Sequence Stratigraphy of the Ordovician System in the Ordos Basin

Key words: 
• Ordos Basin /
• sequence stratigraphy /
• isochronous stratigraphy framework /
• sequence boundary /
• carbonate rocks /
• Ordovician

Abstract: Through comprehensive analysis of  various outcrops, drilling and seismic sequence stratigraphy, Ordovician sequence stratigraphic framework in Ordos Basin has been established. In view of poor quality of Ordovician seismic data in the basin, a set of suitable sequence stratigraphic analysis method is very necessary. So,we meticulously researched Ordovician sequence stratigraphy of Ordos Basin in the eleventh five-year plan, and put forword "the five elements analysis method of carbonate strata sequence stratigraphic classification and correlation". Using the analysis method, 3 secondary sequence interface and 6 tertiary sequence interface have been identified in the Ordovician, and the Ordovician can be divided into 2 secondary sequences and 8 tertiary sequences. OMsq1 including Osq1 (equivalent to Yeli Group), Osq2 (equivalent to Liangjiashan Group), Osq3 (LST: equivalent to Majiagou 1 Formation；TST: equivalent to Majiagou 2 Formation；HST: equivalent to Majiagou 3 Formation), Osq4 (TST: equivalent to Majiagou 4 Formation；HST: equivalent to Majiagou 5 Formation) and Osq5 (equivalent to Majiagou 6 Formation /Kelimoli Group)；OMsq2 including Osq6 (equivalent to lower Pingliang Group/Wulalike Group + Lower Lashizhong Group), Osq7 (equivalent to Upper Pingliang Group/Upper Lashizhong Group + Gongwusu Group) and Osq8 (equivalent to Baiguoshan Group). Different basin tectonic environment formed different sequence stratigraphic framework. Ordovician of the northern portion of western narrow continental margin in the basin can  be divided into Osq3-Osq7 of sequence stratigraphy  of  which geologic time is from the Floian of Early Ordovician to the Sandbian of Late Ordovician. Ordovician of the southern portion of western narrow continental margin in the basin can  be divided into Osq1-Osq7 of sequence stratigraphy of which geologic time is from the Tremadocian of Early Ordovician to the Sandbian of Late Ordovician, The sequence stratigraphy's thickness is 0~1 600 m which exhibit a bolt in shape extend in a north-south ward direction on the norrow edge which is sloped gently toward west and overlapping and pinching out to Yimen-Qingyang Ancient Uplift . In the southern wide continental margin of the basin, we have discoved eight sequence stratigraphy in Ordovician  of  which geologic time is from the Floian of Early Ordovician to the Early Katian of Late Ordovician. The sequence stratigraphy's thickness is 0~1 800 m which exhibit a bolt in shape extend in a west-east ward direction on the wide edge which is sloped gently toward south and overlapping and pinching out to Qingyang Ancient Uplift. In the eastern platform inner sag of the middle-eastern Ordos Basin, we only divided Ordovician into three sequence stratigraphies (Osq 3-Osq5 TST) which thickness is 800m in the east of the depression and overlapping and pinching out to Qingyang Ancient Uplift. Northem Yimeng Ancient Uplift and Qingyang Ancient Uplift have been uplifted in eroding state, which developed the only Osq4 TST ,and show that the Qilian Sea in western Ordos Basin and the North China Sea in Eastern Ordos Basin have been fleetingly connected in the late of Middle Ordovician.