Geological Controls on Marine Cavernous Landforms along Japanese Pacific-side Rocky Coasts

Abstract

Marine cavernous landforms, including notches, caves, arches and tunnels, characterize the rocky coast landscape and are strongly influenced by the geology (e.g., rock strength and structure) of sea cliffs. Geological controls on the development and shape of marine cavernous landforms are evaluated based on field investigations along the Pacific coastlines of Honshu Island, Japan. Morphological parameters, width (w), depth (d), and height (h), of cavernous landforms were measured directly or through images taken with an unmanned aerial vehicle, together with geological factors, such as rock types, strength (Schmidt hammer rebound values: R), dips and strikes of bedding, major joints, and fault planes. In total, 76 caves are investigated in six coastal areas: Sanriku Coast, Joban Coast, Boso Peninsula, Miura Peninsula, Izu Peninsula, and Kii Peninsula. According to shape index, d/w, and presence of the open end, cavernous landforms are classified into notches (d/w < 1, closed end), caves (d/w ≥ 1, closed end), arches (d/w < 1, open end), and tunnels (d/w ≥ 1, open end). An analysis shows that the major geological controls differ between notches and the other three forms. Low rock strength (R < 40) and sub-horizontal bedding are, respectively, the primary and secondary controls on the formation of notches, whereas weak rock structures (joints, faults and bedding) with a dip steeper than 30° (vertical or steeply inclined structures) and a strike trending perpendicular to the cliff face are the major controls on the formation of caves, arches and tunnels. Rock strength also affects planar forms defined by the shape index, promoting a deepening of cavernous forms, particularly when the cliff has a medium rock strength (R = 30-50) that provides an optimal balance between erosion force and resisting force. In general, vertical or steep structures contribute to the deepening of holes, whereas horizontal or gentle structures favor widening.