Characteristics of sediments and regolith alterations in the Plio-Pleistocene succession, coastal cliff sections, St Vincent Basin, South Australia

Syn- and post-depositional alterations are distinguished in a detailed lithological and mineralogical study of the largely unconsolidated Plio-Pleistocene non-marine succession in coastal cliff sections in the St Vincent Basin south of Adelaide. At its base, the sequence interfingers with Late Pliocene estuarine marine sediments in places but mostly unconformably overlies older Cenozoic marine sediments or Neproterozoic bedrock. The fluvial and alluvial siliclastics have bioturbation, blocky-prismatic macro-peds and subtle Fe-mottling indicators of hiatuses in deposition, and imprints of soil and shallow groundwater environments. A relative abundance of kaolinite, illite and randomly interstratified illite-smectite reflects both sediment source and conditions in the local depositional environment. A thick deposit of aeolian calcareous silt with associated pedogenic calcretes blankets the succession. Conspicuous bleached Fe mega-mottled intervals and zones of alunite-halloysite within the sequence record post-depositional, groundwater-related alterations in regolith environments. These formed during incision and erosion of the sedimentary fill in the basin in response to regional falls in base level. Each marks a different time and specific geomorphic environment according to the chemistry of the discharge of local groundwaters from aquifers that were intersected by incision and scarp retreat.