A potential earliest trilobite-brachiopod fossil assemblage from the Sichuan Basin, South China

Abstract

Parabadiella is considered to be the earliest representative of trilobites from southern China, and was first recovered from the middle and upper part of the Guojiaba Formation in the Liangshan area of Hanzhong, southern Shaanxi Province, where the oldest trilobites in China and various fossils associated with them were also produced. Previous studies have suggested that the earliest brachiopods coexisted with the oldest trilobites, i.e., the Parabadiella-Eoobolus assemblage in the XYB section of the Zhenba area in southern Shaanxi Province is considered the earliest trilobite-brachiopod assemblage in southern China. The occurrence of both brachiopods and trilobites in the JS103 well extends the first occurrence of this assemblage in the Cambrian stratigraphy to the central part of the Yangtze Platform. The JS103 is a shale gas exploration well targeting the Lower Cambrian Jiulaodong Formation in the southern part of the Sichuan Basin, which is about 350 m thick in the well, and is in a paralleling unconformable contact with the underlying Terreneuvian Maidiping Formation and a conformably contact with the overlying Yuxiansi Formation. This well confirms that the Jiulaodong Formation has been continuously deposited and is divided into 2 sections and 11 layers, which belong to 3 sedimentary gyres, among which the lower 1-9 layers (3310-3592 m) with a total length of 282 m are fully coring layers, and the uppermost fossil layer is a siliceous clastic rock with a thickness of nearly 10 m. The fossil layer is a siliceous clastic rock with a thickness of nearly 10 m, and it has been found that two different trilobites have been discovered here. Two different trilobite range zones are found here, including the lower Parabadiella zone and the upper Wutingaspis-Eoredlichia zone. In addition, conventional studies consider the lowermost stratigraphic part of the Guojiaba Formation to be a unfossiliferous zone, with a thickness ranging from 200 to 250 metres. It is worth pointing out that fossilised shells of Linguliform brachiopod were found nearly 200 m below the zone of the earliest trilobite, Parabadiella, in the Well JS 103. The earliest brachiopod fossils were previously thought to be co-occurred with or slightly later than the earliest trilobites, e.g., Liangshan Mountain, Hanzhong, and Xiaoyangba, Zhenba, Shaanxi, etc. The present discovery of brachiopods may be the lowest known brachiopod fossil horizon in South China. Thus, the discovery of the brachiopod-trilobite assemblage provides new insights into the biostratigraphy of Sichuan and the surrounding area, and the biostratigraphy of the siliciclastic clastics along the western margin of the Yangtze Platform also contrasts with great potential.