The upper Permian reefs in West Hubei, China

Abstract

Upper Permian reefs are widely distributed in the Lichuan district, West Hubei, The reefs occur within the Changxing Formation (zone withPalaeofusulina). Main frame-building organisms are sphinctozoan calcisponges, which together with various binding organisms (lamellar blue-green algae andTubiphytes) from the organic framework of the reefs. In addition early diagenetic fibrous carbonate cements are important constructional elements. Four stages can be recognized in the growth of the Jiantianba reef: The sediment of the first stage represents deep-water basinal facies (Wujiaping Formation, thinbedded cherty limestones, siliceous rocks and shales), follwed upwards successively by slope, fore-reef, and reef core facies, which in turn were overlain by a back-reef facies. This sequence indicates a shallowing-up sedimentation pattern and a gradual progradation of the carbonate platform basinwards. Slope facies and fore-reef facies are characterized by thick-bedded and massive sponge-skeletal limestones with intercalations of limestone breccias composed of reef-derived material. The reef core facies consists of non-bedded framestones with abundant sphinctozoan and inozoan calcisponges, hydrozoans, and encrusting and binding algae.Tubiphytes is found within quiet-water environments as well as in high-energy environments. The lagoonal deposits consist of dolomitized limestones with echinoderms, limestones with abundant gymnocodiacean algae, and coarse-grained calcidolomites. Mixing of saline water with freshwater due to subaerial exposure of the reef resulted in dolomitization and dissolution. The reef dolomites are effective productive reservoir rocks. The composition of the frame-building fauna and also the depositional fabric of the reef framework can be compared with the Permian Capitan reef in North America and with Triassic reefs in the Alps.