SEDIMENTARY RECORD OF THE BEACH EROSION DUE TO SURF-ZONE LOWERING IN RESPONSE TO RAPID SEA-LEVEL FALL, PACIFIC COAST OF JAPAN

The coastal process occurring in response to sea-level fall is poorly understood. This study documents lowering of the surf zone (i.e., the upper shoreface) leading to beach erosion following two rapid relative sea-level falls along a tectonically-uplifted coast during the Holocene, and the characteristics of the resultant sedimentary record. These findings are based on high-resolution analysis and radiocarbon dating of six drill cores within a 1.5 km obtained from the Kujukuri strand plain, Pacific coast of eastern Japan, combined with an information on modern shoreline profile adjustments. A shallowing-upward sandy succession composed of lower and upper shoreface facies, foreshore and backshore facies was recognized in the drill cores. Two rapid falls in relative sea level at 2.3–2.6 ka and 1.8–2.0 ka are recorded by downstepping of the base of the foreshore facies, and farther seawards by the lowering of an erosional boundary between the upper and lower shoreface facies. Superimposed bed profiles of an adjacent modern beach define an envelope, the base of which reflects shore-normal migration of longshore bars and troughs. The base of the envelope represents an erosional surface that divides the surface mobile layer above from preserved deposits beneath. The surface is concave upwards and steeper than the mean beach profile, and exhibits a flat platform approximately at the lower limit of the upper shoreface equating to the storm surf zone. The geometry of the erosional surface formed in response to the rapid sea-level fall is controlled by the envelope base, rather than the mean profile. The Bruun equilibrium beach profile is considered too rough to account for the decimeter scale architecture of the studied sedimentary succession that reflects processes of longshore bars and troughs.