Dynamic of drowning of the carbonate ramp and sponge build-up in the early stage(Carnian)of Longmen Shan foreland basin,Late Triassic,China
Dynamic of drowning of the carbonate ramp and sponge build-up in the early stage(Carnian)of Longmen Shan foreland basin,Late Triassic,China
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S. De
龙门山前陆盆地是晚三叠世印支期扬子克拉通西部的挠曲前渊，马鞍塘组是挠曲基底不整合面上的第一个地层单元，首次记录了碳酸盐斜坡和海绵建造的建立和淹没。沿着海盆前隆缘向上，西部马鞍塘组钻孔可观测的最大厚度大于250 m，剖面上，西部为深水盆地，中部和东部为碳酸盐缓坡边缘的浅滩相和礁斑，西部为马鞍塘组卡尼期灰岩和海绵沉积，东部为前陆盆地，西部为前陆盆地，中部为前陆盆地，东部为前陆盆地，西部为前陆盆地，东部为前陆早前陆盆地远缘东部发育NE-SW向碳酸盐岩斜坡，其相线与龙门山冲断带走向大致平行，厚度30~ 100 m，向东南方向逐渐变薄，马鞍塘组由三部分组成，下部为鲕粒灰岩和生物碎屑灰岩，下部为泥质灰岩，下部为泥质灰岩。中部由块状海绵礁组成，上部由暗色页岩夹泥岩组成，呈向上变细的序列，因此碳酸盐浅滩相和块状礁记录了龙门山前陆盆地碳酸盐斜坡的存在。在前人盆地模拟的基础上（Li et al.，2003），计算了卡尼期前陆盆地的沉降速率、沉积速率、海绵生物礁的生长速率和相对海平面上升速率，得到沉降速率为0.10mm·a-1，沉积速率为0.04mm·a-1，海绵生物礁的生长速率为0.03mm·a-1，相对海平面上升速率在0.01 ~ 0.05mm·a-1之间。早卡尼期前陆盆地沉降速率大于沉积速率，处于欠充填状态，沉积物供给不足，碳酸盐岩斜坡沉积的鲕滩和生物礁滩相对于相对海平面的上升速率持续上升，卡尼中期相对海平面的上升速率与海绵礁的生长速率相当，海绵礁垂直生长，塔礁高度达100米以上，卡尼晚期相对海平面上升速率大于海绵礁，因此，马鞍塘组建立了一个由鲕粒灰岩组成的向上变细、变深的沉积序列，生物碎屑-海绵礁-页岩是前陆盆地早期碳酸盐岩斜坡和海绵礁生长与淹没的典型模式，本文认为前陆盆地基底不整合和碳酸盐岩斜坡和海绵礁的淹没是卡尼期造山带负荷系统向扬子克拉通快速推进驱动挠曲变形的结果，与松潘-甘孜残留洋盆快速闭合有关。
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.