衡阳盆地始新世含鱼地层的成因分析

Microstratigraphy of the fish-bearing Eocene lacustrine deposits from Hengyang Basin, Hunan

衡阳市郊五马归槽始新世鱼化石保存在黑色湖相纹层中。根据纹层序列的对比,发现该地的鱼化石均为同一层位;根据鱼化石在纹层序列中的具体位置及鱼化石的保存状况,笔者推测鱼群的死亡并非由于季节更替,而是由气候干旱引起的湖水盐度升高所控制;黑白纹层的变化是由大气降雨所致。

As the Green River Fr. in North America, we also have quite a lot of so called oil shale which are proved to be varved lacustrine dolomite in China. In Hengyang Basin in South China, the Eocene lake deposits can be subdivided into two series: the red rock series and the yellowish gray rock series, the former consists of sandstone and silt which represent the early deposits of the lake; the latter are mainly dolomite varves and gypsum alternations which are the result of brackish or salt lake. Usually we have two kinds of varves: light varve and dark varve. One light varve and one neighbouring dark varve are regarded as one varve couplet. Varve couplet is the basic unit of varve sequence. If light varves dominate the sequence, light varve bands will be formed; reversely, dark varve bands will be formed. Light varves and dark varves represent different climates respectively, so varves and varve bands are indicative of paleoenvironments. In thin sections, the thickness of varves varies from 0.05mm to 1mm. The analysis of the samples with probe electronic microscope shows that light varves are mainly composed of magnesium and calcium, and including potassium, sodium and iron; dark varves are organic rich and only silicon and phosphorus have been recognized. Through line scanning analysis with PEM, it was found that all elements change with the alternating between light varves and dark varves. Under polarized microscope, it was found that only in the dark varves existed the continental clastic materials, such as sand grains and clay. Fish fossil is the only one found in varves in Hengyang Basin, and only in the dark varves could the fossils be found. With the microstratigraphic correlation, it was confirmed that the two fish-bearing beds separated by 20m, are exactly the same laminae. In conclusion, during Eocene, Hengyang Basin is a pulse salt lake modified by rainfall, because it's only rainfall that can change lake deposits greatly without disturbing the tectonic feature of the lake.