Abstract
Six oyster-shell beds of exclusively Crassostrea sp. are exposed in a thick estuarine unit in the Campanian (Late Cretaceous) upper part of the Shiroyama Formation, Izumi Group, along the Doki River, Mannou area, Kagawa Prefecture, SW Japan. Five modes of fossil occurrence are observed in the oyster beds: (I) Relay type, (II)single-generation cluster type, (III) mixed type in upright and lying positions, (IV) closely packed side-lying type, and (V) scattered side-lying type. Five of the shell beds consist of more than one type of fossil occurrence, indicating a heterogeneous origin (i.e., both autochthonous and allochthonous), with the beds being of variable size, thickness, and shape. The sixth bed yields only allochthonous oysters (type IV). The former five shell beds were formed on a sandy tidal flat by the intermittent accumulation of autochthonous and allochthonous shells. The combined effects of (1) clustered oyster growth, (2) subsequent destruction by tidal currents or storms and short-distance transportation, and (3) renewed clustered growth over several generations resulted in the formation of thick shell beds. These modes of fossil occurrence, formed under high-energy conditions on a sandy tidal flat, are in contrast to the oyster reefs of modern Crassostrea, which dwells in a sheltered muddy estuary environment. The thick estuarine unit in the section along the Doki River suggests that in this part of the Izumi Basin, the rates of subsidence and sediment supply were in balance during the Campanian. Moreover, the heterogeneous oyster beds may have formed in an estuarine environment during occasional stagnation phases during a transgression related to syn-depositional subsidence of the Izumi Basin.
