Subaerial volcanism in the Palaeoproterozoic Hekpoort Formation (TransvaalSupergroup), Kaapvaal craton

Within a 30 km strike length of the Palaeoproterozoic Hekpoort Formation (P retoria Group, Transvaal Supergroup), approximately equal proportions of ba saltic-andesitic flows and lenses of volcaniclastic rocks are preserved. An overall intracratonic and subaerial setting is inferred for these rocks. A complex volcanic environment is thought to have existed, with relatively q uiescent amygdaloidal, massive lava flows and violent pyroclastic eruptions either succeeding each other or, at times, coexisting within the same sett ing. Identified physical processes of effusive volcanism include coarser an d finer pyroclastic flows (massive pyroclastic breccia and lapilli-tuff bre ccia facies), sedimentary reworking of inferred ash-cloud fine material (la pilli-tuff facies), distal sheetflood (stratified lapilli-tuff breccia faci es) and debris-flow (massive reworked lapilli-tuff breccia facies) reworkin g of pyroclastic debris. Two prominent lenses of volcaniclastic rocks prese rved within the study area may lie either side of an eruptive centre and a palaeovalley may have confined lahar deposits. The predominance of pyroclas tic flow and lahar deposits in the Hekpoort Formation resembles the success ion formed from the 1991 Mount Pinatubo eruption. (C) 2000 Published by Els evier Science B.V.