Palynological analyses have been undertaken on four fully cored boreholes in the central west Murray Basin (Woodlands-1, Manilla-I, Piangil West-2 and Hatfield-1 holes) to provide a biostratigraphic framework for geological factors related to groundwater. Three of these boreholes penetrated the marine sequence of the Geera Clay and its associated facies, which acts as an aquitard confining groundwater flow within the aquifers of the Renmark Group. This study has been augmented by analysis of cores and cuttings from boreholes in the areas east of the main marine influence, and by reference to previously studied sequences at Oakvale-1, on the western margin of the basin. Early Cretaceous sediments of non-marine character are present at Manilla-1 and at Mitfords Corner in the Wentworth Trough. Within the Cainozoic sequence, application of zonal schemes modified from those developed in the Gippsland Basin shows that aquifers in the lower partof the Renmark Group can be assigned to the Middle and Upper Nothofagidites asperus Zones, of mid-Eocene to earliest Oligocene age. They contain dinoflagellates which suggest that the effects of the marine incursion reflected by the Buccleuch Beds in the southwest of the Murray Basin were probably felt over a much wider area of the basin than hitherto believed. The Geera Clay and its associated facies were deposited during the interval of time represented by the Proteacidites tuberculatus and Triporopollenites bellus Zones or parts thereof, during the Late Oligocene and Early to Middle Miocene. Resolution of the palynological zones is too poor to identify pulses of marine transgression with those represented on the global sea-level curve; phases of onlap including TB1.4 and TB2.3 (22 Ma and 16 Ma) may both be represented. There is a strong likelihood of condensed sequences in this interval. Pollen assemblages from the Murray Basin contain a number of taxa, many of them with affinities to modern tropical plant species or groups which are unknown in other southeastern Australian basins. Many of these are of potential stratigraphic value. Broad scale shifts in vegetation, presumably in response to climatic change, are discernible in the Late Oligocene to earliest Miocene over much of the basin. There is a suggestion that the initiation of this shift, which reflects a change from year-round to seasonal rainfall patterns, was diachronous for west to east, but patterns of change in space and time are complex, so the correlative value of changes in pollen frequencies remains limited.
