Lacustrine and Sublacustrine Microforms of Lake Ogawara with Relation to the Lake-level Changes after the Maximum Postglacial Transgression

Abstract

This paper explains the formation of lacustrine and sublacustrine microforms of Lake Ogawara, one of the brackish-water lakes in Northeastern Japan, with relation to the lake-level changes after the maximum phase of Postglacial Transgression. In addition to morphological observation, surface-geological survey and bore-hole record analysis were made on land and echo sounding and sediment sampling were made at the lake bottom. Some of the sediments were analysed mineralogically.
Various microforms are recognized and mapped and they are classified into the four groups, each of which was formed at the different lake level from others. Chronology of the groups is established on the basis of morphostratigraphy with the aid of tephrochronology and archeology. The results are summarized in the following table.
All the microforms discussed were successively formed near the shore line which shifted corresponding to changing lake level within the height range between +4.0m and -1.5m above the present sea-level. The small difference in height causes the partial superposition of younger microforms to older ones. It contributes to the development of apparent littoral shelves which are in fact composed of several microforms of different ages in late Holocene. The fact that the littoral shelves of Japanese brackish-water lakes are wider in general than those of inland lakes will be partially explained by the processes as above.