Origin of widespread, exceptionally thick basaltic phreatomagmatic tuff breccia in the Middle Jurassic Prebble and Mawson Formations, Antarctica

Eruption of the Middle Jurassic Kirkpatrick Basalt flood lavas of Antarctica was preceded by a significant episode of basaltic pyroclastic activity. The resulting deposits include minor tuff and lapilli tuff of base-surge and fall origin, but consist dominantly of massive, poorly sorted tuff breccia in beds up to 170 m thick, which form unbroken successions as much as 370 m thick. Tuff breccia consists of clasts of igneous and sedimentary rock up to several meters long plus basaltic bombs and spatter (≤1 m) dispersed in an ash-and lapilli-size matrix of basaltic pyroclasts and abundant, disaggregated sedimentary debris. Pyroclast characteristics indicate production by phreatomagmatic processes. Sedimentary particles were produced by explosive disruption of underlying strata; some clasts were derived from depths >1 km. Tuff-breccia deposits are attributed to eruption as high concentration mixtures of basaltic tephra, accidental sedimentary debris and water; the ensuing lahars filled valleys and low-standing areas within a tectono-magmatic rift zone associated with Gondwana break-up.