Reconstruction of nearshore chemical conditions in the Mesoproterozoic: evidence from red and grey beds of the Yangzhuang formation,North China Craton     

Yu Zou  Dongna Liu  Hongwei Kuang  Changgui Song  Yuxiang Sun 《International Geology Review》2020,62(11):1433-1449

ABSTRACT

Mesoproterozoic red beds near ancient coasts have not aroused extensive interest. A new geochemical study of the alternating red and grey dolostones from the Yangzhuang Formation provides a better understanding of the redox conditions of nearshore sedimentary environments. In this contribution, whole-rock samples are characterized by positive correlations of rare earth elements (REE) vs. Th and Fe_T vs. Th and flat-type REE distribution patterns, indicating massive terrigenous input, which is considered to be inherited from felsic rocks. Relatively high (Fe_mag+Fe_ox)/Fe_T and Fe³⁺/Fe²⁺ ratios in red beds indicate more oxidized conditions in supratidal environments compared with the lower oxygen contents in intertidal environments. Under these two distinct chemical sedimentary conditions, acetic acid-leached red and grey samples both have HREE-depleted distributions, suggesting significant freshwater invasion. Moreover, limited terrigenous redox-sensitive elements (RSEs) can reach the coast where the red beds are deposited, whereas relatively high RSE enrichment factors originating from shallow oceans are recorded in grey beds. In the Mesoproterozoic, limited oxidative weathering, shallow seawater desalination, and low organic production occurred near the coast. Meanwhile, a prolonged period of low Mo and U availability preserved in carbonate minerals confirmed that marine oxygen levels failed to satisfy the deposition of offshore red beds. During the regression, potentially exposed sediments connected to atmospheric oxygen guaranteed the oxidation of iron and the formation of red beds, and these events were coupled with negative δ¹³C_carb shifts in the Yanliao rift zone.  相似文献   

Study of the trophic web of San Simón Bay (Ría de Vigo) by using stable isotopes     

R. Filgueira  B.G. Castro 《Continental Shelf Research》2011,31(5):476-487

Based on the stable isotope composition in ¹⁵N and ¹³C of different potential sources of organic matter and consumers of an intertidal Zostera marina meadow located in San Simón Bay (Ría de Vigo, NW of Spain), a simplified food web of this community was reconstructed. For this purpose, some alternatives in different steps of the most used methodology of stable isotope dietary analysis were developed that cope with some of the limitations associated to the interpretation of isotopic signals for food web analysis, those of uncertainty on the fractionation value, mathematical model to use for the diet resolution and shortage of the isotope number for discriminating many food sources. The application of this protocol to the studied community reported similar results to those from other studies based on similar trophic webs, emphasizing the importance of local primary producers, especially microphytobenthos, which could be available for several primary consumers through resuspension forced by tidal hydrodynamic. The good agreement with previous results suggests that the proposed protocol is a feasible alternative to elucidate the most plausible trophic relationships in complex trophic webs using stable isotopes analysis.  相似文献   

Is most hummocky cross‐stratification formed by large‐scale ripples?     

JAMIE G. QUIN 《Sedimentology》2011,58(6):1414-1433

Although normal isotropic hummocky cross‐stratification is commonly interpreted to be the deposit of large‐scale ripples, there are many reasons why this may not usually be the case. These reasons include: (i) that the stratification produced by large‐scale ripples does not particularly look like isotropic hummocky cross‐stratification; (ii) that it is difficult reconciling the abundance of HCS with the restricted hydraulic stability of large‐scale ripples in silt to fine sand (i.e. the grain sizes in which hummocky cross‐stratification is usually found); (iii) that the distribution of hummocky cross‐stratification within ancient storm beds is not the distribution that would be expected from large‐scale ripples; (iv) that the flows calculated to have formed ancient examples of hummocky cross‐stratification would be expected to generate an upper stage plane bed rather than ripples; and (v) that it is difficult to explain why large‐scale ripples would predominate in the proximal parts of storm beds when modern storm flows commonly exceed the threshold for entrainment. In contrast to the various hypotheses which propose that isotropic hummocky cross‐stratification is generated by ripples, an alternative hypothesis which suggests that it is generated by instabilities, does seem to adequately explain the origin of hummocky cross‐stratification. However, it is difficult to accept this hypothesis given that the origin of the proposed instabilities is unproven. These conclusions highlight the continued uncertainty regarding the process, which generates hummocky cross‐stratification.  相似文献   

中国南方湿润区红层地貌及相关问题探讨   总被引：5，自引：1，他引：4  

彭华 《地理研究》2011,30(10):1739-1752

我国红层出露面积广大,但地学界对红层关注不足,各学科对红层问题的研究缺乏融合,存在着＂多张皮＂的问题;尤其是对红层这个特殊的脆弱地理单元的土地退化及生态环境问题缺少关注。南方湿润区是我国红层集中分布区之一,大部分红层区存在着严重的地貌灾害和水土流失问题,甚至分布着许多＂红层荒漠＂。红层区的土地退化虽与人的不合理利用关系...  相似文献   

煤层层间距对比法在岩煤层对比中的应用     

魏润相 《陕西地质》2011,29(2):96-99

在煤炭地质勘查中,对各勘查工程所揭露的各煤层进行编号,通常采用煤层层间距,岩、煤层自身特征其及组合关系、标志层、生物化石、测井地球物理特征、地震物性特征等要素进行综合对比。其中煤层层间距对比方法是一种重要的有效对比方法。  相似文献   

