Mega-shoaling in carbonate platform of the Middle Triassic Leikoupo Formation,Sichuan Basin,southwest China

Shoaling is a common type of sedimentation in the evolution of carbonate platform, and commonly has poor continuity. This paper presents a newly discovered and rare type of shoaling, i.e., mega-shoaling in nearly basin scale, which is developed in the Middle Triassic Leikoupo Formation of the Sichuan Basin, southwest China. During the Leikoupo time, the studied Sichuan Basin experienced hot and dry climate conditions and developed a carbonate platform within a restricted epicontinental sea. In B sub-layer of the Lei-1-1 sub-member of the Leikoupo Formation a series of grainstones of shoal facies accumulated throughout almost the entire basin, thereby generating features associated with basin-scale mega-shoaling. By detailed core examination and microscopic observation of thin sections, it is shown that the lithology of this set of grainstones is dominated by doloarenite (calcarenite) followed by oolitic dolomite (limestone). In addition, it contains three types of sedimentary sequences characterized by upward-coarsening and upward-shallowing as the followings: restricted lagoon to platform interior beach; restricted lagoon to platform interior beach and to platform flat; and tidal flat to peritidal beach. Subsequently, a multicyclic stratigraphic division and correlation revealed that this set of grainstones can be well traced and compared horizontally, and is generally isochronous. In addition, a template for logging facies, established based on core calibrations and logging data, was employed to analyze the 235 wells in the basin. The results demonstrate the shoal grainstones to be 10–40 m thick with a 15×10⁴ km2 continuous distribution area. These findings indicate that the carbonate platform developed mega-shoals within a short period of time. The genesis of such a mega-shoaling was investigated by focusing on various shoaling conditions, such as paleo-tectonics, paleo-geomorphology, paleo-climate, sea-level changes, and palaeo-hydrodynamics. A specific combination of independent geological factors creates beneficial geomorphologic conditions for the mega-shoaling including a quiescent paleo-tectonic environment, relatively flat paleo-geomorphology and evaporites filling up and leveling off. In addition, a stably settling carbonate platform underwent sea-level fluctuations through swift transgressions and protracted regressions, which is not only conducive to continuous, multicyclic and superimposed vertical development of grain beaches but also beneficial for the horizontal migration, coalescence and superimposition of individual grain beaches. As a consequence, large-scale and continuously-distributed grain beach sedimentation emerges and mega-shoals develop.