Restudies on the Depositional Environments of the Ding jiazhai Formation in Shidian of Yunnan and the Genesis of Its Bottom Pebble-bearing Layer

ZHOU Zhi-cheng; FANG Zong-jie; WANG Yu-jing; XIAO Yin-wen

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Article Navigation > Acta Sedimentologica Sinica > 1999 > 17(S1): 691-698

ZHOU Zhi-cheng, FANG Zong-jie, WANG Yu-jing, XIAO Yin-wen. Restudies on the Depositional Environments of the Ding jiazhai Formation in Shidian of Yunnan and the Genesis of Its Bottom Pebble-bearing Layer[J]. Acta Sedimentologica Sinica, 1999, 17(S1): 691-698.

Citation:

ZHOU Zhi-cheng, FANG Zong-jie, WANG Yu-jing, XIAO Yin-wen. Restudies on the Depositional Environments of the Ding jiazhai Formation in Shidian of Yunnan and the Genesis of Its Bottom Pebble-bearing Layer[J]. Acta Sedimentologica Sinica, 1999, 17(S1): 691-698.

Restudies on the Depositional Environments of the Ding jiazhai Formation in Shidian of Yunnan and the Genesis of Its Bottom Pebble-bearing Layer

1.
Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences Nanjing 210008;
2.
Regional Surveying Team of Geological Bureau of Yunnan Yuxiyunnan 653100

Received Date: 1998-10-21
Rev Recd Date: 1999-03-17
Publish Date: 1999-12-10

Abstract

The Dingjiazhai Fm. aged Asselian-Sakmarian pseudoconformably overlies the Pumenqian Fm. aged the Lower Carboniferous. The Dingjiazhai Fm. in the Dingjiazhai section at Shidian County mainly comprises 3 parts in ascending order:1. the pebble-bearing layer at the lower part;2. siltstones and black shales at the middle part in which trace fossils Palaeophycus sp.,Taenidium serpentinum?,have been found,3. muddy siltstones and biocalcarenites at the upper part. A thin purple mudstone occurs at its topmost in which the trace fossil Dendrotichnium sp.,is present. According to the palaeontological, petrological and ichnological features, six main sedimentological facies are distinguished from the upper part of Pumenqian Fm. to the Dingjiazhai Fm. . They are tidal flat fades,margin facies and lagoon facies which were located in the restricted platform,sand bar or shoal facies,basin facies of shallow water and open marine platform which belonged to open marine environments. The Dingjiazhai Fm. in the Jinji section of Baoshan region consists of the pebble-bearing layer at its lower part and mudstones and muddy siltstones at its middle and upper part. Palaeo-phycus sp. B and Tisoa jinjiensis are main trace fossils present in the mudstones and muddy siltstones overlying the pebble-bearing layer. The vertical U-shaped structures of Tisoa jinjiensis are similar to those of Areni-colites. The trace fossil assemblage can be attributed to the Skolithos ichnofacies. As indicated by Frey&Pemberton (1984),the environments which the,Skolithos ichnofacies represents are characterized by shifting substrates, moderate to high-energy conditions and episodic erosion or deposition. On the basis of petrological studies on the pebble-bearing layer,it can be divided into two beds:the lower pebble bed and the upper brecdated limestone. The pebbles with better rounding in the former are elliptical and flat and mainly consist of siltstones,quartzites,silicolites and lithic sandstones. The breccia in the latter are briefly composed of lime-stones whose carbonate microfacies are similar to those of the underlying Pumenqian Fm. . The cementation of the brecciated limestone is much better than that of the lower pebble bed. The authors consider that the origin of the pebble-bearing layer at the lower part of the Dingjiazhai Fm. is not related to glaciation. The lower pebble bed was deposited in the normal littoral environments and the upper brecciated limestone was de- rived from the debris flow. The palaeoecologic features of biota (corals, fusulinids,spores) also indicate that the organisms lived in warm marine environments. Many geologists have regarded striated clasts, dropstones, three-fold sequence and periglacial marine biota as main evidences of glacial origin of the Dingjiazhai Fm. .This paper puts forward different viewpoints against their conclusions. The striated clasts are not only prod- acts of glacial process. They can be produced by many other geological processes such as faults,debris flow,landslip,landfall, mountain flood and even wind action. The photos of striated clasts (Fan&Fang,1992) do not show any typical characters generated by glaciation. They have better roundness and lack evident worn planes. The direction of striates is different from that of long axis of clasts. Some Chinese and foreign geologists misunderstand that the dropstones are able to stab the stratification when they fall down. In fact,when the dropstones fall down and come into contact with the surface of deposits,the power of dropstones is not strong enough to crash the stratification and only can bend it. The local concentration of pebbles and coarse clasts and the occurrence of till pellets can be regarded as products of ice-berg raft. Unfortunately,these phenomena have not been found in the Dingjiazhai Fm. Wopfner (1994) suggest that the three-fold sequence consisting of 1. diamictites and coarse elastics;2. pebbly mudstones and laminites and 3. black pyritic mudstones respectively correspond to the deposits in glacial advance,glacial retreat and deglacialtion which are observed in many Gondwana areas. Howerver,the stratigraphic sequences and regional dstribution of the Dingjiazhai Fm. in Baoshan region are so simple that they can not be attributed to the mentioned above three-fold sequence related to glaciation. Eurydesma-Deltopecten fauna characterized by lower diversity are typical periglacial marine biota. The main members of this fauna include bivalves Megadesmus;Myonia,braehiopods Trigonotreta, Cyrtella,Arc-titreta, Tomiospsis;gastropods Keeneia, I'eruispira. The specimen Eurydesma in the Dingjiazhai Fm, is an error of identification. It has been acted as an important proof of the presence of periglacial marine biota. Fang and Lin (1990) restudied this specimen and identified it as Schizodus. The most members of brachiopods are attributed to the Tethyan type. Only Cyrtella belongs to the Gondwana type. As mentioed above,the hypothesis of glacial origin of the Dingjiazhai Fm. at Baoshan region is truthless.
- pebble-bearing layer of the Dingjiazhai Frn.,
- littoral facies,
- debris flow,
- glaciomarine fades

