Author(s): An Taixiang Wang Xinping An Zuoxiang
Pages: 158-169
Year: 1982 Issue:  2
Journal: Oil & Gas Geology
Keyword:  奥陶系; 华北地区; 地理变迁; 寒武系; 钙质白云岩; 泥质白云岩; 竹叶状灰岩; 白云质灰岩; 马家沟; 徐庄阶;
Abstract: There are many oil showings in Cambrian and Ordovician systems in No-rth China,which have drawn the attention of petroleum geologists for years.In this paper,lithologic identification is made up of 8 formations:theHoujiashan Stage,the Mantou-Xuzhuang Stage,the Changxia Stage,theUpper Cambrian,the Lower Ordovician,the Lower Majiagou Stage,theUpper Majiagou Stage and the Fengfeng Stage.Based on formation thicknessand paleontologic data,paleogeographic changes are reviewed.Marine trans-gression of limited extent took place along the depression zone in the innerflanks of the Inner Mongolia,the Jiaoliao and the Huaiyang oldlands duringthe Houjiashan age.Clastic rocks and mudstones were well developed at theoutset.Till the Changxia age carbonate rocks began to be widespread.Duringthe late Cambrian,dolomites occured progressively from the north and reach-ed their greatest thickness to the south,accompanied by a gradual loweringof horizons.In the Early Ordovician,the southern boundary of the sea basinshifted northward,but it expanded again during the early Middle Ordovician,more dolomites among carbonates were deposited.Clay and gypsum beds be-came widespread during the late Middle Ordovician when the sea area beingcontracted.All the isopach contours extend in an E-W direction.