Taphofacies models for Upper Cretaceous ammonoids from the Kotanbetsu area, northwestern Hokkaido, Japan

Abstract

The taphonomic attributes of ammonoid shells in the Upper Cretaceous of the Kotanbetsu area, northwestern Hokkaido, Japan, are elucidated with reference to sedimentary facies. The major attributes are: (1) fragmentation rate and fragment-separation rate; (2) patterns of ammonoid-size distribution and mean size; (3) shell concentration within calcareous concretions; (4) the occurrence of heteromorphic ammonoids; and (5) the total number of individuals and species diversity. These attributes are classified into four taphofacies, analysis of which has lead to the following insights: the fragmentation rate and fragment-separation rate increase toward shallower environments; thus, these indices would chiefly reflect the energy level and the frequency of reworking events. Shells are so abundantly concentrated in storm-dominated shelf environments that shell concentrations would be formed by storm and following reworking events. The total number of individuals and the diversity differ among the four taphofacies: these indices are the largest in the uppermost part of outer shelf. These tendencies are inferred to complexly reflect environmental conditions, such as energy levels, frequency of reworking events, and sea-level changes.