Correlation of Aptian-Albian sediments in the central part of North Caucasus and Ciscaucasia. Paper 2: Stratigraphy

A complex correlation between outcrops and well sections was performed for the central part of North Caucasus and Ciscaucasia using geophysical, biostratigraphic, and lithostratigraphic methods with new substage and zonal division of the Aptian and Albian sediments taken into account. Previously recognized local subsidiary stratigraphic units (members) were traced into the covered areas, and the stratigraphic position of productive layers was specified.     

Organic matter in Oligocene Maikop Sequence of the North Caucasus

The results of the thorough study of organic matter (OM) in the Oligocene-Miocene Maikop sequence of the North Caucasus are considered. It is shown that its distribution within the Maikop sequence is characterized by irregular patterns. Despite that these rocks show the elevated TOC content, their hydrocarbon-generating potential is, as a rule, low. The low quality of OM is partly related to its intense anaerobic decomposition in anoxic environments with terrigenous sedimentation. The elevation of OM concentrations and hydrocarbon-generating potential could be connected with the reinforced phytoplankton influx into sediments. Sediments deposited in the second half of the Early Oligocene were synchronously enriched in OM. This enrichment chronologically corresponded to the restoration of connection with the World Ocean of the formerly partly isolated and freshened basin. Most probably, this event resulted from the sharp increase in biological productivity.     

Neogene-Eopleistocene shelly limestones in the North Caucasus and Ciscaucasia regions

In the North Caucasus and Ciscaucasia, seven distribution zones of Middle-Upper Miocene, Pliocene, and Eopleistocene shell-detrital limestones are defined: Lower Don, Tsimlyansk-Manych, Western Ciscaucasia, Western Caucasus, Central Ciscaucasia, Kabarda-Chechen, and Eastern Dagestan. The shell-detrital limestones are present among sandy-clayey sediments in the form of intercalations of variable thickness traceable over different distances (up to a few kilometers). The maximum thicknesses of such limestones are characteristic of the Neogene sections in the northwestern pericline of the Greater Caucasus: Taman-Adagum area, where their distribution comprises the stratigraphic interval from the Chokrakian to early Cimmerian regional stages (approximately 10 m.y. long). Their most spacious distribution area is confined to the Central Ciscaucasia zone. The shell-detrital limestones in the Neogene sections mark shallowwater shelf areas (ancient coastal bars, banks, and shoals) with intermittently forming environments favorable for accumulation of significant volumes of shelly detritus. In such paleogeographic settings, the sediments were subjected to repeated erosion with subsequent formation of limestones. Now, they represent building material widely used in the region for different purposes.     

Correlation of Aptian-Albian sediments in the central part of North Caucasus and Ciscaucasia. Paper 1. Composition and rock distribution features

The lithology change patterns between sections on the northern slope of Caucasus and in Ciscaucasia wells must be taken into account in addition to geophysical and biostratigraphic characteristics during detailed correlation of Aptian and Albian sediments. This paper discusses rock lithologic types (LT), rock distribution patterns in the covered territories, and their formation conditions. It includes a facies profile showing these patterns.     

Lithological-stratigraphic characteristics of the Aptian-Cenomanian sediments of the Abkhazian zone of the Western Caucasus

For the first time, a complex lithological-stratigraphic and facial study of the Aptian-Cenomanian sediments of the Abkhazian zone of the Northwestern Caucasus was carried in sections of the Mzymta and Khipsta river valleys. As a result, the areas of distribution of potential oil-source rocks in these sediments, which correspond to the events of the OAE-1 and OAE-2 global paleoecological crises, oil reservoir rocks and rocks-caps were defined.     

