Reevaluating the Guam Welded Tuff, Southern Goheung Peninsula, Korea: Evidence for Pyroclastic Origin, Rheomorphic Deformation, and Complex Ignimbrite Facies

The Guam Welded Tuff in the southern Goheung Peninsula, Korea, exhibits a striking lava-like appearance, historically causing misinterpretation as an effusive rhyolitic lava flow. However, detailed field investigations, microstructural observations, and volcanic facies analyses indicate that this deposit originated from a pyroclastic density current generated by explosive silicic volcanism, subsequently modified by rapid welding, intense rheomorphic deformation, and crystallization. The planar basal contact with underlying units such as the Dohwa Andesite or Hado Formation and the absence of basal vitrophyre strongly argue against a lava flow origin. Extensive and uniform welding, accompanied by a continuous parallel fabric, supports rapid compaction and high-temperature adhesion of pyroclastic material. Microscopically, the presence of strongly elongated fiamme, distinct parataxitic textures, and low-angle stretching lineations confirms significant ductile deformation under hot conditions. Additionally, systematic vertical variations in volcanic facies—from the basal lithic-rich layers to the upper vitric zones—reflect progressive changes in depositional mechanisms, welding intensity, and cooling history. Regional geological context, including the Yujusan caldera setting and a SHRIMP U-Pb age of 83.2 Ma, further supports an explosive eruptive origin. These comprehensive observations necessitate reinterpretation of the Guam Welded Tuff as a high-temperature ignimbrite, whose lava-like textures result from complex post-depositional processes rather than effusive emplacement.