Research On Reef Characteristics And Palaeogeography Of The Middle And Late Ordovician In Fuping-Yaoxian Region

Situated in the central and east part of the southern margin of Ordos Basin,Yaoxian-Fuping region developed comparatively complete in its middle and upper Ordovician layers and possessed various sedimentary types,in particular,the reefs there have become the focus during the southern Ordovician gas exploration in recent years.Therefore,taking the reefs and palaeogeography characteristics as the research objects,this paper,through observations and measurements as well as sampling tests towards the field sections of the middle and upper Ordovician in Yaoxian-Fuping region and in its neighboring regions,had studied such aspects there as the layers,rocks,sedimentary facies,palaeogeography,reefs,controlling factors,etc.By multiplied layers classification and comparison within the research area in rock,age,organism,geochemistry and unconformity,a plan in this regard had been designed for the researched area and even for the middle and upper Ordovician layers in the whole south.According to observations and measurements within the area for the outcrop sections in the middle and upper Ordovician,the rock types were identified as follows:carbonate rocks with normal sedimentation accounted primarily,and then carbonate rocks by events,as well as other clastic rocks,siliceous rocks,pyroclastic rocks,etc.Also,researches towards the primary rock types had been made from the perspectives of petrology,sedimentary environment and spatial distribution characteristics.Based on the sedimentary characteristics,the middle and upper Ordovician of the area was classified into three types of sedimentary facies and seven types of subfacies,whose characteristics and spatial distribution had been analyzed one by one for drawing its paleogeography planar graphs and for establishing the sedimentary models.Drawn from these,distribution of the main sections with reef-outcrop had been studied,and in turn the development age of the reef sections within the area in Taoqupo Reservoir,Jiangjunshan Mountain and Fenghuangshan Mountain,as well as the reef characteristics had been analyzed,further with analysis towards the reef controlling factors and reef-forming mode.Research suggested that the west part of the area in Pingliang phase of the late Ordovician developed reef facies sedimentation while the development trend in the east part was the “Fuping Rift” with north-east trend,and the whole south,speaking from the south-north trend and from the east-west trend,was featured by rimmed platforms and by platform-and-basins respectively;the Beiguoshan phase inherited the sedimentary pattern from the Pingliang phase with the development of “Yaoxian Rift”under the background of marine transgression;the reef-layers within the area developed early-middle Katian,with its main reef-forming organisms being bacillus-algae,calcareous algae,corals and stromatoporoids,its main rocks being baffle stones,bound-stones,framestones and bioclast limestones,and its controlling factors being six;however,the dominate factors were the amid-sedimentary fractures,palaeogeography pattern distribution and sea level fluctuation.