Aragonite loss in a cold‐water coral mound: mechanisms and implications     

TRACY D FRANK  JÜRGEN TITSCHACK  MIEKE THIERENS 《Sedimentology》2011,58(3):670-690

Selective dissolution of aragonitic grains is emerging as a volumetrically significant process that affects a broad range of modern carbonate settings. This study explores mechanisms and implications of aragonite loss in Challenger Mound, a giant cold‐water coral (Lophelia pertusa) mound of Pleistocene age, which lies on the continental slope off south‐west Ireland. A comprehensive sampling scheme allowed the integration of petrographic data with geochemical analyses of sediment and pore water. The mound remains virtually unlithified and consists of stacked, fining‐upward cycles of silty coral floatstone–rudstone and bafflestone grading into wackestone. Whereas calcitic grains appear unaltered, aragonitic grains are corroded and fragmented. Aragonite dissolution is attributed to organic matter oxidation at/near the sediment–water interface and, at greater depths, to the initial stages of bacterially mediated sulphate reduction, when alkalinity production is outpaced by the generation of H⁺. Pore water profiles indicate that undersaturated waters are diffusing towards the mound interior from two centres of sulphate reduction: one located in the upper 10 m of the sediment column and a second that lies below an erosional unconformity which marks the base of the mound. Continued aragonite dissolution is expected to gradually lower the diagenetic potential of the Challenger Mound and delay lithification until deep burial, when solution‐compaction processes come into play. Despite a fundamental role in predestining the final taphonomic and textural characteristics of Challenger Mound, the processes described here are expected to leave little trace in the geological record due to a lack of cementation and calcitization. Assuming that similar processes have been active throughout the Phanerozoic, results imply that the understanding of diagenetic processes in carbonate systems may be incomplete.  相似文献   

Spatio‐temporal analysis of groundwater recharge and mound dynamics in an unconfined aquifer: a GIS‐based approach     

Ritesh Vijay  Nikhil Panchbhai  Apurba Gupta 《水文研究》2007,21(20):2760-2764

Groundwater recharge and mounding of water‐table is a complex phenomenon involving time‐ and space‐dependent hydrologic processes. The effect of long‐term groundwater mounding in the aquifer depends on soil, aquifer geometry and the area contributing to recharge. In this paper, a GIS‐based spatio‐temporal algorithm has been developed for the groundwater mound dynamics to estimate the potential rise in the water‐table and groundwater volume balance residual in an unconfined aquifer. The recharge and mound dynamics as predicted using the methodology recommended here were compared with those using the Hantush equation, and the differences were quite significant. The significance of the study is to assess the effectiveness of the basin in terms of its hydrologic and hydraulic properties for sustainable management of groundwater recharge. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

Growth history of a cold-water coral covered carbonate mound — Galway Mound, Porcupine Seabight, NE-Atlantic     

Markus Eisele  Dierk Hebbeln  Claudia Wienberg   《Marine Geology》2008,253(3-4):160-169

During the past decade, knowledge about the ecology and the environment of giant carbonate mounds has been growing continuously. However, still little is known about their growth dynamics. Three gravity cores from Galway Mound, Belgica Mound Province in the Porcupine Seabight off Ireland, were investigated for their sedimentological, geophysical and geochemical properties to get insight into the long-term development of this cold-water coral covered carbonate mound. These data were supplemented by radiometric age determinations on planktonic foraminifera and coral skeletons. The records from three different settings on Galway Mound reveal a coherent growth history that in general is similar to what is known from other carbonate mounds at the Irish margin. However, whereas other cores are often disturbed by numerous and not correlateable hiatuses, Galway Mound, in contrast, appears to be characterised by only one major hiatus representing a time gap of ~ 250 kyr. Several mechanisms are discussed in this study as possible causes for the observed stratigraphic record at Galway Mound. The most likely explanation is that the hiatus has its origin in a major mass wasting event on an instable, possibly glacial, unit that could have acted as a slip plane. The overall Late Quaternary growth history of Galway Mound fits well into existing cyclical mound development models, pointing to Galway Mound being an ‘actively growing’ mound (“coral bank stage”) at present.  相似文献   

