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

摘要

The weathering rates of bedrock of the datederosional marine terraces were examined for two definitions: (1)the rates at which the thicknesses of weathered zones with differentstrengths increase with weathering time and (2) the rates at whichthe strength of weathered materials at an arbitrary depth decreasewith time. The bedrock of terraces in the study area consists mainlyof Tertiary sandstone and mudstone. The weathering profiles wereobserved 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 in regarded as the weathering time of bedrock. The degree ofweathering grade was described by the residual strength ratio,which is defined as the ratio of the needle penetration hardness ofweathered part to that of fresh part of bedrock. The weatheringrates of the first definition do not decrease monotonically, butdecelerate logarithmically with time. Zones with lower weatheringgrades have faster weathering rates than those with higherweathering grades. With respect to the weathering rates of thesecond definition, mudstone starts of weather earlier thansandstone, but ix exceeded by the latter after a certain time haselapsed, because mudstone has a higher slaking susceptibility.