珠江口盆地流花碳酸盐台地灰岩坑的地震反射特征及成因探讨

早中新世东沙隆起带上发育了大规模的流花碳酸盐台地，上覆巨厚未固结-弱固结泥岩。基于三维地震数据，利用相干体及可视化等方法，在台地上发现了由溶蚀垮塌作用形成的数量众多的椭圆形的灰岩坑。灰岩坑直径一般为200~700 m，最大超过900 m，深度10~90 m；灰岩坑在地震反射上表现为碳酸盐岩顶部垮塌，并导致上覆泥岩地层塌陷；灰岩坑一般沿NWW向断层分布，这些NWW向断层切割中新世碳酸盐岩和上覆海相泥岩地层，绝大多数终止于T₁₀的不整合面。综合分析东沙海区构造和沉积过程认为：发生在晚中新世末期的东沙运动形成了NWW向张扭性断层，为酸性流体提供了运移通道和物质交换场所；东沙运动伴生的基底岩浆活动可能产生了酸性热液流体；东沙运动末期发生强烈的构造抬升和大规模海底暴露，造成地表水沿断层注入，加快了碳酸盐岩的溶蚀速率。在台地底部珠海组砂岩层的高压流体、地表水和岩浆活动可能产生的酸性热液流体的共同溶蚀作用下，碳酸盐岩地层内部产生溶洞，并且越来越大，最终顶部无法承载上覆巨厚泥岩而发生垮塌，形成灰岩坑。流花碳酸盐台地灰岩坑的形成不是简单的暴露溶蚀，而是构造活动影响下深埋藏溶蚀作用的产物。

Seismic Characteristics and Generations of Sinkholes in the Liuhua Carbonate Platform,Pearl River Mouth Basin

Liuhua carbonate platform was developed in Dongsha uplift in Early Miocene with maximum thickness of more than 600 m. It was overlain by huge thickness of mudstones. A lot of sinkholes were imaged in upper carbonate platform based on coherent slices and three dimension visualization from seismic data. The sinkholes were larger depressions with diameter generally ranging from 200 to 700 m, the maximum more than 900 m, and depth varying from 10 to 90 m. Based on the characteristics that sinkholes developed along the NWW oriented faults which terminated at T₁₀ unconformity or continued to transform into syn-sedimentary faults, and depressions of mudstone stopping at T₁₀ unconformity, Dongsha Movement is considered to be the key factor leading to the occurrence of sinkholes. Concretely, basal magmatic activities might provide acidic hydrothermal fluid to dissolve the carbonate rock. NWW oriented faults produced by Dongsha Movement served as pathways for fluid flow and rooms for fluid exchange. At the end of Late Miocene, exposure caused by Dongsha Movement accelerated the dissolution rate of carbonate rocks. With continuous corrosion, the dissolved cavern became larger and larger until the carbonate strata over the cavern couldn't hold the upper loading, and then carbonate strata collapsed and the sinkhole formed.