Yongningzhen Fm

Synonym: Shuiningzhen Fm

Lithology and Thickness:

It is a carbonate rock assemblage, which can be divided into four units from bottom to top. First unit is composed of gray, dark gray thin to medium thick micritic limestone, sand, gravel, aggregated limestone, bioclastic limestone and oolitic limestone. Second unit is composed of purplish red, yellowish green sandy claystone, calcareous claystone and marl, micritic dolomite, salt laminated pseudo-crystalline dissolution breccia with unequal thickness interbedded, claystone sometimes has mud crack, wave-mark and acicular sub-gypsum. Third unit is dominated by light gray and gray medium to thick micritic limestone. Fourth unit is composed of light gray, thin gray to medium thick layers of argillaceous dolomite, dolomite, dissolved breccia dolomite and a small amount of argillaceous limestone, purple red and gray green claystone, dolomitic claystone, and often develops paste salt pseudo-crystals, karst caves and thin layers of gypsum. The total thickness is 300 ~ 875 m.

Lithology-pattern: Dolomitic limestone

Relationships and Distribution:

Lower contact:

The bottom is characterized by the conformable contact between thin to medium-thick gray micritic limestone and the underlying Feixianguan Fm purplish red shale of Induan. Locally, it overlies the Dongchaun Fm of Induan (Tong et al., 2019).

Upper contact:

The top boundary to the overlying Guanling Fm is marked by the disappearance of pint-salt pseudo-crystalline dolomite.

Regional extent:

western Guizhou

GeoJSON:

Fossils:

Ammonoids: Tirolites spinosus-Pteria cf. murchisoni zone; Tirolites sp., Dinarites sp. Bivalves: Entolium discites microtis. Conodonts: (1) Parachirognathus delicatulus- Pachycladina obliqua zone; (2) Neospathodus homeri-N. triangularis zone; (3) Neogondolella cf. jubata- Neospathodus timorensis zone; (4) Neogondolella navicula-N. constricta zone.

Age:

Tong et al. (2019, Triassic integrated stratigraphy and timescale in China, Science China: Earth Sciences, v. 62, no. 1) assign as Olenekian.

Age span:

Beginning stage: Olenekian

Fraction up in beginning stage: 0.0

Beginning date (Ma):

Ending stage: Olenekian

Fraction up in ending stage: 1.0

Ending date (Ma):

Depositional setting:

Depositional-pattern:

Additional Information

The former Yongningzhen Formation is a stratum distributed in the western Guizhou between the Feixianguan Fm of the Lower Triassic and the Guanling Fm of the Middle Triassic, which is dated as the late Early Triassic. The stratigraphic group (1976) of the Stratigraphic Ancient Stone Biological Team of Guizhou Geological Bureau expanded the rock stratigraphic range of Yongningzhen Fm from the original two members to four members, and the "mundouyan" ("mung bean rock" or "Green Bean" rock) layer was taken as the overbearing marker, and the age was defined as the Early Triassic Olenekian. Shouren Yang and Qingchun Chu (1992) found the conodont Neogondolella navicula, N. constricta layer in the fourth member of the formation, and revised the age of the formation to the Early Triassic Olenekian-Middle Triassic Anisian. Wang Zunzhou et al. (1992) further expanded the lithostratigraphic range of Yongningzhen Fm and assigned a large set of argillaceous dolomite and dolomite interbeds above the "mundouyan" ("mung bean rock" or "Green Bean" rock) layer to this formation. This book adopts the meaning of Yongningzhen Fm defined by Stratigraphic Group (1976) of Stratigraphic Paleontology Team of Guizhou Geological Bureau.