【作者】 杨柯；

【导师】 万晓樵；

【作者基本信息】 中国地质大学（北京） ， 海洋地质， 2006， 硕士

【摘要】 西藏南部岗巴盆地位于位于特提斯构造域的东端,发育有完整的晚白垩世至古近纪地层,地层中化石丰富,前人根据化石研究准确地进行了地层时代的划分。认为该区白垩纪/古近纪界线位于宗山组和基堵拉组之间,以底栖大有孔虫Orbitoides-Omphalocyclus动物群的消亡和Rotalia-Smoutina-Lockhartia动物群的始现为标志。界线上下岩层为平行不整合接触,期间有一短暂的暴露面。古新世的砂砾岩直接覆于晚白垩世的陆棚碳酸岩盐沉积之上,沉积的重大转变代表一次区域构造运动。西藏南部晚白垩世的海平面从坎潘晚期到丹尼早期是逐渐降低的,生物消亡特点显示出与海平面变化的一致性,可能原因主要受海平面变化的影响。锶同位素曲线与全球温度变化和全球海平面变化有很好的对应关系。在第22层位上,锶同位素曲线出现一处明显的负偏波谷,⁸⁷Sr/⁸⁶Sr比值达到了0.707605,此时全球温度也出现下降,对应坎潘期印度洋脊南北向迅速扩张,全球海平面上升,大规模海泛导致盆地中陆地剥蚀物减少。其余各点均表现出明显的正偏升高趋势,全球温度也表现为一个小的升高波动,对应马斯特里赫特晚期和丹尼初期,印度洋脊扩张速度减慢,海平面下降,印度板块北缘的陆壳及洋壳向下俯冲抬升遭受风化剥蚀,大规模的造山运动导致陆壳物质的输入增多。磁化率受古代海平面变化的控制。特别当陆源物质供应低时,磁化率曲线变化和海平面变化相一致,磁化率主要由自生矿物的种类决定。如果陆源物质供应高时,平均磁化率的值也会相应增高,但是与海平面变化还是有关联。海平面下降会导致陆源物质含量的增加,并且磁化率也会升高。西藏岗巴盆地质量磁化率高的层位颜色较深,大都为铁质胶结,含石英和岩屑较多,颗粒较粗,多为砂岩。可以看出其生成条件是氧化环境,陆源输入多,搬运距离近,沉积在高能环境里。白垩纪/古近纪界线由下往上岩性变化从灰岩到砂岩,可以推测出海水是由深变浅的。当时印度板块北移碰撞,导致藏南岗巴盆地局部隆升,造成海退。根据岩石薄片鉴别出的生物组合及岩性特征,认为白垩纪/古近纪界线时期,岗巴盆地海洋环境是处于温暖的热带、亚热带浅海陆棚相-开放循环环境中,分为碳酸盐缓坡相和碳酸盐浅滩相。更多还原

【Abstract】 The Gamba basin of southern Tibet, is located in the east of Tethys tectonic region, whereexposes complete strata from late Cretaceous to Paleogene. The strata yield abundantfossils, and accurate biostratigraphic works have been done by previous studies. TheCretaceous/Paleogene boundary was recognized by the extinction of larger benthicforaminifora Orbitoides-Omphalocyclus fauna and the first appearance ofRotalia-Smoutina-Lockhartia fauna. The boundary is between the Zongshan and Jibulaformations. Both formations are disconformity. There is a short time exposing facebetween them. At the contact, the Paleocene clastic rock of Zongshan Formation overliesdirectly on the latest Cretaceous shelf carbonate rock. It indicates a regional tectonicprocess. The boundary extinction is mainly influenced by sea level changes in southernTibet. The regression occurred from late Campanian of Cretaceous to early Danian ofPaleogene. Sr isotopic curve is well correlated with global temperature and sea levelchanges. An apparent negative excursion at bed 22 is correlated with the globaltemperature decline. The ⁸⁷Sr/⁸⁶Sr ratio of 0.707605 can be related to the tectonic processand sea level rising resulting from the rapid Indian Ocean ridge expansion in Campanian.Decrease of detrital weathering materials is resulted from ingression. Other horizonsshow apparent positive excursions, which are correlated with the global temperature shift,we relate to the sea level regression resulting from the Indian Ocean ridge expansion inlate Masstrichtian and early Danian stages. Continental crust and ocean crust at northmargin of Indian Plate subducted and caused continental uplift. Increase of detritalweathering materials is resulted from orogeny activity. Magnetic susceptibility variationis controlled by ancient sea level changes. It is particular if the supply of detrital materialwas low, when the magnetic susceptibility was mainly controlled by the content ofauthigenic minerals. If the supply of detrital materials was high, the average susceptibilityvalue would be higher, but there is still a correlation between susceptibility and sea levelcurves. A sea level depress would lead to an increase in the content of the detritalmaterials, and therefore to an increase in the susceptibility values. Beds with highermagnetic susceptibility are darker in color in the Gamba basin of southern Tibet. Most ofthem are ferruginous cement, more quarts and rock fragments, fine grain sandstone. It isconcluded that they occurred in oxidation environment, more detrital materials, longdistance transport and deposited in high-energy environment. There are carbonate rocksbelow the Cretaceous/Paleogene boundary and sandstone above the boundary. It isconcluded that the sea level was dropping down. The Gamba basin uplifted partly andseawater withdraw were the result of continental collision when Indian plate moved tonorth. According to the fossil assemblages and lithology, it is suggested that, during theCretaceous/Paleogene boundary time, Gamba basin was in a warm tropical, subtropicalshallow shelf-circulating environment. It can be recognized as carbonate gentle slope andcarbonate shallow bank facies.更多还原