Bioerosive structures formed by Miocene rock-boring bivalves in Hokkaido, Japan

Abstract

Fossilized rock-boring bivalves and associated borings in the Miocene Sankebetsu Formation in northern Hokkaido, Japan, are useful for reconstructing paleoenvironmental changes. The sediments are typical of shallow-marine facies in Hokkaido, and are rich in rock-boring bivalve traces. The Miocene host formation (Sankebetsu Formation, 23–19 Ma) unconformably overlies an earlier Eocene formation (Sakasagawa Formation). The inferred paleoenvironmental changes associated with deposition of the Miocene Sankebetsu Formation at the study locality started with development of a wave-cut bench of stiff mud or soft rock that was extensively bored by the rock-boring bivalve Platyodon nipponica to form unnamed ichnofossils. The bench was subsequently covered by sediments associated with a transgression and were gradually lithified. A later regression lead to the development of new bench of hard rock that was bored by the rock-boring bivalve Penitella kotakae to form the boring ichnofossil identified as Gastrochaenolites turbinatus. Finally, the new bench was covered by sediments during a major transgression, forming a shoreface environment characterized by storm-generated shell beds. The analysis of bioerosion on marine rocky surfaces is a useful tool for interpreting geological episodes such as relative oscillations of sea level, erosion/sedimentation rates, and the intensity of physical disturbance.