Oyster reef

Abstract of ref. 1610:
The Member U-III a of the Upper Himenoura Subgroup distributed in Kumamoto and Kagoshima Prefectures is composed mainly of cross-bedded sandstone, which yields two oyster beds. Autochthonous oyster reefs can be observed at two outcrops exposed at Onoura in Amakusa-Shimojima and Hirasezaki in Shimokoshikijima (Fig.1). These oysters are 'mudstickers' (Seilacher et al., 1985), and are assigned to one of the Crosssostrea species. They are divided into two distinct morphotypes, namely, a stocky form (Onoura type; about 30 cm in shell height) and a stick-like elongated form (Hirasezaki type; less than 100 cm in height). The Onoura type seems to adopt 'relay-strategy', and one clump usually consists of three or four generations. On the other hand, the Hirasezaki type resembles morphologically solitary mudstickers such as Konbostrea. However, this Hirasezaki type also make the clump composed of three or four generations. The Hirasezaki type may, therefore, adopt not only 'elongated-strategy' but also 'relay-strategy'. This is an intermediate mode of life between Crassotrea and Konbostrea. Sedimentological analysis suggests that the Cretaceous strata exposed at Onoura represent more nearshore facies than those at Hirasezaki. The Onoura type might be adapted to nearshore, lagoonal environments, while the Hirasezaki type might be an adaptive form to offshore bar in open marine environments. Cretaceous gigantic oyster beds are confined to three stages in Japan: middle Cenomanian, upper Turonian to lowest Santonian, and upper Campanian.