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沈阳化工大学材料科学与工程学院沈阳 110142

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Restudies on the Depositional Environments of the Ding jiazhai Formation in Shidian of Yunnan and the Genesis of Its Bottom Pebble-bearing Layer

1. Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences Nanjing 210008;

2. Regional Surveying Team of Geological Bureau of Yunnan Yuxiyunnan 653100

Received Date: 1998-10-21

Rev Recd Date: 1999-03-17

Publish Date: 1999-12-10

Key words:

pebble-bearing layer of the Dingjiazhai Frn. /
littoral facies /
debris flow /
glaciomarine fades

Abstract: The Dingjiazhai Fm. aged Asselian-Sakmarian pseudoconformably overlies the Pumenqian Fm. aged the Lower Carboniferous. The Dingjiazhai Fm. in the Dingjiazhai section at Shidian County mainly comprises 3 parts in ascending order:1. the pebble-bearing layer at the lower part;2. siltstones and black shales at the middle part in which trace fossils Palaeophycus sp.,Taenidium serpentinum?,have been found,3. muddy siltstones and biocalcarenites at the upper part. A thin purple mudstone occurs at its topmost in which the trace fossil Dendrotichnium sp.,is present. According to the palaeontological, petrological and ichnological features, six main sedimentological facies are distinguished from the upper part of Pumenqian Fm. to the Dingjiazhai Fm. . They are tidal flat fades,margin facies and lagoon facies which were located in the restricted platform,sand bar or shoal facies,basin facies of shallow water and open marine platform which belonged to open marine environments. The Dingjiazhai Fm. in the Jinji section of Baoshan region consists of the pebble-bearing layer at its lower part and mudstones and muddy siltstones at its middle and upper part. Palaeo-phycus sp. B and Tisoa jinjiensis are main trace fossils present in the mudstones and muddy siltstones overlying the pebble-bearing layer. The vertical U-shaped structures of Tisoa jinjiensis are similar to those of Areni-colites. The trace fossil assemblage can be attributed to the Skolithos ichnofacies. As indicated by Frey&Pemberton (1984),the environments which the,Skolithos ichnofacies represents are characterized by shifting substrates, moderate to high-energy conditions and episodic erosion or deposition. On the basis of petrological studies on the pebble-bearing layer,it can be divided into two beds:the lower pebble bed and the upper brecdated limestone. The pebbles with better rounding in the former are elliptical and flat and mainly consist of siltstones,quartzites,silicolites and lithic sandstones. The breccia in the latter are briefly composed of lime-stones whose carbonate microfacies are similar to those of the underlying Pumenqian Fm. . The cementation of the brecciated limestone is much better than that of the lower pebble bed. The authors consider that the origin of the pebble-bearing layer at the lower part of the Dingjiazhai Fm. is not related to glaciation. The lower pebble bed was deposited in the normal littoral environments and the upper brecciated limestone was de- rived from the debris flow. The palaeoecologic features of biota (corals, fusulinids,spores) also indicate that the organisms lived in warm marine environments. Many geologists have regarded striated clasts, dropstones, three-fold sequence and periglacial marine biota as main evidences of glacial origin of the Dingjiazhai Fm. .This paper puts forward different viewpoints against their conclusions. The striated clasts are not only prod- acts of glacial process. They can be produced by many other geological processes such as faults,debris flow,landslip,landfall, mountain flood and even wind action. The photos of striated clasts (Fan&Fang,1992) do not show any typical characters generated by glaciation. They have better roundness and lack evident worn planes. The direction of striates is different from that of long axis of clasts. Some Chinese and foreign geologists misunderstand that the dropstones are able to stab the stratification when they fall down. In fact,when the dropstones fall down and come into contact with the surface of deposits,the power of dropstones is not strong enough to crash the stratification and only can bend it. The local concentration of pebbles and coarse clasts and the occurrence of till pellets can be regarded as products of ice-berg raft. Unfortunately,these phenomena have not been found in the Dingjiazhai Fm. Wopfner (1994) suggest that the three-fold sequence consisting of 1. diamictites and coarse elastics;2. pebbly mudstones and laminites and 3. black pyritic mudstones respectively correspond to the deposits in glacial advance,glacial retreat and deglacialtion which are observed in many Gondwana areas. Howerver,the stratigraphic sequences and regional dstribution of the Dingjiazhai Fm. in Baoshan region are so simple that they can not be attributed to the mentioned above three-fold sequence related to glaciation. Eurydesma-Deltopecten fauna characterized by lower diversity are typical periglacial marine biota. The main members of this fauna include bivalves Megadesmus;Myonia,braehiopods Trigonotreta, Cyrtella,Arc-titreta, Tomiospsis;gastropods Keeneia, I'eruispira. The specimen Eurydesma in the Dingjiazhai Fm, is an error of identification. It has been acted as an important proof of the presence of periglacial marine biota. Fang and Lin (1990) restudied this specimen and identified it as Schizodus. The most members of brachiopods are attributed to the Tethyan type. Only Cyrtella belongs to the Gondwana type. As mentioed above,the hypothesis of glacial origin of the Dingjiazhai Fm. at Baoshan region is truthless.

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Restudies on the Depositional Environments of the Ding jiazhai Formation in Shidian of Yunnan and the Genesis of Its Bottom Pebble-bearing Layer

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