Shallow-marine fan delta slope deposits with large-scale cross-stratification: the Plio-Pleistocene Zaimokuzawa formation in the Ishikari Hills, northern Japan

• Published in: Sedimentary geology 2003-05, Vol.158 (3), p.195-207
• Main Authors: Tamura, Toru, Masuda, Fujio
• Format: Article
• Language: English
• Subjects: Brackish; Fan delta slope; Large-scale cross-stratification; Sediment density flow; Slump scar; Storm-wave base; Tidal current
• ISSN: 0037-0738; 1879-0968
• DOI: 10.1016/S0037-0738(02)00266-X

The Plio-Pleistocene Zaimokuzawa Formation comprises marine to fluvial deposits in one of the Neogene back-arc basin around the Japanese islands. Five sedimentary facies (facies A–E) and two facies associations (FA 1 and FA 2) were distinguished in the lower Zaimokuzawa Formation. Facies A is cross-stratified sandstone induced by tidal currents. Facies B is a couplet of wave-rippled fine sandstone and siltstone. Facies C is pebbly sandstone interpreted as the deposit from a concentrated density flow and a turbidity flow. Facies D is pebbly mudstone deposited from a mud flow or debris flow. Facies E is a slumped alternation of sandstone and mudstone. FA 1 and FA 2 are typical of the lower and upper parts of the lower Zaimokuzawa Formation, respectively. FA 1 is composed of alternations of facies A and C, including facies B, D and E. Facies C and D have lobe-like geometries in FA 1. FA 2 is composed of facies B and C, and lacks facies A. Facies B and C fill concave-upward slump scars in FA 2. The constituent facies, shapes of the deposits and paleocurrent directions suggest that the lower Zaimokuzawa Formation was formed by progradation of a shallow-marine fan delta slope influenced by tidal currents, and that its lower (FA 1) and upper part (FA 2) corresponds to middle, lower and upper slope deposits, respectively. Wave and tidal currents reworked and redeposited the sediments supplied by gravity-driven processes on the delta slope to form the multiple facies associations. Grain sizes of facies A and B, which are mainly composed of sand-sized particles in spite of the presence of intervening pebbly deposits of sediment density flows, imply competence of the reworking processes. The distribution of cross-stratified sandstone is restricted to the western area, indicating a local inequality in the influence of tidal currents.