On the Relationship between the Formtion of the Small-relief Erosion Surface and the Weathering of Granite

Abstract

The purpose of this article is to investigate the relationship between the formation of geomorphic surfaces and the weathering of bedrock in the Takehara area, Hiroshima Prefecture, where granite rocks are dominantly distributed on the small-relief erosion surface. This erosion surface, known as a part of the Setouchi Surface, is subdivided into three levels; the Higher, the Middle, and the Lower Setouchi Surfaces. The Higher Setouchi Surface is identified with the undulating summits of the small-relief mountains, the Middle one with the piedmont benchlands, and the Lower one with small hills composed of granite or gentle piedmont slopes. The author distinguished the following five horizons in a vertical profile of weathered granite in this area, based mainly on the friability of weathered material in situ and on the degree of preservation of the parental rock structure in weathered material; the Soil layer, the Weathered horizon I, the Weathered horizon II (subdivided into IIa and IIb), the Weathered horizon III (subdivided into IIIa and IIIb), and the basal surface of weathering. Weathering profiles of bedrock in this area can be classified into eight types which are established by combining all or some of the above five horizons. The distribution of these types is closely related to the topography of the region as is shown below: Type 1 is characterized by the deep weathering and distributed in the Higher and the Lower Setouchi Surfaces; Type 2 occupies small eminences projected from the Higher and the Lower Setouchi Surfaces; Type 3 develops on the piedmont benchland of the Middle Setouchi Surface; Types 4 and 5 are distributed in the restricted area near Yomokuro-ike belonging to the Higher Setouchi Surface; Types 6, 7 and 8 occupy projected peaks composed of rhyolite, Paleozoic sandstone, Paleozoic clayslate and fine-grained granite on the Higher Setouchi Surface. The last three types are also found at the knickpoints in valley floors cutting the small-relief erosion surfaces. The above mentioned occurrence indicates that the profile types having the upper weathering horizons distribute surrounding the areas with the profile types having the lower weathering horizons, as is shown in Fig. 5. The relief of the area may have been accentuated by the removal of the upper horizons and the exhumation of the lower horizons. In other words, the relief of this area may have been formed by the exhumation of irregularly rolling basal surface of weathering caused by the difference in susceptibility of bedrocks to the weathering process. The process of the formation of erosion surfaces in this area is such that the small-relief surface was deeply weathered in the stable stage, then it was denuded in the following cycle until the basal surface of weathering nearly parallel to the former geomorphic surface appeared. Based on the above view, it is concluded that the Higher Setouchi Surface and the Lower one may be parts of the irregularly undulating basal surfaces of weathering formed in different stages, and that the Middle one may be rocky peaks formed simultaneously with the Lower one by the exhumation of protruded parts of the basal surface of weathering.