Xh. Meng et al., SEQUENCE STRATIGRAPHY, SEA-LEVEL CHANGES AND DEPOSITIONAL SYSTEMS IN THE CAMBRO-ORDOVICIAN OF THE NORTH CHINA CARBONATE PLATFORM, Sedimentary geology, 114(1-4), 1997, pp. 189-222
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.