Middle jurassic trace fossils from the ridang formation in sajia county, south Tibet, and their palaeoenvironmental significance

摘要

An ichnofauna consisting of 18 ichnospecies (one of them new) in 14 ichnogenera are described for the first time from the Middle Jurassic Ridang Formation in Sajia County, South Tibet. The ichnofauna can be subdivided into two ichnoassemblages in ascending stratigraphic order: the Palaeophycus-Megagrapton ichnoassemblage in the lower and middle parts of the Ridang Formation, followed by the Cosmorhaphe-Nereites-Paleodictyon ichnoassemblage in the upper Ridang Formation. Overall, the trace fossils occur in a middle-distal turbidite fan sequence, as evidenced by both sedimentological analysis and the composition of the trace fossils. Several subenvironments of the middle-distal fan system have been recognized on the basis of the spatial distribution of the trace fossils. Typically, the channel-fill deposits in the middle part of the turbidite fan lack trace fossils, the interchannel and upper channel-fill (levee) subenvironments of the middle turbidite fan contain abundant facies-crossing trace fossils, in contrast to the distal part of the turbidite fan where deep-water trace fossils are dominant. The ichnofauna is similar to typical flysch ichnofaunas from Europe and North America in characteristics, and is interpreted to represent a typical deep-sea Nereites ichnofacies. The presence of these deep-sea trace fossils therefore would suggest that a continental slope environment existed in southern Tibet during the Middle Jurassic and the study area was located in a slope-abyssal plain setting.