Recognition of a Floodplain within Braid Delta Deposits of the Oligocene Minato Formation, North-East Japan: Fine Deposits Correlated with Transgression

Summary

The fluvial Oligocene Minato Formation, north-east Japan, consists mostly of clast-supported conglomerate beds interpreted to represent deposition by a braided river system. Tuffaceous sandstone beds, a facies interpreted to indicate a nearshore sedimentary environment, however, are interbedded with conglomerates at the southern basin margin, suggesting that the distal Minato Formation represents a coalescing braid delta. Moreover, laterally extensive fine-grained sediments that include in situ lignite beds were recently exposed at a large quarry in the northern part of the basin, and are interpreted to be of floodplain origin. The recognition of fine-grained sediments within thick conglomerates enables the correlation of alluvial plain and marine strata, and provides an excellent chronostratigraphical basis for interpreting the two different depositional facies in terms of sequence stratigraphy.

The Minato Formation is interpreted to represent part of a transgressive systems tract, and correlations suggest that base-level rise can have a profound effect on inland alluvial sedimentation in a coarse-grained braided system. As a result of sea-level rise, widespread but thin shallow-marine sediments may extend a few tens of kilometres inland over fluvial deposits. The generally high gradient of braided rivers, however, means that shallow-marine sediments are unable to spread over gravelly braided systems. Instead, in the case of the Minato Formation, a slight change of stream gradient produced by the sea-level rise caused fining of the river gravels, resulting in floodplain formation.