Abstract
The remains of sand-eruptions are preserved in the limuro Formation exposed in the floor of the Tama River. This formation, a member of the Lower Pleistocene Kazusa Group, is distributed in the Tama Hills district. It is composed of massive sandy mudstone with several intercalations of fine to medium sandstone. The total thickness is about 45m. The remains of sand-eruptions are located near the middle of the formation. Two different types of sand-eruption structures are recognized. Type I occurs sporadically, and shows a tubular to columnar form. Typell occurs along small faults or fissures. Type I are subdivided into A〜D subtypes, based on the shape of the transverse section, and the presence of a central hole. Thus, a tube with ellipsoidal profile comprises subtype I-A; a tube with circular profile subtype I-B; a multi-layered tube, subtype I-C; and columnar, subtype I-D. Type II is subdivided into an elongate lenticular subtype (II-A), and subtype II-B clastic dykes. Some members of subtype I -C have an upper funnel-shaped form, which might result from the sudden release of pressure during the final stage of sand-eruption. Subtype I-C with a multi-layed inner structure indicates several episodes of sand-eruption. As these structures are often accompanied by small faults or fissures, it is assumed that sand-eruption might occur repeatedly at the same time as fissuring of sediments by earthquake activity, which controlled the form of sand-eruptions. The sedimentary facies and fauna of the limuro Formation show that these eruptions happened in shallow water near the coast, during the Early Pleistocene.
