论“太湖冲击坑溅射物”——来自太湖现代沉积物中铁质结核的证据

针对太湖发现由菱铁矿组成的“铁质溅射物”和“贫铁溅射物”是太湖形成于陨石冲击的观点,本文对这些实为铁
质结核样品进行了详细的矿物学、地球化学和年代学测试。数据表明,这些样品是在太湖现代沉积淤泥层中形成的菱铁矿
和褐铁矿结核。结核中含有大量水生植物碎片、植物蛋白石、细菌和少量动物壳体碎片。菱铁矿高温灼烧后全部变成赤铁
矿,说明太湖地区不存在菱铁矿熔融状的高温相。菱铁矿14C测年结果表明各个样品形成于年代,不是瞬间形成的。δ13C 数
据指示为淡水无机碳来源,并非湖区碳酸盐岩直接来源。所谓的“贫铁溅射物”实际是黄土层的成岩钙结核。这些证据表明,
太湖发现的这些结核是沉积成岩期的产物,并非冲击高温溅射熔融形成的溅射物。     

Shimajiri Group equivalent sedimentary rocks dredged from sea knolls off Kume Island,central Ryukyus: Implications for timing and mode of rifting of the middle Okinawa Trough back-arc basin

The late Cenozoic geohistory of the Ryukyu arc is closely related to the rifting history of the Okinawa Trough. The submarine geology and stratigraphy of areas around Kume Island, which is situated near the eastern rifted margin of the middle Okinawa Trough, provide key constraints to understand the timing and mode of Okinawa Trough rifting. Here we report the lithology of sedimentary rocks dredged along slopes of ~1000-m-deep sea knolls located north and northwest off Kume Island, and their depositional ages determined by calcareous nannofossils and strontium (Sr) isotope analyses. Various types of sedimentary rocks, such as siltstone, very fine-grained sandstone, medium-grained sandstone, fossiliferous coarse-grained sandstone, and tuffaceous sandstone, were recovered at two dredge sites. These sedimentary rocks are lithologically similar to those in the Aka Formation and a part of the Maja Formation of the Shimajiri Group in nearby Kume Island. Calcareous nannofossils and Sr isotope analyses indicate their depositional ages from the early Pliocene to the early Pleistocene, which are generally consistent with those of the Aka Formation. The finding of the dredged rocks similar in lithology and ages to the Aka Formation indicates that marine deltaic area continued toward north and northwest from Kume Island during these periods. The presence of the Shimajiri Group equivalent sedimentary rocks at the dredge sites are likely related to the main rifting of the Okinawa Trough after ca. 2 Ma in the central Ryukyus.     

北美西部内陆海盆Niobrara组的钙质超微化石及其环境意义——Ⅰ：钙质超微化石及其分带

北美西部内陆海盆上白垩统Niobrara组中的钙质超微化石十分丰富,且多保存良好,呈现典型的晚白垩世远洋钙质超微化石组合面貌,经系统鉴定,计有60属100余种和亚种。为适应不同的环境,Kansas西部和South Dakota东部的钙质超微化石组合面貌稍有不同。经与白垩纪颗石藻化石带对比,可将Kansas西部的Niobrara组划分为6个化石带(CC13～CC18)和8个亚带,其中,根据本文研究地区的化石序列,CC15和CC16带被进一步划分。根据与同一剖面所建立的无脊椎动物化石带对比,钙质超微化石CC17带的时代被重新厘定,即该带应始于中Santonian的晚期,结束于晚Santonian的早期,并据此将Santoniatr／Campanian的界线划在CC18带之内。     

Late Barremian to early Aptian calcareous nannofossil paleoceanography and paleoecology from the Ocean Drilling Program Hole 641C (Galicia Margin)

Calcareous nannofossil assemblages at Site 641C (Galicia Margin, North Atlantic) were investigated in order to determine changes in fertility and temperature of surface waters. Taxa such as Zeughrabdotus spp. <3.5 μm, Biscutum constans, Discorhabdus rotatorius and Diazomatolithus lehmanii, which thrived in higher fertility conditions, are particularly abundant across the CM0 interval as opposed to those with oligotrophic affinities such as Watznaueria spp. and Nannoconus spp., which are generally reduced in abundance. The abundances of nannoconids are much lower than those observed in Tethyan sections, indicating higher fertility conditions. Slumpings and low recovery prevent the identification of the onset of the “nannoconid crisis”, but a sharp drop in nannoconid abundances, observed prior to the CM0 interval, correlates with the “nannoconid decline” observed in several Tethys sections.The normalized ratio between low and high fertility taxa (Fertility Index) was used to characterize the nannofossil assemblages in terms of productivity changes. The highest values of the Fertility Index were observed across magnetic chron CM0. The paucity of cold water taxa such as Seribiscutum spp. and Repagulum parvidentatum suggests warm water conditions throughout the deposition of upper Barremian–lower Aptian sediments on the Galicia Margin.     

