1986 年 25 巻 3 号 p. 139-163
The Miyazaki Plain has the best-developed late Quaternary terraces and deposits in Kyushu. But the landform evolution of this plain has not been investigated in detail in previous works. This study tephrochronologically describes the landform evolution in late Pleistocene.
Thick weathered tephra layers overlying the terrace surfaces are subdivided into an older and a younger groups. The younger tephra group, which has been deposited in the last ca. 100, 000 years, contains many marker tephra layers, in ascending order, Ata, K-Tz, FkP and Aso4 of 90, 000-70, 000y.B.P., ; AyP, IwtP, IwP, AwS, HnS II and OtP of 60, 000-30, 000y.B.P.; OsP, ItoPfl, AT and KbP of 22, 000-15, 000y.B.P.; and Ah of 6, 000y.B.P.
The older tephra group covers the higher terrace group of middle Pleistocene. The younger group covers the following lower terrace surfaces, in descending order: the Sanzaibaru surface, the Karasebaru surface, the Nyutabaru I, II, III surfaces, the Saitobaru I, II surfaces, the Toyobaru I, II surfaces, the Oyodo surface, the Kunitomi I, II surfaces, the Mikazukibaru I, II surfaces, and the Holocene terrace surface group.
Of many terrace surfaces in the Miyazaki Plain, the Sanzaibaru surface is the most extensive one. It is mostly of marine origin, composed of thick transgressive deposits called the Sanzaibaru Formation and lithologically subdivided into three members. The lower member is fluvial gravelous deposit in the regressive stage. The middle member consists of alternating beds of sand and silt with fossils of mollusccs that lived in the embayment of a warm sea in the transgressive stage. The upper member is sandy deposit of deltaic and beach conditions in the maximum stage of the transgression. On the Sanzaibaru surface, the upper member forms sand ridges which are inferred to have been bars, barriers and dunes.
The oldest marker tephra layer on the Sanzaibaru surface is the Ata ash which erupted from the Ata caldera ca. 90, 000-80, 000y.B.P.. The Sanzaibaru surface underlying the Ata ash is thought to have emerged ca. 100, 000y.B.P., in the Last Interglacial Stage. It is estimated by subtracting the height of the base of the Sanzaibaru Formation from the height of the shoreline of the surface that sea level rose more than 100m in the Sanzaibaru stage.
The Nyutabaru I, II, III surfaces are characterized by gentle gradients and wide distribution. They are of fluvial origin, although the Nyutabaru II and probably III surfaces are partly of marine origin in the northern part of the plain. The Nyutabaru III surface is an accumulation terrace formed during the little transgression. These surfaces are thought to have emerged ca. 90, 000-60, 000y.B.P., when a relatively high sea level was maintained because regression was slow and debris supply from the Kyushu Mountains increased or discharge in rivers decreased.
The Saitobaru I and II, Oyodo, and Kunitomi I and II surfaces, which are characterized by steeper and more linear longitudinal profiles, are erosional terraces and are mostly of fluvial origin. Their surfaces came out ca. 50, 000-10, 000y.B.P., in the Last Glacial Stage, when rapid regression occurred and sea level stayed relatively lower. The discharge of the rivers tended to increase gradually.
The Karasebaru, Toyobaru I and II and Mikazukibaru I and II surfaces in the northern part of the plain are fluvial fans. They were formed in three regressional stages: the late Last Interglacial Stage, the early Last Glacial Stage and the late last glacial stage. These fans were formed under conditions in which the river bed gradient was steeper than that of the continental shelf when the sea level went down.