Outcrop analogues of pockmarks and associated methane-seep carbonates: A case study from the Lower Cretaceous (Albian) of the Basque-Cantabrian Basin, western Pyrenees Luis M. Agirrezabala a, ⁎, Steffen Kiel b , Martin Blumenberg b , Nadine Schäfer b , Joachim Reitner b a Estratigrafia eta Paleontologia Saila, Euskal Herriko Unibertsitatea UPV/EHU, 644 P.K., 48080 Bilbo, The Basque Country, Spain b Georg-August University, Göttingen Geoscience Center, Geobiology Group, Goldschmidtstr. 3, 37077 Göttingen, Germany abstract article info Article history: Received 25 May 2012 Received in revised form 4 October 2012 Accepted 27 November 2012 Available online xxxx Keywords: Paleo-pockmark Outcrop analogue Methane-seep carbonate Fossil biota Biomarker Lower Cretaceous Late Albian deep-water sediments of the Black Flysch Group in the Basque-Cantabrian Basin (western Pyrenees) preserve a fossil pockmark field including methane seep carbonates and associated macrofauna. The geometry of the pockmarks is reconstructed from repeated lens-shaped turbidite deposits with centrally located carbonate bodies. Early diagenetic carbonate phases such as clotted micrite and yellow calcite with δ 13 C values as low as -41.6‰, and hydrocarbon biomarkers (e.g. 2,6,10,15,19-pentamethylicosane) with strong depletions in 13 C indicate that the carbonates precipitated due to anaerobic oxidation of methane. The pockmarks probably formed due to subsidence induced by dewatering and degassing of the gas-charged seabed perhaps enhanced by the weight of the carbonate bodies. The macrofauna resembles that of other late Mesozoic deep-water methane-seeps world-wide, and is dominated by large lucinid and Caspiconcha bivalves, and hokkaidoconchid gastropods. During late diagenesis the carbonate δ 18 O values were reset to a narrow range of -12 to -10‰, the remaining pore spaces and fissures were filled with pyrobitumen, and additional carbonate phases precipi- tated, potentially due to thermochemical sulfate reduction processes. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Methane seepage on the seafloor is often associated with pockmarks: craters resulting from the rapid expulsion of water and/or gas through the seabed. When seepage of methane-rich fluids continues after the initial expulsion it can fuel distinctive communities of chemosymbiotic animals such as siboglinid tube worms, bathymodiolin mussels and vesicomyid clams (Hovland et al., 1987; Dando et al., 1991; Ondréas et al., 2005; Sahling et al., 2008). Furthermore, the anaerobic oxidation of the methane (AOM) mediated by microbial consortia results in the precipitation of carbonates with characteristic petrographic and geo- chemical features (Hovland et al., 1985; Peckmann et al., 2001; Peckmann and Thiel, 2004) and forms the hardground for the associated macrofauna. Pockmarks are concave depressions with different sizes, ranging from 1 to 700 m in width and several centimeters to 45 m in depth (Hovland et al., 2002), although larger ones can result from the ag- glomeration of several pockmarks. They were first discovered on the Scotian shelf (King and MacLean, 1970) and subsequent mapping of the seabed confirmed their occurrences in most oceans and seas (Judd and Hovland, 2007). They occur on both passive and active conti- nental margins at various depths and geodynamic contexts, where fluids discharge upward through muddy seabed sediments (Hovland and Judd, 1988). Many pockmarks are related to methane cold seepage, but pockmarks formed by hydrothermal fluid flows have also been described in volcanic areas (Pickrill, 1993). Fossil methane seep communities range back into Devonian time and have been reported from ancient forearc, backarc, and continen- tal shelf settings (Goedert and Squires, 1990; Gaillard et al., 1992; Peckmann et al., 1999; Majima et al., 2005; Campbell, 2006; Metz, 2010). Although ancient pockmarks have been recorded in seismic profiles from different continental margins (Hovland, 1982; Cole et al., 2000; Gemmer et al., 2002; Andresen et al., 2008), there is only one exam- ple of cropping ancient pockmarks (Hovland, 1989). In this paper we present a multidisciplinary study (sedimentology, petrology, paleontolo- gy and geochemistry) of Cretaceous pockmarks and related authigenic carbonates and fossil fauna cropping out in the northern margin of the Basque-Cantabrian Basin. The excellent exposure of these ancient pock- marks on a present-day wave-cut platform and adjacent seacliffs allowed examination of the entire seep system, including the sub-seabottom plumbing system, on a lamina by lamina resolution that is 2 to 3 orders of magnitude better than state-of-the-art seismic profiles. 2. Geological setting The studied paleo-pockmarks and lenticular authigenic carbonate bodies crop out within Albian rocks of the northern margin of the Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2013) xxx–xxx ⁎ Corresponding author. Tel.: +34 946015425; fax: +34 946013500. E-mail address: l.agirrezabala@ehu.es (L.M. Agirrezabala). PALAEO-06343; No of Pages 22 0031-0182/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.palaeo.2012.11.020 Contents lists available at SciVerse ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo Please cite this article as: Agirrezabala, L.M., et al., Outcrop analogues of pockmarks and associated methane-seep carbonates: A case study from the Lower Cretaceous (Albian) of the ..., Palaeogeography, Palaeoclimatology, Palaeoecology (2013), http://dx.doi.org/10.1016/j.palaeo.2012.11.020