摘要： 晚白垩世,东亚地区风成沉积分布广泛,具有显著的纬向分带特征,然而,与华南地区处于相同古纬度的青藏高原中部地区却鲜有风成沉积的报道。近年来,前人依据磁性地层学研究结果将藏东贡觉盆地原古近纪风成沉积时代重新厘定为白垩纪末期,这为研究该时期青藏高原隆升前的古地理和古气候条件创造了新的契机。通过野外剖面考察/实测、样品薄片偏光显微镜岩矿鉴定、扫描电子显微镜观察和能谱分析等,本研究对贡觉盆地贡觉组风成沉积序列、风成界面和沉积体系开展了详细研究。研究结果表明,贡觉盆地风成沉积组合主要位于贡觉组底部,集中分布在盆地西缘,主体由沙漠、冲积扇与河流沉积组成。其中,风成沉积以大型槽状层理、楔状层理和高角度板状交错层理为特征,层系中可见典型的颗粒流层、颗粒飘落层和风成沙波层,层面上可见风成波痕。风成砂岩中的碎屑颗粒以中、细砂为主,普遍表现出高结构成熟度和高成分成熟度特征,石英颗粒边缘具有“沙漠漆”结构。扫描电镜下,石英颗粒表面发育典型的新月形和碟形撞击坑,很好地指示了风成沉积成因。垂向上,风成沉积组合中显示出明显的下部风成与水成沉积交互、中部风成沉积主导、上部水成沉积增多的沉积序列特征,反映了干旱气候背景下突发性的降水/洪水引发地下水位的波动。认为贡觉盆地白垩纪末期风成沉积的形成演化与冈底斯山晚白垩世隆升形成的“雨影”效应密切相关,同时也与全球马斯特里赫特中期升温事件存在着一定联系。研究结果对于理解白垩纪末期青藏高原隆升前的古地理格局和古气候条件具有重要意义。

关键词: 贡觉盆地, 晚白垩世, 风成沉积, 古气候, 古地理

Abstract: During the Late Cretaceous period,aeolian deposits were widely distributed in East Asia,exhibiting significant zonal variation. However,there are few reports on aeolian deposits in the central region of the Tibet Plateau,which shares the same paleo-latitude as South China. In recent years,previous magnetic stratigraphy studies have redefined the Paleogene aeolian sedimentary age in the Gonjo Basin of eastern Tibet as Late Cretaceous. This finding presents a new opportunity to investigate the palaeogeographic and palaeoclimatic conditions prior to the uplift of the Tibet Plateau during this period. The aeolian sedimentary sequence,aeolian interface and sedimentary system of the Gonjo Formation were investigated in detail through field profile investigation,rock and mineral identification using a polarizing microscope,scanning electron microscopy and energy spectrum analysis. The findings indicate that the aeolian sedimentary association of the Gonjo Basin is primarily situated at the base of the Gonjo Formation,predominantly concentrated along the western margin of the basin,and is mainly composed of desert,alluvial fan and fluvial deposits. Aeolian sediments are characterized by large trough,wedge-shaped cross-bedding,and steep-angled plate cross-bedding. Additionally,typical features such as grainflow strata,grainfall laminea,wind-ripple strata and wind-ripple marks are observable within the layers. The clastic particles of the eolian sandstone are primarily medium to fine sand,exhibiting high structural maturity and compositional maturity. The edges of the quartz particles display a “desert varnish” structure. Under a scanning electron microscope,typical crescent and dish-shaped impact craters were observed on the surface of quartz grains,indicating the origin of aeolian deposition. Vertically,the aeolian sedimentary association demonstrates a clear interaction between lower eolian and hydrogenic deposits,with central aeolian deposits prevailing and an increase in upper hydrogenic deposits. This pattern suggests fluctuations in the groundwater level caused by sudden precipitation or flooding in an arid climate context. The formation and evolution of aeolian sediments at the end of the Cretaceous in the Gonjo Basin is closely linked to the “rain shadow” effect resulting from the Late Cretaceous uplift of the Gangdise Mountains,as well as to the global mid-Maastrichtian warming event. These findings are significant for understanding the palaeogeographic patterns and palaeoclimatic conditions preceding the uplift of the Tibetan Plateau at the end of the Cretaceous.

Key words: Gonjo Basin, Late Cretaceous, aeolian deposition, palaeoclimate, palaeogeography

中图分类号: 

• P512.2+1