不整合面に残された古地形 : 岡山県南西部に分布する中新統浪形層の層序

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An important issue in the Miocene paleogeography of the Southwest Japan arc is whether the Early/Middle Miocene transgressions from the Pacific and Japan Sea met across the arc. Some clues could be found in the Namigata Formation, which is distributed in the border zone between the two transgressions. A stratigraphical study of this formation reveals the high-relief morphology of the basal unconformity and its paleogeomorphological origin in addition to providing basic data for examining the meeting of the two transgressive events. The 120m thick Namigata Formation consists of well-sorted massive sandstone with conglomerate and shelly limestone, that overlies a basal unconformity of high relief. Gravel beds distributed on the top of mountainous to hilly land in and around the study area (so-called "mountain gravels") were considered to be Pliocene to Pleistocene fluvial gravel beds. However, the mountain gravel beds in the study area show no evident discordance with the underlying Namigata Formation and contain shallow-marine molluscan fossils. This suggests that the mountain gravel beds are the uppermost part of the formation. The morphology of the basal unconformity of the Namigata Formation shows that the preserved paleolandform consists of both subaerial and marine erosional landforms. The former is a mountain slope higher than 120m, dissected by a fluvial system which flowed southward. The latter is a staircase rocky coast, comprising three wave-cut terraces separated by two lines of sea cliffs. Because the Namigata Formation is of intertidal to upper neritic origin, the staircase landform is interpreted to have been formed by a marine erosional process controlled by intermittent rise of relative sea level. Mountain gravels which are extensively distributed to the north of the study area, have similar lithology and distributional altitude to those of the uppermost Namigata Formation. This indicates that an uplifted peneplain extended to the north of the rocky coast, bordering the northern margin of the marine area. Essential in understanding the meeting of the two-directional transgressions is the relationship between the altitude of the low-relief mountains and sea level at that time, an issue that will be discussed from a synthetic view point in subsequent papers.
地学団体研究会の論文
1994-09-25

不整合面に残された古地形 : 岡山県南西部に分布する中新統浪形層の層序

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