Ordovician Lithofacies Paleogeography of North Bohai Bay Basin

Key words: 
• Dagang region /
• Ordovician /
• karst reservoir /
• distribution law /
• control factors

Abstract: North Bohai Bay Basin includes the Huanghua Depression, Cangxian Uplift, Central Hebei Depression and Xialiaohe Depression. In the study area, the Ordovician is 600～800 m thick and mainly consists of limestones and dolostones. The Lower Ordovician is divided into the Yeli Formation and Liangjiashan Formation, and the Middle Ordovician is divided into the Lower Majiagou Formation which is further divided into the upper member and lower member, Upper Majiagou Formation which is further divided into the upper member and lower member, and Fengfeng Formation which is further divided into the upper member and lower member. The Upper Ordovician is missing. The Ordovician is conformable with the underlying Cambrian, and disconformable with the overlying Middle Carboniferous. The contact between the Lower and Middle Ordovician is also disconformable. In the Yeli Age, the study area was mainly a shallow restricted platform, in which there were 2 tidal flats and 5 banks. The deposits are mainly medium to thick bedded limemud limestones intercalated with some penecontemporaneous dolostones and pebble-sized intraclast limestones, with rare fossils.In the Liangjiashan Age, the lithofacies paleogeography was similar to the Yeli Age, and was still mainly a restricted platform, but tidal flats increased to 7. At the end of the Liangjiashan Age, sea level fell quickly, and the whole study area was exposed and the regional disconformity between the Lower and Middle Ordovician was formed. The lithofacies paleogeography throughout the Middle Ordovician showed strong cyclicity. In the early Lower Majiagou Age, transgression occurred, and the whole study area was occupied by tidal flats which included dolomite flats, gypsiferous dolomite flats, limy dolomite flats, limemud flats and dolomitic limemud flats. The deposits are mainly well laminated muddy penecontemporaneous dolostones and limemud limestones. Fossils are rare. In the late Lower Majiagou Age, sea level rose further and the whole study area changed to a shallow restricted platform, in which there were 3 small dolomitic limemud tidal flats. In the early Upper Majiagou Age, sea level fell, and tidal flats again dominated the area, which included limemud flats, dolomitic limemud flats, limy dolomite flats and dolomite flats. In the late Upper Majiagou Age, sea level rose quickly and the whole study area changed to a relatively deep open platform. The deposits are mainly thick bedded fossiliferous limestones. The fossils include cephalopods, brachiopods, echinoderms and gastropods. In the early Fengfeng Age, sea level fell again, and the area changed to tidal flats. In the late Fengfeng Age, transgression occurred, and the whole study area changed to a relatively deep open platform again. During the Ordovician, the study area underwent 4 marine transgression-regression cycles, and in the late Upper Majiagou Age marine transtression reached maximum.