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Field surveys and numerical simulations were conducted to examine lithostratigraphic cyclicity in strike-slip basins, which is still poorly understood due to its complexity. The basin-fi lling processes in strike-slip basins are closely associated with regional tectonics represented by confi guration of faults and spatial/temporal variations in the slip rate. We attempted to bridge the gap between qualitative sedimentary facies analyses and quantitative numerical models in order to better understand the formation of these sedimentary successions. This paper focuses on the Izumi Group (Upper Cretaceous), southwest Japan, which was deposited in an elongate basin (300 km long by 10-20 km wide) along the Median Tectonic Line, which at the time of deposition was a sinistral strike-slip fault related to oblique subduction along a forearc margin. The depositional environments of the group were deduced from fi ve lithofacies associations (LAs): submarine channel-fi ll facies (LA I), proximal facies of lobes or frontal splays (LA II), distal facies of lobes or frontal splays (LA III), slope-apron facies (LA IV), and basin fl oor facies (LA V). LAs I-III represent point-sourced submarine channel-fan successions in the axial facies, with unidirectional paleocurrent directions from ENE to WSW, and LAs IV-V constitute the marginal facies, the paleoslope of which dipped to the SSW. Two units of submarine channel-fan successions are stacked with ~10 km of eastward (backward) shift. Each unit shows a cyclic lithostratigraphy of rapid upward coarsening and thickening in the lower part (~350 m thick) and gradual upward fi ning and thinning in the upper part (1-3.5 km thick). It is estimated to have taken 5-7 × 10 5 yr for 10 km offset on each stratigraphic unit based on the depositional ages. Although many processes can control the stratigraphic architecture, such as global and local sea level, climate, and tectonics, the stratigraphic cyclicity observed in the study area is closely related to the depocenter migration, suggesting that fault movement was the primary control on the stratigraphy. On the assumption that the formation and fi lling processes of the Izumi sedimentary basin were basically controlled by strike-slip faults, a numerical simulation suggests that episodic changes in fault-slip rate or sediment-supply rate could control the stratigraphic cyclicity. In this paper, we propose a model where cyclic stratigraphy is ascribed to temporal variations of fault activity controlling accommodation generation, sediment supply, and relative sea level, which could generate cyclic stratigraphy associated with depocenter migration in strike-slip basins.

The Izumi Group, which occurs from western Shikoku to the Izumi Mountains, Kii Peninsula, comprises sediments deposited in a strikeslip basin during the Late Cretaceous. Some differences exist in the lithostratigraphy and geological structure of the group where it occurs in western and eastern Shikoku; consequently, outcrops in central Shikoku may record evidence of the transition between the two areas. We conducted stratigraphical and geochronological surveys as part of compiling the "Niihama" geological map, and defined the following new formations: the Kussaki, Isoura, and Niihama formations. Fission-track dating of felsic tuff beds in the Niihama Formation yielded an age of . . Ma. The radiolarian assemblages within mudstone of the Niihama Formation are DK assemblages, corresponding to the earlymiddle Campanian. The combined age data indicate a middle Campanian age for deposition of the Niihama Formation. The present results suggest that the Izumi Group was simultaneously deposited in western and central Shikoku. The lithology and geological structure in the study area are comparable with those in western and eastern Shikoku, indicating that the analyzed rocks represent strata deposited during a transitional stage of basin development and basin fill. Abstract