Textural characteristics and genesis of the aeolian sediments in the Kuwaiti desert

Several types of aeolian deposits have been recognized in Kuwait: (a) smooth sand sheets that resemble desert floor sand, (b) immobile sands that include rugged vegetated sand sheets and wadi fill deposits, and (c) mobile sands that form active sand sheets and sand dunes. Simple size frequency curves illustrate the genetic relationship between the various aeolian sediment types. The four size parameters, namely, mean size, sorting, skewness and kurtosis, were calculated. Scatter plot diagrams of sorting versus mean size and sorting versus kurtosis are effective in differentiating smooth sand sheet deposits from dune sands. Active sand sheet deposits can also be recognized because they are usually located between the two end members–smooth sand sheets and dune sands. Size parameters change with location regardless of their types. Coarsening and positive skewness usually increase downwind. Mineralogical and textural characteristics of the aeolian deposits in Kuwait revealed that they are mostly derived from the lower Mesopotamian muddy flood plain deposits, the sand fraction of the Al-Dibdibba gravelly deposits and the disintegrated material from calcretic and gypcretic duricrusts. Distribution of depositional and deflational areas indicates that the northern desert of Kuwait is characterized by a positive sand budget, whereas the southern desert has a negative sand budget.     

Age of metamorphic rock masses in Laba Basin,North Caucasus

A Neoarchaean sanukitoid pluton that was intruded into the base of the Guyang greenstone belt in the Yinshan Block of the North China Craton hosts a number of hornblendite enclaves. Geochemically, the pluton is characterized by high MgO, Mg#, Cr, Ni, large-ion lithophile element (LILE), and heavy rare earth element (HREE) contents and low TiO₂ and high-field strength element (HFSE) contents, and has relatively low Sr/Y ratios and negative Eu anomalies. Whole-rock Sr–Nd isotopic analyses indicate that it has ε_Nd(t) values of +1.4 to +2.0. These geochemical and isotopic characteristic suggest that the sanukitoid was formed under a low-pressure and high-temperature environment by slab melting and assimilation of hornblendite enclaves. The hornblendite enclaves show high MgO, Mg#, Cr, and Ni contents, high and variable K₂O, LREE, and Th contents, enrichment in LILEs and LREEs, and depletion in HFSEs. They have high Y contents and relatively low Sr/Y values, strongly negative Eu anomalies, and whole-rock ε_Nd(t) values of +1.0 to +1.9. The geochemical and isotopic characteristics indicate that these enclaves might have been derived from a mixed source of an enriched mantle, metasomatized by melts expelled from subducted sediments in a high-temperature, low-pressure environment. To explain these characteristics, a ridge subduction model is proposed for the formation of the sanukitoid and hornblendite in the Yinshan Block in the Neoarchaean.     

Rain-induced debris and mudflow triggering factors assessment in the Santiago cordilleran foothills,Central Chile

Debris and mudflows are some of the main geological hazards in the mountain foothills of the Chilean capital city of Santiago. There, the risk of flows triggered in the basins of ravines that drain the range into the city increases with time due to the city growth. A multivariate statistical study based on the logistic regression method is presented. The model provides equations that allow the computation of combined meteorological triggering factors associated with a probability of rain-induced flow occurrence. Daily rainfall, accumulated rainfall and the snowfall level, traditionally considered as the relevant factors, are analysed for a 25-year period. The results show a strong relevance of the rainfall on the day of the flow event over the other factors. However, the relatively low probabilities returned for some real flow events suggest that the model does not capture all the significant variables and the problem is more complex than as it has been traditionally assumed, and further investigations are needed to develop predictive models of flow triggering.     

Isotopic characteristics of potassic rocks: evidence for the involvement of subducted sediments in magma genesis

