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Tube Fossils in the Lelet Formation, New Ireland: Evidence for Miocene Hydrocarbon Seepage in the New Ireland Basin?

Abstract

The Lelet Limestone formation is a thick buildup of neritic platform carbonate rocks exposed on the island of New Ireland. Sedimentary facies of the Lelet include coralgal biostromal calcarenite, calcirudite and foraminiferal biomicrite (Hohnen, 1978). In situ coral and biohermal reef facies are also reported as present but are relatively uncommon. The base of the Lelet Formation lies unconformably over Oligocene volcanic rocks of the New Ireland island arc. It is estimated that 1,000 m of carbonate buildup occurred following the cessation of island arc volcanism in the Early Miocene (Exon and Marlow, 1988). In the Late Miocene, a regional microplate reorganization event linked to the arrival of the Ontong Java Plateau at the Pacific-Australian plate boundary, caused rapid tectonic inversion along the New Ireland basin margin. This resulted in the former volcano-plutonic arc rocks and the overlying carbonate cap emerging above sea level throughout the Pliocene-Pleistocene and created the extensive high relief, karstic terrain on present day New Ireland. Logging activity on the island has provided new access to the densely forested central highland region, and a recent geological expedition to fresh roadcut and quarry exposures has led to the discovery of two tube fossil locations within the Lelet Formation. These fossil tubes are comparable to ancient (Peckmann et al., 2005) and modern (Southward et al., 2002) chemosynthetic vestimentiferan worm fauna found at hydrocarbon seep deposits. Worm tubes in the Lelet localities occur in both weathered biomicrite and massive calcareous concretions in various orientations. The tubes are infilled with matrix sediment with no soft tissue preservation. Mouldic casts of gastropod and mollusk shells are found adjacent to tubes. The tubes are non-Previous HittaperedTop and have diameters ranging from 20-30 mm, which are some of the largest diameters reported in the fossil literature (Peckmann et al., 2005). It was not possible to determine original lengths since no unbroken specimens were observed. The tube walls are up to 2 mm thick, dark brown to tan color relative to the whitish matrix limestone, and show a concentrically laminate structure. SEM images of wall cross-sections show that the lamellae consist of ultrafine (2-4 µm) and fine (5-12 µm) calcite crystals. The roughly equant crystal sizes and curviplanar grain boundaries suggest that the tubes have experienced diagenetic recrystallisation. Age dating of the worm tubes was carried out via in situ laser ablation 87Sr/86Sr mass spectrometry. Values for the two sites were 0.70837-0.70843 and 0.70845-0.70846, indicating roughly similar Upper Miocene ages of 20-21 Ma and 19 Ma, respectively. The Miocene Lelet worm tubes can be compared to chemosynthetic fauna discovered at active hydrocarbon seeps in the New Ireland basin, such as at the Edison Seamount (Herzig et al., 1994) and Mussel Cliff (Herzig et al. 1998) localities located south of LIhir Island. The Mussel Cliff locality is characterized by methanotrophic tube worms (Southward et al. 2002) and Bathymodiolus mussels (von Cosel and Jansen. 2008), and authigenic carbonate cements generated by thermogenic hydrocarbons (Schmidt et al. 2002) seep up the Mussel Cliff fault structure. The modern tube worms described by Southward et al (2002) were identified as a new species (Paraescarpia echinospica), larger (1.0-1.5 mm diameter) than earlier varieties described from Gulf of Mexico seep localities. The Miocene Lelet worm tubes at 20-30 mm diameter, are clearly larger than the modern New Ireland worm tubes. Carbon and oxygen isotope studies of the samples are underway, and ∂13C values will shed light on whether the carbon in the Miocene worm tubes originated from methane seepage. Previous studies of carbonates from northwestern New Ireland with ∂13C of -35 to -37 per mil PDB led Rigby (1986) to hypothesize that methane seepage has been occurring since the Pliocene. If the Miocene worm tubes reported here are also confirmed to have had a methane food source, then the Lelet Limestone, with up to 25% porosity (Sandy 1986), can be considered a potentially significant hydrocarbon reservoir facies, both offshore and onshore (Exon and Marlow, 1988).