Geochemical paleoredox indicators in organic-rich shales of the Irati Formation, Permian of the Paraná Basin, southern Brazil

ABSTRACT: Paleoredox indicators were used to assess the depositional environment of the Permian Irati Formation, and discuss the controls on the accumulation of organic-rich sediments. Geochemical data were measured for 43 drill-core samples in southern Paraná Basin. We infer that redox boundary was located at the sediment-water interface during the accumulation of these sediments. Trace-element data demonstrate that Irati shales were similar to average shales, except during deposition of two organic-rich beds in which U and Mo are enriched, suggesting fully anoxic conditions. The variable degree of oxygenation of bottom waters is also attested by DOPT, FeT/Al, and sedimentological indicators of episodic ventilation. DOPT and Fe and S correlation points to an iron-limited environment, where the main sulfur sink is organic matter rather than pyrite. Cu and Cu/Mo ratio suggests bottom-water anoxia driven by increased organic flux. The Irati Formation was probably deposited in a salinity-stratified body of water, with positive water balance. The main control on the accumulation of the organic-rich sediments was the high organic carbon flux resultant from high primary productivity in surface waters. Anoxia arose as a consequence of increased productivity, not as the primary driver in the formation of these organic-rich rocks.

摘要：本研究采用古氧化还原指示剂（Paleoredox indicators）评估二叠纪伊拉蒂组（Permian Irati Formation）的沉积环境，并探讨富有机质沉积物堆积的控制因素。研究对巴拉那盆地（Paraná Basin）南部的43个岩芯样品开展了地球化学数据测试。我们推断，在上述沉积物堆积期间，氧化还原界面位于沉积物-水界面。微量元素数据显示，伊拉蒂页岩的地球化学特征与普通页岩基本一致，仅在两套富有机质岩层沉积阶段出现铀（U）、钼（Mo）富集，指示完全缺氧环境。底层水氧化还原状态的变化还通过DOPT、总铁与铝之比（FeT/Al）以及反映间歇性通风的沉积学指标得到证实。DOPT与铁、硫的相关性表明研究区为铁限制环境，此时硫的主要汇为有机质而非黄铁矿。铜（Cu）含量与铜钼比（Cu/Mo）表明，有机通量提升驱动了底层水体缺氧。伊拉蒂组大概率沉积于盐度分层且水均衡为正的水体环境中。富有机质沉积物堆积的主要控制因素为表层水体高初级生产力所产生的高有机碳通量。缺氧环境是生产力提升的结果，而非此类富有机质岩石形成的首要驱动因素。