Last Interglacial Sea Level Changes Deduced from Coral Reef Terraces in Southwest Bohol, Central Philippines

Abstract

High precision α-spectrometric U-series dates for fossil corals from raised marine terraces in Pamilacan Island and from the neighboring Panglao Island and Punta Cruz in southwest Bohol were combined with previous data to re-examine the sea level history during the Last Interglacial and its implications to the tectonic setting of the area. Hermatypic corals were collected at elevations between 3-6m, 9-13m, and 20-27m apmsl (above present mean sea level) from three successive terraces in Pamilacan which have maximum elevations of 6m, 13m, and 27m apmsl. The dates of these corals cluster within 79-83ky, 101-108ky, and 122-131ky (from lowest to highest terrace) which correspond respectively to the marine isotope stages (MIS) 5a, 5c, and 5e. A constant rate of tectonic uplift ranging from 0.18-0.21m/ky in the past 125ky was estimated based on the published mean sea level data of 3-6m apmsl during MIS 5e. Using these local uplift rates, the respective paleo-sea levels during 5c (105ky) and 5a (82ky) are calculated to be 9-11m and 6-9m below present, which are consistent with uplift-corrected reef data estimates from Haiti, Barbados, and Huon Peninsula. The 5a paleo-sea level also coincides closely with the results from the submerged reefs of the tectonically stable Florida margin. The substantial agreement between the Pamilacan paleo-sea levels with those from different localities worldwide support the assumption of a constant uplift rate in the past 125ky. In contrast, the altitude of the sea level indicators correlated to the MIS 5e in Panglao and Punta Cruz are much lower, ≥12m apmsl, while that of 5c are ca. 5-6m apmsl. Minimum local uplift rates since MIS 5e ranges from 0.06-0.09m/ky, three times lower than that of Pamilacan. The estimated mean sea level during 5c is only a few meters (ca. 3m) below present indicating that uplift is not constant. Significant variation in the elevations of contemporaneous terraces at the three neighboring islands may have been greatly influenced by localized tectonic activity which puts the reported relative vertical stability of the region into question.