Characteristics Of Weathering And Ancient Weathered Crust Of The Red Bed Mudstone In Heifangtai Area

Heifangtai area is the important part of Meso-Cenozoic lanzhou-Minhe basin. Loess has an unconformable contact with overlying red bed mudstone. The special geological structure together with Yellow River irrigation causes the frequent occurrence of landslides in heifangtai area. Especially, the loess-red bed mudstone landslides closely related with the unconformity surfaces, usually have large scale and great harm, greatly restricting the local economy development. In this paper, based on the field investigation, taking two typical ancient weathered crusts of cretaceous red bed mudstone and one modern slope weathering profile as the research objects, we analyzed the change law of particle size, microstructure, major elements and mineral composition, analyzed the spherical weathering mechanism of red bed mudstone and explored the developmental regular pattern of ancient weathered crusts. The results show as follows.(1) Modern weathering of red bed mudstone slope mainly concentrates on the shallow surface (3-6cm), which is characterized by continuous, gradual destruction process and particles crushing and spalling. The weathering is closely related by material composition and microstructure of red bed mudstone. From the outside in, cracks decrease, crushing size increases, and the orientation of particles becomes poorer. Both apparent porosity and amount of pores reduce significantly, and macropore decreases while small pores increase. The contents of elements of Si, Al, Ca slightly decrease, while Fe increases. The relative content of illite increases, while the content of illite smectite mixed layer decreases.(2) Different test results in the ancient weathering crust show different laws. Structural morphology varies apparently. Howe ever, from top to bottom microstructure becomes denser. The shape of pores is nearly round and the directionality is very poor for both profiles. The count of pores and apparent porosity gradually decrease. In addition, the maximum apparent porosity value of sample J-04is6.8%and sample of FN-05is9.93%, respectively. Grain size analysis shows that the content of clay particles in samples of profile1is22.33%-26.93%and the values in the upper section is greater than the lower. On the contrary, for profile2, clay particle content is24.67%～36.42%and the values in the upper section are smaller than the lower. The contents of major elements and clay mineral show different laws in two profiles.(3) The vertical zoning of ancient weathered crust. The two profiles are along to the leaching and eluvial ancient weathered crusts under the condition of dry and hot paleoclimate, but they are not preserved completely. The ancient weathered crust of profile1contains two period weathering, denudation, local burial and re-weathering, and then the discontinuity is located at sampling point J-04. The ancient weathered crust of Profile2conforms to the positive evolution sequence of ancient weathered crust. However, under the intense leaching and dissolution, fine grained material migrating induces that particle size distribution presents upside down. The interface of relative strong and weak weathered red bed mudstone is located at sampling point FN-05or FN-06. The vertical zoning of ancient weathered crust presents that the layer within about30cm below the unconformity is the relative strong weathered zone; the layer from30cm to60cm is the relative weak weathered zone, while the wreathing of the layer excesses60cm is rather weaker.(4) The relationship between ancient weathered crust and landslide types. The main controlling factors of landslides are special stratigraphic structure and ground water. But because of nearly horizontal layers, the occurrence of unconformity has a restrict effect on landslides. While the structure and material characteristics of ancient weathered crust, to some content, control the location of shear outlet of landslides.