This is a raster representing the base surface of the Nulla Basalt Province, inferred from sparse data available, dominated by private water bore records. This interpretation was conducted by a hydrogeologist from Geoscience Australia.
• This is just one model, based on sparse data and considerable palaeotopographic interpretation
• This model relies on the input datasets being accurate. However it is noted that substantial uncertainty exists both in the location of private bores and the use of drillers’ logs for identifying stratigraphic contacts.
• The location of palaeothalwegs is imprecise, and often it is only indicative of the presence of a palaeovalley.
• The purpose of this model is for visualisation purposes, so should not be considered a definitive depth prediction dataset.
Maintenance and Update Frequency: asNeeded
123 bores with strata logs were used to interpolate an initial raster surface using the ArcGIS Topo to Raster command. In addition, the edge of the province was converted to a series of points attributed with the surface elevation for an additional input to Topo to Raster.
The resultant surface raster was converted to vector contours using the Contour spatial analyst tool in ArcGIS
Iteratively altered the generated contours and used these modified contours as an input to the next Topo to Raster run and subsequent contour generation.
After several iterations, bores that were not fully penetrating the basalt, and points along the edge of province were dropped from inputs, and later the contours were given priority over the full-depth bores
Later iterations use extra dummy points to constrain the shape of interpreted features
The interpretation used the idea of sequences of spurs/re-entrants to form ridgelines/valleys.
Contours were modified to ensure that areas of outcropping basalt had a positive thickness. Areas of outcropping basement rocks were used to adjust contours to ensure a small or negative thickness (i.e. with above ground elevation) of basalt remained. Areas mapped as having sedimentary units could be either covering basalt or outside the extent of basalt, so therefore not used to constrain thinking.
Several iterations were required to ensure that (almost entirely) the basalt had positive thickness.
This method is based on the assumption that the base of basalt surface reflects the palaeotopographic surface, and so a topographic method for creating the surface is appropriate. The shapes of contour lines therefore reflect those that could be found in a topographic map.
As the method is iterative, the final product is therefore prone to bias from previous iterations and ultimately the initial Topo to Raster result. It has inherited features from these previous iterations, such as the need for a spur/re-entrant in a subregion.
The resultant model is one possible interpretation of the base of basalt. (No uncertainty has been presented.) Other models are possible.
Future drilling with accurate spatial (X, Y) and depth + strata (Z) data would constrain possible models – this would either suggest modifications to the model presented here, or suggest different ideas for building another model.