Abstract Lacustrine organic-rich fine-grained rocks are widespread in the second member of the Permian Lucaogou Formation (P2l2) in the Malang Sag, Santanghu Basin, with complex lithology and significant changes of lithofacies due to frequent volcanic activity. In this study, the mineralogical and petrological characteristics, types of organic matter (OM), and effect of volcanism on the sedimentary environment and OM accumulation were analyzed using organic, carbon, and oxygen isotopes, as well as major and trace element data. The results revealed that the P2l2 was characterized by mixed deposition of volcanic ash and carbonate minerals, with four main rock types: tuff, dolomitic tuff, tuffaceous dolomite, and dolomite. The fine-grained rocks dominantly contain type I and II1 kerogens, which were in a low-mature stage and exhibited excellent original hydrocarbon generative potential. When volcanic activity was strong, large amounts of nutrients were released by the volcanic ash, which then entered the lake, in turn promoting the blooming of phytoplankton, and causing the δ13C values in the tuff to decrease. In addition, the paleotemperature decreased for a short time, and the suboxic-euxinic environment was conducive to the preservation of OM, resulting in high total organic carbon (TOC) content in the tuff. In contrast, during intermittent or weak periods of volcanic activity, the climate was hotter and drier, and evaporation resulted in a sustained increase of salinity. Consequently, the growth of algae was restricted, which brought about an increase in the δ13C values and relatively low TOC content in the dolomite. The relationships among primary productivity, redox conditions, clastic influx proxies, and TOC content illustrate that primary productivity was the most vital control on the accumulation of OM. High primary productivity was mainly driven by volcanic ash falls. Meanwhile, high salinity and suboxic-euxinic conditions were beneficial to the preservation of OM, giving rise to a high degree of OM enrichment.

中文翻译：

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西北三塘湖盆地玛琅凹陷二叠系芦草沟组火山活动的环境响应及其对有机质富集的影响

摘要 三塘湖盆地玛琅凹陷二叠系芦草沟组二段（P2l2）广泛分布湖相富机质细粒岩，岩性复杂，火山活动频繁，岩相变化显着。本研究利用有机、碳、氧同位素以及主要和微量元素数据，分析了矿物学和岩石学特征、有机质（OM）类型、火山作用对沉积环境和有机质积累的影响。结果表明，P2l2以火山灰和碳酸盐矿物混合沉积为特征，主要岩石类型为凝灰岩、白云质凝灰岩、凝灰质白云岩和白云岩四种。细粒岩以Ⅰ型和Ⅱ1型干酪根为主，处于低成熟阶段，具有优良的原始生烃潜力。火山活动强烈时，火山灰释放出大量养分进入湖中，进而促进浮游植物的大量繁殖，使凝灰岩中的δ13C值降低。此外，古温度短时间内下降，弱氧-富氧环境有利于OM的保存，导致凝灰岩中总有机碳（TOC）含量较高。相比之下，在间歇性或弱火山活动期间，气候更加炎热干燥，蒸发导致盐度持续升高。因此，藻类的生长受到限制，导致白云岩中δ13C值增加和相对较低的TOC含量。初级生产力、氧化还原条件、碎屑流入代理和 TOC 含量之间的关系表明初级生产力是对 OM 积累最重要的控制。高初级生产力主要是由火山灰下降驱动的。同时，高盐度和低氧-富氧条件有利于OM的保存，导致OM高度富集。