Cambrian system are found at the southeast margin of Ordos basin

which distributed from Lishi in Shanxi province to Hancheng in Shaanxi province. From bottom to top

they are as follows:Lower Cambrian Mantou F. (∈

1m

);Middle Cambrian Maozhuang F. (∈

2m

);Xuzhuang F. (∈

2x

);Zhangxia F. (∈

2z

);Upper Cambrian Gushan F. (∈

3g

);Changshan F. (∈

3c

);Fengshan F. (∈

3f

)Various trilobites are found

totalling 23 families and 56 genera. Type section of the area is in Xiweikou of Hejin

Shanxi province.Discovery of Redlichia chinensis in 1964 in Hancheng proved not at all absence of Mantou sediments in this area. Take the Maozhuang F. as the beginning of the middle Cambrian

the strata are characterized by abundant Ptychopariidae. The Shabtungaspis aclis

among the 6 important genera

is the most widespread one.The Xuzhuang F. contains 4 fossil zones

such as Kochaspis hsuchuangensis

Sunaspis laevis

Porigraulos abrota and Honanaspis.The Zhangxia F. shows the climax of development of trilobites

which comprises 50% of the total amount of t

he genera found in Cambrian system in the area.The Gushan F. is considered as a marker bed for distinguishing the middle from the upper series

due to the stability in trilobite groups

lithology and thickness.Remarkably rare trilobites are found both in the Changshan and Fengshan Formations. Moreover

the top of the latter is characterized by Calvinella."Huoshan sandstones" to the south of lat. 36 N. correspond to the base of the Mantou F.

but to the north of it

most of them match the Maozhuang F. In Zhongyang region

they can be traced up to the base of the Xuzhuang F.

while they were completely missed along Sanchuan river.It is rational to place the boundary of the Cambrian and Ordovician in between earlier Ordovician dolomitic limestones with cherty bands (containing Aotiaspis cf. karaipsis) and Fengshan dolomitic limestones (containing Calvinella).