SEQUENCE STRATIGRAPHY, SEA-LEVEL CHANGES AND DEPOSITIONAL SYSTEMS IN THE CAMBRO-ORDOVICIAN OF THE NORTH CHINA CARBONATE PLATFORM

The Cambro-Ordovician strata in North China were deposited over a very extensive craton, extending some 1500 km east-west and 1000 km north- south. The dominantly shallow-water carbonate succession reaches up to 2000 m in thickness and two megasequences (transgressive-regressive c ycles) can be distinguished: Lower Cambrian through Lower Ordovician s trata, and Middle through Upper Ordovician strata, separated by a majo r palaeokarst. The first megasequence consists of nine sequences which are generally 50-150 m in thickness. The Lower Cambrian sediments con sist of phosphorites and phosphatic sandstones, deposited during the f looding of the craton. Carbonates, mudrocks and evaporites were deposi ted in the Early Cambrian under an arid climate, laying the foundation for the subsequent long period of shallow-water carbonate deposition which lasted some 70 m.y. The Middle and lower Upper Cambrian sequence s consist predominantly of mudrocks and storm deposits ('tempestites') in the lower part and oolitic grainstones and tidal-flat lime mudston es in the upper part; these represent outer-mid and mid-inner-ramp dep ositional systems, respectively, of the transgressive and highstand sy stems tracts. These sequences have strong similarities with the 'Grand Cycles' in the Cambro-Ordovician successions of North America. In the Upper Cambrian, there is a distinctive unit of glauconitic purple-red mudstone several metres thick which is interpreted as the deposits of the maximum flooding of the first megasequence. In the Upper Cambrian , there was a phase of tilting of the North China Carbonate Platform t o the north, and storm deposits, especially intraclastic conglomerates and hummocky cross-stratified grainstones-packstones, were very commo n at this time. Also common in upper Middle and lower Upper Cambrian s trata are stromatolitic-thrombolitic bioherms, several metres in diame ter. The upper Upper Cambrian through Lower Ordovician strata are domi nantly fine-grained limestones and dolomites deposited in shallow-subt idal and inter-to supra-tidal environments on a low-energy epeiric-sea platform. This part of the succession is the regressive part of the f irst megasequence, so that overall the platform shows an evolution fro m platform initiation to platform foundation, to a ramp-depositional s ystem and then an extensive epeiric platform. On a small scale, the su ccession is composed of metre-scale shallowing-upward cycles (parasequ ences) arranged into cycle sets of 10-30 m thick. The Middle-Upper Ord ovician megasequence consists mostly of shallow-water carbonates with several thick evaporite units. Minor palaeokarsts and palaeosoils sepa rate the sequences, which are composed of metre-scale cycles, Regional uplift affected the North China Platform in the Ordovician and sedime ntation did not resume until the Carboniferous, There is a good correl ation of the two transgressive-regressive megasequences described here with the global 2nd-order relative sea-level curve for the Cambro-Ord ovician.