X‐ray tomography of tsunami deposits: Towards a new depositional model of tsunami deposits

X‐ray computed microtomography is used to obtain high resolution imagery of a historical tsunami deposit in Andalusia, Spain (1755 Lisbon tsunami). The technique allows characterization of grain‐size distribution, structures, component analysis and sedimentary fabric of fine‐grained unconsolidated tsunami deposits at resolutions down to particle scale. The results are validated by comparing to data obtained using other techniques such as laser diffraction, anisotropy of magnetic susceptibility and X‐ray microfluorescence on the same deposits. Specific technical details such as sampling, scanning and image processing methods, and further improvements are addressed. The use of X‐ray computed microtomography provides new insights into the stratigraphy of the deposits and gives access to significantly more detailed view of key sedimentary features such as mudlines, rip‐up clasts, crude laminations, convolutions, floating outsized clasts and contacts between successive units. This analysis of the 1755 tsunami deposits using X‐ray computed microtomography allows the proposal of new hypotheses for the sedimentary processes forming tsunami deposits. Deposition by settling is limited and the section analysed here is dominated by a high shear stress leading to the development of traction carpets, with laminated mudlines corresponding to the basal frictional region of these carpets. The onset of the tsunami backwash is marked by a micro‐vortex resembling Kelvin–Helmoltz instabilities.     

Origin of Exceptionally Abundant Phonolites on Ua Pou Island (Marquesas, French Polynesia): Partial Melting of Basanites Followed by Crustal Contamination

On the basis of the first systematic mapping of Ua Pou, longknown for its exceptionally abundant phonolites, we estimatethat these rocks cover 65% of the surface of the island whereasmafic lavas cover 27% and intermediate ones 8%. The silica-undersaturatedsuite was erupted in a restricted time span (2·9–2·35Myr), following the emplacement of tholeiites derived from ayoung HIMU-type source at c. 4 Ma. Primitive basanites, derivedfrom a heterogeneous mantle source with a dominant EM II + HIMUsignature, represent likely parental magmas. The series is characterizedby a Daly gap defined by a lack of phonotephrites. We considerthat the most likely model for the origin of evolved lavas ispartial melting at depth of primitive basanites, leaving anamphibole-rich residuum and producing tephriphonolitic magmas.These tephriphonolitic magmas may have evolved by closed-systemfractional crystallization towards Group A phonolites. Threeother groups of phonolites could have been derived from tephriphonoliticmagmas by open-system fractional crystallization processes,characterized respectively by seawater contamination (GroupB), assimilation of nepheline syenite-type materials (GroupC) and extreme fractionation coupled with assimilation of theunderlying oceanic crust (Group D). The prominence of evolvedlavas is a consequence of their origin from partial meltingof mafic precursors followed by crustal contamination. KEY WORDS: Marquesas; French Polynesia; phonolite; partial melting; contamination     

塔里木盆地早、中志留世沉积序列及其对构造一海平面变化的响应:以柯坪露头区为例

志留纪是晚加里东运动对塔里木盆地古地理和古构造格局产生重要影响的构造变格阶段,以全盆地规模的不整合面为界,前志留纪以海相碳酸盐岩沉积为主的古环境转变为志留纪以海相碎屑岩沉积为主的古环境.开展志留纪沉积充填序列及岩矿特征研究,有助于进一步认识塔里木盆地该变格阶段构造运动特征及其沉积充填响应.为此,精选柯坪露头区志留系出露...     

重力流主导的深水沉积特征及其模式——以共和盆地下三叠统为例

深水沉积具有良好的油气勘探前景,近年来已成为沉积学研究的前缘和油气工业界关注的焦点。以共和盆地下三叠统为例,通过野外剖面的详细观察描述,结合室内镜下薄片鉴定与粒度分析,对深水沉积类型、沉积特征、垂向组合、成因机制以及沉积模式进行系统研究。共和盆地早三叠世深水沉积类型主要有滑塌型深水重力流沉积、底流沉积以及深水悬浮沉积,其中滑塌型深水重力流沉积又可识别出滑塌沉积、砂质碎屑流沉积与浊流沉积三种类型。滑塌沉积常见以同沉积褶皱为代表的多种软沉积物变形构造;砂质碎屑流沉积以块状砂岩为主,内部可见砂质团块、泥砾或泥质撕裂屑,块状砂岩顶底与相邻岩层均为突变接触;浊流沉积普遍发育正粒序,可见不完整的鲍马序列,底面常见多种类型的底模构造;底流沉积发育多种牵引流沉积构造。研究区软沉积物变形构造类型丰富,其中滑塌层内的软沉积物变形构造主要在斜坡滑塌过程中形成;未发生滑塌层内的软沉积物变形构造主要因地震使沉积物发生液化作用及流体化作用而形成。在综合分析盆地构造背景、深水沉积分布规律、重力流触发机制的基础上,建立了共和盆地早三叠世滑塌型重力流主导的深水沉积模式。滑塌型重力流主要由地震以及火山事件触发,水道化的地...     

滑塌型深水重力流沉积特征及沉积模式:以渤海湾盆地临南洼陷古近系沙三中亚段为例*

综合运用钻井岩心、测录井等资料,通过岩心观察、薄片鉴定和粒度分析等方法,对渤海湾盆地临南洼陷古近系沙三中亚段深水重力流沉积类型及其沉积特征、沉积模式展开研究。研究表明,临南洼陷沙三中亚段深水重力流沉积主要发育滑塌沉积、碎屑流沉积和浊流沉积3种成因类型。滑塌沉积以包卷层理、液化砂岩脉、阶梯状小断层、变形岩层与未变形岩层叠置为典型特征。碎屑流沉积中砂质碎屑流沉积分布较广,以突变的底部接触面、块状层理、泥岩撕裂屑、土黄色泥砾、突变或不规则的上接触面为典型识别标志。浊流沉积则以正粒序层理、底部冲刷面和槽模、薄层砂泥互层、不完整的鲍马序列为典型识别标志。滑塌沉积主要发育在三角洲前缘斜坡根部,在滑塌沉积前方形成碎屑流沉积,碎屑流向前搬运的过程中,流体被稀释逐渐转化成浊流。滑塌型深水重力流沉积整体分为近源沉积、中部沉积和远源沉积3个部分: 近源沉积主要发育具变形构造的滑塌沉积和厚层块状砂质碎屑流沉积;中部沉积主要发育砂质碎屑流沉积及浊流沉积;远源沉积以薄层浊流沉积为主。