| Based on the analysis of regional tectonic background, the understanding of tectonic-sedimentary
characteristics and sedimentary facies evolution of Leikoupo Formation in northwestern Sichuan Basin has been deepened
through drilling core observation, outcrop profile investigation, thin section identification, outcrop-well profile correlation
and typical seismic profile interpretation. The results show that: (1) The uplift of the Xuefengshan ancient land during the
Leikoupo period of the Middle Triassic intensified the geographical background of “low west, high east” in the Upper
Yangtze sea. With the continuous uplifting of the Luzhou palaeohigh and Kaijiang palaeohigh, the subsidence center and
depocenter during the Leikoupo period was formed in Jiangyou-Mianzhu area of northwestern Sichuan Basin, and in Late
Triassic the depression extended southward to the Kuahongdong area of Emei County. (2) Controlled by the geographical
background and sea level change, three major sedimentary facies, i.e., platform margin facies, limited platform facies, and
evaporite platform facies, are developed from west to east in Leikoupo Formation. Shoal subfacies is developed in the
platform margin facies, which mainly consists of sandy shoal and oolitic shoal microfacies. The limited platform facies
consists of shoal, tidal flat and lagoon subfacies, including sandy shoal, algae limestone flat, calcareous dolomite flat, tidal
channel, desalinated lagoon, saline lagoon, brackish lagoon, and other microfacies. The evaporite platform facies is
composed of evaporite flat and salt lake subfacies, with microfacies of gypsum flat, gypsum-dolomite flat, dolomitegypsum
flat, dolomite flat, gypsum-salt lake and halite lake developed in them. (3) The platform margin shoal has a
significant scale with densely packed internal clastic and oolitic grains. Conversely, the internal shoal has a smaller scale
primarily characterized by algae debris particles. We propose a sedimentary model of "platform margin facies-limited
platform facies-evaporite platform facies" in the Leikoupo Formation, and the sedimentary changes are controlled by the
sedimentary base tilting northwest and the transgression-regression cycle of the sea level. |
