利用滩坝砂体规模研究古风力的定量恢复*

古风力是一项重要的古气候指标,其定量恢复是一个难题。风作用于水体产生的波浪大小间接地反映了风力,能够为古风力的恢复提供思路。发育于破浪带和冲浪回流带的破浪沙坝、沿岸沙坝分别记录了破浪和冲浪过程,作者分别介绍利用古湖泊中发育的破浪沙坝和沿岸沙坝进行古波况和古风力恢复的原理和操作流程。(1)根据破浪沙坝的几何形态,可以将其厚度与破浪水深建立函数关系,而破浪水深又由破浪波高决定,因此破浪沙坝厚度可以恢复破浪波高,据此可以进一步根据波浪统计关系恢复有效波高、根据风浪关系恢复风力。此方法依托以下3个参数: 单期次的破浪沙坝厚度、破浪沙坝的基座坡角、古风程。(2)沿岸沙坝厚度近似记录了冲浪的极限高度,后者受控于有效波高,据此也可以恢复有效波高和风力。此方法依托以下5个参数: 单期次的沿岸沙坝厚度、古(平均)水深、古风程、古风向相对于岸线的入射角、组成沿岸沙坝的沉积物粒度。上述2种方法综合性较强,涉及古风向、古地形坡度、风程或盆地直径、古水深等参数的恢复,需要综合运用古地貌恢复、去压实校正、古岸线识别、古水深恢复等技术,并需要结合波浪理论。古湖泊滨岸带地层中保存有大量的滩坝沉积,利用其恢复古波况和古风况具有一定的应用前景,能够有助于更详细地重建沉积盆地的古地理背景。

Determination of palaeowind strength by using scale of beach-bar sand bodies

The strength of palaeowind,which is an important indicator for palaeoclimate,is difficult to be quantitatively reconstructed.The scale of wind-generated waves reflects wind strength,which may become a new idea for the reconstruction of palaeowind strength. The longshore bar and coastal bar that formed in the swash zone and surf backflow zone,can record wave breaking and swash processes,respectively. In this paper,the principal and workflow for reconstruction of palaeowave and palaeowind conditions by using longshore bar and coastal bar in ancient lakes were introduced. Two methods are used in this paper: (1)There is a functional relationship between the thickness of longshore bar and the water depth of breaking waves(i.e.,breaking depth)according to geometrical analysis. The breaking depth,in turn,is determined by breaking wave height. Thus the breaking wave height can be reconstructed by using ancient longshore bar thickness. The significant wave height and palaeowind strength can also be calculated according to empirical wind-wave relationship. Three parameters are required in this method: The thickness of the isolated longshore bar,the slope angle of the basement where the longshore bar sits,and the palaeo-fetch. (2)The thickness of coastal bar records the swash height,which is determined by significant wave height. The palaeowind strength can also be reconstructed according to empirical wind-wave relationship. Five parameters are required in the latter method: The original thickness of the isolated coastal bar,the average depth of the paleowater,the palaeofetch,the angle between the palaeowind direction and the normal to the shoreline,and the particle size. All these two integrated methods,which include the reconstruction of palaeowind direction,palaeoslope,palaeo-fetch,palaeo-water depth etc.,should apply techniques such as palaeogeomorphy,decompaction,location of the palaeoshoreline,reconstruction of palaeowater depth and the wave theory. Many beach-bar deposits are preserved in ancient lacustrine strata,which can be applied in the reconstruction of ancient wave regime and ancient wind regime. These can also be helpful in reconstruction of the palaeogeographic setting of the sedimentary basin.