The beast burrowed, the fluid followed – Crustacean burrows as methane conduits

Abstract An extensive pockmark field with associated methane-seep carbonates has recently been reported from the late Albian (Lower Cretaceous) Ubidepea Mudstone of the Black Flysch Group in the Basque Country, northern Spain, but the exact pathways of the migrating methane-rich fluids remained elusive. Here we provide petrographic, stable carbon and oxygen isotope evidence that abundant crustacean burrows in the surrounding mudstone, preserved as the trace fossil Thalassinoides, and the seep carbonates themselves have acted as long-lasting fluid conduits in this system. The Thalassinoides infill generations show a diagenetic parasequence often starting with a distinctive lining, followed b…