| 概要 |
The Nichinan Group in the Aburatsu-Nango and the Honjo areas, along the Nichinan coast, South Kyushu, shows a very chaotic nature of lithology and geologic structures and is referable to a large-scale... olistostrome. It contains scatteredly many olistoliths of various lithologies, sizes and ages within the argillaceous formation, showing very chaotic depositional facies and deformations, and ranging in age from Middle Eocene to Earliest Miocene, predominantly of Late Oligocene. As a clue to elusidate the original stratigraphy and sedimentary environments, and the mechanism of emplacement of them, this paper describes largely ages, lithologies and deformational styles of four representative olistoliths, here named the Aburatsu, Izaki, Nango and Honjo Olistoliths. These olistoliths can be assigned to Late Oligocene to Earliest Miocene, mainly based on planktonic foraminifera and molluscan fossils contained. They can be divided into two types with very different litho-and bio-facies suggesting shelf and deep-sea fan environments, respectively. All of the olistoliths have experienced the flexural folding, accompanied with reverse, strike-slip and normal faults, occasionally. Because these deformations occur restrictedly in olistolith and never extend into the argillaceous formation surrounding them, they are attributed to the deformation related to the mechanism of their emplacements. Two structural patterns are discernible among the olistoliths, in which the geometric features and spatial relation of folds to other deformations accompanied are very different from each other. The Aburatsu, Nango and Honjo Olistoliths represent intense contractive deformations, characterized by the asymmetric, overturned flexural folds, facing southerly. They are associated commonly with strike-slip faults showing local bend and modification of trend. Besides, extentional fractures, parallel to the trend of fold axes, occur behind the folded zone of the Honjo Olistolith, exclusively. These structural features indicate that a series of deformation within olistoliths have been formed while the olistoliths decelerated and terminated their sliding motions. On the other hand, the Izaki Olistolith lacks noticeable deorfmations due to contractive strai and shows a gentle basin structure with radially distributed fold hinges at its margin. It is overlain by incoherent slump beds with decollement zone at base and is, furthermore, intruded by mud diapirs in the axial part of basin structure. Deformations observed and structural relationship between the olistolith and the other adjointing argillaceous formations may suggest that this structural pattern had been formed probably as a results of disequilibrium in density between the coherent sheet-like olistolith and the underlying unconsolidated argillaceous formation after the termination of sliding motion. Those varieties in structural pattern can be explaned in terms of a dislocation model for the olistoliths with lateral displacements and a subsiding sheet model for those with vertical displacements. The first-order structures in ferred from the mesoscopic second-order structures in each olistolith show approximately horizontal surface. Combined with other structural data, such as fold asymmetry, spatial relation between contractive and extentional deformations, and profiles of basal slip, it is presumed that the direction of emplacement of olistoliths had been from present northwest to southast.続きを見る
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