Intra‐shoreface erosion in response to rapid sea‐level fall: depositional record of a tectonically uplifted strand plain, Pacific coast of Japan

Abstract

Abstract This study documents lowering of the surf zone (i.e. the upper shoreface) leading to intra‐shoreface erosion, following two rapid relative sea‐level falls along a tectonically uplifted coast during the Holocene, and the characteristics of the resultant prograding shoreface deposits. These findings are based on high‐resolution analysis and radiocarbon dating of three new drill cores obtained from the Kujukuri strand plain, Pacific coast of eastern Japan, combined with previously published borehole data and 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 to 2·6 and 1·8 to 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 seaward transition of this surface, rather than the mean equilibrium profile, controls the metre‐scale to decimetre‐scale internal structure of the Kujukuri shoreface deposits. Depositional models for sea‐level fall based on an exponential equilibrium profile do not adequately account for the presence and migration of longshore bars and troughs.