Abstract

The 90 × 200 km coastal plain of southeastern South Australia provides a setting which throughout Quaternary time was ideally suited for the sedimentation of dolomite and other carbonate minerals. Comparative studies between various present-day sedimentary environments and cores taken from regions of active dolomite formation near the Coorong Lagoon have defined a typical Holocene regressive sedimentary cycle for the area. A basal restricted marine and lagoonal aragonite and Mg-calcite unit, approximately 6,500 yr old, passes upwards into ephemeral lake units with carbonate mineralogies ranging from protodolomite +Mg-calcite to ordered dolomite. Hydrologic data show that the present coastal zone is the main area where seaward-flowing shallow ground waters emerge from unconfined carbonate aquifers beneath the coastal plain. This fact, and the observation that modern dolomite is also forming in isolated areas of ground-water discharge and evaporation remote from the coast, suggests that dolomite ideally forms from ions provided by ground waters, whereas aragonite, Mg-calcite, and protodolomite assemblages form from a marine or mixed sea-water–ground-water reservoir. The observed Holocene cycle is therefore explained by progressive seaward movement of a sea-water-ground-water interface, due to a slight relative fall of sea level about 6,500 yr ago combined with lateral sediment accretion. That the bulk of the dolomite formation is presently occurring near the coast is explained by the fact that this is the site, near base level, of most ground-water discharge and evaporation. Possible sources of Mg ions for the extensive formation of dolomite throughout Quaternary time are Mg depletion of high Mg-calcite allochems in the carbonate aquifers and leaching of ash deposits around Quaternary volcanic centers near ground-water source areas.

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