海洋地质与第四纪地质

<正> 雷州半岛和海南岛北部新生代以来是一沉陷区,第四系发育较完整,从老到新有早更新世湛江组、中更新世北海组、晚更新世八所组、全新世早期石岛组、全新世中期灯楼角组、全新世晚期鹿回头组及烟墩组.第四纪以来曾有四期火山岩:早更新世湛江火山岩、早更新世晚期石峁岭火山岩、晚更新世湖光岩火山岩和全新世雷虎岭火山岩.现仅就第四系中的北海组进行论述.本文是我队共同劳动成果,由薛万俊同志执笔,文中谬误之处,请批评指正.一、北海组的岩性北海组主要分布于雷州半岛北部、广西北海地区及海南岛沿岸.前人最早在北海市发现该地层,故称“北海组”.北海组呈棕红、棕黄色,有的地方可见到网纹,按岩性分为两层,上层为粘土质砂,下层为砂砾.各地岩性略有差异,详见以下剖面.

Quarternary strata are well-developed in coastal areas of the northern part of the South China Sea, particularly in the Leizhou-Hainan area and several big river estuaries. From old to new, the Quaternary strata include the upper early Pleistocene Zhanjiang Formation, middle Pleistocene Beihai Formation, late Pleistocene Basuo Formation, early Holocene Shidao Formation, middle Holocene Dengloujiao Formation, late Holocene Luhuitou Formation and Yandun Formation, as well as four stages of volcanic rocks, i. e. Zhanjiang stage of early Pleistocene, Shimaoling stage of late early Pleistocene, Huguangyan stage of late Pleistocene and Leihuling stage of Holocene. Taking the Beihai Formation as an example, the author introduces briefly the results of the study on its geologie age and sedimentary environment as follows. I. THE DISTRIBUTION AND LITHOLOGIC CHARACTERS OF THE BEIHAI FORMATION The Beihai Formation mainly occurs in the northern Leizhou Peninsula, the Beihai area of the Guangxi Autonomous Region as well as the coastal areas of the Hainan Island. This formation was firstly found in the Beihai City and in this reason was named as the Beihai Formation. The lithologic characteristics of the Beihai Formation is extremely outstan-ding, characterized by that the upper layer is composed of brownish red and dark red clayey sands and the lower layer composed of sands and gravels. The sorting of the clayey sands is very poor while that of the sands and gravels is medium. Clastic heavy minerals are mainly stable ones, such as zircon, tourmaline, rutile, ilmenite, etc. Few unstable minerals are found. Quartz, feldspar, debris and some tekite dominate the light minearls. Kaolinite, a little illite and diasporite are the main clay minerals. The thickness of the Beihai Formation is about 20m with the maximum near 80 m. II. BASES FOR ASCERTAINING THE GEOLOGIC TIME OF THE BEIHAI FORMATION Being situated between the Zhanjiang and Basuo Formations, to ascertain the geologie time of the Beihai Formation must be of great significance for the determination of the geologie time of its upper and lower strata. A great amount of sporo-pollen fossils were found in the Beihai Formation. Of the sporo-pollen assemblages, the xylophyta pollen comprises Cycadaceae, Palmaceae, Annonaceae, Proteaceae, Sapindaceae, Cinnamomum, Moraceae and Asclepiadaceae. And Gramineae dominates the herbaceous pollen. The spores comprises Cyatheaceae and Glaicheniaceae. In the lower part of several sections of the Beihai Formation along the Guangxi coast, the doninance of the xylophyta indicates that there were tropical forest vegetation landscape and hot and humid climate at that time. In the upper part of the sections herbaceous pollen is dominant, reflecting the tree-rare steppe environment and the hot and dry climate. The fluctuation of the climate in this area was affected by the global climatic variations in middle Pleistocene. 1.Dating of the Beihai Formation Thermoluminescence determination and fission-track dating were carried out, especially on the tektite, for the Beihai Formation. It is determined that the Beihai Formation is approximately 700,000 years old. 2.Paleomagnetic determination A paleomagnetic determination was made on the Meichaipo Section in the northern Hainan Island. The lower part of this section is composed of the Zhanjiang Formation, which shows the negative magnetization, whereas the upper part is the Beihai Formation, which shows the positive magnetization. The lower part should be of Matuyama magnetic reversal epoch, the upper part is of Brunhes normal magnetic epoch. The age of 730,000 years was taken as the time boundary between them and the bottom of the Beihai Formation is just on this boundary. Data from various sources correspond well to each other. III.SEDIMENTARY ENVIRONMENT OF THE BEIHAI FORMATION The sedimentary environment of the Beihai Formation is an unresolved subject which has long been argued by geologists. Much work has been done in recent years on the sedimentation process and the environment of t