TOKUHASHI, SHUICHI, Geological Survey of Japan
Abstract: The Relationship Between the Three-Dimensional Cross-Sectional Forms of Individual Turbidite Sandstone Beds and Those of Time-Sliced Turbidite Successions of the Neogene Forearc Basin Fills, Central Japan
The turbidite succession can be divided into time-sliced units usually from tens of meters to hundreds of meters in thickness. The units is sometimes bordered by intercalation of relatively thin mudstone-dominated alternation in the sandy flysch succession, when the activity of turbidity currents have ceased or weakened temporally. It is very important to correlate individual turbidite sandstone beds in the thick turbidite succession widely and to disclose their three-dimensional cross-sectional forms not only to clarify the depositional mechanism of constituents grains from the turbidity currents but also to analyse the superimposing process of individual turbidite beds getting to the time-slice units. Usually it is very difficult due to the monotonous repetition of the enormous number of turbidite beds without useful marker-beds or due to the complicated structure of the turbidite succession.
However, many useful tuff markers are intercalated in the Neogene turbidite succession which are distributed along the gently undulated anticline and syncline more than 40 km along the major folding trend and more than 5 km across the trend from west to east coasts of the Boso Peninsula, central Japan. These turbidite successions were formed as an sandy submarine fan deposits in an ancient forearc basin. Many individual turbidite sandstone beds were correlated bed by bed in many horizons of the succession throughout the basin. Detailed depositional process of each turbidite sandstone bed has been discussed base on the sedimentological data of these widely correlated turbidite sandstone beds.
Here I discuss mainly on the characteristics of the three-dimensional cross-sectional forms of individual turbidite sandstone beds and the relationship between the forms of individual turbidite sandstone beds and time-sliced turbidite successions. Main conclusions are as follows:
1) Thicker turbidite sandstones can be traced more widely, i.e. throughout the total distribution area of turbidite succession, though thinner turbidite sandstones are distributed in the narrower or restricted areas in the proximal areas, e.g. near the main feeder channel.
2) The forms of individual turbidite sandstone beds are strongly controlled by the texture of constituent grains through their depositional process. The thicker turbidites are mostly composed of coarser-grained massive sandstones with no or minor finer-grained laminated sandstones in the uppermost part of the bed, while the thinner turbidites are mostly formed of finer-grained laminated sandstones with no or minor massive sandstones in the lowermost part of the bed.
3) Regardless the difference of thickness of individual turbidite sandstone beds, the same or common prototype can be observed throughout the three-dimensional crosssectional forms of individual turbidite sandstone beds which belong to the same time-sliced unit.
4) The prototype in forms of individual sandstone beds can be maintained within the same timesliced unit, usually of several tens to a few hundreds of meters in thickness.Therefore, the cross-sectional forms of time-sliced turbidite successions are basically the same as those of individual turbidite sandstone beds in the same time-sliced unit. This means that the turbidity currents, regardless their magnitude, tend to maintain the same depositional or distribution pattern of transporting grains along the same main course during the deposition of a time sliced unit. The shift of main course of turbidity currents which deposit the turbidites of the.different unit result in the shift of area of the main depositional center in each turbidite sandstone bed, therefore result in the different three-dimensional cross-sectional forms of individual turbidite sandstone beds from those of other units.
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