RTK-Based Survey of Marine Terrace Morphology, Holocene Coastal Highstands and Neotectonics Segmentation in Southern Java, Indonesia

The southern coast of Java, Indonesia, lies along the active Sunda subduction margin, where coastal landforms record the interaction between sea-level change, wave erosion, sedimentation, and tectonic uplift. Marine terraces and raised coastal surfaces are important geomorphic indicators of vertical deformation, but their interpretation remains difficult where chronological control is limited and where coastal surfaces have been modified by erosion, deposition, karstification, or human activity. This study presents new Real-Time Kinematic Global Navigation Satellite System (RTK-GNSS) topographic profiles from four coastal sites: Pantai Ajah, Kalijali, Kulon Progo, and Wingko. The profiles were measured from the beachward side toward the landward side and were used to identify terrace treads, risers, slope breaks, residual topographic highs, and possible raised coastal platforms. These field data are integrated with published information on Holocene sea-level change, marine terraces, coastal uplift, and forearc deformation along the southern Java margin. The RTK profiles show variable terrace morphology between sites. Pantai Ajah preserves a prominent riser and a probable terrace tread at approximately 7–8.5 m elevation. Kalijali records a lower terrace-like surface at approximately 4–5 m, an upper surface at approximately 7–9 m, and a higher local topographic high near 12–13 m. Kulon Progo shows a subdued low-elevation raised coastal surface, while Wingko contains a distinct slope break at approximately 1450–1500 m from the beachward end and a broad landward surface at approximately 5–6.5 m elevation. The profiles suggest two tentative morphostratigraphic terrace groups: a lower group at approximately 4–6.5 m and an upper group at approximately 7–9 m. Higher local peaks, including the 12–13 m high at Kalijali and comparable elevated points at other sites, may represent remnants of older or more strongly uplifted coastal features. One possible interpretation is that some of these higher surfaces originated near the mid-Holocene sea-level highstand, when relative sea level in parts of Indonesia and Sundaland was higher than present, and were subsequently uplifted to different elevations according to local uplift rates. However, this hypothesis requires direct chronological and sedimentological confirmation. The raised terrace ridges and topographic highs may also act as partial natural barriers that reduce tsunami flow penetration inland, although they should not be treated as complete protection. Overall, RTK profiling provides a useful field method for recognizing coastal terrace morphology and identifying priority sites for future dating, tsunami-inundation modelling, and coastal-hazard planning.