Influence of sedimentary setting on the use of magnetic susceptibility: examples from the Devonian of Belgium   总被引：1，自引：0，他引：1  

A.-C. DA SILVA  C. MABILLE  F. BOULVAIN 《Sedimentology》2009,56(5):1292-1306

Bulk magnetic susceptibility measurements on sedimentological samples from all geological periods have been used widely in the last two decades for correlations and as a proxy for sea‐level variations. This paper explores the link between magnetic susceptibility, depositional setting and environmental parameters. These environmental parameters include distal–proximal transects, microfacies successions and fourth‐order trends on different carbonate platform types (platform, ramp, carbonate mound or atoll) during different Devonian stages (Eifelian, Givetian and Frasnian). Average magnetic susceptibility values over a distal–proximal‐trending facies succession vary markedly with depositional setting. On carbonate platforms, average magnetic susceptibility generally increases towards the top of shallowing‐upward sequences. On a distal–proximal transect, average magnetic susceptibility is intermediate for the deepest facies, decreases for the reef belts and increases to a maximum in the back‐reef zone. In ramps and atolls, magnetic susceptibility trends clearly differ; average magnetic susceptibility generally decreases towards the top of shallowing‐upward sequences and is highest in the deepest facies. The strong relationship between magnetic susceptibility, facies and sequences implies a strong environmental influence. However, the different responses in the different platform types suggest that sea‐level changes leading to variation in detrital input is not the only parameter controlling average magnetic susceptibility values. Other primary or secondary processes also probably influenced magnetic mineral distribution. Primary processes such as carbonate production and water agitation during deposition are probably key factors. When carbonate production is high, the proportion of magnetic minerals is diluted and the magnetic susceptibility signal decreases. High water agitation during deposition will also selectively remove magnetic minerals and will lead to low average magnetic susceptibility values. These parameters explain the lowest values observed on the reef platform, inner ramp and atoll crown, which are all in areas characterized by higher carbonate production and greater water agitation during deposition. The lowest values observed in the lagoon inside the atoll crown can be related to detrital isolation by the atoll crown. However, other parameters such as biogenic magnetite production or diagenesis can also influence the magnetic signal. Diagenesis can change magnetism by creating or destroying magnetic minerals. However, the influence of diagenesis probably is linked strongly to the primary facies (permeability, amount of clay or organic matter) and probably enhanced the primary signal. The complexity of the signal gives rise to correlation problems between different depositional settings. Thus, while magnetic susceptibility has the potential to be an important correlation tool, the results of this investigation indicate that it cannot be used without consideration of sedimentary processes and depositional environments and without strong biostratigraphical control.  相似文献   

浙江大榭史前制盐遗址人工土台的堆土特征及来源分析^*          下载免费PDF全文

栗文静  雷少  王张华  李月  岳伟  郑童童 《古地理学报》2018,20(6):1102-1112

位于浙江省宁波市大榭岛的大榭遗址,是迄今所发现的中国最早的史前海盐业遗址,与一般遗址沉积地层不同,其主体区系人工堆筑土台。本研究在遗址发掘过程中,采集了泥质和砂质2种类型堆土、废弃陶器堆中的白色硬化物和盐灶坑表层白色硬化物,同时采集本岛自然堆积物,进行了粒度、无机元素地球化学和重矿物分析,以探讨堆土的原料构成、古人的制盐工艺及制盐活动对堆土地层产生的影响。研究结果显示: 来自盐灶遗迹附近的7个泥质堆土样品,其中5个具有陆相元素富集特征,但是后期受海水影响,另外2个显示滩涂盐泥特征;废弃陶器堆中的白色硬化物元素组成与盐泥相似,盐灶坑表层白色硬化物具高Ca、Sr、Ba含量,推测是烧煮卤水产生的沉淀物。砂质堆土的粒度和重矿物组成显示其属于河流冲积物,且具强烈的化学风化特征,推测其原始地层不仅限于全新统。该研究显示了钱山漾文化时期古人利用岛屿上的潮上带、山麓黄土、河流冲积物等原材料在滩涂营建土台,并在土台上利用“淋卤煎炼”工艺从事制盐活动。  相似文献   

