| Research Abstract |
Composition of brackish-water fossil assemblages and stratigraphy and sedimentary facies of the Cretaceous in southwest Japan were investigated. Study are includes katsuura area in Tokushima Prefecture, Yuasa area in Wakayama Prefecture and Tanoura coast in Kumamoto Prefecture. In these areas, the Cretaceous sedimentary successions yielding brackish-water bivalves, consist of thick sandstone of tidal channel origin overlain by finer sediments occasionally displaying wavy- and flaser bedding indicationg tial current. Habitats of brackish-water bivalves were reconstructed in relation to the cyclicity originated from tidal channel formation.
Detailed paleoecological analysis was conducted in Tatsugatani section in Katsuura area. In-situ preserved brackish-water bivalves were identified firstly in this study for Isodomella shiroiensis, Costocyrena radiatostriata, Tetoria sanchuensis, Pulsidis sp.aff.nagatoensis and Pharella sp. Five associations, comprising Crassostrea, Ostrea-Pulsidis, Ost
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rea Pyrgurifera, Isodomella-Pharella and Costocyrena-Ostrea Associations, were identified, on the basis of the relative abundances of the constituent species and in-situ preserved species The most common are Costocyrena-Ostrea Association, consisting mainly of infaunal bivalve, Costocyrena radiatostriata, along with epifaunal Ostrea sp. The ratio of individuals of infaunal bivalves is highest, and the habitat is interpreted as most unstable, being characterized by frequent erosion and deposition. In contrast, Crassostrea Associations are mostly monospecific associations, consisting of epifaunal Crassostrea sp.having elongated and flat shell. The habitat of this association is interpreted as stable substrate with relatively high salinity, probably of meso- and brachyhaline.
Formerly, brackish-water benthic associations have been understood in terms of salinity regime, from oligohaline, mesohaline and brachyhaline. However, our study indicates that physical factors such as substrate unstability, such as erosion and deposition acted as a serious limiting factor, being similar to those in marine benthic associations. Less
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