Reworked ammonoids and their taphonomic implications in the Upper Cretaceous of northwestern Hokkaido, Japan

Taphonomic attributes of ammonoids occurring in Late Cretaceous calcareous concretions in northwestern Hokkaido are examined. These are as follows: patterns of fragmentation of body chambers, condition of the outer shell surface, and sedimentary structures within calcareous concretions, as well as direct evidence of reworking, such as geopetal structures and sediment infill of chambers. Reworked ammonoids were recognised based on geopetal structures showing two different upward directions and types of sediment infill of successive phragmocone chambers. Each ammonoid assemblage from the study area is interpreted as a within-habitat time-averaged association. The amount of time represented by these fossil assemblages may range between 100 and 10,000 years. Storm-events are the most likely cause of reworking and time-averaging, the Upper Cretaceous of the study area having been a storm-dominated shelf environment. (C) 2001 Academic Press.