Abstract
Formative processes of conglomerates intercalated in interbedded siltstones and sandstones of the Upper Pliocene Shiramazu Formation in the southern Boso Peninsula were investigated. The study was based on detailed outcrop analyses of geometry, composition and shape of component clasts, and faunal features of the deposits. Conglomerates contain both articulated and fragmented Calyptogena shells in component siltstone clasts and scoriaceous muddy matrix, and were interpreted to be a debris flow deposit. However, the conglomerates show the following external and internal features : 1) the conglomerates show a locally developed anticlinal or dome-like structure compared with the associated interbedded siltstones and sandstones that exhibit monoclinal structure ; 2) most of component clasts show lithologies similar to those of interbedded siltstones and sandstones of the Shiramazu Formation ; 3) contacts between the conglomerates and the associate interbedded siltstones and sandstones are not sharp or erosional and are commonly transitional associated with brecciation of some parts of surrounding interbedded siltstones and sandstones due to injection of scoriaceous muddy matrix ; 4) locally, conglomerates show dyke-like and sill-like occurrence compared with the surrounding interbedded siltstones and sandstones ; 5) occurrence of Calyptogena shells shows a spectrum from articulated, through partly fragmented, to intensely fragmented condition due to the intensity of injection of scoriaceous muddy matrix ; 6) the geologic age of Calyptogenabearing siltstones (4.8-3.8 Ma) is older than that of the interbedded siltstones and sandstones of the Shiramazu Formation (3.4-2.55 Ma). These features of conglomerates are interpreted to indicate that they developed in response to hydraulic fracturing of some parts of the lower Shiramazu Formation with Calyptogena colony and intruded into interbedded siltstones and sandstones of the upper Shiramazu Formation as mud diapirs under overpressure in the landward slope of the paleo-Sagami Trough.
