Lugou Fm

Lithology and Thickness:

It is a set of thick-very thick-bedded of coarse clastic deposits. Lowre part is dominated by the pale rose-colored sandstone, pebbly sandstone, and conglomerate interbedded with fine sandstone. Upper part is dominated by the pale gray-green sandstone and pebbly sandstone. Its thickness is 423 m in the type section. The thickness changes little, generally about 400-600 m thick.

Lithology-pattern: Coarse-grained sandstone

Relationships and Distribution:

Lower contact:

At the bottom, the pale rosy conglomerate is in disconformable contact with the underlying Upper Permian Sunan Fm. In other regions (central Gansu) the next older unit can be the Daquan Fm of uppermost Changhsingian. In south-central Gansu (TJ27), the regional stratigraphic columns indicate an absence of Permian (?), therefore the next older unit might be early Paleozoic.

Upper contact:

Its top with light gray-green medium-thick medium-grained feldspar and quartz sandstone is in fault contact with the overlying Dingjiayao Fm

Regional extent:

This formation is distributed in the open basin in the Wuwei-Jingtai area in eastern Gansu, and the small and narrow mountain and piedmont basins in the Sunan-Minhe and Jiuquan-Yumen areas in the west.

GeoJSON:

Fossils:

No fossils have been seen.

Age:

Schematic stratigraphic column assigns as spanning Early Triassic

Age span:

Beginning stage: Induan

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:

It is interpreted as fluvial-lacustrine deposition under a semi-arid-arid climate.

Depositional-pattern:

Additional Information

From 1936 to 1942, Sun Jianchu called the “green sandstone bed” about 1000 m thick above the red sandstone of the Yaogou Gr in the Hexi Corridor and Qilian Mountains as the “Xidagou System” (no specific section). In 1936, it was assigned to Permian-Triassic and then was assigned as the Triassic in 1942. In 1962, Si Xingjian and Zhou Zhiyan classified the lower part of this system in western Gansu into the Permian Yaogou Group. The upper part and the lower part of the overlying Lower Jurassic Longfengshan System were merged into the Xidagou Gr, and the age was determined to be the Triassic. In 1980, the Stratigraphic Table Compilation Group of Gansu Province published the Triassic of the Shezigou section in Sunan County, and separately called the Xidagou Gr for the lower part and the Nanyinger Gr for the upper part. In 1987, Li Peixian and others re-divided the section into 4 parts: the original Middle-Lower System of the Xidagou Gr and the lower part of the overlying original Nanyinger Gr with about 300 m thickness and the Angra flora were assigned to the Upper Permian as Xidagou Gr; The rosy-colored strata of the original Nanyinger Gr with a thickness of about 423 m above the disconformable contact boundary was assigned to the Lower Triassic, named Lugou Fm; the strata above with a thickness of nearly 300 m was assigned to the Middle Triassic Dingjiayao Fm; the remaining part with about 400 m thickness was still called the Nanyinger Gr. In 1989, "Regional Geology of Gansu Province" basically agreed to this division plan. However, only the Lower Series was used as the Xidagou Gr rather than separately Lower Series the Lugou Fm and the Middle Series Dingjiayao Fm. In 1993, Cai Kaidi regarded the Shezigou section as the representative section of the Triassic in the North Qilian-Hexi Corridor but still represented the Xidagou Gr as the middle-lower Triassic. In addition, Cai Kaidi called the Lower Series of Xidagou Gr as the Wufosi Fm and the Middle series of Xidagou Gr as the Dingjiayao Fm. And the Wufosi Fm was originated from the “Wufosi System” and the “Wufosi System” was named by Wang Dexu (1973) for a set of variegated strata with fossil plants. However, according to the lithology and fossil assemblage of this formation, it is the same as the Middle Triassic Dingjiayao Fm rather than the Lower Triassic. So, this book adopts the division scheme of Li Peixian and others.