The aim of this geological synthesis is to present a homogeneous geological map covering all seven Andean countries and favouring a stratigraphic delimitation. As can be imagined, with seven countries in an area of 3.83 million km² that extends for some 8500 km from north to south, it is not possible to provide an exact correlation of the multiple regional/local formations; nevertheless, the habit of referring to stratigraphic formations is general within the geological domain and so it was essential to maintain the access to this information.

In this light, the formations were first grouped according to age and general type (e.g. sedimentary, volcanic, plutonic, metamorphic, etc.). This made it possible to pass from more than 1500 listed formations to 308 preliminary divisions presented as polygons (see Figure 1). Each of these divisions includes an "age and general rock type" code, for which the list of corresponding formations differs from one country to another, and even within the same country because formation names are commonly only local designations.

Figure 1

Figure 2

This first grouping still being too large for a clear synthetic picture at a scale of 1:2 million, the preliminary divisions were regrouped into 69 final stratigraphic/rock type divisions (Figure 2) giving a level of detail compatible with the synthetic map scale. This synthesis was achieved by simplifying the stratigraphic divisions at the same time as grouping similar rock types.

The base information behind the synthesis is easily accessed by 1) zooming-in on the area of interest on the map, which will display the relevant polygon(s) within that area, 2) clicking on the relevant polygon(s), which will then display the names of the formations grouped within this division. The geological data related to each polygon is stored in tabular form in which the attributes are shown according to the original map data (Sample extract from the Peru Geologic Table). The geological boundaries of individual formation, however, cannot be accessed.

The synthesis map was plotted onto the DCW (ESRI) geographic base using the Transverse Mercator (WGS 84, -72°) projection. The geology was compiled from various geological maps at different scales (from 1:100,000 to 1:2,500,000) and commonly in different projections (UTM, Lambert conformal conic, Lallemand modified polyconic, Gauss-Krüger, etc.) for which the parameters were generally not given.

Because the synthesis was prepared at a scale of 1:2,000,000, the precision of the boundaries can never be more accurate than ± 2 km. Consequently the authors take no responsibility for any consequences resulting from using the map data at a larger scale.