鄂尔多斯盆地中部奥陶系马五盐下沉积环境与海平面变化^*          下载免费PDF全文

席胜利  熊鹰  刘显阳  雷晶超  刘明洁  刘灵  刘耘  文汇博  谭秀成 《古地理学报》2017,19(5):773-790

鄂尔多斯盆地中部奥陶系马家沟组赋存有丰富的天然气资源,前人研究多集中于顶部与风化壳相关的储集层。文中基于岩心、薄片及阴极发光分析,将鄂尔多斯盆地中部奥陶系马五盐下碳酸盐岩-蒸发岩划分为10种岩石类型。根据各岩类的宏观、微观特征以及它们在纵横向上的组合与分布特点,利用岩石类型组合的方法,将研究区划分为局限-蒸发潟湖、台内滩、滩间海、微生物丘和台坪5种沉积环境。根据各亚段岩石类型发育的差异性及统计结果发现,研究区马五盐下具有蒸发岩与碳酸盐岩间互的旋回性沉积演化与海平面升降特征,每个亚段都代表一次海侵或海退的旋回沉积。其中,马五6、马五8和马五10亚段为海退期沉积,以纹层状云质膏岩、泥晶云岩构成的局限-蒸发潟湖沉积为主,蒸发岩矿物含量高,反映沉积环境相对闭塞、能量较低且盐度较高;马五7和马五9亚段为海进期沉积,以相对高能的台内滩或微生物丘沉积为主,并与相邻的局限-蒸发潟湖和台坪沉积构成较完整的向上变浅米级沉积旋回,表明该阶段水体循环相对更好,整体沉积环境开阔且能量较高。  相似文献   

南海北部陆坡东沙海域海底丘状体气体与水合物分布   总被引：1，自引：0，他引：1  

刘斌 《海洋学报》2017,39(3):68-75

海底丘状体在天然气水合物发育区是一种常见的微地貌,对丘状体的研究有助于理解海底流体渗漏模式以及水合物的赋存规律。本文研究南海北部陆坡东沙海域天然气水合物发育区海底丘状体的特征及其与水合物的关系。研究所用的数据包括准三维多道地震数据、多波束数据以及浅地层剖面数据。在多波束海底地形图上,丘状体表现为局部的正地形,直径大约为300 m,高出周围海底约50 m。浅地层剖面上存在明显的声空白以及同相轴下拉现象,指示了海底丘状体气体的分布以及流体运移的路径。丘状体周围明显的BSR表明局部区域可能发育有水合物,水合物钻探结果也证实了这一推测。三维多道地震剖面上,丘状体正下方存在空白反射区域,这与泥火山的地震反射特征类似。但空白反射区域内存在强振幅能量,而且丘状体正下方存在连续的反射层,这表明该丘状体并非泥火山成因。综合钻探结果以及三维地震成像结果,认为水合物形成过程引起的沉积物膨胀以及海底碳酸盐岩的沉淀是形成该丘状体的主要原因。  相似文献   

Reservoir characterization of the Pennsylvanian Caddo Limestone in Stephens County,Texas: A case study of Komia-dominated algal mounds     

《Marine and Petroleum Geology》2017

The Caddo Limestone forms economic carbonate reservoirs in Stephens County, northern Texas. This study demonstrates that, in the Caddo Limestone of the Eliasville and Breckenridge fields, porosity and permeability are best developed in phylloid-algal wackestones and packstones, as well as Komia wackestones and packstones prevalent within the uppermost interval (i.e., Cycle A) of the formation in the study area. The main reservoirs formed in the upper and middle intervals of the Caddo algal mounds because of meteoric dissolution related to subaerial exposure (which created a large volume of secondary pores) and early cementation that prevented mechanical compaction. A great portion of the secondary pores remain open, providing the principal pore spaces of the reservoir interval. Vugs (including moldic voids) are abundant, and dissolution-enhanced intragranular pores are very common within widespread Komia. Intercrystalline pores are prevalent in dolomitized and neomorphised lithofacies where micrite was converted to microsparitic and sparitic calcite. Micropores are abundant in the matrix and within grains (especially Komia fragments). The lower or basal interval of Cycle A is commonly much less porous owing to the substantial loss of primary pores by physical compaction and lack (or rare presence) of secondary pores. Laterally, wells in the areas with thicker Cycle A (interpreted as algal mounds) have higher porosity and thicker net reservoir than those in intermound areas. This work provides a case study of carbonate reservoirs in which Komia wackestones and packstones are the major reservoir rocks.  相似文献   

