Field measurements of weathering rates of bedrocks under erosional terrace surfaces

目的是建立基岩风化速率的经验方程，定义为dZ/dt，其中Z为风化物质在垂直于地表方向的厚度，t为新基岩表面最后一次被侵蚀的时间。这些数据是在日本东京附近的河流和海洋侵蚀阶地表面下获得的基岩风化物质数据。研究的阶地分为21个序列，年龄在70 ~ 70000 b.p.之间，年龄假定等于时间t， 2)由第三纪沉积岩组成，3)覆盖着0.2 ~ 3 m厚的砾石或砂饰面。采用试坑、钻孔和地震勘探相结合的方法，对14组梯田的56个测点进行了风化剖面调查，将其划分为高风化带(H)、中等风化带(M)、轻度风化带(S)和轻度风化带(F) 4个不同风化等级的风化带。孔径分布和穿透硬度的差异确定了四个区域之间的边界。从梯地贴面基底到4个带各基底的风化物质厚度分别记为Z_H、Z_M、Z_S、Z_F。这些Z和t之间的关系表示为Z= α ^< β >，其中α和β是常数。Z_H、Z_M和Z_S的slpha值分别为0.48、1.2和3.2 mm/yr，而beta值几乎一致，均为0。三个不同的区域有75个。这一事实表明，风化速率dZ/dt随时间t而减小。

The aim is to establish the empirical equations to express the rates of weathering of bedrocks defined as dZ/dt, where Z is the thickness of weathered materials in the direction perpendicular to ground surface and t is the time elapsed since the surface of fresh bedrock was last eroded. The data were obtained for the weathered materials of bedrocks under the dated fluvial and marine erosional terrace surfaces arount Tokyo, Japan. The studied terraces are 1)divided into the 21 sequential series ranging from 70 to 70,000 yr.B.P.in the age assumed to be equal to the time t, 2)composed of the Tertiary sedimentary rocks, and 3) covered with gravels or sand veneer ranging from 0.2 to 3 m thick.The weathering profiles were investigated at 56 sites on 14 series of the terraces with the combined methods of test pits, borings and seismic prospectings, and were divided into four weathering zones different in weathering grades : highly weatheredzone (H), moderately (M), slightly (S) and faintly (F). The boundaries between the four zones were confirmed by the differences in the pore-size distribution and penetration hardness. The thickness of weathered materials from the base of terrace veneer to each base of the four zones are denoted as Z_H, Z_M, Z_S and Z_F, respectively.The relationships between these Z's and t are expressed as Z=alphat^<beta>, where alpha and beta are constatns. The values of slpha are 0.48, 1.2 and 3.2 mm/yr for Z_H, Z_M and Z_S, respectively, whereas beta shows an almost uniform value of 0..75 for the three different zones. This fact indicates that the weathering rates, dZ/dt, decrease with the time t.