Characteristics and genesis of the mixed siliciclastic-carbonate successions in the Upper Sinian Dengying Formation in the Nanjiang region, Sichuan

The mixed siliciclastic-carbonate successions represent a special type of depositional mechanisms and significant deposition. They are well developed in the Upper Sinian Dengying Formation in the Xinli section, Nanjiang region, Sichuan. Four lithologic associations have been distinguished, including (1) fine-grained sandstone-dolomicrite association; (2) siltstone-dolomicrite association; (3) silty sandstone-silty dolostone association, and (4) gravelly sandstone-dolostone association. Four sedimentary subfacies types are also recognized on the basis of the rock types and associations, including the mesotidal flat, subtidal shoal, tidal channel and lower tidal flat subfacies. The mixed siliciclastic-carbonate successions occur primarily in the mesotidal flat and tidal channel subfacies. The trace element data show that the studied strata are generally formed in the oxidation environments. However the mixed siliciclastic-carbonate successions are formed in the anoxic environments. In the light of the sea-level changes and tectonic evolution, the development of the mixed siliciclastic-carbonate successions in the Dengying Formation in the Xinli section, Nanjiang region, Sichuan may be companied by the siliciclastic input caused by the negative movements following the phase I of the Tongwan Movement.