The potassic igneous rock suite (with molar K₂O/Na₂O > 1) can be divided into an “orogenic” subgroup that occur in subduction-related tectonic settings and an “anorogenic” sub-group that are confined to stable continental settings. Representatives of both sub-groups possess trace element and isotopic features consistent with the contamination of their magma sources by incompatible element rich and isotopically evolved “metasomatic” components. It is argued here that these metasomatic components are principally derived from subducted lithosphere, including subducted sediments. Most examples of orogenic potassic magmatism (e.g. Italian potassic rocks, Spanish lamproites, Sunda arc leucitites) have trace-element and Sr, Nd and Pb isotopic characteristics consistent with the contamination of their mantle sources by a component derived from marine sediments. Anorogenic sub-group potassic magmas have generally similar incompatible trace element and Sr and Nd isotopic characteristics to those of orogenic potassic magmas, but many examples have unusual Pb isotopic compositions with unradiogenic ²⁰⁶Pb/²⁰⁴Pb. Modern marine sediments characteristically have low U/Pb ratios and the unradiogenic ²⁰⁶Pb/²⁰⁴Pb of anorogenic potassic magmas may have evolved during long-term storage of subducted sediments (or components derived from them) within the subcontinental lithosphere. These unusual Pb isotopic compositions require substantial time periods (> 1 Ga) to have elapsed between the fractionation events lowering the U/Pb ratio (i.e. erosion and sedimentation at the Earth's surface) and subsequent potassic magmatism and it is therefore not surprising that most examples of anorogenic potassic magmatism are not associated with recent subduction processes. Although the eruption of potassic magmas is commonly related to rifting or hotspot activity, these processes do not necessarily play an important role in the genesis of the unusual sources from which potassic magmas are derived.     

A prediction of minimum age for the Weichselian maximum glaciation in North Iceland

The Weichselian glaciation in Norht lceland is locally divided into three main stages:(1) The maximum stage, when North Iceland was ice-covered northwards to the island of Grimsey; (2) the ice-lake stage, when a series of ice-dammed lakes were formed in Fijóskadalur; and (3) the Langhöll Stadial, ¹⁴C age about 10,000 B.P., an advance restricted to the valleys on both sides of Eyjafjördur, after the final emptying of the youngest lake in Fnjöskadalur, By combining changes in strandine gradients with time, an age of about 20,700 B.P. for the oldest ice-dammed lake is predicted. As this a ge is greater than the assumed age, 18,000 B.P., of the maximum extent of the Weichselian glaciation, it is unlikely that the maximum occurred at that time. Possibly, the maximum extent of the Weichselian glaciation in North lceland took place concurrenly with some of the early s tadials that have been identified in Arctic Canada, in East Greenland and on Svalbard.     

Issue of the genesis of recent sediments on the Barents Sea shelf

This paper is a reply to the comment on our two previous publications (Krapivner, 2009a, 2009b) devoted to the genesis of recent sediments on the Barents Sea shelf in (Epshtein et al., 2011a, 2011b). It is substantiated that the physical nature of the reflection of recent sediments in seismoacoustic records, a very important point for the lithofacies analysis, is incorrectly interpreted by the above opponents. The paper presents geochronological and paleomagnetic data confirming invalidity of the popular concepts about the link between the cover of poorly consolidated sediments and the epoch of the last deglaciation. We show incorrectness of the statement of the opponents about the redeposited character of Pliocene-Quaternary marine biota of diamictons and the glacial processing of coarse-clastic material therein. Diverse properties of recent sediments and their topography artificially united into a complex of indicators of the glacial paragenesis are either simply explained in terms of the natural (for the Barents Sea) icemarine sedimentation or attributed to postsedimentary processes during intense neotectonic activity of the Barents Sea shelf.     

Geochemistry of Cenomanian/Turonian boundary sediments in the mountainous part of Crimea and the northwestern Caucasus

Detailed data obtained on the chemistry of sedimentary rocks from the mountainous part of Crimea and the northwestern Caucasus that were dated at the Cenomanian/Turonian boundary and were formed during Oceanic Anoxic Event 2 (OAE 2) make it possible to calculate the dissolved oxygen concentration in the bottom waters of the sedimentation basin. The enrichment coefficients of trace elements in the black shales are revised and an explanation is suggested for the genesis of the rocks with regard for unusual climatic changes.     

