THE EXPOSURE INDICATORS FORMED DURING THE DEPOSITION OF THE UPPER SINIAN DENGYING FORMATION IN SOUTHWESTERN SICHUAN AND THEIR SIGNIFICANCE

This paper deals with large amount of the subaerial exposure indicators recognized in the Upper Sinian Dengying Formation in southwestern Sichuan, including vadose pisolites, ruiniform houizons, tepee structures and palaeokarst sediments. The dolostones comprising these subaerial exposure indicators in the Dengying Formation have long been considered as oncolitic dolostone, snowflate dolostone, comb-crust algal dolostone and husklike algal dolostone. They are also interpreted as having formed in the tidal flat environments and as being of biological origin under the influence of sea water. The discovery of the subaerial exposure indicators leads us to the following new explanations.
(1)There must be some important depositional omissions in the Dengying Formation and subaerial exposure environments during the Sinian.
(2)The botryoidal dolostones may be interpreted to be of palaeokarst origin.
(3)Some so-called algal fossils or algae-related fossils such as oncolite, clotted stone and deformed stone are now recognized as a kind of secondary rock structures.
(4)According to the theory of sequence stratigraphy, two sequences and associated depositional systems tracts can be distinguished for the Upper Sinian strata: the one is from the Guanyinya Formation to the lower part of the second member of the Dengying Formation, the other is from the top of the former sequence to the top of the Maidiping Formation.