Mississippian pyroclastic flow and ash-fall deposits in the deep-marine Ouachita flysch basin, Oklahoma and Arkansas

摘要

Two pumiceous vitric-crystal tuffs, the Hatton Tuff Lentil and Beavers Bend tuff, occur in the deep-marine Mississippian Stanley Group. These widespread rhyodacitic tuffs range in thickness from 7 to 40 m and are separated by tens of metres of non-tuffaceous quartzose and feldspathic turbidite sandstone and shale. The tuffs consist of varying proportions of ash-sized embayed quartz crystals, plagioclase crystals (oligoclase to andesine), relict shards, volcanic dust, and altered flattened pumice fragments. Each pyroclastic unit consists of two or more tuff lithologies, including a thick lower unstratified pumiceous vitric-crystal tuff with a density-graded crystal-rich base overlain by thin-bedded pumiceous tuff and an upper massive fine-grained siliceous vitric tuff. These tuffs were probably formed by highly explosive eruptions of vesiculating acidic magma from a vent or fissure that produced incandescent avalanches of pyroclastic debris and accompanying ash clouds. The hot turbulent suspensions were rapidly quenched by sea water to form steam-inflated density slurries that flowed into the Ouachita basin. Pyroclastic flows created thick, density-graded, pumiceous vitric-crystal tuff. Numerous smaller density slurries following the main flow in rapid succession deposited the overlying bedded pumiceous tuff. Toward the end of each volcanic eruption, continuous settling of fine ash formed thick, fine-grained upper vitric tuff. Isopach maps of tuff thicknesses, an isopleth map of pumice sizes, logarithmic plots of crystal size versus distance, paleocurrent indicators, and Late Mississippian paleogeography suggest a southern volcanic source that may have been part of a magmatic arc formed at a continental margin during plate convergence between the North American plate and a southern continental(?) plate.