An abrupt backreef infilling in a Holocene reef, Paraoir, Northwestern Luzon, Philippines
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An abrupt backreef infilling in a Holocene reef, Paraoir, Northwestern Luzon, Philippines Shou-Yeh Gong • Tso-Ren Wu • Fernando P. Siringan Ke Lin • Chuan-Chou Shen
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Received: 20 June 2012 / Accepted: 1 November 2012 / Published online: 19 November 2012 Ó Springer-Verlag Berlin Heidelberg 2012
Abstract We describe a sudden backreef infilling at the west coast of Luzon, Philippines, which occurred after 324 ± 12 yr ago (year BP, before 1950 AD). Results of 30 230 Th-dated fossil corals from the surface and 5 cores, 17–29.1 m in length, recovered from a Holocene reef at Paraoir show that the reef flat developed in two stages. The reef margin is dated at 10,256 ± 50 (2r) yr BP at 23.9 m below mean sea level (MSL) and about 6,654 ± 29 yr BP at 3.7 m below MSL with ages increasing with depth. The reef flat was formed with sediments of 818–324 yr BP old, which do not follow an age–depth correlation. The evidence suggests that a backreef moat remained empty throughout the buildup of the reef for about 6 kyr and was filled abruptly with a 26-m-thick succession of rubble and bioclastics by an extreme wave event (EWE) after 324 ± 12 yr BP. Field evidence, historical records, and tsunami simulation suggest
Communicated by Geology Editor Prof. Bernhard Riegl S.-Y. Gong Department of Geology, National Museum of Natural Science, Taichung 40419, Taiwan, ROC T.-R. Wu Institute of Hydrological and Oceanic Sciences, National Central University, Jhongli City 32001, Taiwan, ROC F. P. Siringan Marine Science Institute, University of Philippines, 1101 Quezon City, Philippines K. Lin C.-C. Shen (&) High-Precision Mass Spectrometry and Environment Change Laboratory (HISPEC), Department of Geosciences, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan, ROC e-mail: [email protected]
the EWE sedimentation was likely caused by a single severe tropical cyclone, although the possibility of tsunami is not ruled out. The Paraoir reef flat was built up in a mode different from previously reported cases of Holocene reefs. Keywords Holocene reef Backreef infilling Extreme wave event Tropical cyclone Philippines Coral U–Th dating
Introduction The relationship between reef framework growth and backreef infilling in Holocene reefs is complicated (Kennedy and Woodroffe 2002; Montaggioni 2005; Purdy and Gischler 2005). The classic view is that the balance between reef growth and backreef deposition is primarily controlled by sea level changes and growth rates along the reef margin, and lagoons will remain unfilled as empty buckets (Schlager 1993; Yamano 2000; Purdy and Gischler 2005). Most case studies show that a reef crest mainly was initiated by coral growth in the early Holocene and reached present levels in association with a Mid-Holocene highstand (Yamano 2000; Kennedy and Woodroffe 2002; Montaggioni 2005), while backreef areas are filled by bioclastic deposits when sea level stabilized after 6000 yr BP (relative to 1950 AD) and mostly remain unfilled (Schlager 1993; Purdy and Gischler 2005; Hop
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