Alternating seismic uplift and subsidence in the late Holocene at Madang, Papua New Guinea: Evidence from raised reefs

Well-preserved mid-late Holocene coral reefs are exposed in low coastal cli ffs in the vicinity of the Madang lagoon on the north coast of Papua New Gu inea. Results from U/Th and C-14 dating of corals, surveying, and field map ping indicate several major changes in relative sea level over this period. Specifically, there is evidence for a relative sea level fall of greater t han or equal to 4.5 m about 3000 calendar years B.P followed by relative se a level rises of similar to 1.5 m about 2400 calendar years B.P. and greate r than or equal to 0.5 m about 1200 calendar years B.P. and a subsequent re lative sea level fall of greater than or equal to 3 m some time in the past 1000 years. Since regional eustatic sea levels are believed to have been d ropping gradually over this time frame, these observed changes in relative sea level are interpreted as reflecting alternating tectonic uplift and sub sidence. Furthermore, the detailed structure and age relationships of the c oral deposits indicate that both uplift and subsidence occurred rapidly, mo st probably as coseismic events with vertical displacements of 0.5 to 4.5 m . These events may be related to rupture on NW-SE trending reverse faults w hich have been mapped in the nearby Adelbert Range and possibly on NE trend ing cross faults which have been inferred from seismicity. This interpretat ion implies a much greater degree of tectonic instability and potential sei smic hazard in the region than previously recognized, although the inferred coseismic vertical displacements are shown to be consistent with present-d ay local seismicity. In a broader context, the study illustrates how detail ed analysis of vertical changes in coral reef structure and assemblages may be used as a sensitive indicator of changing relative sea level, capable o f resolving century timescale events and reversals.