西秦岭岷县地区红层砂岩成因分析

Genesis of red-bed sandstones in the Minxian region, western Qinling Mountains

关键词:  西江盆地  SEM  粒度  石英颗粒表面特征

Key words:  Xijiang Basin  SEM  grain size  surface feature of quartz grains

摘要:

岷县西江盆地红层地层为上白垩统磨沟组,这套地层底部为一套紫红色厚块状砾岩,砾岩上部为厚度巨大的红层砂岩。本文试图通过对这套砂岩的研究,揭示其成因背景,对恢复晚白垩世西秦岭岷县地区古地理具有重要意义。根据岷县地区红层砂岩良好出露剖面测绘和取样资料,从沉积序列特征、粒度特征、石英颗粒表面特征方面对其成因进行分析。研究表明,这套砂岩厚度大,岩层中发育薄砾石层与干河床沉积物夹层,薄砾石层中砾石具沙漠漆等特征。粒径相对集中,缺乏粉砂和粘土组分,以中砂、细砂、极细砂组分为主。石英颗粒磨圆度好,表面发育碟形和新月形坑、溶蚀坑、硅质薄膜等典型特征。综合各种特征,认为这套砂岩为干热气候环境下风沙活动的产物。

Abstract:

The red-bed strata represented by the Upper Cretaceous Mogou Formation in the Xijiang Basin, Minxian are made up of purplish red thick massive conglomerates at the base, followed upwards by red-bed sandstones with tremendous thickness. The genetenic analysis based on depositional sequences, grain sizes and surface features of quartz grains shows that this succession of sandstones has tremendous thickness interbedded with dry channel deposits and thin gravel beds in which the gravels have the features of desert varnish. The sandstones are well sorted, and dominated by medium-grained, fine-grained and very fine-grained sands without clay and silt. The quartz grains are well-rounded, and have well-developed wind scoop, trittkarren, morel basin and siliceous films. All the above-mentioned features suggest that the red-bed sandstones in the study area should be generated from the eolian deposition in the xerothermic conditions.