晚三叠世龙门山前陆盆地早期(卡尼期)碳酸盐缓坡和海绵礁的淹没过程与动力机制

晚三叠世龙门山前陆盆地分布于扬子克拉通西缘,属于印支期造山楔构造负载驱动的挠曲型前渊凹陷.其中卡尼期马鞍塘组是分布于底部不整合面之上的第一套地层单元,记录了前缘隆起边缘碳酸盐缓坡和海绵礁的构建和淹没过程.据钻孔揭示马鞍塘组的最大厚度超过250m,显示为西北厚东南薄的楔形结构,从北西向南东依次分布了深水盆地、碳酸盐缓坡和海绵礁和浅水滨岸带等沉积物类型.其中碳酸盐缓坡和海绵礁分布于前陆盆地的远端,呈面向西的条带状展布,其走向线与龙门山冲断带的走向大致平行.碳酸盐缓坡和海绵礁的厚度介于30～100m之间,由北西向南东变薄.在垂向上,马鞍塘组由3部分构成,下部为鲕粒滩和生物碎屑滩,中部为海绵礁,上部为黑色页岩,显示为向上变细、变深的沉积序列.在Li et al.(2003)盆地模拟的基础上,本次对卡尼期前陆盆地的沉降速率、沉积速率、海绵礁生长速率、相对海平面上升速率进行了定量计算,其中沉降速率为0.10mm·a-1、沉积速率为0.04mm·a-1、海绵礁生长速率为0.03mm·a-1、相对海平面上升速率介于0.01mm·a-1～0.05mm · a-1之间.研究结果表明:在卡尼期早期,相对海平面处于初始上升阶段,相对海平面上升速率较小,盆地处于欠补偿状态,沉积了碳酸盐缓坡型鲕粒滩和生物碎屑滩；在卡尼期中期,相对海平面上升速率等于海绵礁生长速率,海绵礁持续保持垂直向上的生长状态,形成了高度达100余米的塔礁；在卡尼期晚期,相对海平面上升速率大于海绵礁生长速率,礁顶的水深逐步变大,导致礁体被淹溺致死,从而在卡尼期形成了鲕粒灰岩滩-生物碎屑滩-海绵礁灰岩-页岩的向上变细、变深的沉积序列,显示了前陆盆地早期碳酸盐缓坡和海绵礁生长并被淹没的特有模式.本次研究成果表明龙门山前陆盆地的底部不整合面和碳酸盐缓坡、海绵礁的淹没过程是扬子板块西缘印支期造山楔逆冲构造负载的挠曲变形的产物,显示了在卡尼期松潘-甘孜残留洋盆的迅速闭合和造山楔构造负载向扬子板块的推进过程.

Dynamic of drowning of the carbonate ramp and sponge build-up in the early stage (Carnian)of Longmen Shan foreland basin, Late Triassic, China.

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.