珠江三角洲北部晚第四纪构造运动的新证据

New Evidences for Late Quaternary Tectonic Movement in the North Pearl River Delta

位于珠江三角洲北部的广州和佛山之间的西淋岗,近来发现了被抬升、掀斜及断裂切割的晚第四纪地层,经初步考察,地层由老到新分A、B、C三层:A层下为白色中细砂,向上过渡为灰黑色粘土,总厚约50 cm,其下花岗岩已深度网纹红土化;B层为灰黑～褐红色砂与粘土互层,总厚约4 m;C层为浅桔红色粗砂～棱角状砾石层,层理不发育。A、B、C三层总厚大于10 m,其中A、B二层均向南倾斜,前者倾角约30°;后者倾角4°左右,B层不整合地覆盖在A层之上,两者沉积间断清楚;C层则嵌入A、B层中。A、B、C三层构成顶面拔河约22 m基座阶堆积。若与区域网纹红土及临近的西、北江阶地对比,B层很可能属于中更新世晚期至晚更新世早期。B层明显被NNE向广从断裂东支断层以正断层方式错断53 cm,错断进入了C层。断层上方的氡气测量值高出背景值近3倍。西淋岗南面的珠江三角洲平原见全新世泥炭层(D层),厚约2 m,含大量水松古木,覆盖在棕红色～灰白色薄层砂层之上,再下为深度网纹红土化风化壳。上述现象不仅包含了珠江三角洲断裂活动的信息,也留下了新构造运动、海平面变化和古环境变化的记录,具有显著的理论意义和实际意义,值得进一步深入研究。

It was discovered recently that late Quaternary layers,located at the Xilin Hill between Guangzhou and Foshan cities in the North Pearl River Delta,were uplifted,tilted and faulted.The layers could be divided into A,B and C from old to young.Layer A is composed of grayish white sand and silt in lower part and brownish red silt in upper part with total thickness about 50cm,underlying on vermiculated laterite weathering granite.Layer B is composed of grayish and brownish interbeds sand and clay with total thickness of about 4m.And layer C is composed of poor bedded light brownish coarse sand and fine gravel.Both layer A and B were tilted southward but with different dip angles of 30°for former and 4°for latter,suggesting their angular unconformity relation.Layer C,however,interest into both layer A and layer B.All of three layers make up 22m high pedestal terrace.By comparing ages of other adjacent similar river terraces and vermiculated laterite weathering crust in southern China,age of layer B is supposed to be the late middle Pleistocene to early late Pleistocene.A NNE fault,eastern branch of the Guangzhou-Conghua Fault,offset layer B by 53cm in normal fault and cut into layer C.Radon measurement showed three times abnormal of background across the fault.On the south the Xilin Hill on the north in the Pearl River Delta plain,two meter Holocene black carbonaceous mud with sand intercalated,containing Chinese cypress old tree,and underlying interbeds sand and clay, cover on vermiculated laterite weathering granite.Thus,the strata above contain not only the information of faulting,but also the records of neotectonic movement,sea level changes and paleo -environmental changes in the North Pearl River Delta area,which is also of great significant in civil engineering construction,city planning,and geo-hazard prevention and worth studying further.