云南思茅盆地景谷地区下白垩统曼岗组风成砂岩沉积学特征及其古气候意义

Sedimentology Characteristics of Eolian Sandstones in the Lower Cretaceous Mangang Formation in the Jinggu Area,Simao Basin,Yunnan and the Paleoclimate Significance

本文通过野外剖面的观察和实测,室内薄片鉴定、阴极发光分析、粒度分析、扫描电镜及能谱测试分析,研究了云南思茅盆地景谷地区下白垩统曼岗组风成砂岩沉积学特征,详细了解了风成砂岩的发育特征,以期为研究区白垩纪古气候研究提供新的资料,为我国钾盐勘探提供一定的地质依据和新的思路。研究结果表明:1研究区曼岗组主要出露一套中—细粒石英砂岩、岩屑石英砂岩,发育中—大型交错层理、平行层理,碎屑颗粒分选中等-好,以次圆状-圆状为主,孔隙不发育,杂基含量少;2曼岗组中段发育具平行层理的石英砂岩,粒度分布呈双众数,带状分布,石英颗粒为浑圆状,颗粒表面见Fe、Mn质薄膜,颗粒表面具明显的碟形坑等特征,属于典型的风成砂岩,初步判定其沉积环境为风成沙丘和丘间沉积;3景谷地区下白垩统曼岗组风成砂岩,整体表现为干热型气候,曼岗组中段为丘间沉积,曼岗组上段和下段为沙丘沉积。其在搬运沉积成岩过程中经历了强烈的物理、化学风化作用,整体表现为氧化环境,风力较大,风速稳定,持续时间长。此外,可能还经历了一段时期的湿润性气候环境,这可能与大气降水、大气环流带的迁移,以及白垩纪海侵作用等有关。

This paper,based on observing and measuring the field section,the thin section analysis,cathodoluminescence analysis,particle size analysis,scanning electron microscope analysis,andenergy spectrum analysis,has studied sedimentology characteristics of eolian sandstones in the Lower Cretaceous Mangang Formation,in the Jinggu area,Simao Basin, Yunnan,and then,and the paleoclimate significance of eolian sandstones of this area.This paper has summarized the development characteristics of eolian sandstones.The purpose of this article is to provide some base dates for further studies of the paleoclimate significance of eolian sandstones.And meanwhile,it may be open up a new way of thinking for explorations of potash resources.It shows as follows：（1） The lithology of eolian sandstones of this area is mainly middle-fine lithic quartz sandstone and quartz sandstones.In this area,it developed middle-large scales cross-bedding,parallel bedding.The sorting of grains is medium to well,and the psephicity is subround to round,and the pores do not develop,and the content of matrix is low.（2） The grains of quartz sandstones,which shows parallel bedding in the middle member of the Mangang Formation,is perfectly round and has two modal scores.There are some features of the particle surface,such as dish pits,which is the typical chareacteristics of eolian sandstones.According to this,it can be speculated that the sedimentary environment was eolian sand dunes and eolian indunes. （3） During the process of the transportation,the sedimentation and the diagenesis,it underwent a setting of dry-warm climate in in the Mangang Formation,Lower Cretaceous,in Jinggu area.The sedimentary environment of the middle member of the Mangang Formation was the eolian interdune deposition, while the sedimentary environment of the upper member and the lower member of the Mangang Formation were the eolian dune deposition.The formation of eolian sandstones experienced long physical weathering and chemical weathering.During those processes,it had been in oxidation environments,and the wind strength was strong as a stable speed.In addition,it probably experienced a period of time of humid climate environment.And it was probably related to meteoric waters,the migration of atmospheric circulation zones and the progress of transgression during the Cretaceous.