Abstract: The Longmen Shan foreland basin developed as a flexural foredeep at western Yangtze craton during the Late Triassic Indosinian orogeny. The Maantang Formation is the first stratigraphic unit overlies a flexural basal unconformity and initially records the establishment and drowning of a carbonate ramp and sponge build-up along the forebulge margin of the marine basin. The observable maximum thickness of Maantang Formation in boreholes is greater than 250m in the west, thinning southeastward, so the geometry of the basin is wedge-shaped. In a transverse section, there was a deep-water basin in the west, shoal facies and patch reefs which were on the edge of a carbonate ramp in the middle and the eastern part of the foreland basin. The Carnian limestone and sponge built-up of the Maantang Formation was deposited on a westward, NE-SW orientated carbonate ramp in the east of distal margin of the early foreland basin. The strike line of facies is approximately parallel to that of Longmen Shan thrust belt. The carbonate ramp attains a thickness of 30~100m and thins to the southeast. The Maantang Formation is composed of three parts, the lower part consists of oolitic and bioclastic limestone, the middle part is made up of patch sponge reefs, the upper part is comprised of dark shales intercalated mudstones in an upward-fining succession, so carbonate shoal facies and patch reefs record the presence of a carbonate ramp on Longmen Shan foreland basin. In the basis of previous basin modeling (Li et al., 2003), we have done the calculation of the subsidence rate, depositional rate, the growth rate of sponge reefs and the rising rate of relative sea level of the foreland basin in Carnian, consequently, the subsidence rate is 0.10mm·a-1, the depositional rate is 0.04mm·a-1, the growth rate of sponge reefs is 0.03mm·a-1, and the rising rate of relative sea level is between 0.01mm·a-1 and 0.05mm·a-1. In the Early Carnian, the subsidence rate was greater than the depositional rate in the foreland basin, which was in a underfilled condition and insufficient sediments supplied, the oolitic and bioclastic bank of carbonate ramp deposited relative to the rate of relative sea level continued to rise. In the Middle Carnian, the rising rate of relative sea level was equivalent to the growth rate of sponge reefs, the sponge reefs grew vertically, and the height of tower reef reached to more than 100 meters. In the Late Carnian, the rising rate of relative sea level was greater than that of sponge reefs, so the sponge reefs were drown to death gradually. So the Maantang Formation shows the establishment of an upward-fining and deepening sedimentary sequence made up of oolitic limestone-bioclastic-sponge reefs-shales, which is a typical pattern of the carbonate ramp and growth and drowning of the sponge reefs in the early stage of the foreland basin. This paper suggests that the basal unconformity of the foreland basin and the drowning of carbonate ramp and sponge reefs result from the flexural deformation driven by the orogenic load system advanced rapidly towards the Yangtze craton in the Carnian, associated with the rapid closure of Songpan-Ganzi remnant ocean basin.