Sedimentological Features and the Genesis of Lower Permian Nodular and Thin-Bedded Siliceous Rocks in Southern Anhui

Based on rock types and fossil assemblages, the Qixia and Gufeng Formations may be distinguished for the Lower Permian in southern Anhui. The Qixia Formation is composed dominantly of a suite of nodular and striped siliceous limestones which contain abundant neritic benthonic organisms such as algae, fusulinids, benthonic foraminifera, corals, brachiopods, echinoderms and bryozoa. The Gufeng Formation is dominated by a suite of sedimentary sequences consisting of thin-bedded siliceous rock/muddy shale couplets which comprise thinshelled brachiopods, ammonites, planktonic foraminifera, sponge spicules and radiolaria.
The emphases in this paper are as follows.
(1)Sedimentary environments of the siliceous rocks
The carbonate rock types with siliceous nodules in the study area include micritic limestones, muddy limestones and wackestones, indicating moderate to low-energy sedimentary environments. It can be seen from Figs. 3 and 4 that sedimentary environments of the nodular or striped siliceous rocks are assembled by open-sea carbonate platform and semi-restricted platform, whereas the thin-bedded siliceous rocks were formed in intraplatformal basin environments. The shallow-sea carbonate sediments were of widespread occurrence in southern Anhui and its adjacent area during the Qixiaian. Till the Gufengian, the southern part of the Susong-Guichi-Tongling-Jingxian-Jingdezhen zone remained to be the shallow-sea carbonate sediments, whilst in the northern part of it, the deeper-water intraplatformal basin sediments predominated as a result of the crustal differential movement, leading to the formation of the intercalations of the thin-bedded siliceous rocks and muddy shales.
(2)The genesis of the siliceous rocks
The thin-bedded siliceous rocks in the study area are thought to be organogenic deposits which were formed through organic chemical sedimentation and early diagenetic differentiation, whereas the nodular siliceous rocks were resulted from early diagenetic silicification along the bedding plane of the carbonate rocks.
(3)The sources of SiO₂
SiO₂ in the siliceous rocks was principally derived from terrigenous mud and siliceous organisms. By and large, SiO₂ in the nodular siliceous rocks was originated from terrigenous mud, i.e. the chemical. weathering products of the continental Al-silicates, whereas SiO₂ in the thin-bedded siliceous rocks was mainly stemmed from siliceous organisms which are made up of 30—60% of siliceous sponges and radiolaria.
(4)The diagenesis of the siliceous rocks
A series of diagenetic processes once took place after the deposition of the abundant siliceous sediments. Three diagenetic stages include: 1)the solution of silica gel or crystalline opal(Opal-A); 2)the reprecipitation(including silicification)and redistribution(i.e. differential deposition)of the disordered cristobalite(Opal-CT), and 3)the formation of micro-crystalline quartz and chalcedony from Opal-CT by solid-solid reaction.