Tectonic setting and its effect on organic matter enrichment of the lower Cambrian shales in the Sichuan Basin (SW China)

Abstract

The lower Cambrian shale, a significant source rock, has been witness to substantial tectonic and environmental shifts on Earth. However, the correlation between early Cambrian tectonic history and organic matter enrichment (OME) remains elusive. The Sichuan Basin in the northwestern Yangtze Block hosts the largest Precambrian-Cambrian gas field in China. The primary source rocks contributing to this reservoir are mainly the lower Cambrian shales. The organic matter enrichment of the lower Cambrian shale is commonly ascribed to paleoproductivity, redox conditions, localized hydrothermal activity, upwelling currents, and terrigenous inputs. However, organic matter enrichment varies across the Yangtze Block due to different influencing factors. Particularly, little information has been employed to elucidate the tectonic impact on OME in the lower Cambrian shales within the Sichuan Basin. This study presents detrital zircon U-Pb ages and geochemical data obtained from late lower Cambrian shales in the Sichuan Basin, SW China. The black shales exhibited similar chondrite-normalized REE patterns. They display a slightly right-dipping trend in LREE, a gentle slope in HREE, and a significant Eu anomaly in some samples. Additionally, trace elements generally exhibited a slightly right-leaning pattern, with distinct depletions observed in Nb, Ta, Sr, and Ti. These suggest a complex provenance and origin of the Cambrian shales. The maximum deposition ages of YB-10, NHX-22, and O1H-4 samples were 541Ma, 580Ma, and 523Ma, respectively. These samples were analyzed in cumulative scale maps (crystallization and deposition ages), and it was found that the samples all fell into the collision environment. The detrital zircon U-Pb ages signify a novel magmatic activity in a continental arc setting from the Ediacaran to the early Cambrian, acting as a major sedimentary provenance. It suggests that a weakening retreating subduction-related weak extensional basin rather than a passive marginal or foreland basin of the early Cambrian Sichuan Basin, which had led to diminished extensional and broad trough-platform shales. The compiled geochemical data indicate that OME during the early Cambrian was strongly linked to high primary productivity, redox conditions, and detrital input. However, the influence of these factors on organic matter enrichment varied spatiotemporally, likely due to the unstable and variable tectonic-sedimentary environment associated with the retreating subduction event. This case study illuminates the effects of retreating subduction on the differential organic matter enrichment in the platform shales.

Keywords: Cambrian; shale; retreating subduction; organic matter enrichment; tectonic effect; Sichuan Basin