Holocene sea-level highstand and its implications for the vertical stability of Panglao Island, southwest Bohol, Philippines

摘要

Emerged tidal notches in Panglao Island, southwest Bohol, central Philippines, were used to investigate relative sea-level changes during the Holocene. The elevation and profile of a laterally continuous notch were surveyed at 22 sites around Panglao Island. The tidal notch documents higher-than-present mean sea level (pmsl), consistent with regional field observations and predictions for mid-Holocene time. The reconstructed paleo-shoreline elevations range from 0 to 1.6 m above pmsl, exhibiting a general increase to the northeast, the more protected part of the coast. This lateral variation in the Holocene paleo-mean sea level (msl), along with changes in notch depth and profile, suggests tilting of the island best explained by differential land movement. Last interglacial msl estimates by previous workers also show tilting to the southwest. Moreover, the Holocene notch data suggest segmentation of the island into several blocks that tilt consistently to the southwest, suggesting the presence of faults. Lineations in aerial photographs, trending northeast-southwest, confirm the presence of these faults. These data indicate differential land movements during the Holocene and Late Pleistocene. Calculated rates of uplift are 0.3 and 0.06 mm/yr for the Holocene and for the Late Pleistocene, respectively. These values, relative to the high uplift rates from tectonically active regions, imply that Panglao is relatively vertically stable. After accounting for tectonic tilting, mid-Holocene sea level is estimated at approximately 0.3-0.6 m.