Tongyusi Fm

The Tongyusi Fm is the highest formation of the Zhashui Gr. The type section is located at Tongyu River of Shanyang County, Shaanxi. It was named by Shaanxi Regional Geological Survey in 1980, and Du Dinghan et al. formally quoted it in 1986.

Metamorphic sandstone. Lower part is characterized by thin- to medium-bedded metamorphic fine-grained quartz sandstone and siltstone intercalated with sandy slate and thin-bedded limestone. Middle part is composed of silty slate and slate intercalated with siltstone, thin-bedded metamorphic fine-grained quartz sandstone and limestone. Upper part is dominated by metamorphic fine-grained quartz sandstone, feldspathic quartz sandstone and siltstone intercalated with silty slate and a few sandy limestone beds.

In the Tongyu River section of Shanyang, the total thickness is more than 2480 m; in the Eryu River section, the total thickness is more than 3090 m; and in Zhangjiaping section of Zhashui, the thickness is more than 7046 m.

The lower part of Tongyusi Fm yields brachiopods Chonetes and crinoid stems (Tongyu River), Cyrtospirifer (Zhangjiaping); and the upper part yields plant fossil Leptophloeum rhombicum (Qingyangou of Eryu River), and above this fossil bed occur abundant spore fossils Arachaeozonotriletes variabilis, Discernisporites micromanifestus, Samarisporites comcinnus, etc., and acritarchs, chitinozoans, scolecodonts, etc.

There are two interpretations for its sedimentary environment: one is a progression from shallow-marine to nearshore deposits in a regression sequence (Huishan to Shanyang Map, 1980; Du Dinghan et al, 1986; Shangxian Map, 1988; Yang Zhihua et al., 1991), and another one suggests gravity-flow deposits on a slope and considered to be a prograding sequence. (Liang Jinzhe et al. 1991).

The Tongyusi Fm is bounded by a fault from Hongyansi to Huishan, and has two zones of south and north that are different in lithology and facies. The maturity of the north zone is lower, commonly contains tuffaceous beds, and in Xiaowangjian of Zhouzhi to Houzhenzi area is composed of tuffaceous sandstone, feldspathic quartz sandstone, siltstone and slate, with a thickness of 2587 m. At its base is 12.5-m-thick conglomerate and sandy conglomerate that disconformably overlies on metamorphic rocks of Early Paleozoic. In Lujiayan of Taibai area, the lower part of Tongyusi Fm is mainly composed of feldspathic quartz sandstone and siltstone, yields plant fossil Sublepidodendron, its upper part is intercalated with tuffaceous sandstone and slate, and has a thickness of 1550 m.