西宁盆地晚始新世石膏成因研究

Petrogenesis of late Eocene gypsum in Xining Basin

西宁盆地保存了一套晚始新世(大约42～34 Ma)沉积的以红色泥岩和石膏为主交互产出的地层,其中石膏是该地重要的非金属矿产。石膏和红色泥岩具有明显的旋回特征,剖面下部和上部旋回厚度分别为约1m和2～5m,在盆地内具有良好的空间可对比性。这些旋回所跨越的时间与黄赤交角周期(4.1万年)和/或地球轨道周期(10万年)一致。在西宁盆地存在三种不同形态特征的石膏:微晶厚层状石膏、紧密堆积薄片状石膏和粗晶薄层状石膏。晚始新世西宁盆地气候干旱,水的补给量是西宁盆地石膏沉积的控制性因素。晚始新世地球黄赤交角和/或轨道周期性变化驱动了盆地内相对干旱和相对湿润环境的交互出现,相对湿润的气候为石膏沉积提供了有利条件,由水的补给量控制的潜水面高低导致了三种形态石膏层的形成。

A series of late Eocene（about 42-34 Ma） sedimentary succession is conserved in Xining Basin,which consists of basic cyclicity of alternating red mudstones and gypsum,and the gypsum in the succession is an important nonmetallic deposits.The thicknesses of the lower and upper cyclicity in outcrops are 1 m and 2-5 m,respectively,which could be well compared in space.The span of time for each cyclicity is concordant with obliquity cycle（41 kyr） and/or eccentricity cycle（100 kyr）.Three kinds of gypsum beds,massive fine-grained gypsum beds,densely-packed laminated gypsum beds and the beds of prismatic gypsum crystals,are preserved in Xining Basin.Aridity was developed in Xining Basin during late Eocene,and the controlling factor for the gypsum formation was water availability.Astronomical factors（obliquity cycle and/or eccentricity cycle） forced the oscillations of drier and wetter conditions in Xining Basin,and wetter condition provided advantage for the late Eocene gypsum deposition.The depth of ground water level controlled by the availability of water led to the formation of three gypsum facies.