Discovery of a Fossilized Chemosynthetic Molluscan Community in Liog-Liog Point, Tabango Municipality, Northwest Leyte, Philippines

Tomoki Kase

Department of Geology, National Science Museum, Tokyo, Japan

Ryuichi Majima

Faculty of Education and Human Sciences, Yokohama National University, Yokohama, Japan

Yolanda Maac-Aguilar

Petrolab, Mines and Geosciences Bureau, Quezon City

Chemosynthetic autotrophic communities identified in hydrothermal vents, in plate spreading areas and in cold-seeps at plate convergences have attracted much attention from biologists and geologists since its first discovery in the 1970’s. Rich fossil chemosynthetic molluscan communities of cold-seep origin have recently been reported from younger Cenozoic fore- and back-arc basin-fill deposits in Japan. Such unique communities are considered good indicators of ancient cold-seep environments. We herein report the discovery of fossilized chemosymbiotic molluscan communities from Liog-liog Point in Tabango municipality, northwest Leyte, Philippines. This records the first discovery of fossil cold-seep community in Southeast Asia. The assemblage was recognized in several huge (< 5 meters in diameter) possibly drifted calcareous block concretions full of beautifully preserved large vesicomyid bivalve Calyptogena spp. exposed near the beach at Liog-liog Point. Found admixed in these associations are bivalve species of Lucinoma and Thyasira sp. These types of associations are typical of modern chemosynthetic cold-seep communities. In the blocks, Calyptogena spp. were found seemingly deposited in situ being ubiquitous, densely packed, aligned in linear trends, the valves mostly conjoined and sometimes with colonies of annelid tubes covering the posterior part of the shells. The matrix of the concretions is composed exclusively of authigenic carbonates, calcite and aragonite, with carbon and oxygen stable isotope ratios, δ¹³C = -42.03 ‰ and δ¹⁸O = 0.91‰ (vs. PDB: the Pee Dee Formation belemnite), respectively. Authigenic carbonates precipitated at methane-seep sites are extremely depleted in ¹³C as a reflection of the methane-sourced carbon. The δ¹³C value of the concretions at Liog-liog Point evidently confirms that the community was sustained by methane seepage. The fossil-bearing blocks are apparently derived from an adjacent cliff consisting of early-middle Pliocene deep-water sediments. Finding of this type of fossilized molluscan community in northwest Leyte has a probable bearing on the sedimentary and tectonic setting of the region.