Tempestites and the Environmental Paleoecological Significance of Early Silurian, Nvv Sichuan Area

Several Early Silurian storm-wave genesis of bioclastic limestone beds which deposited on the muddy shelf have been recognized from NW Sichuan area. The Early Silurian deposited at the NW margin of the S-China Paleo-plate tectonically and it has the very thick sequence of shale,silty shale,muddy shale and silty sandstone with some reefs, bioherms and stratigraphically discontinuous lenticular and irregular bioclastic limestone beds. The fossils (trilobites, pelmatozoas, brachiopods and some small solitary rugose corals) which are dispersed in surrounding siliclclastic deposites,are the source of the bioclasts for tempestites. The bioclasts result from storm wave-caused winnowing effects and they are lag concentrates. It is one of the very important characteristics of the shallow muddy shelves. Detailed petrographic and stratigraphic analysis reveals 5 distinct tempestite (bioclastic limestone beds) patterns which located at 3 stratigraphic levels, according to 5 different stratigraphic sections. Pattern A:Thick lenticular or irregular bioclastic limestone beds (1-3m thick) with erosion at the bottom. Generally, the beds show three gradding subdivisions from bottom to top (at the Middle-Upper Ningqinag Formation). Pattern B:The small lenticular bisclastic limestone beds with l-20cm thick and 10-50cm long. The petrography of this pattern is similar to the third subdivision of the Pattern A (at the Upper Ningqiang For-mation). Pattern C:The lenticular or irregular bioclastic limestone beds with erosion at the mastem. The lower parts of the beds are mud-teariag bioclaslic limestone and gradually cosfer to the bioclastic grains upwardly. The eds are 5-15cm thick and locate at the gjianwan Formation. Patterm D:The small enticular limestone beds with 2-6cm thick and over 10cm lung, which can be divided into 4 petrographic subdivisions vertically. The first containing thin-shell brachiopod and ostracoda shells.is lag concentrates .the second is rizental lamunar zone,the third is the hummocky cross laminar stratification zone and the woverh is parullet laminar zone(at the Cuijiagou Formation). Pattern E: It is very similar to the Pattern D on the size, the distribution and from the second to the fourth subdivisions of Pattern D petrographically. Because the storm wave is different on energy .frequency and the span of the storm pro cess.several different types of tempestite deposited in this area. The petrographic and paleoe-cologic analysis reveals that the Pattern A and B are proximal and Pattern C.D.E .are distal. Form Pattern A to E,the petrographic characteristics show that the storm energy became weaker and the depth of water increase gradully. The storm wave accumulation of bioclasts can directly influence the structure and dynamics of nature of the sea floor and can change the physical characteristics of benthis habitats. Along with the bioclast accumulations in abundance,the soft muddy sea floor would be transformed into a coarser,firmer and topographically more complex benthic habitat. Many coarser substratum-like and firmer substratum-like epi-organisms,such as encrusting organisns,can settle and develop on it and form the biological hardgrounds. Many reefs and bioherms developed on the biological hardground through the process of taphonomic feedback. Usually, reefs developed on Pattern A tem-pestite.a firm bioherms can be seen on Pattern B,biostrome on Pattern C and Pattern D,E are covered by shale,because the depth of water increased gradually.