酒西盆地青南凹陷柳沟庄—窟窿山地区下白垩统震积岩的发现及意义

在甘肃省酒西盆地青南凹陷柳沟庄—窟窿山地区下白垩统下沟组及中沟组地层序列中,首次发现发育一套地震事件沉积物,是一套含微裂缝或同生变形构造及振动液化泄水构造等震积特征的泥质白云岩、白云质泥岩及泥质粉砂岩的组合。本区震积作用及震积岩的主要识别标志为振动液化卷曲变形及其伴生构造、重荷模及伴生构造、脆性和塑性两类沉积物相间的滑塌变形作用及滑塌岩、软沉积物的液化作用及泄水构造、层内阶梯状断层及地裂缝。柳沟庄—窟窿山地区下白垩统震积岩的发现,填补了本区震积岩研究的空白,反映了本区在早白垩世存在一个强烈的构造活动时期,可以推测其影响范围可能还会波及青南凹陷的其他地区,也有望在酒西盆地类似环境的其他凹陷发现震积岩,开辟震积岩研究的新领域。区域构造背景的研究同样表明,青南凹陷青西Ⅰ号同生断裂带及509断阶带的强烈构造活动正是本区早白垩世震积作用的直接诱发因素,为本区震积岩的存在提供了科学依据。

Extreme Flood Events over the Past 300 Years Inferred from Lake Sedimentary Grain Sizes in the Altay Mountains,Northwestern China

Understanding the temporal variations of extreme floods that occur in response to climate change is essential to anticipate the trends in flood magnitude and frequency in the context of global warming. However, long-term records of paleofloods in arid regions are scarce, thus preventing a thorough understanding of such events. In this study, a reconstruction of paleofloods over the past 300 years was conducted through an analysis of grain sizes from the sediments of Kanas Lake in the Altay Mountains of northwestern China. Results showed that grain parameters and frequency distributions can be used to infer possible abrupt environmental events within the lake sedimentary sequence, and two extreme flood events corresponding to ca. 1736–1765 AD and ca. 1890 AD were further identified based on canonical discriminant analysis(CDA) and coarse percentile versus median grain size(C-M) pattern analysis, both of which occurred during warmer and wetter climate conditions by referring to tree-ring records. These two flood events are also evidenced by lake sedimentary records in the Altay and Tianshan mountains. Furthermore, through a comparison with other records, the flood event from ca. 1736–1765 AD in the study region seems to have occurred in both the arid central Asia and the Alps in Europe, and thus may have been associated with changes in the North Atlantic Oscillation(NAO) index.

南海北部海域北宋年间的古海啸记录及其潜在触发机制

鉴识南海北部历史上大规模海啸灾害事件及其指示特征与影响区域,对沿海地区海啸灾害的预测、评估和防范具有重要意义.现有南海北部古海啸灾害研究多集中于沿海陆地或海岛上零星保存的事件沉积物,而对沉积环境相对稳定且不易受后期改造破坏的浅海沉积物关注很少.本文以汕头海门镇和惠州三门岛外海B01和D02钻孔的浅海岩芯为研究对象,通过沉积物粒度分析、地球化学指标、沉积环境和加速器质谱(accelerator mass spectrometry,AMS)^(14)C年代测定等,分别在B01和D02岩芯的195~228和72~105 cm深度鉴别出“海啸事件沉积层”.两个岩芯的事件层有着相似的形成年龄,介于1048~1108年的北宋年间.该层平均粒径较上下层位显著增加,显示为正粒序单层沉积结构、未见平行或交错纹层.层内贝壳碎屑丰富、“新老”共存,其沉积物特征性地球化学指标明显不同于上下层位,且在界面附近地球化学元素含量或元素比值呈现突变规律.综合南海内部及其周缘同期海啸或疑似海啸记录与历史文献记载,我们认为南海北部上千公里沿岸一带可能发生了与菲律宾、西沙和南沙群岛近于同期的海洋巨灾事件.如此空间规模的极端灾害很可能代表了北宋年间的超级海啸事件,其触发机制应主要受控于该时期马尼拉俯冲带M_(w)>8.0的超强地震.

A new paleoenvironmental index for anoxic events—Mo isotopes in black shales from Upper Yangtze marine sediments

This paper investigates the high-solution of Mo isotopes and uses trace-element analyses for fresh and representative black shales and siliceous shales collected from the transition between the Late Ordovician and the Early Silurian at the Wangjiawan section in Yichang and the Late Permian Dalong Formation in the Shangsi Section of Sichuan. The applicability of different geochemical parameters used as paleo-oxygenation indices are also compared. The preliminary results show that V/（V＋Ni）, Uauth （auth U）, V/Cr, Ceanom and U/Th have a scattered variation range, but most samples plot within the suboxic-anoxic fields. The suboxic-anoxic environment was dominant during the deposition and formation of the two anoxic facies. These redox indicators show little correspondence to the δ98M0 values. The U/Mo ratio can be used as a potential proxy for the paleo-redox conditions due to the possibility that Mo is enriched relative to U at different redox gradients during early diagenesis. This evidence is more significant for the euxinicity condition and corresponds to positive δ98M0 （〉1.5%o） values with low U/Mo ratios. This evidence is likely related to the depositional conditions near the boundary between anoxic and euxinic environ- ments, which are characterised by low bioturbation or water circulation. Other samples reveal a wide scatter of U/Mo ratios and δ98M0 〈1.5%0. These results are likely due to punctuated improvements in oxygenation with intense bioturbation or water circulation, which led to the redistribution of trace element.