对合肥地区“下蜀土”时代、形成环境的探讨

PRELIMINARY STUDY ON THE TIME AND FORMING SETTING OF THE XIASHU SOIL, HEFEI

合肥GK061孔剖面的"下蜀土"共有24层,主要是一套青黄色重粘土与浅棕黄色粉质轻粘土重复出现的粘性土。GK061孔经古地磁测定布容/松山界限在13.87米,热释光测年为18.2万年。1.00-13.87米为中更新世地层,地层名称沿用戚家矶组和下蜀组,13.87-31.88米为早更新世地层,建议命名为合肥组（Q1 h）。从"下蜀土"的粒度组成、矿物成分、化学组分推测其形成时的古气候是早更新世时偏温湿,逐渐向干而温和方向发展,及至晚更新世以来为半干旱环境,其中又有过几次冷暖干湿的波动变化,故而有若干层古土壤形成.从合肥地区"下蜀土"的粒度、化学成分所反映的特征来看,其母质与西北黄土有某种物源上的联系.合肥及江淮丘陵地区普遍分布着厚从几米到十几米的灰黄、黄褐色亚粘土,长期来被称作"下蜀粘土",并往往与南京等邻区作对比,时代定为晚更新世。近年来对合肥下蜀土的工作获得一些成果。对其地层时代、形成环境有一些新的认识,提出来和大家讨论。

Hefei GK061 borehole transection of the Xiashu soil has 24 beds which are dominated by a series of repeadly occurring blue-yellow heavy clay and brownish-yellow light silty clay. GK061 borehole through paleomagnetic measurements indicates the boundary between the Burong and Songshan Forma- tions at a depth of 13. 87 metres, dated 182 thousand years by means of heat light release. The mid- Pleistocene strata include the formerly named Qijiaji and Xiashu Formations in depth range of 1. 00 to 13. 87 metres. The early Pleistocene part from 13. 87 to 31. 88 metres is suggested being named the Hefei Formation(Q_1h). It is postulated from the grain size, mineral and chemical compositions of the Xiashu soil that the paleoclimate was slightly warm and moist at the early Pleistocene stage, evolving toward dry and warm, and formed a semi-drought setting from the late Pleistocene on. During this period there existed several times of climate flunctuations which resulted. in an occurrence of some paleosoil beds. The well- shown characteristics of the grain size and chemical composition of the Xiashu soil in Hefei area implies that its parent material has something to do with that of the Northwest Yellow Soil Highland of China.