Evolution Of Late Permian Reef Communities And Its Response To Episodic Volcanic Activities-Evidence From Ziyun In Guizhou Province

Most of the researches on the end-Permian global event are concentrated near the Permo-Triassic boundary (PTB), but studies appear to be relatively lack concerning the evolution of marine biota and the environment throughout the whole Late Permian Changhsingian. Marginal platform reef succession of Changhsingian, however, was well developed in Ziyun of Guizhou Province. This created the conditions for researching the relationship between Late Permian geological events and the reef evolution. Because modern reefs are peculiar ecosystems extremely sensitive to any environment change, reef succession in the earth history becomes the unique section documenting the detailed paleoecological evolution of ancient shallow marine environment.Late Permian Changhsingian reefs are paleogeographically located in Yangtze Platform and Nanpanjiang basin. Ziyun Shitouzhai reef, which was located along the southern margin of Yangzi Platform, becomes one of the most typical reefs. The Changhsingian reef of Shitouzhai section in Ziyun was developed on the base of Wuchiapingian clastic sediments, forming140m thick of reef stratigraphic sequence. The alternative occurrence of reef limestone with algae-foraminifer grainstone in the reef stratigraphic sequence indicates several replacements in reef sedimentary environment. The PTB is a topographically fluctuating unconformity, representing typical karst topography at the end of Permian. The Early Triassic taupe mudstone with several layers of volcanic clay at the bottom overlies the unconformity surface and the lithology shifts gradually to thin-bedded argillaceous limestone in the increasing sequence.Magnetic susceptibility value can reflects the quantity of paramagnetic material. Magnetic minerals can be brought into seawater mainly by terrestrial material and volcanism. With the decline of sea level or increased volcanic activities, the magnetic susceptibility value of rock will increase significantly because of the more magnetic minerals brought from terrestrial material and volcanic clastics. Whereas, when the environment becomes stable, reef-building organisms will form pure reef limestone due to the scarcity of terrestrial material and volcanic clastics. As a result, the magnetic susceptibility value of the rock will reduce accordingly. So, magnetic susceptibility data of different kinds of rock in reef stratigraphic sequence can well reflect the evolution history of reef ecosystem, volcanic activity and sea level. The magnetic susceptibility data of Changhsingian reef stratigraphic sequence in Shitouzhai section show:(1) The magnetic susceptibility values of bioclastic limestone with chert in early Changhsingian are higher, with the average values near (2.57～11.10)×10-9m3/kg.(2) The magnetic susceptibility values of reef limestone are generally much lower, with most of the values around Om3/kg or even less.(3) The magnetic susceptibility values of sponge framestone and bafflestone in reef core facies are only about-1.86×10-9m3/kg, but those of the bioclastic limestone (algae-foraminifer grainstone) interbedded in the reef limestone are much higher, about1.99×10-9m3/kg.(4) The magnetic susceptibility values of algae bindstone at the top of reef are higher than those of the sponge framestone and bafflestone, about5.70×10-9m3/kg.(5) The magnetic susceptibility values of mudstone near PTB increase dramatically up to25.61×10-9m3/kg. Based on the analysis of the variation of magnetic susceptibility values in Ziyun section, we infer that high magnetic susceptibility values in the beginning stage of reef are due to the influence of terrestrial material, whereas the lower magnetic susceptibility values of reef limestone reflect favorable marine conditions for marine organisms. However, the magnetic susceptibility values from several beds of the bioclastic limestone (algae-foraminifer grainstone) interbedded in reef limestone are higher, showing periodic influence of volcanic activities at that time. Temporary punctuations of volcanic eruption nearby would result in the disappearance of reef communities and the occupation of reef ecological space by algae-foraminifer fauna. However, the magnetic susceptibility values of bindstone at the top of reef obviously increase, indicating intensified volcanic activities near the end stage of reef development. The abnormally high values of magnetic susceptibility near PTB may be associated with the volcanic activities near PTB as well as the relative concentration of ferromagnetic minerals due to the reef-weathering after large