Unconventional maar diatreme and associated intrusions in the soft sediment-hosted Mardoux structure (Gergovie, France)
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A Miocene age volcanic-hypabyssal structure comprising volcaniclastic deposits and mafic intrusions is exposed with vertical relief of ∼110 m on the side of Gergovie Plateau (Auvergne, France). Three main volcaniclastic facies are: (1) Fluidal tuff breccia composed of juvenile basalt and sediment clasts with dominantly fluidal shapes, with several combinations of basalt and sediment within individual clasts. (2) Thickly bedded lapilli tuff composed of varying proportions of fine-grained sediment derived from Oligocene–Miocene lacustrine marls and mudstones and basaltic lapilli, blocks, and bombs. (3) Planar-bedded tuff forming thin beds of fine to coarse ash-size sedimentary material and basalt clasts. Intrusive bodies in the thickly bedded lapilli tuff range from irregularly shaped and anastomosing dikes and sills of meters to tens of meters in length, to a main feeder dike that is up to ∼20 m wide, and that flares into a spoon-shaped sill at ∼100 m in diameter and 10–20 m thick in the eastern part of the structure. Volcaniclastic deposits and structural features suggest that ascending magma entrained soft, saturated sediment host material into the feeder dike and erupted fluidal magma and wet sediment via weak, Strombolian-like explosions. Host sediment and erupted material subsided to replace the extracted sediments, producing the growth subsidence structure that is similar to upper diatreme facies in typical maar diatremes but lacks evidence for explosive disruption of diatreme fill. Irregularly shaped small intrusions extended from the main feeder dike into the diatreme, and many were disaggregated due to shifting and subsidence of diatreme fill and recycled via eruption. The Mardoux structure is an “unconventional” maar diatreme in that it was produced mainly by weak explosive activity rather than by violent phreatomagmatic explosions and is an example of complex coupling between soft sediment and ascending magma.
KeywordsMaar Diatreme Phreatomagmatic Dike Sill
This work was conducted during faculty and student exchange under the joint EU-US INVOGE project. The authors thank Jamal Amin, Kelly Wooten, Andrew Harp, and Matthew Sweeney for assistance in the field and Alison Graettinger for reviewing an early version of the manuscript. Pierre-Simon Ross, Gerardo Carrasco, Volker Lorenz, and associate editor Steve Self, all provided helpful reviews that improved the manuscript.
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