Sedimentary environments of the Middle Permian phosphorite–chert complex from the northeastern Yangtze platform, China; the Gufeng Formation: a continental shelf radiolarian chert

Abstract

Radiolarian chert deposited on a continental shelf occurs in the Middle Permian Gufeng Formation on the northeastern Yangtze platform, China. The sedimentary environments of radiolarian cherts from accretionary complexes have been well studied; however, there are few studies about radiolarian chert deposited on continental shelves. Therefore we have completed a sedimentological and geochemical study of the Gufeng Formation. The Gufeng Formation is subdivided into the Phosphate Nodule-bearing Mudstone Member (PNMM) and the Siliceous Rock Member (SRM) in ascending order. The basal PNMM consists of glauconite-bearing mudstone, which indicates deposition under aerobic shallow-marine conditions, whereas the upper PNMM is composed of mudstone including abundant phosphate nodules, suggesting deposition near the outer shelf in suboxic conditions. The SRM consists mainly of alternating beds of black chert, mudstone, and siliceous mudstone, with minor tuffaceous mudstone and porous chert beds. The black chert contains abundant radiolarians, sponge spicules, and organic matter. Framboidal pyrite occurs in the black chert. Porous chert in the SRM includes abundant rhombohedral cavities, which are dolomite moulds surrounded by quartz, suggesting dolomitization before silicification during early diagenesis. The Gufeng Chert is characterized by high Si, Mo, Ni, Cu, and Zn contents, and extremely low Mn content. The Gufeng Chert has high normal paraffin concentrations with petroleum-like markers. These geochemical features suggest that the Gufeng Chert was deposited under sulfate-reducing conditions; and is not hydrothermal, but rather biogenic in origin. The sedimentological and geochemical data suggest that the Gufeng Formation was deposited mainly on the outer shelf in suboxic–anoxic conditions caused by organic matter produced during upwelling. The upwelling probably led to high radiolarian productivity. Abundant silica and organic matter were deposited in the Gufeng basin. Compared with chert from other sites, the Gufeng Chert shows many similarities with chert from platformal basins, but not with chert from accretionary complexes. Especially, with respect to age, rock type, stratigraphy, and geochemical features, the chert of the Gufeng Formation is similar to that of the Phosphoria Formation in the western U.S.A., which is a world-class phosphorite giant. These similarities suggest that upwelling occurred around the eastern margins of Panthalassa and the Paleotethys concurrently during the Middle Permian. In general, shelf-type radiolarian chert, represented by the Gufeng Chert, usually shows some of the following features: dark color, lack of rhythmical bedding, association with phosphorite, platformal limestone, felsic tuff, abundant organic matter, and sulfides. Shelf-type radiolarian chert is deposited in a poorly aerated restricted basin, or in an oxygen-minimum zone.