Genesis of Deep Weathering Crusts and Morphogeny in Neogene and Quaternary Periods

(II) Deterioration of Rocks and Morphogeny

Abstract

Weathering crusts of various sedimentary rocks are described and the relation between geomorphology and deterioration of rocks are discussed in part II.
In folded zones, which are composed mainly of deformed sandstone, slate, chert and schalstein, weathering crusts are made up of two layers : upper leached zone and lower precipitate zone.
In the upper layer, leaching is predominant and the top of crusts are truncated by aeolian surfaces. Because a sandstone mainly composed of clastic quartz and feldspar, the reaction to weathering is similar to granitic rocks. Halloysite is found as replaced products of feldspar in sandstone, and the density of weathered regolith attains often 1.2g/cm³.
Black slates, consiting of illite, chlorite and noncrystallised silica (a kind of chert), beside clastic quartz and feldspar, are stable to chemical reaction of ground water, but they are leached and lightned. Chert is one of the most stable rock against weathering, but also leached in the top of the layer. Density of a fresh chert (2.7g/cm³) is reduced to 1.6g/cm³ in the top of the layer. In this case more than 40% of the total mass is leached out. In X-ray defraction, removed kaolinite is identified beside residual illite and chlorite.
In the second layer, the leaching is less dominant, and density of the crust is about 2.3 g/cm³ and larger than the upper layer. In the bottom of the layer montmorillonite or a kind of mixed-layer mineral of illite/montmorillonite are deposited. Along the clay layer, often total mass of the weathering crust is slided as bed rock slide (Fig. 2).
In NE Japan, the Neogene contains acidic to intermediate tuffs and volcanics, which are easily weathered. The weathering crust is also composed of two layers as folded zones. Because Neogene beds are nearly flat, the aquifer is flat, wide-spread and forms very wide topography of weathering. As Neogene rocks are less deformed and therefore more porous (originally 1.8-2.0g/cm³ in density) than folded rocks, reaction of weathering is more penetrated to form thicker crusts in late Neogene, under hot and humid climates. Leached and porous regolith, containing halloysite and kaolinite is predominant in the lower precipitate layer.
During dry and cold glacial ages, aeolian surface truncates the upper leached layer of the crust, and downward caving of valley is predominant due to lowering of the sea level. After the glacial age, where the lower layer sunk once more in the ground water due to upheaval of the sea level, large bed rock sliding occurred in NE Japan. (Fig. 1 5 on opening plate of this volume).
Flat-topped weathering crusts, deep-carved valleys and large bed rock slides are characteristic in the Quaternary and recent geomorphogeny of Japan.