粤西河流阶地的分布与特征

The Distribution and Characteristics of the River Terraces in the West of Guangdong Province

依据广东西部超过18条河流、39处河流阶地、至少35个14C、热释光的冲积层测龄数据等情况,可知粤西最多有4级河流阶地(不包括湛江组和北海组阶地);多数河流最高阶地靠近现代主河床分布,深切曲流中有河流阶地,反映近数十万年来河床改道不大;西江在封开有广东高度最高(76 m)的河流阶地,而广东高度最高的地下河阶地是111 m,大致显示出地面与地下剥蚀强度的差异;通常山区河流长度越大,河漫滩和河流阶地的高度越大,河流阶地的级数和级别也常增加;在河流上游的下段和中游的上段,河流阶地的高度最大且级数也最多;在晚更新世之前与后,河谷中下游地区的新构造运动趋势由上升变为稳定或沉降;连滩盆地是广东全新世构造沉降最典型的山间盆地。

Over 39 terraces along 18 rivers in west Guangdong are studied and at least 35 data of the ^14C and TL ages of the alluvial layer are analyzed. It is believed that there are at most 4 stages of river terraces （ except the Zhanjiang Formation and Beihai Formation terraces） in the west of Guangdong. The alluvial layer of flood land belongs to Holocene Series. Mostly the first stage terrace is accumulation terrace, of which the upstream segment is the normality terrace that can not be flooded, the alluvial layer is mainly Pleistocene Series, there is possibly deposit of late Middle Pleistocene Series at the bottom; the mid-downstream segment is semi-buried terrace that can be flooded, the alluvial layer is similar to that of normality terrace, but its superstratum is covered intermittently by thin layer of Holocene Series, the height is equivalent to that of the flood land formed in Holocene Epoch, the terrace and the flood land are adjacent and form together a broad alluvial plain; and the delta segment near the sea is a buried terrace covered by the delta facies of Holocene Series. The alluvial layer of the second stage terrace is mainly Middle Pleistocene Series, in which the minority part is covered by the early phase deposit of Upper Pleistocene Series, mostly it is a bedrock-seat terrace, but there is also hidden bedrock-seat terrace. The alluvial layers of the third and the fourth stage terraces are Middle Pleistocene Series, they both are bedrock seat terraces, but alluvial layers of the fourth stage terrace at Fengkai belong inferentially to last phase of Early Pleistocene Epoch to early phase of Middle Pleistocene Epoch. Most of the highest river terraces are distributed near recent main riverbeds and there are river terraces at the incised meanders, that reflects the riverbed has not changed its course obviously in last hundreds thousands years. Before and after Late Pleistocene Epoch, the trend of New Geotectonic Movement in the midstream-downstream areas of the valley was first rise and then stability or subsidence.