Kwangsian tectonic–paleogeographic reconfiguration and associated hydrographic reorganization during termination of the TOC >3 wt.% shale system in the lower Silurian Longmaxi Formation, South China

The lower Silurian Longmaxi Formation is one of the most important marine shale gas-bearing successions in the Upper Yangtze region of South China. Although organic-rich shale development in this formation has commonly been attributed to euxinic preservation and elevated productivity, the mechanism responsible for termination of the TOC >3 wt.% shale system remains poorly constrained. To address this issue, we analyzed TOC, major-element, and trace-element data from 85 core samples from the Banye (BY), Andi (AD), and Fudi (FD) wells in the Zheng’an area of northern Guizhou, using graptolite biostratigraphy to provide a chronostratigraphic framework. The Zheng’an area lies near the southeastern margin of the Upper Yangtze region, close to the paleogeographic transition between the Yangtze shelf and adjacent Cathaysia-related paleoland, making it sensitive to regional shoreline migration and paleogeographic reorganization. The studied successions record a clearly expressed two-stage TOC pattern, in which a lower TOC >3 wt.% interval is overlain by a TOC <2 wt.% interval, producing an apparently abrupt transition within the available sampling resolution. No samples with TOC values between 2 and 3 wt.% were identified, although the presence of a thin transitional interval cannot be entirely excluded. Stratigraphic comparison with the XK and CN wells further shows that termination of the TOC >3 wt.% interval is consistently constrained within the Rhuddanian Cystograptus vesiculosus Biozone across the Zheng’an–Xishui–Changning inner shelf, although the thickness and exact termination level of this interval vary among localities. Across the studied wells, redox-sensitive and preservation-related proxies, including Mo, U, V and TOC/TP, shift at or immediately adjacent to the sampled intervals that bracket TOC termination. In contrast, productivity-related proxies, including Porg, Cuxs, and Baxs, show less synchronous and more stratigraphically dispersed changes, with some productivity signals persisting into the overlying low-TOC interval. This relative stratigraphic relationship supports preservation deterioration as the most plausible proximate trigger for termination of the TOC >3 wt.% shale system, rather than an immediately preceding collapse in organic-matter supply. We therefore propose a three-level preservation–restriction–Kwangsian (PRK) framework consisting of preservation deterioration, restriction intensification, and Kwangsian-related tectonic–paleogeographic reconfiguration. Within this framework, Kwangsian-related tectonic–paleogeographic reconfiguration is interpreted to have shifted the basin from relatively less restricted and sufficiently replenished euxinic waters to relatively more restricted and poorly replenished conditions, thereby weakening sustained euxinia and contributing to termination of the TOC >3 wt.% shale system. This framework provides a stratigraphically constrained interpretation of the regional termination of organic-rich shale deposition and has implications for predicting high-quality intervals in low-pressure shale gas systems.