Shallow-marine phreatomagmatic eruptions through a semi-solidified carbonate platform (ODP Leg 144, Site 878, Early Cretaceous, MIT Guyot, West Pacific)

Leg 144 (1995) of the Ocean Drilling Program recovered basalts and volcaniclastic material from the volcanic basement of several guyots in the northwest Pacific, including MIT Guyot in site 878. Tectonic reconstruction suggests that this seamount originated as an intraplate volcano on the Pacific Superswell in the Early Cretaceous. Tephra emplaced by mass flow within the sedimentary cap is the subject of this study. The tephra unit consists of palagonitized, moderately to strongly vesicular basaltic clasts of apparent phreatomagmatic origin, carbonate mud- to grainstone clasts, and armoured lapilli. Armoured lapilli are inferred to have been formed during eruption within a steam envelope that developed above the vent due to magma–water interaction and high gas content of the magma. Another distinctive feature of the tephra unit are abundant truncation surfaces that cut the lithotypes due to subsidence, and are juxtaposed to the same lithotypes. These truncation surfaces are inferred to represent intra-eruption slip surfaces, which formed during eruption of the volcano that generated the tephra pile. The resulting slumps initiated grainflows and dilute turbidity currents, whose deposits are interspersed with those of high-concentration density currents. A subaqueous setting of deposition and eruption is indicated by the abundance of sideromelane glass shards, cauliflower-chilled clasts, the underlying marine carbonates, the lack of tachylite within the deposits, and sedimentary structures within the breccia.