Abstract

Fossil shell beds of the shelf can be used as the good tools in the analysis of paleoenvironments because they include many features clearly associated with sedimentary processes. Thier usefulness, however, has not been evaluated adequately. The upper Miocene to lower Pliocene Horokaoshirarika Formation is distributed in central Hokkaido. The upper part of the Horokaoshirarika Formation is composed mainly of sandy and fossiliferous deposits and is interpreted to be deposited in the inner-shelf. Analysis of the sedimentary facies suggests it as a successive upward-shallowing sequence. Based on the shell arrangement in the deposits, the shell beds of the upper part of the formation can be divided into two types. Type 1 shows regularly arranged shells which are stacked, imbricated, and arranged parallel to the bedding plane and is formed by physical reworking. Type 2 represents randomly arranged shells and is formed by bioturbation. We subdivide the upper part of the formation into even stratigraphic unit I to IV. Number of both types increases gradually from I to III, but abruptly decreases from III to IV. The ratio between Types 1 and 2 does not change from I to IV. The increase of the type 1 from I to III reflects the increase frequency of storm reworking in ascending order, and its abrupt decrease from III to IV reflects severe amalgamation of the shell beds. The constant ratio of Types 1 and 2 indicates the constant extent of bioturbation in each unit. Thus, in the inner-shelf deposits of the Horokaoshirarika Formation, the physical reworking increases upward with shallowing, while extent of biological reworking is almost the same throughout the successtion.