江汉平原东北缘末次冰消期沉积物粒度特征及环境意义

麻城剖面位于江汉平原东北缘山地丘陵与平原过渡带半封闭的宽浅碟形洼地,是江汉平原北缘末次冰消期代表性沉积剖面.沉积环境主要由山麓坡洪积相和沼泽相组成,形成年代为距今18.00～8.60 ka.麻城剖面粒度组成主要以粉砂为主,达60.73％,其中,粗粉砂达40.83％.剖面粒度变化可划分为7个变化阶段.结合剖面的岩性与沉积...

Grain Size Characteristics and Climatic Environmental Significance of Sediments in the Northeastern Margin of the Jianghan Plain During the Last Deglaciation Period

The Macheng section is a semi?closed wide， shallow saucer?shaped depression located in the transition area between the Dabie Mountains and the Jianghan Plain. It is a typical section of deposition during the last deglaciation at the northern margin of the Jianghan Plain. The depositional environment of the area is made up of torrential floodplains and swamps， with a formation age between 18.00 and 8.60 ka. The major component of sediments in the Macheng section is silt， which represents 60.73% of the grain size， including a significant 40.83% of coarse silt. The grain size variation of the section is vertically divided into seven stages of change. Judging by the grain size composition and variation of sediment， combined with lithological and depositional environment analysis， it was found that the climate change during the last deglaciation was in four stages: the first was 17.90?17.02 ka B.P.， after the last glacial maximum， when a warm climate returned with increased precipitation； the intensity of surface runoff at this stage was medium to strong. The second stage， from 17.02 to 12.02 ka B.P.， was a period of rapid warming， generally warm and humid， when swamps were being developed along the torrential plains along the northern margin of the Jianghan Plain. The variation of grain size data indicates an unstable climate， with intense precipitation during the earlier and later phases of this stage， causing erratic surface runoff. In the metaphase （14.47?12.64 ka B.P.） precipitation was significantly reduced， but with a balanced stability， and the surface runoff was also relatively stable. During the later phase， precipitation intensified again and surface runoff became stronger. This stage is the most significant warm?humid period in the area during the last deglaciation， which is consistent with the stalagmite record of the middle and lower reaches of the Yangtze River. The third climatic stage was 12.02?10.80 ka B.P.， recognized as layer ④ of the Macheng section （depth 1.65?1.10 m）， with an obvious sedimentation break with its underlayer⑤， indicating the Younger Dryas （YD） event， followed by rapid warming. The values of average grain size and sorting coefficient are the largest in the entire section during this stage， which also indicates an unstable climate， but with rich precipitation. The fourth climatic stage was 10.80?8.60 ka B.P.， which was warm and humid in general， with less surface runoffs than the previous stage， but with heavy precipitation during the early phase， becoming the least intensive of both these phases in the later phase. The records of two climatic events are present in the Macheng section sediment: （1） the Younger Dryas event， represented by the short break in swamp facies deposition between layer ④ and layer ⑤； and （2） drought conditions from 9.80 to 9.20 ka B.P.， evidenced by a 15 cm loess?like depositional layer in the section. This study has shown that the sediment in the depressions along the northern margin of the Jianghan Basin， which was not affected by flooding of the Yangtze and Hanjiang Rivers， is a good indicator of climatic conditions in the Jianghan Plain region.