有孔虫壳内充填物的意义

FILLINGS INSIDE FORAMINIFERAL TESTS AND THEIR EARLY DIAGENESIS

南海陆架第四纪表层样及新第三纪钻井样中的有孔虫壳常被海绿石、黄铁矿及碳酸盐矿物充填,说明有孔虫壳为早期成岩阶段自生矿物的形成提供了良好的微环境。根据有孔虫壳内矿物的种类、多少及结晶程度,可以判断早期成岩环境及成岩阶段。这些充填矿物的形成对有孔虫壳体有不同程度的破坏,从而不利于化石保存。

Many foraminiferal tests in sediments accumulated before Holocene were filled with sed-iments or mineral crystals. Glauconite,pyrite and calcite are the most common minerals inforaminiferal tests. The infilled sediments and minerals can provide evidence of early diagene-sis. On the other hand,chamber infillings also influence the preservation of foraminiferaltests. 1. GLAUCONITE In relict sediments of the outer shelf and upper slope of South China Sea,almost half thenumber of tests are filled with glauconite,usually black and dark green in colour. Colours ofglauconite are very good indicators of paleoenvironments and maturity of the mineral itself;black glauconite only appears in relict and pre--Holocene sediments. Nevertheless,the ques-tion of glauconite colour is still left open,and we are not quite sure whether the colour is in-dicative of chemical composition,maturity or redox potential (Eh)and acidity(pH)of the envi-ronment in which glauconite once formed. The origin of glauconite is also a complicated sub-ject. Based on the fact that glauconite is more frequent in foraminiferal tests than in sandsand most of the tests are preserved with organic lining, we propose that the formation ofglauconite should have undergone two processes: infilling of very fine clay minerals beforeburial and glauconitization during diagenesis. The organic lining and the microenvironmentinside foraminiferal tests are the most favourable conditions; besides,the supply of very fine-grained clay is also very important. 2. PYRITE Pyrite is one of the most common minerals filled in the cavities of animal skeleton bothin surface sediments and in ancient strata. Its formation is completely controlled by the con-tent of oxygen and organic matter. Three types of pyrite can be recognized inside foraminifer-al tests, namely, steinkern, framboidal and euhedral types; the last one only appears in thelower part of the sediments. As established in this study,pyrite infilled foraminiferal tests never appear in relict sedi-ments, but are very abundant in areas with a large,steady supply of sediments,such as theLedong Well in which 30% of the tests are infilled with pyrite. The sedimentary environ-ments of pyrite formation are very favourable to the preservation of foraminiferal tests. Meanwhile,the pyrite infillings in the tests are also a potential factor to destroy the calcare-ous tests when they are oxidized,which has been proved in laboratory. 3. CARBONATES Carbonate overgrowth on the inner chamber wall (or chamber infilling of carbonate)isthe result of early diagenesis; thus its appearance is only related to the degree of lithification.In the South China Sea, crystallization of carbonates inside foraminiferal tests can be found insediments accumulated before Pleistocene (except the Quaternary beach rock). The cementof carbonates has been formed at the expense of small,fragile foraminiferal tests and /or cal-careous nannofossils. In the Ledong Well,the percentage of the tests with carbonate over-growth or infillings increases gradually with depth. On the other hand,the size--frequency dis-tribution changes with increasing depth, and the size of foraminiferal tests tends to becomemore uniform in the lower part of the core,that is to say,the smaller tests have dissolved asa result of pressure solution. Carbonate overgrowth or infilling also can destroy the originaltests as glauconite does.