Identification of suspected paleotsunami deposits study from Karapyak Beach, Pangandaran area, West Jawa, Indonesia

Abstract

Identifying and constraining palaeotsunami deposits can be a vital tool for establishing the periodicity of earthquakes and their associated tsunami events beyond the historical records. However, the deposits can be difficult to establish and date. In this study we used the characteristics of the 2006 Pangandaran tsunami deposit as a reference for identification of paleotsunami deposits in Karapyak Beach, Pangandaran area, West Java, Indonesia. Similar to the 2006 Pangandaran tsunami deposit, the Karapyak Beach paleotsunami deposit is characterized by light brown loose sand materials overlying a dark brown paleosoil layer with erosional contact. A thin layer that varies in thickness is locally found right above the erosional contact, with non-laminated coarser grain in the lower part that gradually change into medium to fine sand-sized in upper part. The base of the lower part is rich with broken mollusc shells and corals, and the mid-top of the lower part may contain several intact molusc shells and corals, rock fragments and anthropogenic products (rooftile). Those types of fragments are absent in the upper part of the thin layer. Grain size analysis shows a mixture of fine and coarse grains in the lower part of 2006 tsunami deposits as well as in the suspected paleotsunami deposits, suggesting uprush high energy flow during sedimentation. Fining upward sequence above mixed grain layers reflects waning flow or pre-backwash deposition. Foraminifera analysis also shows a mixture of shallow and deep marine foraminifera in the two deposits. Based on the characteristics of the 2006 tsunami deposits, there are at least four identified paleotsunami deposits at Karapyak Beach, Pangandaran area.