Shirenzhigou Fm

The type section is located at Shirenzigou about 12 km to the east of Urumqi City, Xinjiang; and it is the Dongzhigou section in the upper reaches of the Jingjingzi trench which was measured jointly by the Xinjiang Regional Geological Survey Party and the Institute of Geology under the Chinese Academy of Geological Sciences in 1977 that had been taken for the reference section of the formation. It was named by the Xinjiang Regional Geological Survey Party in 1965.

Synonym: Shirenzi Fm

Volcaniclastics and Clastics. Lower part is composed of a rhythmite consisting of siltstone, sandstone and breccia, as well as volcanic breccia, tuff and felsite. It contains locally massive breccia, with limestone pebbles being of 1 m in diameter and mingled with the fine-grained groundmass, with a thickness of about 30 m. In the Jingjingzigou area the basal part of the formation is composed of pebble-bearing limestone layer containing occasionally volcanic-rock and sandstone pebbles, with the limestone pebbles yielding the corals and brachiopods fossils from the Qijiagou Fm. Middle and Upper parts of the formation are composed of medium- and thin-bedded siltstone, fine-grained sandstone and thick-bedded sandstone, with the fine-grained sandstone being intercalated with siliceous bands and with the development of microlayers, with a thickness of about 120 m, with the such plant fossils (phytolites) as Paracalamites sp., Ullmannia sp., Walchia piniformis, etc. having been collected from the Jingjingzigou area. The thickness and lithology of the Shizirengou Fm are changeable greatly.

The thickness of the formation in the Jingjingzigou and Qijiagou areas is of 203 m and of 65 m respectively, with its basal part containing a fairly thick collapse conglomerate layer. In the Jijicao area 4 km to the western flank of the trench the formation is composed largely of fine-grained clastic rocks, with a thickness of less than a hundred meters.

In the Sangonghe area the formation is divisible into 4 members, with its Basal part being about 98-m thick, and representing a rhythm consisting of conglomerate and coarse-grained greywacke, and the component part of the sandy pebbles is of volcanic rocks in most cases. Lower part of the formation represents a rhythmic deposit composed of volcanic breccia, conglomeratic tuff and dust tuff, with a thickness of about 258 m. Middle part of the formation is composed of lenticular limestone, psammitic limestone, calcirudite and micrite, with a thickness of about 60 m. Upper part of the formation is similar to the base of the Tashikula Fm, which is composed mainly of siltstone, intercalated with a small amount of thin-bedded siliceous rock, black siltstone and micrite, with thicknesses of 40 m and 90 m, respectively. The total thickness of the Shirenzigou Fm is of 460 m. From the Sangong River eastward to the Baiyang River the thickness of the Shirenzigou Fm is increasing as great as over 700 m.

Plant fossils (phytolites)

It is interpreted as an early-stage deposits formed in a back-arc basin, which used to be recognized as continental-shelf, continental-slope and deep-sea-basin turbidites (Jin Huijuan et al., 1989), or as faulted-trough turbidites (Yu Lian, 1987).