Origin of Lower Carboniferous cherts in southern Guizhou, South China

Abstract

Cherts are generally developed in the Lower Carboniferous Dawuba Formation in southern Guizhou. A further understanding of the origin of cherts is helpful to clarify Early Carboniferous oceanic and depositional conditions. However, the origin of the Lower Carboniferous cherts remains controversial. The Dawuba cherts are rich in sponge spicules (monaxons and triaxon hexactines), radiolarians, and foraminifera (Endothyracea) and can be lithologically divided into the Manchang spicule-bearing chert (MSC), Manchang foraminifera-bearing chert (MFC), Getuhe radiolarian-bearing chert (GRC), and Luogang foraminifera-bearing chert (LFC). The analytical results of redox-sensitive trace elements (Mo, U) covariation pattern and microfossils types indicate that the Dawuba cherts formed in a suboxic–euxinic deep-water environment with some contributions of intermittent calcareous allochthonous sediments from the platform and slope. According to microfossils, Fe/Ti values, Post-Archean Australian Shale normalized rare earth element + Y patterns, Eu anomalies versus Y/Ho cross plot, silicon isotopes (δ 30 Si), and oxygen isotopes (δ 18 O), the Dawuba cherts have biogenic, hydrothermal, and hydrogenetic origins, and the MFCs and LFCs are also characterized by replacement cherts. The silica of Dawuba cherts was mainly sourced from siliceous organisms, hydrothermal fluids, and seawater, followed by river water. The silica diagenesis underwent a sequence of opal-A → opal-CT → quartz via dissolution–precipitation reaction or in-situ replacement without intermediary dissolution steps. Siliceous ooze underwent diagenesis to form biogenic cherts. Silica from hydrothermal fluids, seawater, and river water chemically precipitated to form hydrothermal and hydrogenetic cherts. Silica-rich solutions silicified the calcareous allochthonous sediments to form replacement cherts. Flourishing of siliceous organisms, mixing of hydrothermal, seawater-dissolved, and riverine-dissolved silica, and intermittent transport of calcareous allochthonous sediments, played important roles in the deposition and development of cherts in the trough basin during the Early Carboniferous. • Lower Carboniferous chert formed in a suboxic–euxinic deep-water environment. • Cherts were biogenic and chemical origins, some of them had a replacement origin. • Silica sourced from siliceous organisms, hydrothermal fluids, seawater, river water. • Silica diagenesis via dissolution–precipitation reaction or in-situ replacement. • Chert model on organism, dissolved silica, allochthonous sediment was established.