珠江三角洲古今海蚀地形的高度差异及影响因素

海岸平台之所以能指示海面的高度, 是因为海面在岩石海岸上长时间作用的结果. 位于现代珠江三角洲平原上的古海蚀平台远高于珠江口内外现代海岸的现代海蚀平台, 但普遍认为其高度仍在现代海岸动力范围之内, 并且被作为判断全新世地壳升降或海面变化的依据. 从海岸地貌学、海岸动力学、岩体力学等几个方面对造成珠江三角洲古今海蚀地形高度差异的各种因素的影响程度进行了研究. 理论计算表明: 古今珠江三角洲河口湾内平均高潮高度的差异最大不超过0.3 m; 在广东海岸的波浪条件下, 因波浪差异或海岸岩石强度差异造成的海蚀平台之间的高度差异均不足0.3 m. 但野外实测数据显示, 珠江三角洲古今海蚀平台高度差异在1 m以上, 因此上述因素不是形成古今海蚀地形高度差异的主要原因. 而多年潮汐统计数据则显示,广东各地海岸高于3 m(珠江基面)的风暴潮至少要50 年才有1 次, 几十年一遇的特大高潮和出现频率极低的特大波浪没有足够的时间塑造相应的高海蚀平台和海蚀穴. 因此, 珠江三角洲古海蚀遗迹的高度已超出现代海面的动力作用范围, 更不是风暴潮波浪作用的结果, 只能是构造抬升或海平面下降的结果, 但如何准确地鉴别构造的作用, 还原海面下降幅度, 则需要更进一步的研究.

Factors affecting the heights of ancient and present wave-cut benches in the Pearl River Delta area

The height of marine wave-cut benches and notches is an indicator of the sea level because they are formed by long-term wave action. Although ancient wave-cut benches on the Pearl River Delta plain have heights much higher than those of benches developed on the modern coasts, the heights of the old benches are still widely considered to be within the range of modern wave actions. They are therefore used as indicators of Holocene tectonic uplift or sea level changes in the Pearl River Delta area. In this study, we address the coastal geomorphological, hydrodynamic and rock mechanical factors affecting heights of wave-cut benches. Field surveys showed that the ancient benches on the Pearl River Delta plain are >1 m higher than modern ones developed on coasts within and beyond the current Pearl River estuary firth. We calculated that the difference between ancient and modern mean high tide levels in the Pearl River Delta was no more than 0.3 m. Both of the differences between wave-cut bench heights induced by different wave heights and by differential coastal rock hardness were also no more than 0.3 m. Therefore, these are not influential factors regarding the differences in heights between ancient and current wave-cut benches. Tidal statistics indicate that 3 m-high storm surges occur at least every 50 years, but these cannot explain the high-level benches. The levels of old marine benches have ascended beyond the range of modern wave actions, and are likely the result of structural uplift or sea level fall.