14:45 〜 15:00
[HCG28-05] Ichnodiversity in the Upper Cretaceous submarine channel-levee deposits: examples from the northern and southwest Japan
キーワード：生痕化石、生物撹拌強度、根室層群、姫浦層群、和泉層群
This study aimed to describe spatio-temporal variation of ichnoassemblages in the Upper Cretaceous deep-marine deposits in the northwestern Pacific area. It has been known that composition and number of ichnotaxa vary depending on depositional environments. Therefore, they are important and useful parameters to estimate depositional environments on the seafloor. Generally, ichnoassemblages in the submarine channel or proximal part of frontal splays are dominated in dwelling traces produced by crustacean. In contrast, small-sized, diverse patterned feeding or dwelling structures (graphoglyptids) are dominant in levees or distal parts of frontal splays. However, it is possible that the deep-marine ichnofacies was significantly different between those in Cretaceous and Cenozoic. Indeed, it is considered that some representative ichnotaxa in the Cenozoic deep-marine setting immigrated from the shallow-marine to deep-marine setting during Cretaceous. This study investigated the Upper Cretaceous submarine channel-levee deposits of the Nemuro, Himenoura, and Izumi groups to describe the ichnoassemblages and compare their characteristics with the Cenozoic ichnofacies model.
As a result, the ichnoassemblages in study areas showed low ichnodiversity and were different in composition from the previous ichnofacies models based on Cenozoic ichnoassemblages. In the Nemuro Group, 12 ichnogenera occurred from the levee deposits and no trace fossil was found in the channel-fill deposits. The ichnotaxa in the levee facies of the Nemuro Group consisted of the component ichnospecies of Paleodictyon ichnosubfacies while the number of ichnotaxa was smaller than that known in the Cenozoic models. In the Himenoura Group, 9 ichnogenera occurred from the channel-fill deposits and 7 ichnogenera were found in the levee deposits. The ichnoassemblages of the both of the channel-fill and levee deposits in the Himenoura Group were dominated in the grazing or dwelling traces produced by crustaceans and echinoids, and scarce in the patterned structures that consist of the Paleodictyon ichnosubfacies. In the Izumi Group, 11 ichnogenera occurred from the channel-fill deposits and 9 ichnogenera were found from the levee deposits. The ichnoassemblage in the Izumi Group was mainly composed of grazing or sessile feeding traces. In the Himenoura and Izumi groups, it was also characterized by high bioturbation intensity in the mudstone beds by the dwelling or grazing traces.
The difference among ichnoassemblages occurred in the groups can be explained by variation of preservation potential for each ichnotaxon caused by interactions of the benthic activity. The graphoglyptids, which is a representative group of the deep-marine trace fossils, are generally observable as convex hyporeliefs on the sole of turbidite sandstones. It is considered that they were primarily burrows or tunnels formed in fine-grained deposits near to the seafloor, and cast by subsequent deposition of sand. The high bioturbation intensity in the mudstone beds of the Himenoura and Izumi groups suggest high frequency of the dwelling or grazing behavior by large-sized benthic animals such as echinoids during inter-event[n1] periods of turbidity currents. This intense bioturbation could destroy the primary burrows or tunnels by the graphoglyptid producers and prevent to preserve the graphoglyptids.
The results of this study imply that the ichnoassembalges in the Upper Cretaceous deep-marine deposits may be characterized by high bioturbation intensity and low ichnodiversity. It is possible that decrease of bioturbation intensity in the near-surface deposits and consequent increase of preservation potential of the graphoglyptids may be a cause of the difference between the Cretaceous and Cenozoic ichnoassemblages. In future works, further observation focused on the bioturbation in the mudstone beds and benthic interactions will be needed.
As a result, the ichnoassemblages in study areas showed low ichnodiversity and were different in composition from the previous ichnofacies models based on Cenozoic ichnoassemblages. In the Nemuro Group, 12 ichnogenera occurred from the levee deposits and no trace fossil was found in the channel-fill deposits. The ichnotaxa in the levee facies of the Nemuro Group consisted of the component ichnospecies of Paleodictyon ichnosubfacies while the number of ichnotaxa was smaller than that known in the Cenozoic models. In the Himenoura Group, 9 ichnogenera occurred from the channel-fill deposits and 7 ichnogenera were found in the levee deposits. The ichnoassemblages of the both of the channel-fill and levee deposits in the Himenoura Group were dominated in the grazing or dwelling traces produced by crustaceans and echinoids, and scarce in the patterned structures that consist of the Paleodictyon ichnosubfacies. In the Izumi Group, 11 ichnogenera occurred from the channel-fill deposits and 9 ichnogenera were found from the levee deposits. The ichnoassemblage in the Izumi Group was mainly composed of grazing or sessile feeding traces. In the Himenoura and Izumi groups, it was also characterized by high bioturbation intensity in the mudstone beds by the dwelling or grazing traces.
The difference among ichnoassemblages occurred in the groups can be explained by variation of preservation potential for each ichnotaxon caused by interactions of the benthic activity. The graphoglyptids, which is a representative group of the deep-marine trace fossils, are generally observable as convex hyporeliefs on the sole of turbidite sandstones. It is considered that they were primarily burrows or tunnels formed in fine-grained deposits near to the seafloor, and cast by subsequent deposition of sand. The high bioturbation intensity in the mudstone beds of the Himenoura and Izumi groups suggest high frequency of the dwelling or grazing behavior by large-sized benthic animals such as echinoids during inter-event[n1] periods of turbidity currents. This intense bioturbation could destroy the primary burrows or tunnels by the graphoglyptid producers and prevent to preserve the graphoglyptids.
The results of this study imply that the ichnoassembalges in the Upper Cretaceous deep-marine deposits may be characterized by high bioturbation intensity and low ichnodiversity. It is possible that decrease of bioturbation intensity in the near-surface deposits and consequent increase of preservation potential of the graphoglyptids may be a cause of the difference between the Cretaceous and Cenozoic ichnoassemblages. In future works, further observation focused on the bioturbation in the mudstone beds and benthic interactions will be needed.