Diagenetic processes in carbonate mound sediments at the south‐west Rockall Trough margin     

CEES VAN der LAND  FURU MIENIS  HENK DE HAAS  NORBERT FRANK  RUDY SWENNEN  TJEERD C. E. VAN WEERING 《Sedimentology》2010,57(3):912-931

Cold water coral covered carbonate mounds at the south‐west margin of the Rockall Trough form ridges several kilometres long and up to 380 m high. Piston cores obtained at three mound crests reveal the complex internal structure of the mound build up, with alternating unlithified coral‐dominated intervals and lithified intervals. The most recent lithified interval is covered by corals embedded in a fine‐grained matrix, comprising ca 11 000 years of continuous mound evolution. Before this time ²³⁰Th/U dating shows the presence of several hiatuses in mound build‐up. Aragonitic coral material is absent or only present as mouldic porosity in the lithified intervals and coccoliths display widespread overgrowth. Downcore X‐ray fluorescence scanning, computer tomography scan images and petrographic observations indicate different degrees of diagenetic alteration. The upper boundary of the most recent lithified interval shows some erosional features, but petrographic observations indicate that initial lithification of the sediments is not related to this erosive event or to long‐term non‐sedimentation, but to earlier sub‐surface diagenesis. Organic matter oxidation and the subsequent lowering of the saturation state of the carbonate system drives dissolution of the unstable aragonitic coral skeletons. Depending on the openness of the system, this can lead to precipitation of a more stable low‐magnesium carbonate. A model is presented describing the sedimentary and diagenetic processes leading to the formation of lithified intervals.  相似文献   

基于合成孔径声学深拖调查的海底浅表层流体活动研究——以SAMS DT6000深拖在琼东南海域调查为例          下载免费PDF全文

单晨晨  温明明  刘斌  冯强强  李勇航  黄伟 《地球物理学报》1954,63(12):4451-4462

声学深拖作为一个声学设备搭载平台,主要功能是获取高分辨率的声学数据,精细刻画海底地形地貌特征以及浅层剖面结构,对于研究海底浅表层流体活动系统的类型、形成机制和演化模式有着重要作用.本文介绍的合成孔径声学深拖（Synthetic Aperture Sonar Deep-tow）搭载了合成孔径声呐、浅地层剖面仪以及多波束系统等声学设备,相比于传统的侧扫声呐,合成孔径声呐采用小物理孔径基阵通过信号处理虚拟合成大孔径基阵来获得方位向高分辨率,大大提高了测绘速率,同时结合高分辨率的浅地层剖面和多波束背散射数据,可实现海底浅表层特征的三维立体显示.为查明调查区海底浅层流体活动的声学特征,分析天然气水合物相关的流体渗漏活动性与浅层构造之间的关系,我们利用声学深拖对研究区进行了全覆盖的扫测,获得了高分辨率的合成孔径声呐图像、浅地层剖面资料以及多波束背散射数据,平面上识别出多个呈条带状的海底丘状体,火焰状的流体渗漏,新月形的麻坑构造等流体活动地质构造;浅层剖面上可见气体聚集的声学空白段落,凸起的活跃喷口,以及反射杂乱的柱状浑浊带.通过识别流体活动的特征,我们总结了浅层流体活动演化模式具有周期性：游离气体通过高渗透运移通道上升至海底,首先扩散聚集造成局部沉积物体积膨胀形成丘状体;然后受其各种外界因素影响丘状体崩塌而引起气体渗漏;最后流体逸散剥蚀海底松散沉积物而形成麻坑构造;随着流体排出,喷口重新闭合,流体在地层中再次聚集,聚集的气体又将沉积地层上拱,在麻坑底部又可能生成含气丘状体.海底浅表层蕴藏着丰富的地质信息,这对于研究海底复杂的流体活动有着重要意义.  相似文献