钙质砂中锚定板的极限抗拔力计算

采用室内足尺寸模型试验研究钙质砂中锚板的抗拔破坏面形状,并将破坏模式分为浅埋型、过渡型及深埋型。采用极限平衡理论得到三种不同的破坏模式的极限抗拔力计算公式。     

Shearing response of geotextile-reinforced calcareous soils using monotonic triaxial tests

Abstract

Different types of geosynthetic reinforcements have been widely used for improving shear strength of soil materials in recent decades. In this study, two different calcareous soils reinforced by geotextile layers were evaluated by monotonic compression triaxial tests. The calcareous soils were obtained from Hormuz Island and Boushehr Port located in the north coast of the Persian Gulf. Different reconstituted samples were prepared at dense conditions with and without geotextile layers and consolidated under different effective confining pressures ranging from 100 to 600?kPa. Different layers of reinforcements including one, two and three geotextile layers were placed in the reinforced samples and sheared under drained condition. The results show that confining pressure has a significant effect on the efficiency of reinforcements. Evidently, increasing the reinforcement layers decreased the particle breakage of the calcareous soils used in this study.     

灵台风尘堆积中钙质结核的地球化学研究

通过对甘肃灵台风成黄土－古土壤－红粘土序列中钙质结核的野外分布及形态特征观察,并对主元素和微量元素进行化学分析,发现结核中氧化物活动给分和惰性组分含量的比值在剖面中自下而上逐渐变大,反映了结核形成时其上覆土层所经历的淋溶程度逐渐增强;而ＭｇＯ含量校正后的ＭｇＯ／Ｃａｏ比值则逐渐减小,批示结核形成时环境温度逐渐降低。灵台剖面自下而上不同层位结核中Ｓｒ／Ｂａ比值及ＣａＣＯ３含量的变化同氧化物含量及比值     

Late Oligocene Sedimentary Hiatus in Deep Sea Area of South China Sea, Result of Calcareous Nannofossils from ODP Site 1148

In order to determine the age of the sedimentary hiatus and its geological significance, a study of the calcareous nannofossil biostratigraphy was carried out. Detailed strutigraphical data of the Late Oligocene-Eariy Miocene diagnostic species thus obtained. The nannofossil zonation of this interval was subdivided and the Oligocene-Miocene boundary was further determined. Several last Late Oligocene events were recognized, indicating a long-term sedimentary hiatus in the uppermost Upper Oligocene.The time span of the hiatus was estimated for about 2.2 Ma, at least from 23. 9 to 26. 1 Ma. The lithological and geophysical data from Site 1148 indicate some abrupt sedimentary changes that occurred below and above the hiatus. This hiatus at Site 1148 was probably related to the tectonic change, a major ridg ejmnp during the seafloor spreading in the Late Oligocene South China Sea.     

Coniacian-Santonian Planktic Stratigraphy in central Tunisia

In this study, we describe a new stratigraphy of three exposed sections in central Tunisia, integrating Coniacian and Santonian planktic foraminifera and calcareous nannoplankton, supported by ammonite and inoceramid bioevents. In the three sections, the Coniacian/Santonian (C/S) boundary lies slightly above the lowest occurrence (LO) of the calcareous nannofossil Lucianorhabdus cayeuxii, which marks nannofossil Zone CC16 and matches well with the LO of the planktic foraminifera Dicarinella asymetrica. It also lies ∼4–7 m below the LO of the inoceramid Platyceramus cycloides and the ammonite Texanites (Texanites) sp. Comparing these marker C/S bioevents with the global stratotype section, the Olazagutia section (Spain) shows that the stratigraphic range of the bioevents are variable. This observation must be taken into consideration when making regional chronostratigraphic correlations.     

川北地区下寒武统筇竹寺组钙质结核特征及成因机制

不同成因的钙质结核在研究沉积水体、早期成岩环境及泥页岩的压实程度方面具有重要意义。通过对川北地区旺苍县下寒武统筇竹寺组钙质结核抛光,发现结核内部具有层理、暗色圈层和张裂缝构造。结核和围岩的层理关系和具有漂浮状的微观结构特征表明,该地区结核是形成于压实作用之前的成岩早期结核。对结核中Al、Ti、Ca、K、Na元素分析表明,结核的元素分布受到结核内部裂缝的影响。结核内部富含生物碎屑和其形成深度为数十米的特征,推测结核主要是在硫酸盐还原带生长。结核具有暗色圈层之间层理不弯曲和元素移动的特征,揭示了结核为透入性生长,在整个结核生长阶段就开始大量同时结晶。Mg/Ca和Sr/Ca的比值说明,结核内部钙质成分为富镁方解石-生物成因方解石。结核中富含钙质成分的原因可能是围岩中的生物成因方解石大量向结核运移所致。裂缝的形态特征表明,裂缝为张裂缝。对结核上部地层的沉积特征研究表明,张裂缝的成因很可能与筇竹寺顶部的浊积岩和沧浪铺组底部的似瘤状灰岩快速搬运沉积有关。