The Environmental Implication of the Trace Fossil Op hiomorpha from the Upper Permian of the Yily Basin, Xinjiang

CUI Zhi lin

The Yily Basin,developed in the central Tianshan orogenid belt,accommodat es a huge thickness(about 5 000～9 000 m) of continental Permian strata.The Upper Permian is chiefly composed of lacustrine clastic sedimentary rocks interbedded with limestone 2 700 m in total thickn ess.including units Xiaoshansayi Formation,Hamist Formation,Tiemulike Formation and Bakaluesayi Formation in ascending order.Ophiomorpha is amp l y produced in the Lower part of Xiaoshansayi Formation which consists of thin-be dded limestones interbedded with massive sandstones.Graded pebble-bearing sandst onaes,cross-bedded sandstones and rippled fine sandstones often constitute cycli c fining-upward deposits in section,showing mountainy river fan deposits.The lime stone usually contains sands,bioclastics and mud with few plant and freshwater b ivalve fossils in them revealing shallow lacustrine deposits.It is absent in bla ck laminar limestone and shales of Tiemulike Formation and Bakaluesayi Formation because this lithology represents an exaerobic condition. Ophiomorpha exists in limestone beds without laminae in them,pre served as a set of different burrows in diameter from 0.6 to 1.6 cm,probably rep resenting developing stages of the burrow-maker.The burrow wall,0.5～2.5 cm in thickness,is made up of dark or black pellets ovoid,or irregular in shape,but it is conspicuously double-layered.Their maximum length observed in the section ca n reach 35 cm .They are tube-like or caecum-like,predominantly horizontal and obli que in level,neither showing divergent branching pattern of typical Oph iomorpha So,they are similar to Ophiomorpha nodosa Lu ndgren in size and pellet wall,but different in wall texture and branching patte rn.Infill is passive and its composition is like country rocks. Ophiomorpha was once accepted as a marine indicator(Kennedy and Sellwood,1970),and afterwards documented in continental strata(Stewart,1978;Bown,1982; Merill,1984).But Goldering and Pollard (1995) thought that the continenta l appearance of Ophiomorpha is questionable.Samples in this stud y are collected in absolutely lacustrine sedimentary rocks,This supports the poi nt of view that Ophiomorpha can occur in nonmarine environments. Some samples possess upward grading fill (Plate I-E),reflecting a rapi d depositional condition. It could be also used as top-indicating structure.Abov e characteristics demonstrate that the burrow-maker is inclined to penetrate sof t sedimentary floor where there are relatively a high rate of deposition and a h igh level of oxygen content,and they are scarcely controlled by composition of the substrate,water-depth and salinity.

Department of Geology,Northwest University,Xi'an 710069

Received Date: 1998-03-04

Rev Recd Date: 1998-05-02

Publish Date: 1999-06-10

Key words:

trace /
fossil /
Late

Abstract: The Yily Basin,developed in the central Tianshan orogenid belt,accommodat es a huge thickness(about 5 000～9 000 m) of continental Permian strata.The Upper Permian is chiefly composed of lacustrine clastic sedimentary rocks interbedded with limestone 2 700 m in total thickn ess.including units Xiaoshansayi Formation,Hamist Formation,Tiemulike Formation and Bakaluesayi Formation in ascending order.Ophiomorpha is amp l y produced in the Lower part of Xiaoshansayi Formation which consists of thin-be dded limestones interbedded with massive sandstones.Graded pebble-bearing sandst onaes,cross-bedded sandstones and rippled fine sandstones often constitute cycli c fining-upward deposits in section,showing mountainy river fan deposits.The lime stone usually contains sands,bioclastics and mud with few plant and freshwater b ivalve fossils in them revealing shallow lacustrine deposits.It is absent in bla ck laminar limestone and shales of Tiemulike Formation and Bakaluesayi Formation because this lithology represents an exaerobic condition. Ophiomorpha exists in limestone beds without laminae in them,pre served as a set of different burrows in diameter from 0.6 to 1.6 cm,probably rep resenting developing stages of the burrow-maker.The burrow wall,0.5～2.5 cm in thickness,is made up of dark or black pellets ovoid,or irregular in shape,but it is conspicuously double-layered.Their maximum length observed in the section ca n reach 35 cm .They are tube-like or caecum-like,predominantly horizontal and obli que in level,neither showing divergent branching pattern of typical Oph iomorpha So,they are similar to Ophiomorpha nodosa Lu ndgren in size and pellet wall,but different in wall texture and branching patte rn.Infill is passive and its composition is like country rocks. Ophiomorpha was once accepted as a marine indicator(Kennedy and Sellwood,1970),and afterwards documented in continental strata(Stewart,1978;Bown,1982; Merill,1984).But Goldering and Pollard (1995) thought that the continenta l appearance of Ophiomorpha is questionable.Samples in this stud y are collected in absolutely lacustrine sedimentary rocks,This supports the poi nt of view that Ophiomorpha can occur in nonmarine environments. Some samples possess upward grading fill (Plate I-E),reflecting a rapi d depositional condition. It could be also used as top-indicating structure.Abov e characteristics demonstrate that the burrow-maker is inclined to penetrate sof t sedimentary floor where there are relatively a high rate of deposition and a h igh level of oxygen content,and they are scarcely controlled by composition of the substrate,water-depth and salinity.

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Reference (17)

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The Environmental Implication of the Trace Fossil Op hiomorpha from the Upper Permian of the Yily Basin, Xinjiang

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