海南岛神尖灯塔剖面海相更新世地层与沉积环境

PLEISTOCENE MARINE STRATA AT SHENJIAN LIGHTHOUSE PROFILE OF HAINAN ISLAND AND THE SEDIMENTARY ENVIRONMENTS

海南岛西北岸神尖灯塔附近的海蚀崖 ,出露了一套厚逾 40m、富含海相生物化石的更新世地层 ,为除台湾省外的我国沿海和岛屿所仅见。该剖面的古地磁测量结果显示出布容正极性时和松山反向极性时及其中的加拉米洛和奥杜威事件 ,其时代跨越了约 2 5 0～0 2 6MaB .P .的早、中更新世。这一结论得到了电子自旋共振测年结果和浮游有孔虫组合的验证。而14 C年龄测定结果 ( 2 80 0 0～ 2 30 0 0aB .P .间 )则因贝壳和碳酸盐样品经过淡水交换和重结晶作用而偏新。沉积学研究表明 ,这套地层由 8个水下和水上的沉积旋回所组成 ,反映了该地区在早、中更新世时期至少发生了 8次海平面的升降变化。由于这套地层已受到明显的变位 ,呈和缓的穹隆状 ,且多处有两组正断层发育 。

Northwest Hainan Island is a transitional area between continental South China and marine South China Sea in Quaternary, characteristic of changes in neotectonics, climate and environments. Marine sedimentary outcrops of the Pleistocene strata are less exposed in China and Asian Pacific areas, and being absent in systematic studies in synthesis of stratigraphy, geobiology and radioactive isotope chronology. A 40m thick Shenjian lighthouse profile from Northwest Hainan Island provided an entire marine sedimentary section of the Early Middle Pleistocene, with marked stratigraphic records, abundant marine fossils and terrestrial pollen, and strong neotectonic events of volcanic eruptions. Palaeomagnetic reversals of the B/M and the events of the Jaramillo and the Olduvai can be found in the profile, which showed that the strata were deposited during ca 2.50～ 0.26MaB.P. This chronology is consistent with ESR dating (0.3±0.06 MaB.P. at the 12th unit, and greater than 1.0 MaB.P. at the top of the 4 unit in the profile) and ages of the planktonic foraminifer assemblages ( Globorotalia exlis, G. fohsi, G. plesiotumida, Globigerina ampliapertura and Globorotalia miocenica ). Results of identifying the carbonate structures and the stable oxygen and carbon isotopes showed that carbonate in the strata had been leached by freshwater and has recrystallized possibly due to the neotectonic uplift in the region, leading to the radiocarbon dates (concentrated between 28 000～23 000aB.P.) too young. Studies in sedimentology show that the profile contains submarine and on marine characteristic sedimentary formations in eight cycles, suggesting that there were at least eight veridical changes in sea levels. A series of strong neotectonic activities including two sets of uplift faults led to a change in marine environments into land conditions. The Shenjian lighthouse profile can provide a good sequence for a type section of marine strata in the Pleistocene and be a key site of China to compare with Quaternary marine strata of the world.