京西地区寒武系凤山组地球化学特征及古环境意义

为重建京西地区芙蓉世古海洋环境演化、更好地理解沉积环境对碳酸盐岩沉积的影响，选取下苇甸剖面凤山组为研究对象，在沉积环境分析基础上，运用碳-氧同位素、主-微量元素和稀土元素等分析手段，探讨地球化学特征对沉积古环境的指示意义。下苇甸剖面凤山组主要发育泥岩、薄板状泥晶灰岩、薄板状泥晶灰岩与泥岩互层、灰泥丘、生物扰动藻黏结泥晶灰岩、砾屑灰岩、粉屑砂屑灰岩和白云质灰岩8种岩相类型，垂向上表现为多个向上变浅的米级旋回，构成了一个三级层序，其中海侵体系域以潮下低能带沉积为主，高位体系域以潮下高能带-潮间高能带藻席沉积为主。凤山组碳酸盐矿物含量以方解石为主，Al和Si等陆源碎屑元素含量较低。δ13C_car介于-0.71‰~1.51‰，反映沉积期海平面逐渐上升、末期短暂下降的变化趋势；Ce/Ce*和氧化还原敏感微量元素特征表明凤山期沉积水体以弱氧化性为主。

Geochemical Characteristics and Their Paleoenvironmental Significance for the Cambrian Fengshan Formation in the Western Beijing Area

For a better understanding of the environmental evolution in the Furongian Series and its influence on depositional systems, the present study analyzed the carbon and oxygen isotopes, the major and trace elements and the rare earth elements in the carbonates of the Fengshan Formation at the Xiaweidian section. Eight lithofacies were classified as shale, thinly-bedded lime mudstone, shale-thinly-bedded lime mudstone alteration, bioturbated boundstone, biohermal microbialites, calcisilitite, pebbly limestone conglomerate and dolomitic limestone, all occurring in high-frequency sedimentary cycles. The transgressive system tract was dominated by low-energy subtidal deposits; the highstand system tract was composed mainly of microbial biostromes. The elemental composition of micritic limestone was influenced weakly by detrital inputs and diagenesis for low contents of terrigenous Al₂O₃ and SiO₂. The δ13C_car values ranged from -0.71‰ to 1.51‰, suggesting a progressively rising sea level during the Fengshan period, ending with a lowering phase. The δ13C_car shared a distribution pattern with the relative sea-level curves obtained from the outcrop analysis, implying that global sea-level fluctuation determined the development of the third-order sequence. The seawater during the Fengshan period had a lowto moderate oxygen content because of the low enrichment factors for the redox-sensitive trace elements and the weak negative Ce anomaly(Ce/Ce* < 1). The lower and middle parts of the Fengshan Formation were deposited in more highly oxygenated seawater, but the seawater oxygen content was lower towards the end of the Cambrian.