To portray or infer depth to crystalline basement

To portray or infer depth to crystalline basement

Four data sources were combined to produce the Delamerian Basement Elevation Grid: 1. Drillholes with stratigraphic logs that encountered units defined as basement within the project area. 2. Drillholes with lithological logs that encountered...

Four data sources were combined to produce the Delamerian Basement Elevation Grid: 1. Drillholes with stratigraphic logs that encountered units defined as basement within the project area. 2. Drillholes with lithological logs that encountered lithologies considered as basement units. 3. Selected drillholes that stopped short of basement units, but assisted with the depth interpolations (these are not included in the final data package) 4. 100K mapsheet polygons of outcropping basement units were used to clip a 3 second SRTM Digital Elevation Model (DEM) and converted to point data. 

Using Intrepid Geophysics, the variable density gridding method was applied to compute the final basement elevation (relative to sea level) grid, with a resolution of 800m cells. This was determined to be the optimal resolution due to the sparsity of drillholes over the entirety of the project area. A direct comparison of the drillhole point data and the basement elevation grid, in some instances, may diverge due to interpolation effects over large distances.  For this reason it is recommended that interpretations utilise the drillhole point data in tandem with the interpolated grid product.

The cover thickness grid was calculated using the DEM, resampled to 800m to match the resolution of the basement elevation grid. In ArcGIS, the Raster Calculator tool was used to subtract the basement elevation grid from the DEM to produce the final output.

Two reliability maps are also provided. A point spacing reliabilty map which provides an overview of the distances between basement data points; and a nominal grid resolution reliability map. In general terms, sparse data points produce outputs that are considered less reliable than close-spaced data points.

Source data include information submitted by mineral and petroleum exploration companies as part of statutory reporting requirements, information from academic institutions, and information collected by Department for Energy and Mining (DEM) and its predecessors. Hardcopy data were transcribed manually into SA_GEODATA or manually via project databases which were later uploaded. Data received in digital form were checked and corrected as necessary before upload into SA_GEODATA. Source history: Locational coordinates and The data is updated in SARIG on a daily basis.

The Geological Survey of South Australia's SA_GEODATA database holds stratigraphic logs for over 43,000 drillholes, coded with an age based Map Symbol system which is also used for corporate digital map polygons, including the Mineral Geological Provinces layer. Therefore, a robust query of the SA_GEODATA log data based on the stratigraphic units, has enabled production of point data of stratigraphic units and top depths of crystalline basement, as well as the top depths of eight time slices which highlight the major phases of the State's geological development based on separation by major breaks in the rock record. The eight time slices identified are · Archaean to early Mesoproterozoic · Middle Mesoproterozoic · Late Mesoproterozoic · Neoproterozoic · Cambrian to Late Carboniferous · Late Carboniferous to Triassic · Jurassic to Cretaceous · Cenozoic

In the eastern portion of SA, where Ordovician and older rocks are affected by the Cambrian-Ordovician Delamerian Orogeny and older events, crystalline basement includes the Adelaide Geosyncline, Kanmantoo Province and basement to the Murray Basin. This latter affected area excludes the cratonic region of the Curnamona Province. In the Warburton Basin, Cambro-Ordovican sediments are included, despite minimal evidence of Delamerian influence, as basement to these rocks has only been encountered in a few drillholes and may not be detectable with geophysics.

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