2019 Volume 125 Issue 10 Pages 737-757
A series of outcrops, 250 m long and 20−40 m high, were exposed during construction of a factory at Sajima, Yokosuka City, Kanagawa Prefecture, central Japan. Before these outcrops were covered with concrete, we described the stratigraphy and structures in detail. The upper part of the Misaki Formation (Aburatsubo Member) of the Miura Group (roughly 3−5 Ma in age) dips gently northeastward and is 60 m thick. The sequence consists mainly of marine pyroclastic sediments, including mafic scoria, lithic fragments, tuffaceous sandstones, and felsic pumice−ash beds, which are centimeters to several meters thick. Several characteristic tephra layers are determined as key beds and numbered from Sj 1 to Sj 21, in stratigraphically ascending order, among which the tephra key bed Sj 20 is correlated with the previously known key bed So. Several thrust-duplex-like structures were recognized in parts of the outcrop, and we discuss here the formation processes of four representative duplex-like structures. One group of duplex structures developed along thrust faults that are orientated oblique to the bedding and are spatially associated with cataclasites. These duplex structures were tectonically induced and of late-stage origin. The other three duplex-like structures developed along the bedding surfaces and were associated with submarine landslides. These gravitationally induced structures are associated with smaller duplex-like structures and show slump-like folds. The directions of thrusting and submarine sliding are different from the modern attitude of depositional beds in the Aburatsubo Member, and suggest an original transport direction from NW to SE.
