Geological boundary dispute: reflecting on the ability of the traditional classified geological map to fully represent geology

This work seeks to encourage reflection and discussion on the ability and suitability of traditional classified geological maps to represent the full complexities of geology in the wild, and to consider why this is important to think about in order to serve 21^st century geological mapping purposes.

The key components of traditional classified geological maps are boundary lines (in 2D), and boundary surfaces (in 3D); both of which must be ‘closed’ to form polygons or volumes representing the various classes of the map. These lines, polygons, surfaces, and volumes carry geological meaning, but what exactly?

The boundary lines that we traditionally construct geological maps from represent changes in geology, such that the geological properties on one side of the line should be different from the geological properties on the other. But, at any point along a drawn boundary line, which geological properties are changing, and by how much, and how sharply is this change occurring?

The line-based construction of the traditional classified geological map gives a restrictive view of geology. A line gives an on/off binary indication of a change in geological properties. Are we to believe that the change in geological properties is equal at all points along the perimeter of any geological polygon? Logically, the magnitude of change in geological properties (perhaps assume the sum of magnitudes of change for all properties, but it could also be for an individual property) must have a maximum somewhere along the perimeter of the polygon – perhaps this is the point that is most deserving of being represented by the line, but does the entire perimeter deserve to be represented by that line?

The use of a line to indicate a boundary also implies infinite sharpness; that the change in geological properties is instantaneous on crossing the line. Whilst this may be appropriate for faults and unconformities, lines leave us unable to fairly represent the many gradational processes that are inherent to the geological system, examples of which include partial melting, fractional crystallisation, gradational sediment deposition and diagenesis.

So where do these limitations of traditional line-and-polygon based geological mapping leave us? Representing geology in its true complexity requires mapping the individual geological properties themselves through space rather than only delineating where they significantly and collectively change. If we map the geological properties as a collection of scalar fields (as in implicit geological modelling), then all changes – big and small – for all properties are revealed in the magnitude of their gradients. Correspondingly, it appears that traditional hand-drawn geological maps attempt to approximate the sum of the magnitude of the gradients of commonly considered geological properties (age, composition, texture), albeit with a thresholded presentation owing to the line-based approach (the line doesn’t have an intensity, it either is or is not) and some necessary inconsistencies to enable polygon closure. When we consider these points, going beyond the traditional classified geological map seems crucial for progressing the completeness of our geological knowledge in the 21^st century.