Seiichi Toshimitsu

Backward stacking of submarine channel–fan successions controlled by strike-slip faulting: The Izumi Group (Cretaceous), southwest Japan

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-filling processes in strike-slip basins are closely associated with regional tectonics represented by configuration 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 five lithofacies associations (LAs): submarine channel-fill 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 floor 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 fining 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 filling 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.

Ammonoid biodiversity changes across the Cenomanian-Turonian boundary in the Yezo Group, Hokkaido, Japan

Ammonoid biodiversity changes from shallow to offshore environments across the Cenomanian—Turonian (C–T) boundary are reconstructed in the Yezo Group, Hokkaido, Japan. This group was probably deposited at approximately 35–45°N along a westward subduction margin in the northeastern Asian continent. Temporal changes in species richness in the Yezo Group, which show persistently high values during the middle Cenomanian and then decline stepwise from near the middle—late Cenomanian boundary, resemble those in Europe, but not those in Tunisia and the Western Interior. These differences suggest that the Cenomanian—Turonian “mass extinction” was not a global event for ammonoids but was restricted to mid-palaeolatitudinal regions (Europe and Japan). Sea level and climate changes probably influenced ammonoid faunas in the Yezo Group as well as those in Europe. However, it is unlikely that a single, simple cause led to the C—T boundary “mass extinction” because various abiotic changes in the Cenomanian—Turonian transition have been detected, and biotic and abiotic change are interrelated.

北海道添牛内地域産白亜紀セノマニアン初期の アンモナイト フォーナ

The ammonoid fauna from the main part of the Lower Cenomanian sedimentary series in the Soeushinai area of northwestern Hokkaido is prolific. It forms the assemblage of species here called the Stoliczkaia (Lamnayella) japonica Assemblage Zone. It is situated above the basal Cenomanian Graysonites wooldridgei Zone and below the well traced Mantelliceras saxbii Zone. It, thus, represents the main part of the Lower Cenomanian Substage in the studied area. The correlation of this zone with otherwise defined zones, home and abroad, is discussed. Systematic descriptions are given for the zonal indices and several selected species.

Database and species diversity of Japanese Cretaceous ammonoids

Abstract A total of 790 ammonoid species are recorded from the Cretaceous of Japan using a database of Japanese Cretaceous ammonoids. The species diversity changes of these ammonoids over the course of 31 Japanese Cretaceous substages are described. Seven peaks of high diversity and six events of minimal diversity are elucidated for the Cretaceous of Japan. The times of ammonoid species diversity minima are approximately correlated with oceanic anoxic events, OAE1 to 3, which seems to indicate that species diversity for the Japanese Cretaceous ammonoids was influenced by oceanic anoxic events/subevents.