Sequence Sequence stratigraphy, sea-level changes and depositional systems in the Cambro-Ordovician of the North China carbonate platform

Abstract

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 cycles) can be distinguished: Lower Cambrian through Lower Ordovician strata, and Middle through Upper Ordovician strata, separated by a major palaeokarst. The first megasequence consists of nine sequences which are generally 50-150 m in thickness. The Lower Cambrian sediments consist of phosphorites and phosphatic sandstones, deposited during the flooding of the craton. Carbonates, mudrocks and evaporites were deposited 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 sequences consist predominantly of mudrocks and storm deposits (‘tempestites’) in the lower part and oolitic grainstones and tidal-flat lime mudstones in the upper part; these represent outer-mid and mid-inner-ramp depositional systems, respectively, of the transgressive and highstand systems 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 to the north, and storm deposits, especially intraclastic conglomerates and hummocky cross-stratified grainstones-packstones, were very common at this time. Also common in upper Middle and lower Upper Cambrian strata are stromatolitic-thrombolitic bioherms, several metres in diameter. The upper Upper Cambrian through Lower Ordovician strata are dominantly fine-grained limestones and dolomites deposited in shallow-subtidal and inter- to supra-tidal environments on a low-energy epeiric-sea platform. This part of the succession is the regressive part of the first megasequence, so that overall the platform shows an evolution from platform initiation to platform foundation, to a ramp-depositional system and then an extensive epeiric platform. On a small scale, the succession is composed of metre-scale shallowing-upward cycles (parasequences) arranged into cycle sets of 10–30 m thick. The Middle-Upper Ordovician megasequence consists mostly of shallow-water carbonates with several thick evaporite units. Minor palaeokarsts and palaeosoils separate the sequences, which are composed of metre-scale cycles. Regional uplift affected the North China Platform in the Ordovician and sedimentation did not resume until the Carboniferous. There is a good correlation of the two transgressive-regressive megasequences described here with the global 2nd-order relative sea-level curve for the Cambro-Ordovician.