Abstract
As the role of stratigraphic trap becomes greater in the exploration of oil and gas, it becomes more important to disclose the principal characteristics of the extent and form of individual turbidite sandstone beds in flysch sediments, especially in the ancient-submarine-fan sediments. Generally, however, it is very difficult to correlate each turbidite sandstone bed, bed by bed, in a wide area because of the lack of useful marker-beds in flysch sediments. So far, only a few results on such study have been reported.
As many useful pyroclastic tuff marker-beds are intercalated in the Pliocene Kiyosumi and Anno Formations in the Boso Peninsula, central Japan, composed of flysch-type alternations of sandstone beds and mudstone beds, these formations are very suitable for the detailed study on flysch and turbidite sedimentation. Tokuhashi23) clarified the whole sedimentary processes of these formations based on a three-dimensional analysis (Figs. 7-10), and that these are the ancient submarine-fan sediments.
He succeeded in the detailed bed-by-bed correlation of individual turbidite sandstone beds in terms of many tuff marker-beds at the four horizons in these formations, and clarified the extent and form of individual turbidite sandstone beds at each horizon26)-28) (Figs. 11-29). Especially at one horizon in the Kiyosumi Formation, he clarified the depositional mechanism in constructing the form of individual turbidite sandstone beds26), 27) (Fig. 20).
Main conclusions on the principal characteristics of extent and form of individual turbidite sandstone beds are as follows:
(A) The individual turbidite sandstone beds of the same horizon have a common and specific prototype or a basic configuration peculiar to its horizon in their forms.
(B) Such a prototype or a basic configuration in forms of individual turbidite sandstone beds is maintained through succession of several tens to several hundreds of meters in thickness in flysch sediments.
(C) Forms of individual turbidite sandstone beds have a close relationship with grain size of the constituent sediments and sedimentary structures in them. On the other hand, volume, i.e., thickness and extent of the beds, exactly reflects the magnitude of turbidity currents which deposited them.