粤东地区的河流阶地

The River Terraces in Eastern Guangdong

依据广东东部超过32条河流、55处河流阶地、至少25个14C、热释光的冲积层测龄数据等情况,可知:粤东最多有6级河流阶地,最大阶地高度70 m且靠近现代主河床分布。从上游向下游第一级阶地明显变形:龙川县黎咀镇上游为常态阶地,黎咀镇下游至博罗县园洲镇为半埋藏阶地,园洲镇下游进入东江三角洲为埋藏阶地。而且该冲积物时代有渐新趋势,反映晚更新世河源地区的构造抬升较早或速率较快,中下游地区构造逐渐稳定或下沉。粤东至少有12处较典型的剥夷面砾石层。其特怔:①分布海拔20～210 m,与当地剥蚀台地高度相似;②位于当地级别最高、分布面积大的河流阶地上;③石岩镇玉律、平陵镇、东坑镇、水唇镇、龙母镇张坊等剥夷面砾石层由昔日河流形成,其谷中谷成为今日小河(只有狭窄河床河漫滩,或许有第一级河流阶地),今昔河流发育相隔数十万年;④剥夷面有两个倾斜方向:流域内向河谷和向下游倾斜;总体上由南岭向南海倾斜。

55 terraces along 32 rivers in eastern Guangdong and 14C and TL ages from 25 samples arestudied in this paper. There are at most 6 stages of river terraces in the region, of which the highest one is 70 m in height, and is distributed near present-day main riverbed. The first stage terraces include normality terraces （in the upper part of Lijui Town in Longchuan）, semi-buried terraces （from Lijui toYuanzhou Town in Boluo）, and buried terraces （from the downstream of Yuanzhou to the Dongjiang Riv- er Delta）, with decreasing ages. This indicates that the tectonic lift occurred earlier or the lift rate wasfaster in the areas of river head and the tectonic activity was gradually stabilized or sunk in the areas of mid-downstream. There are at least 12 sites where typical denude-planation surface gravel layers were found. They show the following characteristics ： （ 1 ） They are distributed on the highest terraces with ele- vation of 20 ~ 200 m, coincident with local denudation platform. （2） They are located on terraces of thehighest rank with large area in the region. （3） The denude-planation surface gravel layers in Yulu of Shiyan Town, Pingling Town, Dongkeng Town, Shuichun Town, and Zhangfang Town are formed fromformer rivers. The valleys-in-valleys became today＇s little rivers with narrow riverbed and flood land, maybe with the first terraces, during the past hundreds of thousands years. （4） The denude-planation surfaces have two slope directions, towards the valley and the downstream in the basin, but generally towards the South China Sea.