Sequence stratigraphic architecture of a differentially subsiding bay to fluvial basin: the Eocene Ishikari Group, Ishikari Coal Field, Hokkaido, Japan

The Eocene Ishikari Group, deposited in a bay to fluvial basin in central Hokkaido, Japan, provides important information on fluvial sequence stratigraphy in a differentially subsiding ocean-connected setting. The Ishikari Group consists of four mill ion-year-order depositional sequences (Isk-1, Isk-2, Isk-3 and Isk-4), composed mainly of meandering/braided fluvial systems. Each depositional sequence contains marine or lacustrine incursion beds, which show lake or bay to bay-margin tidal facies, at maximum flooding surface horizons. This indicates that marine or lacustrine incursion into fluvial environments took place when the increase rate of accommodation space due to relative sea-level rise exceeded the rate of accumulation. Fluvial channel stacking density varies in response to stratigraphic position within a depositional sequence, namely, high channel density in the lower part of the transgressive interval (Trs: LST to TST) and the upper part of the regressive interval (Rgr: HST), and low channel density in upper Trs and lower Rgr. Multistory channels tend to be developed in lower Trs, whereas single-story channels predominate in upper Trs and Rgr. These patterns in the distribution of fluvial channels resulted from changes in accommodation space in response to relative sea-level changes. Sequence boundaries are recognized at fluvial incision surfaces at the base of Trs, but the development of incised valleys is minor because of the relatively high subsidence rate, which obscured the effect of relative sea-level fall. Syndepositional differential subsidence within the paleo-Ishikari basin resulted in selective development of relatively downstream depositional systems in the more rapidly subsiding area. High-frequency depositional cycles are also prominently developed in the subsiding area where sedimentation rate was sufficiently high to record high-frequency relative sea-level cycles or autocycles of deposition. (C) 2002 Elsevier Science B.V. All rights reserved.