The Kii peninsula, located on the Pacific coast of central Honshu, is fringed with coastal terraces in its southern part, while its east and west coast are characterized by ria shorelines. The purposes of this paper are to clarify coastal development of the peninsula, especially the history of vertical changes in sea level in its coastal region, and secondly, to examine the relation between the mode of Quaternary crustal movement (especially its vertical component) and recent crustal movement associated with contemporary seismic activity.
Coastal terraces and an interpretation of their development
Coastal terraces, developing along the southern coast of the Kii peninsula (Fig. 2), are mostly rocky abraded terraces covered with thin marine beds. In the northern part of the east coast (north of Shingu) and at the mouths of some rivers, however, some marine terraces are composed of marine sediments overlying fluvial beds. The coastal terraces are classified into two levels of the high and low terraces in the surroundings of Shingu, where they are typically developing (Fig. 4). Both levels of the terraces are further subdivided into some sublevels which are dentoted by H₁, H₂, H₃ and H₄ in the high terraces and L₁, L₂ and L₃ in the low terraces respectively in the descending order. The height of each sublevels at Shingu are 125, 113, 95, 61, 49 and 39 meters above sea level. The highest terraces H₁ are dissected into narrow hill ridges, overlaid by round gravel beds more than 20 meters thick. The H₂, H₃ and H₄ terraces are rocky abraded ones. The most extensively developing terraces Li are composed of marine sediments underlaid by fluvial beds more than 40 meters in thickness. The L₂ and L₃ terraces are marine or fluvial terraces which have been formed cutting down the L₁ terraces. Alluvial plains (denoted by A) do not so extensively develop along the coast, except at the mouths of the rivers. Judging from some boring data, alluvial formations at the mouths of the rivers are more than 30 meters thick (Fig. 3). From these facts it is concluded that the Hi and Li terraces and alluvial plains are depositional surfaces composed of thick marine and fluvial deposits, which are filling submerged fluvial valleys and, therefore, that the coastal region, being interrupted by temporary submergence, has been emerged in precess of the formation of the coastal terraces. The other terraces H₂, H₃, H₄, L₂ and L₃ have no feature indicating submergence and, therefore, are considered to have been formed in process of emergence. Judging from the thickness of deposits, amplitude of submergence during the later two periods is considered to be greater than that during the previous period.
As to the process of formation of alluvial plains, rapid submergence of this period is regarded in another regions as being largely due to eustatic rise in sea level caused by world deglaciation from the results of the researches on submerged topography, marine topography, succession of alluvial formations, absolute age determination of alluvial marine beds, climatic changes and etc. The process of formation of alluvial plains in the Kii peninsula is inferred to be the same as in another regions from the submerged fluvial valleys and the thickness of alluvial formation and, therefore, the submergence during the last period was caused by eustatic rise in sea level.