Holocene Relative Sea-Level Changes and Mangrove Response in Southwest Bohol, Philippines

Sediment-core, tidal-notch and14C data establish Holocene relative sea-level changes and the response of mangroves in the lower reaches of the Abatan River in southwest Bohol. Sedimentology, foraminifera and total organic contents of seven cores 4.5 to 17.2 m long define eight lithofacies and the corresponding depositional environments of the Holocene sediments. The lithofacies units are lagoonal clay; subtidal organic-rich clay; low marsh clayey peat; mangrove peat; supratidal organic-rich clay; fluvial mud and gravelly sand; floodplain clay; and beachface silt and sand. Their vertical and lateral distributions, and clustering of14C age dates define three parasequences. Parasequence A, composed of the lagoonal clay, low marsh clayey peat and organic-rich clays, and the lower section of the fluvial mud and gravelly sand was deposited after a rapid sea level rise from 12 to 2 m below present mean sea level (pmsl) approximately 8000 to 6000 yBP. Subsequently and until 4500 yBP, mangroves developed while sea level rose slowly from 2 to 1 m below pmsl. From ca. 4500 to 4300 yBP sea level again rose rapidly, from 1 to 0.7 m above pmsl, translating the shoreline landward and drowning the mangroves. Then a relative stillstand lasted until ca. 2500 yBP, while parasequence B consisting of beachface silt and sand was deposited. At ca. 2500 yBP, sea level started to fall to the present sea level, allowing the deposition of the prograding units of parasequence C, which is composed of peat, fluvial mud and gravelly sand, and floodplain clay. The thick and extensive peats were deposited in the favorable environment provided by a broad, shallow basin that formed during this period. The reconstructed local relative sea-level curve conforms to the general eustatic sea-level trend in the far-field region: a rapid rise followed by a relative stillstand and a subsequent fall to the present. A mid-Holocene higher-than-present sea level that characterizes areas in far-field sites is also evident in an emerged tidal notch.