The basic characteristics,spatial-temporal distribution,and building model of reefs in the western margin of the Ordos Basin of Ordovician

Organic reefs developed in the western margin of the Ordos Basin of Ordovician. By investigating the western margin in the field and identifying well cores of the organic reef in lab,this article concludes that a representative organic reef existed in the Zhuozishan area in the Wuhai( north reef),the Qitan1 Well( middle reef),and the Shijiezigou in the Pengyang County( south reef),and so on. The main reef-building organisms are coral,stromatoporoids,and sponges. The various organic reefs show different characteristics. The northern Wuhai reef has shown 2 ～ 3 growth phases while the middle Qitan reef shows 2 growth phases,and the southern Shijiezigou reef has had at least 4 growth phases. The presence of reef-building organisms suggests that the north and south reefs grew in a relatively shallow marine environment,but the middle reef formed in a relatively deep aquatic environment. The growth,development,and disappearance of these typical organic reefs presented different development features and temporal-spatial distributions. The northern section of the organic reef was developed earlier while the middle and southern developed later. The temporal-spatial distribution of the reefs is related to the law of rotating drift,where the northward drift and clockwise rotation caused by motion of the north china plate around the Equator in the Early Paleozoic. Based on the analysis of the temporal spatial development characteristics and results of carbon and oxygen isotope geochemistry,we establish the reef building model as an open platform margin point reef-beach mode under the marginal sea background near the western side of the central uplift. The death of the organic reef in this area can be attributed to the event of rapid sea-level rise resulting in the drowning of the reef. Furthermore,it can be inferred that an event of rapid sea-level rise occurred above organic reef.