LOWER PALEOZOIC DEPOSYSTEMS AND STRATIGRAPHY OF THE WESTERN NORTH CHINA BLOCK

The Ordos basin is a large, petroleum-bearing sedimentary basin located in north China, set upon the North China Block (NCB). The stratigraphic record of Ordos spans from the Early Paleozoic through the Cenozoic, including periods that the NCB was an isolated microcontinent, accretion to the rest of Asia, and subsequent intraplate tectonics. Throughout the Early Paleozoic, the NCB was the site of a broad carbonate platform that resulted in a thick succession of carbonate and lesser clastic rocks. Recently, Mesozoic stratigraphy, sedimentology and tectonics of Ordos have received much attention due to the reservoir and source potential of this interval. However, comprehensive studies of Lower Paleozoic strata are either limited or poorly documented. We describe the Lower Cambrian through Middle Ordovician carbonate facies of the western margin of Ordos basin in terms of depositional environment and sequence stratigraphic framework within these units. Results are compared to previous work by Meng et al. (1997) which mainly focused on facies east of Ordos, in order to document any sequence or depositional environment variability within the platform.

Lower Paleozoic carbonate facies exposed in orogenic belts surrounding Ordos basin are the primary sources of measured stratigraphic sections. Preliminary results indicate that the Lower Cambrian is composed of three sequences of shales with interbedded lime mudstones which grade into thick oolitic lime grainstones, representing three shallowing-upward sequences. Middle Cambrian facies are predominantly packages of shale, thin-bedded lime mudstone with intraclastic conglomerate and oolitic grainstone; oolitic grainstone content increases upsection, indicating four shallowing-upward parasequence sets. Upper Cambrian strata are characterized by thin-bedded, stromatolitic lime mudstones and upper dolomites. Lower Ordovician facies consist of medium to fine grained sandstone, sandy limestones and dolomites interbedded with thick, fossiliferous lime packstones. On a broad scale, sequences and facies of western Ordos basin are similar to the transgressive Cambrian packages and highstand Lower Ordovician packages of Meng et al. (1997), suggesting that large-scale changes in sea level exhibited similar facies controls throughout the entire platform.