Lacustrine Storm Sediment in Yanchang Tormation of Ordos Basin

GAO Shan-lin; CHEN Hai-hong; DOU Wei-tan; SHAO Dong-bo; LIN Hong-bin

The late Triassic lacustrine in Ordos basin is a large open lake with gentle topography .This paper has firstly reported the lacustrine elastic tempestites in the Triassic Yanchang Formation of the central Ordos basin. The typical tempestite consist of packstones which have a lot of deposition-structural characteristic for differently evolutionary stage of storm. Sedimentary structures of the storm deposits are :a)hummocky cross-stratification are characterized by gentlly curved,low angel cross-lamination. Presently most researchers think that powerful dominant or multidirectional oscillatory flow result in the special character which is most useful and best indicator of the storm sedimentary sequences distinguished from the turbidites., b ) basal division,consisting of one or more of the following:sharp base; oriented directional sole marks ; basal lag of pebbles,or rip-up and deformed resedimented intrabasinal clasts of mudstone. The formation of these structure is related to sediment liquefacion,drainage,reflecting the characteristics of density current. Especially the erosional structure and basal lag of pebbles in turbidite facics are the evidence of storm activity c) planar parallel lamination,d) bioturbated mudstone or fine sandstone .The reservation of escaping structure indicate a quick depositional event. After the storm is over,burrow structure will be developed on the sediment surface,e)lower maturity in texture in sandstone .Lower maturity reflect lower degree of sorting that is a signature of rapid deposit. The typical sequence is concluded. It can be divided into :a) high-discharge form, characterized by block structure,deformed pebble bedding,escaping trace,b ) hiatus, c ) low-discharge,marked by wave-cross ripple mark, burrow trace, lenticular bedding and vein bedding. Storm sedimentary model is given in this paper .We also discuss the structure and paleo-latitude setting of the late Triassic lacustrine tempestites,background of which has been defined as high-level system and was developed in the middle of lake-regressive sequence during the evolution of the basin. The storm deposit transit to deltas or beaches deposits upper and to turbidites offshore. We use the combination of storm facies asso- ciation and geological characters of adjacent areas to modify the former located sedimentary boundary of the Yanchang lake .We futher indicate storm sedimentary ;association developed area as prospective lithologic or structrure-lithological trap.

1. Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing, 100029;

2. Changqing Oil Field,Xi'an, 710021

Received Date: 1999-03-22

Rev Recd Date: 1999-06-08

Publish Date: 1999-12-10

Key words:

Abstract: The late Triassic lacustrine in Ordos basin is a large open lake with gentle topography .This paper has firstly reported the lacustrine elastic tempestites in the Triassic Yanchang Formation of the central Ordos basin. The typical tempestite consist of packstones which have a lot of deposition-structural characteristic for differently evolutionary stage of storm. Sedimentary structures of the storm deposits are :a)hummocky cross-stratification are characterized by gentlly curved,low angel cross-lamination. Presently most researchers think that powerful dominant or multidirectional oscillatory flow result in the special character which is most useful and best indicator of the storm sedimentary sequences distinguished from the turbidites., b ) basal division,consisting of one or more of the following:sharp base; oriented directional sole marks ; basal lag of pebbles,or rip-up and deformed resedimented intrabasinal clasts of mudstone. The formation of these structure is related to sediment liquefacion,drainage,reflecting the characteristics of density current. Especially the erosional structure and basal lag of pebbles in turbidite facics are the evidence of storm activity c) planar parallel lamination,d) bioturbated mudstone or fine sandstone .The reservation of escaping structure indicate a quick depositional event. After the storm is over,burrow structure will be developed on the sediment surface,e)lower maturity in texture in sandstone .Lower maturity reflect lower degree of sorting that is a signature of rapid deposit. The typical sequence is concluded. It can be divided into :a) high-discharge form, characterized by block structure,deformed pebble bedding,escaping trace,b ) hiatus, c ) low-discharge,marked by wave-cross ripple mark, burrow trace, lenticular bedding and vein bedding. Storm sedimentary model is given in this paper .We also discuss the structure and paleo-latitude setting of the late Triassic lacustrine tempestites,background of which has been defined as high-level system and was developed in the middle of lake-regressive sequence during the evolution of the basin. The storm deposit transit to deltas or beaches deposits upper and to turbidites offshore. We use the combination of storm facies asso- ciation and geological characters of adjacent areas to modify the former located sedimentary boundary of the Yanchang lake .We futher indicate storm sedimentary ;association developed area as prospective lithologic or structrure-lithological trap.

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Lacustrine Storm Sediment in Yanchang Tormation of Ordos Basin

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