Mianyang-Changning trough is developed during the transitional period from the Sinian to Early Cambrian in Sichuan Basin.We systematically collected the outcrop samples and drilling data both inside and outside the trough and analyzed its tectonic evolution according to stratigraphic unconformity

lithological variation

log correlation and geochemical data.The results show that there are pronounced onlaps on the Lower Cambiran sequences in the trough

keeping the erosional unconformity between the Lower Cambrian and Sinian Dengying Formation intact.In addition

evidences of early stage karstification occur in the residual Dengying Formation

indicating that the trough has experienced exposure and erosion before the Cambrian sedimentation.Meanwhile

the sedimentary cycle correlation demonstrates that there is an additional sedimentary cycle in the Maidiping Formation with normal proportion of black rocks in the trough.Moreover

elemental logging in Well Ziyang-1 inside the trough shows that there are 20 m-thick siliceous argillaceous dolomites deve-loped in the lower part of the black rocks.Thus

in combination with the stratigraphic division based on drilling data in the adjacent areas

we believed that the black rocks within the trough include not only the Cambrian Maidiping Formation

but also the 3

rd

and 4

th

members of the Sinian Dengying Formation.Besides

the hydrothermal origin of siliceous rocks in the 4

th

member of the Dengying Formation proves that strong regional extension exists during that time.According to the Middle Neoproterozoic-Cambrian magmatic activity records and the characters of the Nanhua sedimentary strata

we considered that the Mianyang-Changning trough inherited from the persistent negative landforms on the basis of Nanhua rift

and it was formed under the combined effect of intermittent extensional subsidence and denudation from exposure.Therefore

the trough is an extensional-erosional trough in tectonics.