Caodi Gr

It was named by Yongxian Xiong in 1941, referring to the strata that distributed around the Songpan County, Sichuan Province, mainly restricted within the west of Songpan, north of Longriba and east of Aba. It was termed as “Caodi Series”, referring to the yellowish green thin-thick metamorphic sandstone and slate dominated strata that distributed in west of Songpan, Sichuan Province, North of Longshiba, East of Aba, and was suggestive of Carboniferous-Permian. It was publicly cited by < Regional Stratigraphic Tables of China> in 1936. But it was renamed as “Caodi Group” by Regional Stratigraphic Table Compiling Group of Sichuan Province in 1978, and suggestive of Middle-Late Triassic. The lower Triassic was assigned to Aba formation. In 1991, the Bureau of Geology and Mineral Resources of Sichuan Province restated the reason for the utilization of Caodi Group, and the group was re-divided into four formations, representing Lower Triassic to Norian of Upper Triassic. This definition was followed herein.

Synonym:

Sediments are dominated by flysch-type clastics, which have been subdivided into four formations. In 1991, the Bureau of Geology and Mineral Resources of Sichuan Province re-divided the Group into four formations, representing Lower Triassic to Norian of Upper Triassic. The lowermost formation is consisted of greyish green massive metamorphic feldspar quartz sandstone and pebbled feldspar lithic sandstone, interbedded with clayey slate, limestone and marlstone. Thickness of this part is more than 1000 m. The second formation of overlying strata is comprised of greenish or greyish metamorphic sandstone, tuffaceous sandstone with interbedded calcareous slate, sericite slate and limestone. Thickness is about 2498-5000 m. The third formation is dominated by greyish green medium-thick tuffaceous sandstone, metamorphic lithic sandstone with conglomerate in the bottom and are more than 700 m. The top formation is consisted of yellowish green and grey tuffaceous sandstone and slate. The slate horizons increased upwards. Top of the fourth formation was not exposed, but in the peripheries of the fault zone, abundant Paleozoic-Middle Triassic fossils with the Colluvial limestones have been found. Thickness was around 2000 m.

First formation: Ammonoids: Flemingites muthensis, Vishnuites decipiers, Anotoceras costatum, IsculItoides globosus; Conodonts: Neospathodus timorensis. Second formation: Bivalves: Costatoria radiata, Mytilus eduliformis, Halobia cf. comatoides, Daonella cf. boeckhi; Brachiopods: Spiriferina tsinghaiensis, Neoretzia fuchsis. Fourth formation: Bivalve: Halobia pluriradiata, H. yunnanensis, H. convewa, H. alaskana. Plants: Neocalamites caomodes, Pityophyllum cf. longifoliun.

Sediments showed turbidite and colluvial features