Although the Mianyang- Changning intracratonic sag is widely recognized as a key petroleum accumulation site, its spatio- temporal distribution and formation mechanisms are still controversial. To address this, systematic stratigraphic and lithofacies investigations were carried out on the Sinian- Cambrian successions in the Guangyuan- Ningqiang area, located at the northeastern margin of the Mianyang- Changning intracratonic sag. Lithofacies indicators and biostratigraphic data suggest that the lower Beiwan Member or upper Algal Dolomite Member are commonly overlain by the Kuanchuanpu Formation, indicating that the Gaojiashan and Beiwan Members are pervasively missing in the Kuanchuanpu, Shujiaba, and Caojiaba- Dongshanzi areas. This indicates that the strata missing of the upper Dengying Formation at the eastern margin of the Mianyang- Changning intracratonic sag extends from the central Sichuan basin and the northern slope of the central Sichuan paleouplift to the Guangyuan- Ningqiang area. A sharp facies transition from tidal flat- lagoonal microbial dolomite and dolomudstone in the study area to slope- basinal chert in the Datan- Yangmu area indicates the influence of extensional faulting. Similar abrupt facies shifts are observed at the southern margin of the middle Yangtze platform and the eastern margin of the upper Yangtze platform, reflecting an extensional tectonic regime characterized by fault- bounded carbonate platforms. The spatial extent of several kilometers, erosion thickness of hundreds of meters, and the depositional break of up to 20 Ma suggest that the Mianyang- Changning intracratonic sag resulted from differential subsidence/uplift rather than karstic morphology. The linear distribution of strata missing area at the eastern margin of the Mianyang- Changning intracratonic sag is interpreted as the result of relative uplift at a tilted block. The climax of extensional faulting occurred at the Sinian- Cambrian transition, causing the strata missing from the upper Dengying Formation to the lowermost Kuanchuanpu Formation.