Paleoenvironments and correlations of interglacial sediments in the North Sea

Benthic foraminiferal faunas suggesting interglacial climatic conditions are present in several shallow drillings in the North Sea and in northern Jutland. Denmark. The interglacial deposits in six of the cores arc correlated by means of isoleucine epimerization, paleomagnetic data and environmental inferences. Four episodes of interglacial circulation are recorded in sediments of Brunhes age and one warm episode is found just below the Brunhes Matuyama boundary. The Holocene and Eemian faunas of the North Sea are richer in species and individuals and contain more thermophilous species than the older interglacial faunas. However. a much more northern distribution of Bulimina marginata in sediments of Eemian age suggests that oceanic circulation was different during this period than during the Holocene. Sediments of the three other Brunhes interglacials (the Holsteinian, the Norwegian Trench and the Devils Hole Interglacial) and the Leerdam Interglacial record periods of relatively cooler conditions and possibly reduced inflow of Atlantic water.     

北京地区泥石流灾害临界雨量阈值分析

对历史上北京市泥石流灾害发生时前3日的雨量、当日激发雨量和最大1小时雨量进行了分析,确定了临界雨量阚值。参照山洪灾害与降雨频率分析法,分3种方案对北京市的泥石流灾害与降雨频率进行分析,计算了各地区不同时段（最大10分钟、最大30分钟、最大60分钟、最大360分钟、最大24小时、最大3天及最大7天）的临界雨量;并利用历史上泥石流灾害发生时的雨量值对3种方案下的计算结果进行了验证和对比,最后确定了准确度较高的一种计算方案。     

三峡沉积物中重金属污染累积及潜在生态风险评估

分析了三峡库区主要支流表层沉积物样品中15种重金属元素（Cd、Cu、Pb、Zn、Cr、Tl、V、Mn、C、Ni、Sr、Ag、Sb、Sn和Mo）的含量水平和分布规律,并采用内梅罗污染指数、地累积指数法和潜在生态风险指数法,初步评价了沉积物中重金属污染状况和潜在生态风险。研究结果表明：（1）三峡库区支流表层沉积物中重金属Cd、Zn和Cu等呈现污染加剧态势,其含量范围分别为0.36-1.22 mg/kg、55.8-182 mg/kg、24.0-93.1 mg/kg;（2）内梅罗单因子污染指数（Ii）和地累积指数（Igeo）指示研究区域中Ag、Sb和Sn等多种重金属污染累积效应显著;（3）研究区域中多种重金属内梅罗多因子污染指数（Pn）和潜在生态风险指数（RI）分别为3.41-11.1和204-568,表明研究区域为重度重金属污染,潜在生态风险较高;（4）三峡库区主要支流表层沉积物呈现出以Cd为主的多种重金属复合污染特征。     

马里亚纳海沟南部海域沉积物的工程地质特性及其成因

大洋27航次中,在马里亚纳海沟挑战者深渊南翼斜坡上获得了箱式插管和重力柱状样品,并对这些样品现场进行了含水率、湿密度、微型扭力十字板、微型贯入和无侧限抗压强度试验,获得了深海表层沉积物的物理力学性质参数,具有较高的含水率、较低的密度、较大的孔隙比、较低的抗剪强度和贯入阻力,各项参数随深度增加呈现规律性变化。归航后通过对沉积物的粒度、碎屑矿物、黏土矿物、微体古生物的鉴定,进一步从物质组成、物质来源和沉积环境等方面对此特殊工程地质性质沉积物的成因进行分析。     

Composition and genesis of the organic matter in the bottom sediments of the East Siberian Sea

The chemical composition of organic matter (C_org, N_org, δ¹³C, δ¹⁵N, and n-alkanes) was studied in the top layer of bottom sediments of the East Siberian Sea. Possible ways were proposed to estimate the amount of the terrigenous component in their organic matter (OM). The fraction of terrigenous OM estimated by the combined use of genetic indicators varied from 15% in the eastern part of the sea, near the Long Strait, to 95% in the estuaries of the Indigirka and Kolyma rivers, averaging 62% over the sea area.