Paleohydrology of an Upper Aptian lacustrine system from northeastern Brazil: Integration of facies and isotopic geochemistry
Paleohydrology of an Upper Aptian lacustrine system from northeastern Brazil: Integration of facies and isotopic geochemistry
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巴西东北部上阿普第阶湖泊系统的古水文学：相和同位素地球化学的整合
DOI：
10.1016/j.palaeo.2006.03.040
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发表时间：
2006
期刊：
影响因子：
--
通讯作者：
D. Rossetti
中科院分区：
文献类型：
--
作者：
J. Paz;D. Rossetti
Codó组记录了Aptian晚期沿巴西赤道边缘形成的陆内裂谷系统的初始演化阶段。该单元出露于Grajaú盆地东缘，矿床包括石膏、沥青黑色页岩和灰岩。这些岩性形成于以干旱/半干旱气候为主的低能、分层良好、缺氧和高盐湖泊体系中。这种湖泊演替在内部被组织成三种浅层向上旋回，一阶和二阶旋回与地震活动有关，与断层重新激活有关，三阶旋回记录了气候波动。强调岩石学研究和地球化学示踪剂Fe、Mg、Sr、Mn、Na和Ca的分析，有助于确定具有原始信号的沉积相，从而适合于以古环境和古水文重建为目标的同位素调查。结果表明，碳酸盐岩中碳、氧同位素值分布广泛，分别为−5.69‰~−13.02‰和−2.71‰~−10.80‰。本文表明，至少在氧的特殊情况下，同位素比率根据地震引起的浅—上旋回而变化，在中心湖泊沉积物占主导地位的底部，其值一般较低，而在边缘湖泊沉积物更广泛的地方，其值逐渐升高。除了确认分析样品的沉积特征外，这种行为还允许开发地震诱发的同位素模型。较轻的同位素值可能与下沉引发的洪水事件有关，而较重的同位素值可能与隆升和/或稳定性增加的短暂湖泊阶段有关。碳、氧同位素总体正方差（+0.42 ~ +0.43）较好，但开放相也存在负协方差（- 0.36），以封闭相为主。在闭湖期，由于停留时间的增加，δ18O变化幅度最大（- 3.63‰~ - 4.89‰），而在湖泊开放期，由于连续入流维持了水同位素组成的平衡，δ18O变化幅度较小（- 0.09‰~ - 1.87‰）。
The Codó Formation records the initial evolutionary stages of an intracontinental rift system formed along the Brazilian equatorial margin in the late Aptian. Deposits of this unit exposed on the eastern margin of the Grajaú Basin include gypsum, bituminous black shales and limestones. These lithologies were formed in a low energy, well stratified, anoxic and hypersaline lake system developed in a dominantly arid/semi-arid climate. This lacustrine succession is internally organized into three categories of shallowing-upward cycles, with the first- and second-order cycles being related to seismic activity associated with fault reactivations, and the third-order cycles recording climatic fluctuations. Studies emphasizing petrography and analysis of the geochemical tracers Fe, Mg, Sr, Mn, Na and Ca helped to identify the sedimentary facies that kept a primary signal, which were thus appropriate for isotopic investigations aiming paleoenvironmental and paleohydrologic reconstructions. The results of this study revealed a wide distribution of dominantly low carbon and oxygen isotope values in carbonates, ranging from −5.69‰ to −13.02‰ and from −2.71‰ to −10.80‰, respectively. This paper demonstrates that at least in the particular case of oxygen, the isotope ratios vary according to seismically-induced shallowing-upward cycles, with values in general lower at their bases, where central lake deposits dominate, and progressively higher upward, where marginal lake deposits are more widespread. In addition to confirming a depositional signature for the analysed samples, this behavior allowed the development of a seismic-induced isotope model. The lighter isotope ratios appear to be related to flooding events promoted by subsidence, which resulted in the development of a perennial lake system, while heavier isotope values are related to ephemeral lake phases favored by uplift and/or increased stability. Furthermore, the results show that a closed lake system dominated, as indicated by the overall good positive covariance (i.e., +0.42 to +0.43) between the carbon and oxygen isotopes, though open phases are also recorded by negative covariance values of −0.36. During closed phases, the δ18O displayed the highest range of variation (i.e., −3.63‰ to −4.89‰) due to increased residence time, while this variation was low (i.e., −0.09‰ to −1.87‰) during open lake phases, when there was a balance in the water isotope composition maintained by continuous basin inflow.