Zhadongcuo Fm
Type Locality and Naming
The type section is located in between the area extending from the Xietongqu to the Zhadongcuo region to the south of the Lagu Village of the Bomi County, Tibet. It was named by Yin Jixiang in 1984.
Lithology and Thickness
Clastics. Representing a suite of fine clastic rocks composed mainly of clayey, silty and sandy slate and pebbled sandy slate. There are four layers of pebbled sandy slates, with an accumulative total thickness of 673 m. The formation is intercalated with many layers of quartz-sandstones, with its lower part consisting of medium-grained feldspar-quartz sandstone; with its middle part consisting of medium- and fine-grained feldspar-quartz sandstone; and with its top part consisting of medium- and fine-grained quartzitic sandstone, with the content of clastic quartz reaching as high as being of 95%, indicating an obvious secondary enlargement of the quartz clastics, which had been proved by a slick and clean surface of grains, demonstrating a fairly complete reformation.
Relationships and Distribution
Lower contact
Uncertain relationship to the other strata (e.g., whether on either Yinga Fm or Xiongencu Fm of Permian or Lagu Fm of upper Carboniferous) – See Additional Information.
Upper contact
Uncertain relationship to the other strata (e.g., whether on either Yinga Fm or Xiongencu Fm of Permian) – See Additional Information.
Regional extent
GeoJSON
Fossils
The formation contains 4 layers of fossils, including brachiopods, bryozoan and a small amount of solitary corals, trilobites, bivalves and crinoid stems, etc. The brachiopods comprise Spiriferella, Cleiothyridina, Neospirifer, “Phricodothyris”, Buxtonia, etc., with only the genus Terrakea being found to occur in the middle Permian strata in Australia. The Penniretepora of bryozoan and the Paracaninia of coral are found to occur in most cases in the early Permian strata.
Age
Depositional setting
Pebbled slate is interpreted as being glacial marine deposits throughout the Sibumasu and Tibet microcontinents.
Additional Information
Upper contact:
Uncertain relationship to the other strata (e.g., whether on either Yinga Fm or Xiongencu Fm of Permian) – See Additional Information.
Regional extent:
GeoJSON:
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Fossils:
The formation contains 4 layers of fossils, including brachiopods, bryozoan and a small amount of solitary corals, trilobites, bivalves and crinoid stems, etc. The brachiopods comprise Spiriferella, Cleiothyridina, Neospirifer, “Phricodothyris”, Buxtonia, etc., with only the genus Terrakea being found to occur in the middle Permian strata in Australia. The Penniretepora of bryozoan and the Paracaninia of coral are found to occur in most cases in the early Permian strata.
Age:
The stratigraphic horizon of the Zhadongcuo Fm should belong to the Chuanshanian Series = early Cisuralian Epoch
From Dong-Li Sun, 1993 " On the Permian biogeographic boundary between Gondwana and Eurasia in Tibet, China as the eastern section of the Tethys", Palaeo-3, 100:59-77: " The Zhadongco Formation is overlying the Xiongenco Fm carbonate unit and consists of pebbly slate of 1000 m thick. A fault can be observed between them. Therefore, this pebbly slate sequence was interpreted as reappearance of the underlying Yinga Fm."
Age Span:
Beginning stage: Asselian
Fraction up in beginning stage: 0.0
Beginning date (Ma):
Ending stage: Asselian
Fraction up in ending stage: 1.0
Ending date (Ma):
Depositional setting:
Pebbled slate is interpreted as being glacial marine deposits throughout the Sibumasu and Tibet microcontinents.
Depositional-pattern:
Additional Information
The structure of the Permian System in the area from the Lagu to the Yinga Pastureland is fairly complicated, so there are different views on whether the whole stratigraphic sequence represents a great reversed stratigraphic sequence (?), or whether the subdivision of the particular sequence concerned into the upper, middle and the lower parts (namely into the Zadongcuo Fm, the Xiong’encuo Fm and the Yinga Fm) is tenable, that is to say whether the horizons containing pebbled slates in the upper and lower parts of the formation represent a repetition of one and the same stratigraphic unit? Yin Jixiang would think that the Permian strata in the Lagu area represent a reversed stratigraphic sequence, which includes in ascending order the Yinga Fm, the Xiong’encuo Fm and the Zhadongcuo Fm.
In addition, a different view has been put forth whether the pebbled slate in the lower part contains a part or the whole stratum be designated to the upper Carboniferous Series. Jin Yugan et al. (1977) would think that in accordance with the common plan of subdivision of the Permian System in the Gondwana, the Permian System here is composed of three lithological formations, namely in ascending order the Pebbled slate-bearing (glacio-marine), the Sandstone-and-slate-bearing, and the essentially Limestone-bearing formations.