methods, whose use is restricted to the rather specific depositional environments of shallow marine and terrestrial carbonates, there is no established geochronometric tool capable of dating more than a fraction of the recent past at a resolution adequate to tackle the environmental issues of this period. This challenge has been met by refining existing procedures (including ¹⁴C, ⁴⁰Ar/³⁹Ar, event chronostratigraphy and optically stimulated luminescence) and developing new ones (such as ³²Si). These offer a means of calibrating the high-resolution environmental records of the last 500 years and answering the critical environmental questions presented by this period.     

国家测震台网中心强震动数据处理系统的设计与实现

为满足国家测震台网海量强震动观测数据的处理时效性、格式标准化、产品丰富度等需求,开发了兼容多类强震动观测站点且具备数据快速汇集、处理及归档等功能的强震动数据处理系统。该系统提供地震波形人机交互数据预处理界面,对加速度记录进行预处理,进一步分析预处理后的加速度事件波形数据,计算得到地震动各项参数,包括峰值加速度(PGA)、峰值速度(PGV)、峰值位移(PGD)、仪器烈度、持时、傅氏谱、反应谱和三联谱等,可以导出地震元数据、地震记录波形,对各类数据进行归档存储。该系统具有平台统一性、功能集成性、数据完备性等特点,有效提升了日常数据处理和管理能力,能在地震应急、震害评估和科学研究中发挥实效。     

近百年来亚洲中部内陆湖泊演变及其原因分析

南疆博斯腾湖湖相沉积物碳酸盐稳定同位素,孢粉及地化元素含量的波动揭示了于１１．０￣１０．０ｋａＢＰ期间的相对冷湿的环境特征,这次变冷事件在年代上可与末次冰消期的新仙女木气候突然变冷事件相对应,虽然目前还无法圆满解释此事件的成因机制,但此事件在极端干昌的南疆博斯腾湖湖相沉积中的发现,无疑对深入认识此事件发生的全球性以及探讨其成因机制具有重要意义。     

爆破、塌陷识别研究进展综述

简要介绍当前国内外关于天然地震与爆破、塌陷等非天然地震特征研究及事件类型识别的进展.对各类事件的定义及主要波形特征进行简要综述,重点介绍了事件类型判定的各类识别方法.与自然界天然地震不同,非天然地震由人工干预或人类活动间接引发.爆破是炸药在爆炸瞬间能量迅速释放,部分能量以地震波形式向外传播,引起地表振动而产生破坏效应的...     

内蒙古地区极端降水事件分布特征

基于内蒙古地区94个气象站1961—2007年逐日降水量资料,利用累积频率法,分析了极端降水事件变化特征。结果表明:(1)近50年内蒙古地区极端降水事件和极端强降水事件发生的强度和出现的频次均呈现出增多趋势,尤其在1977年降水发生突变之后,增加趋势更为明显。(2)近50年内蒙古地区最长连续无降水日数和最长连续降水日数持续时间缩短,表明连续性干旱和降水的持续性减弱,尤其是进入本世纪后,最长连续无降水日数陡升和最长连续降水日数陡降,气候湿润程度下降,加之全球气候变暖,使内蒙古地区进入本世纪后暖而干的气候特征更为明显,这对农牧业生产和生态环境保护极为不利。(3)内蒙古地区近50年小雨和暴雨日数的减少,降水强度的加大,使全区降水不稳定性增加,降水有极端化发展趋势,尤其是在1987年气温发生突变之后,降水强度变化更大。     

甘肃北山南带晚泥盆世岩浆事件:锆石U-Pb年代学、地球化学和Sr-Nd-Hf同位素体系约束SCIEI北大核心CSCD

北山造山带处于中亚造山带南缘,研究其岩浆事件对于推演中亚造山带地质构造演化具有重要意义。在甘肃北山南带的双鹰山-花牛山岛弧带中,分布有大量花岗质岩体。本文选取甘肃北山南带双峰山南复式岩体中的花岗闪长岩和二长花岗岩,开展LA-ICP-MS锆石年代学、Hf同位素、全岩地球化学和Sr-Nd同位素研究。研究结果表明该复式岩体成岩时代、地球化学与同位素特征基本一致,具体如下:(1)锆石年代学结果显示二长花岗岩年龄为367.5±1.7Ma、368.0±1.8Ma,花岗闪长岩年龄为366.3±2.2Ma,均为晚泥盆世;(2)复式岩体SiO_(2)含量为64.39%~74.95%、碱含量为5.01%~9.21%,属准铝质(A/CNK=0.88~1.00),P 2O 5含量低(0.02%~0.17%),且与SiO_(2)呈负相关,具有典型的I型花岗岩特征;(3)样品总体富集轻稀土元素((La/Yb)N=6.23~23.0),具有较强烈的Eu负异常(δEu=0.31~0.55),富集Rb、Th、U等大离子亲石元素,亏损Nb、Ti、P等高场强元素;(4)锆石εHf(t)值为-3.1~+6.0,t DM2为928~1327Ma,全岩(87 Sr/86 Sr)i在0.705607~0.708523之间,εNd(t)在-4.1~-1.9之间。结合区域构造环境,实验数据表明,双峰山南晚泥盆世复式岩体是在活动大陆边缘的环境下,由幔源岩浆底侵加热下地壳使之部分熔融,并与之混合形成的,代表了晚泥盆世柳园洋向北俯冲的构造事件。     

从魁北克2000-世纪湿地大事件活动看21世纪国际湿地科学研究的热点与前沿

介绍了魁北克 2 0 0 0 -世纪湿地大事件活动的主要内容和信息 ,结合收集到的国际上新的湿地研究文献资料 ,总结了国际湿地科学研究的前沿领域、热点、主要研究进展和发展趋势 ,对国际湿地科学研究进展加以简要概述 ,以此透视现代国际湿地科学研究一些新的动向和趋势。     

山西五台山滹沱群四集庄冰碛岩碎屑锆石年龄及其对大氧化事件研究意义

山西滹沱群四集庄组变质陆源碎屑混杂岩的砾石岩性复杂,分选和磨圆性差,砾石表面可见划痕和凹面,发育落石构造,表明其沉积于冰水过渡带环境,记录了华北克拉通古元古代冰期事件。四集庄组不同层位不同岩石的碎屑锆石U-Pb年龄集中于2.5 Ga左右,下部2件陆源碎屑混杂岩样品的碎屑锆石给出的最小年龄分别为(2 440±40) Ma和(2 400±23) Ma,而上部1件砂岩样品碎屑锆石的最小年龄为(2 121±22) Ma,说明冰川沉积作用发生在2.4 Ga之后, 2.1 Ga之前或2.1 Ga左右,与全球性休伦冰川事件(Huronian Glaciation Event,HGE)发生时间一致。砂岩相最小碎屑锆石年龄2.1 Ga左右,说明五台地区冰川事件之后沉积作用仍未结束,冰川沉积发生在陆缘广海盆地。四集庄组之上的大石岭组具有碳酸盐岩碳同位素正漂移,沉积于2.12 Ga之后,2.08 Ga之前,与全球碳酸盐岩碳同位素正漂移事件时间一致。青石村组的红层沉积于2.08 Ga左右,也与全球红层发育记录一致。华北克拉通滹沱群记录了相对完整的大氧化事件序列。