Temporal variations in channel patterns and facies architecture in a gravelly fluvial system: The Paleogene Iwaki Formation on the Joban Coalfield, a forearc basin in Northeast Japan

摘要

This paper examined sequence-stratigraphic features of a gravelly fluvial system of the Iwaki Formation, which developed in a forearc-basin setting in Northeast Japan during the Eocene through Oligocene. On the basis of three-dimensional architectural element analysis, we discriminated three major cycles of channel complexes, which contain ten component channel deposits in total in the fluvial succession. Component channel deposits in the uppermost part of each cycle are sandier and associated with overbank muddy deposits and coal beds as compared with those in the lower part of the cycle. Mean clast-size also decreases upsection in the entire gravelly fluvial deposits. The fluvial succession is interpreted to have been deposited in response to an overall rise in relative sea level that was superimposed by three short-term relative sea-level rises on the basis of vertical stacking patterns and component lithofacies features of channel deposits, and of correlation of the fluvial succession with an age-equivalent marine succession in an area about 50 km offshore. However, geometry and stacking patterns of the channel complexes do not exhibit any distinct temporal variation and amalgamated channel and bar deposits are dominant throughout the transgressive fluvial succession. On the other hand, an overall fining-upward pattern of the entire Iwaki Formation fluvial deposits in association with three component fining-upward patterns is distinct, and is interpreted to be consistent with the tenet of the standard fluvial sequence-stratigraphic models. This indicates that the present example represents one type of variation in the standard fluvial sequencestratigraphic models, possibly reflecting the forearc-basin setting, which is generally represented by higher valley slope, higher shedding of coarse-grained sediments, and shorter longitudinal profiles to the coastal area as compared with a passive-continental-margin setting.