Diagenesis and sea‐level change: lessons from Moruroa, French Polynesia
Diagenesis and sea‐level change: lessons from Moruroa, French Polynesia
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成岩作用和海平面变化：法属波利尼西亚莫鲁罗阿的教训
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发表时间：
2011
期刊：
影响因子：
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通讯作者：
G. Camoin
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作者：
C. Braithwaite;G. Camoin
通过法属波利尼西亚Moruroa环礁边缘的倾斜钻孔，其中单元的年龄受到放射性测年的限制，为研究海平面变化和成岩历史之间的关系提供了独特的机会。遇到的最古老的岩石是在355 - 840年的BP和覆盖的继承记录5个周期的沉积和出现，最终在沉积的全新世序列。 连续出现的间隔为成岩变化提供了驱动力，促进了化学侵蚀性淡水沃茨的流动，这些水有时是普遍存在的，有时被限制在毫米到米尺度的相对孤立的通道内。这些局部变化的流量和流速定义了限制区，其特征在于水化学和成岩特征的差异。在其他地方的一些序列相比，Moruroa环礁继承的上半部分是典型的交替的间隔中，无论是几代的水泥生长的代表或只有一个或两个水泥存在。可识别出三种不同的成岩特征：（1）淡水潜水带反映为新生变质作用和胶结物的沉积，胶结物封闭了孔隙，从而阻止了后期胶结物的沉积，并赋予了相对稳定性;（ii）包气带的特征是反映溶解和胶结物生长的多个阶段的更复杂的成岩序列，证明暴露的相对持续时间和地下水流的条件;以及（iii）在岩心深处，层序反映了遭受海底成岩作用的低位沉积物，生成孔隙，其中多层海相胶结物显然比陆上过程形成的间隔更频繁。所有的水泥似乎都是严格限制的年龄。这些结果突出了一个有争议的问题，即前成岩环境的分布是否与特定的海平面和侵蚀面有关。似乎在Moruroa岩心中，一些海平面变化的周期反映在成岩特征的独特包中，这些特征在后来的海平面期间没有被水泥生长所覆盖。
An inclined borehole through the margin of Moruroa Atoll, French Polynesia, in which the ages of units are constrained by radiometric dates, provides a unique opportunity to examine the relationship between sea‐level change and diagenetic history. The oldest rocks encountered are dated at 355 840 yr bp and the overlying succession records five cycles of deposition and emergence, culminating in the deposition of the Holocene sequence. Successive intervals of emergence provided the driving force for diagenetic change, promoting the through‐flow of chemically aggressive fresh waters that were at times either pervasive or confined within relatively isolated pathways on a millimetre to metre scale. These locally varying flows and flow rates defined restricted zones characterized by differences in water chemistry and diagenetic signatures. In contrast to some sequences elsewhere, the upper part of the Moruroa Atoll succession is typified by alternations of intervals in which either several generations of cement growth are represented or only one or two cements are present. Three distinct diagenetic signatures can be recognized as follows: (i) fresh water phreatic zones are reflected in neomorphism and the deposition of cements that occluded porosity, thus preventing the deposition of later cements and conferring relative stability; (ii) vadose zones are characterized by more complex diagenetic sequences reflecting multiple phases of dissolution and cement growth, attesting to the relative duration of exposure and the conditions of ground water flow; and (iii) deep in the core, sequences reflect lowstand deposits that suffered sub‐sea diagenesis, generating pores in which multi‐layered marine cements apparently formed at more frequent intervals than those subject to subaerial processes. All cements appear to be of narrowly constrained ages. These results highlight the contentious issue of whether the distributions of former diagenetic environments can be related to specific sea‐level stands and erosion surfaces. It seems that, in the Moruroa core, some cycles of sea‐level change are reflected in distinctive packages of diagenetic features that have not been overprinted by cement growths during later sea‐level stands.