Paleoenvironmental reconstruction and organic matter accumulation of the lower Cambrian Qiongzhusi Formation in the Sichuan Basin, South China

The lower Cambrian Qiongzhusi Formation in the Sichuan Basin of the Yangtze Platform is one of the most proliferous petroleum sources. However, the basin-wide lower Cambrian (Qiongzhusi) environmental variation in the Sichuan Basin remains unclear, which could have impeded the unlocking of promising areas of organic matter accumulation. This paper presents the first detailed inorganic geochemical investigation of the lower Cambrian environmental reconstruction in the Sichuan Basin. The results of principal component analysis show that the lower Qiongzhusi succession (LQS) is enriched in carbonate-associated elements and rare earth elements (REE), whereas the middle-upper Qiongzhusi successions (MQS-UQS) are enriched in elements affiliated with aluminosilicates (e.g., Al and Ti). The conservative metal associations suggest that the provenance of the sediments was intermediate-felsic igneous rocks and the tectonic setting was a continental island arc. The combination of Sr/Ba and S/TOC values indicates that it was formed under brackish to normal marine conditions. In contrast to the middle Qiongzhusi succession, the lower and upper Qiongzhusi successions had elevated biological productivity (increased Cu/Al), low detrital inputs (decreased Ti/Al), low sedimentation rate (decreased Th/U), and reducing conditions. These attributes probably resulted in higher organic matter productivity and preservation. Temporally, the northern part of this basin had higher organic matter contents and was deposited under more reducing conditions, with a lower sedimentation rate of terrigenous materials.