川东红层丘陵区软弱夹层工程特性   总被引：1，自引：0，他引：1  

谭超  潘国耀  刘宗祥  张恒  安世泽 《四川地质学报》2011,31(2):212-214

在川东丘陵侏罗系红层中,分布有多层厚度稳定的软弱夹层,这些软弱夹层在很大程度上控制着斜坡的结构特征和稳定性。在进行工程建设时,若基础持力层范围内存在软弱夹层,必须对软弱夹层进行详细研究。以川东地区工程实践经验为基础,进行总结,并进行其工程特性和力学参数取值方法的分析。  相似文献   

江西丹霞地貌演化阶段与分布及其构造控制探讨          下载免费PDF全文

姜勇彪  郭福生  黎广荣  李蓉  楼法生  汪震  陈留勤  郄海满  闫罗彬  李益朝  凡秀君 《地质论评》2024,70(1):44-58

晚白垩世至古近纪期间,江西省内沿区域性断裂构造发育30余个断陷盆地,沉积了巨厚的红色陆相碎屑岩,为丹霞地貌的形成提供了物质基础。已发现的228处丹霞地貌点主要分布于江西东部武夷山北麓和西麓的红层盆地内。利用面积—高程积分法,对江西省9个盆地的丹霞地貌演化阶段进行了定量分析,结果表明省内丹霞地貌景区的面积—高程HI为0. 17~0. 52,平均为0. 38,即其演化阶段处于壮年晚期至老年早期,在空间分布上具有由南向北,由壮年期转为老年期的趋势。该趋势可能与武夷山的构造隆升有关。构造分析显示,节理与武夷山的隆升对江西丹霞地貌演化起控制作用,具体表现为：① 龙虎山、九仙湖、仙岩、蛤蟆坞、赭亭山、龟峰等地发育的崖壁、一线天、巷谷、石墙、石寨、峰丛、峰林等丹霞典型景观均与节理有关。② 流水侵蚀等外营力的作用对丹霞地貌的剥露、削平起重要作用,统计表明距离武夷山越近,丹霞地貌山峰越密集、崖壁越陡峻,指示武夷山的隆升与丹霞地貌的发育有直接关系。总体来看,江西省内的红层为丹霞地貌的发育提供了物质基础,武夷山的隆升控制了丹霞地貌的分布。值得注意的是,丹霞地貌区的节理构造是否与武夷山的隆升存在直接因果关系还需进一步研究。  相似文献   

Heterogeneous deformation recorded by magnetic fabrics in the Pyrenean Axial Zone     

《Journal of Structural Geology》2013

Magnetic fabrics studies (AMS) are a useful tool in order to describe the distribution of deformation in orogenic areas where conventional techniques are difficult to apply, especially due to the lack of proper strain markers. In the present study, AMS and structural analysis are used to define the distribution of deformation in the Central Axial and Nogueres Zones, an area of strong structural changes (i) in the geometry of the antiformal stack defining the Pyrenean Axial Zone and (ii) the distribution of Alpine cleavage. The studied rocks are Lower-Middle Triassic red beds that crop out in three different stacked thrust units (Bielsa, Nogueres and Orri). Primary sedimentary fabrics are preserved in the uppermost thrust units (Nogueres Zone), but a high percentage of the sampling sites shows an overprint of Alpine compression on magnetic fabrics, with the magnetic lineation mostly parallel to the tectonic grain defined by compressional structures and the magnetic foliation showing different orientations between the poles to cleavage and bedding. The development of compressional fabrics strongly depends on the structural position of the sites, and two deformation gradients can be inferred: the southern margin of the Axial Zone (Orri and Bielsa units) shows strong internal deformation, increasing towards the North; farther north (in the restored cross-section), deformation is in general terms lower, but increases towards the basal thrust of the Nogueres Zone. The heterogeneous distribution of Alpine internal deformation indicates a preferred development of cleavage in the Variscan basement and overlying units of the southern margin of the Axial Zone (mainly in the Orri unit), that could be partly controlled by the tectonic load resulting from the stacking of thrust sheets.  相似文献   

动床条件下粘性泥石流沟道淤积实验研究   总被引：1，自引：0，他引：1  

林雪平  游勇  柳金峰  赵彦波  刘曙亮 《山地学报》2013,31(3)

泥石流淤积是危害建筑物的主要方式之一.为了探讨动床条件下泥石流沟道淤积规律,开展了不同泥石流重度、不同沟床物质重度、不同沟道坡度以及不同泥石流总量共4组单次淤积实验和1组不同沟道坡度下的泥石流连续淤积实验.实验结果表明,单次淤积时,泥石流淤积厚度随着沟道坡度的增加而减小,随着泥石流重度的增大而增大,随泥石流总量的增大而增大,受沟床物质重度变化的影响较小;连续淤积时,随着沟道坡度的增加,第2次淤积相对于第1次淤积结果依次表现为完全淤积、冲淤交替以及完全冲刷,且冲刷始于沟道前缘,逐渐向后缘发展.在分析泥石流淤积厚度的影响因素基础上,通过回归分析建立了动床条件下粘性泥石流沟道淤积厚度的经验预测公式:H=0.005x+ (1.63E-11)e(rc)-0.003ln(sinθ) +0.22V-0.02.  相似文献   

Hydrochemistry of ice stream beds—evaporitic or microbial effects?     

Mark Skidmore  Martyn Tranter  Slawek Tulaczyk  Brian Lanoil 《水文研究》2010,24(4):517-523

  相似文献