Rates of weathering and temporal changes in strength of bedrock of marine terraces in Boso Peninsula, Japan

The weathering rates of bedrock of the dated erosional marine terraces were examined for two definitions: (1) the rates at which the thicknesses of weathered zones with different strengths increase with weathering time and (2) the rates at which the strength of weathered materials at an arbitrary depth decrease with time. The bedrock of terraces in the study area consists mainly of Tertiary sandstone and mudstone. The weathering profiles were observed from drill cores on three levels of terrace surfaces, i.e. 2850, 290 and 70 years B.P. in the emergence age of terraces, which is regarded as the weathering time of bedrock. The degree of weathering grade was described by the residual strength ratio, which is defined as the ratio of the needle penetration hardness of weathered part to that of fresh part of bedrock. The weathering rates of the first definition do not decrease monotonically, but decelerate logarithmically with time. Zones with lower weathering grades have faster weathering rates than those with higher weathering grades. With respect to the weathering rates of the second definition, mudstone starts to weather earlier than sandstone, but is exceeded by the latter after a certain time has elapsed, because mudstone has a higher slaking susceptibility.