A new Lower Cretaceous hydrocarbon seep locality from the Czech Carpathians and its fauna

• First confirmed hydrocarbon seep fossil association from the Western Carpathians
• Hydrocarbon seep associated with teschenite
• Early Cretaceous seep fauna reported
• Other alleged Cretaceous seep faunas from Western Carpathians re-evaluated

A new Lower Cretaceous hydrocarbon seep locality at Baška in the Czech Carpathians is described. The seep carbonate and associated mollusk fauna have been encountered in deep-water claystones and siltstones of the Upper Barremian upper part of the Hradiště Formation associated with pillow lavas of so-called teschenite association. The investigated carbonate displays petrographic characters typical of seep sediments (clotted micrite, isopachous rim cements and layered pyrite aggregations). The carbon isotopic composition (− 20.0 to − 23.5‰ vs. V-PDB) most likely indicates anaerobic oxidation of thermogenic methane. The fossils recovered from this locality are rather poorly preserved but hokkaidoconchid gastropods and lucinid bivalves in association with trochid and zygopleurid gastropods are identified. This finding confirms the existence of seep faunas in the Western Carpathians, which was anticipated based on the presence of seep mollusks and brachiopods in old monographs that cover Lower Cretaceous fossils of this region. The fossils illustrated in the monograph of Ascher (1906) are here briefly re-evaluated and possible seep members are indicated.