Fig. 5

Geologic cross section, route map, and geologic columns of the Anno Formation on the western Boso Peninsula. a Stratigraphic correlation of the uppermost part of the Anno Formation showing east–west (Kobata–Iwamoto–Kunawa) and south–north (Kunawa–Terao, Nagasaki) variations in horizons underlying the Sak slump and the equivalent erosional surface (blue line). b, c Route maps in Nagasaki b and Seki c with outcrops of the upper part of the Anno Formation. d Geologic columns of part of the Anno Formation that shows lateral facies changes, based on tephra bed correlation. In a similar manner to the Ikego area (Fig. 3), a mass-transport deposit named “Sak slump” becomes thinner and pinches out eastward where a tens of cm-thick pebbly sandstone bed covers the erosional surface. The lithologic legend is shown in Fig. 4