Weizhou Fm

Beibu Gulf region. The naming section is located in the Weiqian No. 1 well on Weizhou Island, Beihai City, Guangxi. Named the Weizhou Group by the Fourth Reconnaissance and Exploration Party of the Ministry of Geology in 1964.

Located above the Liushagang Fm and below the Xiayang Fm, this formation is a sequence of alternating beds of variegated mudstone and siltstone and grayish white sandstone, pebbly sandstone and sandy conglomerate, the middle part of which the lithology is relatively fine. The formation is divided into three members: Lower member is non-isopachous alternating grayish white, thick-bedded, pebbly, inequigranular sandstone and brownish red and variegated mudstone. Middle member is mainly gray or grayish green mudstone and light gray siltstone and lime, medium or fine sandstone, intercalated with thin-bedded gravel beds at the base and locally with basalt and diabase intercalations. Upper member is brownish red, purplish red and dark gray mudstone with grayish white sandstone, pebbly sandstone and sandy conglomerate and locally with thin coal beds. The lithology changes little, while the thickness changes greatly. The formation is thick in the center of the fault depression and becomes thin rapidly toward margins and the lithology also becomes coarse. The thickness is 353–800 m in the Beibu Gulf, 34–143 m on the Leizhou Peninsula and generally 500–800 m in Qiongbei and may reach a maximum of 1970 m there. In the seismic profile, this formation is the reflector group between reflectors T₂ and T₄.

This formation contains ostracods, gastropods and sporopollen. The ostracods include Chinocythere inflata, Disauritocypris trapezoidea etc; the gastropods are Georgia pericarinata, Andrussowiella antiqua, Stenothyra nonbasicostata, Sinomelania leei etc.; and the sporopollen is the Magnastriatites-Trilobapollis-Verrutricolporites-Retitricolpites assemblage.

This formation belongs to fluvial-lakeshore and swampy deposits, grading gradually from continental deposits in the lower member to paralic deposits in the upper member.