Asphalt Volcanism versus Asphalt Seepage - Examples from the Campeche-Sigsbee Salt Province, Southern Gulf of Mexico

Asphalt deposits have been described from several places of the ocean floor, like the Gulf of Mexico, the Santa Barabra Basin, the Angolan margin and the southwest Atlantic Ocean offshore Brazil. The term asphalt volcanism has been introduced as a novel type of hydrocarbon seepage after an area of approximately 1 km² solidified asphalt was found on top of one of the Campeche Knolls in the southern Gulf of Mexico named Chapopote. Visual seafloor surveys revealed extensive surface deposits of solidified asphalt and light crude oil, emanating from sites along the rim of a crater-like structure in 2.900 m water depth. Large areas of the asphalt deposits were colonized by vestimentiferan tubeworms, bacterial mats, and other biological communities. Also discovered alongside the asphalt were locations of sediment/gas hydrate interlayering associated with emanating gas and oil bubbles from the seafloor. Seafloor mapping in the Southern Gulf of Mexico revealed various morphological elevations like knolls and ridges which all are related to salt diapirism of the underlying sediment sequences. Most of those features show crater like areas associated with seepage. During recent expeditions, the crater structures of Mictlan Knoll and Tsanyao Yang Knoll were measured in high resolution with an AUV and sampled in detail and visually examined with MARUM ROV QUEST4000. Although large areas are also covered with lava-like asphalt sheets, the asphalt seems not to be flown at higher temperatures. Extrusion of heavy liquid oil in form of whips or sheets were observed at several locations which indicate a slow consolidation of the liquid oil to form firm asphalt. After extrusion, chemical and physical changes in the asphalt generate increasing viscosity gradients both along the flow path and between the flow’s surface and core. This allows the asphalt to form pāhoehoe lava-like shapes and to support dense chemosynthetic communities over timescales of hundreds of years.