scale regression.The marginal platform reef succession in Ziyun is composed of reef limestone interbedded with several layers of algae-foraminifer grainstone, which have long been regarded as back-reef lagoon deposits. However, observations by optical microscopy and scanning electron microscopy (under backscattered mode) show richness of quartz grains in these grainstone. These quartz grains are very small (mostly between30-300μm) and most of them can not be distinguished under optical microscope except a few large ones. Most of the quartz grains are well crystallized, with some bigger ones haveing dim nucleus. Because the backreef lagoon to the north of the reef belt could act as an effective sediment trap to terrestrious weathering sediments, airfalls of volcaniclastic are suggested for the origin of those quartz grains. Well-formed crystals of quartz grains indicate apparent authigenic growth during diagenesis, but their nucleus are suggested to have come from volcanic debris. Some similar authigenic quartz crystals have also been found in the isolated Permian reef islands situated in Bayan Kala region of Tibetan Plateau and Late Permian carbonate in Chongyang region of Hubei Province. Quartz grains are especially concentrated around the shell of foraminifera. This phenomenon may be by the higher porosity inside the fossil shells which may have driven pore water rich in diagenetic dissolution of silicon to be transported near the fossil foraminifera. Besides the richness of authigenic quartzs, a lot of volcanic glass and mica have been found in the Ziyun reef limestone, providing further evidence of volcanic origin for the quartz nucleus. Therefore, the alternative occurrence of reef limestone with algae-foraminifer limestone in the marginal platform succession of Ziyun is regarded as the result of periodic volcanic activities during Late Permian Changhsingian. Temporary punctuations of volcanic eruption nearby would have resulted in the disappearance of reef communities. By the contrary, calcareous algae, foraminifera and Tubiphytes survived the volcanic eruptions. Those organisms may have benefited from the nutrition elements dissolved from the volcanic ash, which eventually leaded to the replacement of sponge reef communities by the algae-foraminifera fauna. In conclusion, the algal-foraminiferal limestone beds preserved in the reef succession are considered as the sediments at the top of reef core facies after the temporary disappearance of reef-building organisms caused by volcanic activities, rather than the deposit of backreef lagoon.The several alternations of reef limestone and algal-foraminiferal grainstone in Shitouzhai section in Ziyun reflect the frequent volcanic activities in this region. The frequent alternation of reef limestone and algal-foraminiferal grainstone created unique reef deposition model under the influence of volcanism. However, abundant volcanic quartz particles are also found from the bindstone in the uppermost of Ziyun reef. Although the existence of volcanic quartz indicates volcanic influence at the top of the reef, only bindstone rich in carbonate cements was formed this time, not algal-foraminiferal limestone. These bindstones not only have a lot of volcanic quartz grains, but also the development of abundant fibrous carbonate cements. The reason for the lack of algal-foraminiferal grainstone is probably due to the multiple influences of volcanic activities as well as the increased temperature in the latest Permian. High temperature may induce higher saturation conditions with respect to calcium carbonate, leading to a large amount of fibrous carbonate cementation.The carbon and oxygen isotope composition of carbonate has carbon and oxygen isotope characteristic of seawater, and undergoes the influence of carbon and oxygen isotope equilibrium fractionation between seawater and carbonate. Both of the previous data of carbon and oxygen isotope and those in this study show dramatic negative excursion of carbon isotope with oxygen isotope rapidly increasing at the end of Late Permian in Shitouzhai section. This result suggests oxygen isotope increasing may be associated with high temperature event, while negative carbon isotope in the top of reef may be associated with fresh water intrusion due to the sea level decline. The discovery of reddish-brown weathering crust at the top of reef fully testifies reef exposed and erosion caused by sea level decline at the end of Permian. Therefore, multiple geological events including volcanism, sea level decline and high temperature event may have been involved for the complete collapse of the reef ecosystem flourishing in Late Permian in South China.