Abstract
The wide, flat and swampy Kushiro plain is surrounded by marine and river terraces, and is separated from the Pacific Ocean by a narrow belt of sand dunes on the southern margin. These terraces are as follows; the marine terraces—Shiranuka (Dl2, early Pleistocene), Nemuro (Du1a, middle or later Pleistocene) and Kushiro (Du2a, later Pleistocene), the river terraces—I (Du1a), II (Du1b, middle or later Pleistocen), III (Du2a), and IV (Du2b, latest Pleistocene), and the Alluvial plain. (Kushiro moor). The formation of these terraces were mainly conditioned by the transgression and regression repeated in the Pleistocene in the present moor where the Tertiary formations have a synclinal axis. Judging from the distribution of the terraces and their topographic features, on the other hand, this region has suffered from the basin-forming crustal movement, of which the centre was located in its western part in the early Pleistocene and moved to the south-east later. Broadly speaking, therefore, the moor is a down-warped depositional basin.
The topographic development of this region is as follows. In the latest Pleistocene the sea-level sank by about 90 meters below the present sea-level, and consequently abrasion platforms were built off-shore at a depth of 80-100 meters and the buried valley under the moor was cut down to about 80 meters below the present sea-level. In the early Holocene the sea invaded the plain up to 10 meters higher than the present sea level and formed an enbayment, the Palaeo-Kushiro Bay, and then retreated to the present shoreline leaving a coastal plain accompanied with sand bars and lakes. During these periods the region suffered from differential elevation, as is inferred by the topographic features of the region.
The formation of a swampy plain was caused by its ill-drained condition owing to its low altitude and the existence of the sand bars on the coast. Warping having its axis coincident with that of the former basin-forming movement has occurred since then, affecting the formation of the small reliefs on the surface of the moor.
