Hekou-Huili Fm

Southernmost Sichuan. The "Hekou" Fm was named by the Limahe Party of Geology, Sichuan Bureau of Geology in 1957. The type locality is the Lixi River mouth, Huili County, Sichuan Province.

Synonym: （河口组）, Hekou Fm. NOTE: There is also a "Hekou Fm" of Cretaceous age in Jiangxi. Therefore, for this Mesoproterozoic "Hekou" Fm, the Huili Gr name was appended to avoid confusion. Some researchers named this formation as Hekou Gr, which is subdivided into Huangcaoling Fm, Luodang Fm and Changchong Fm (equivalent to the three members in this description).

In the typical section, the Hekou-Huili Fm is composed of sedimentary volcanic series. The formation possesses three depositional cycles, based on which it is subdivided into three ascending members. First member is over 1333 m thick: the lower part consists of dark-grey to grey carbonaceous slate, sericite phyllite with lenticular copper-bearing marble; the middle part is grayish metamorphosed medium-grained sandstone and muscovite-quartz schist, in which the meta-sandstone contains magnetite ore body; and the upper part is an interbedding of grey-white quartz albitolite (= rich in authigenic albite) with quartz schist; the base is garnet-biotite schist. Second member is 996 m thick: the lower part is greyish calcareous quartz schist intercalated with thin-bedded marble lenticules and meta-sandstone, containing siderite; the middle part consists of sericite quartz schist, carbonaceous slate with marble lenses and garnet hornblende-biotite schist; and the upper part is composed of rhythmic interbedding of grey-white massive quartz albitolite, garnet-biotite schist (copper-bearing horizon), with magnetite albitolite in lower part. Third member, ca. 2000 m thick: the lower part is grey-white muscovite quartz schist sandwiched with thin-bedded meta-sandstone, lenticular marble and carbonaceous slate; the middle part consists of dark-grey garnet biotite schist, garnet hornblende-biotite schist, with thin-layered iron ore sandwiched on the top; and the upper part is grey-green chloritized quartz-albitolite with muscovite quartz schist and thin-bedded quartz dolomitic marble. The total thickness of the Hekou-Huili Fm is recorded more than 4000 m.

It yields microplants, the main taxa are: Asperatopsophosphaera, Leiominuscula, Trachysphaeridium etc.