不整合面に残された海食地形の形成過程

岡山県南西部,中新統浪形層の基底不整合

抄録

The Miocene Namigata Formation, which accumulated in the Setouchi Province (an intra-arc basin of the southwest Japan arc), has a marine erosional landform preserved on the basal unconformity. The landform is a staircase rocky coast composed of wave-cut terraces and sea cliffs. This paper describes the gravelly sediments intercalated in the formation and reconstructs the processes responsible for the marine erosional landform. The marine erosional landform formed through the repetition of the following cycle, which was controlled by pulsatile, but consistent rising of relative sea level. 1) After a rapid rise of relative sea level, a new sea cliff created at stillstand sea level collapses due to marine erosion, resulting in the generation of a wave-cut terrace. 2) Debris flows from the collapse run down the old sea cliff drowned through the rapid rise of relative sea level, and accumulate as a submarine scree at the foot of the cliff. 3) Subsequent retreats of the active sea cliff widen the wave-cut terrace, abrasion of which degrades the collapsed debris from the sea cliff. 4) By wave and current action, clastic material transported from the wave-cut terrace and from rivers covers the submarine scree and forms a wave-built terrace on the seaward side of the wave-cut terrace. 5) Further supply of sand elevates the surface of the wave-built terrace to the level of the wave-cut terrace, resulting in a state of equilibrium at the surf zone. 6) Another rapid rise of relative sea level sets up the next cycle. The marine eroional landform preserved on the basal unconformity of the Namigata Formation is, thus, a product of the pulsatile rising of relative sea level on a rocky coast. In terms of sequence stratigraphy, the formation seems to be a parasequence set forming a transgressive systems tract.