Elements of the geological maps

From a formal point of view, a geological map is a cartographic document in which information has been projected, over a horizontal plan, in four dimensions: the three dimensions of the ordinary space, to which are added the geological time. It is important to point out that the coordinate Z (vertical) does not only refer to the topographic elevation, but also to the column of rocks that can be found under a determined point, and also to the materials that cover it and which have been eroded.

In a geological map the geological elements are represented, conveniently symbolized: the cartographic units with the epigraphs that identify them, the contacts between these and the geological point features, and also other elements corresponding to geological lines or surfaces..

The “cartographic units” (geological map units) are the essential elements of a geological map. A cartographic unit is the representation of a volume of rock with a lithology or constant lithologic association, of a determinate age, limited by surfaces that make contact with the adjacent bodies of rock. It can be recognized on the terrain and can be differentiated from the cartographic units with which they are in contact (photo and figure 1). Frequently, the units of sedimentary rocks are called Formations (Fm.) if they have been formally defined in a concrete place or locality type (i.e. Girona limestone Fm., Montserrat conglomerates Fm.). The intersection area between these rocky volumes and the topographic surface (figures 1 and 2) are represented in the geological map).

Photo anf figure 1: In the photograph clearly two sets of rocks can be observed: a reddish and soft unit at the bottom in over which a set of ochre-coloured stratum is superimposed, massive at the base. The contact between both units is sharp. In the topographic map these units are represented and the projection of the contact is drawn.

Figure 2: a) Representation of several cartographic units and the different types of contacts that delimit them over the shading of the terrain. Below: the geochronological legend and that of geological symbols. b) Block-diagram in which the same cartographic units of figure 2a have been represented.

Each cartographic unit is characterized in the map by a colour, indicative of the age or type of rock, that could have a superimposed pattern indicating the lithology or some geological process (figure 2). They are identified through the “epigraphs” or map unit labels. An epigraph is a code formed by the combination of alphabetical and/or numerical elements with which a cartographic unit is designated. The formulation informs of the age or of the processes that intervened in their genesis. The epigraph accompanies the cartographic units in all the elements of the geological map (main map, geological cross-sections and sketches) and it is the code that allows us to relate each cartographic unit in the map with the explanation in the legend.

The contacts are the surfaces that delimit the rocky volumes. In a geological map the intersection line is represented between the surface that limits two different volumes of rock and the topographic surface. The geological contacts are classified into four main groups, paying attention to the geological process that has generated them:

• Sedimentary contacts: these are the contacts between sedimentary rocks, or between these and their substratum; the most common are the "normal or conformable contact", the "transitional or gradational contacts" and the "unconformable contacts".
• Igneous contacts: these are the contacts between bodies of intrusive igneous rocks and the rocks where they intrude, or the contact between themselves; in the case of the volcanic rocks the base of the outflows of lava is also considered in this way.
• Limits of metamorphic processes: these correspond to the outlines of the rocky volumes affected by metamorphism, or in other words, the limit of the aureole of contact metamorphism, or the limits between different metamorphic zones of regional metamorphism.
• Mechanical contacts: these are the contacts between rocky units that correspond to fractured surfaces, accompanied by a relative movement of the two blocks that are delimited by them. They can be of several types; the most common are the "normal faults", the "thrust faults" and the "reverse faults".

Each type of contact is represented in the map through specific lines, colours or symbols (figure 2). When it is not possible to accurately draw the trace of a contact on the terrain due to the conditions of the exposure, this is indicated in the geological map as “supposed or deduced contact".

The geological map also represents the traces of other surfaces: those characterizing the fold structures (axial planes) or other geological elements related to the internal geometry of the cartographic units or groups of these (joint, traces of foliation, etc). In contrast to the contacts, these lines do not delimit cartographic units.

The geological maps include, conveniently symbolized, data on the orientation measures of the surfaces (stratification, joints, rock cleavage, etc.) and data related to the linear structures (fold axis, fault scoring, etc) necessary for their correct reading and interpretation. The situation of paleontological sites, the data of which have been used to date the materials included in them, indication of minerals, fountains, test drilling and wells, among others, is also represented.

The representation system of the described cartographic elements has the aim of facilitating the reading of the map, providing indications in a simple way about the distribution of the cartographic units, their age and the spatial and chronological relationship between them, and also indications about their disposition in depth. This capacity to communicate information about additional dimensions (subsurface, the past) and the interpretative component places the geological map on a different level with respect to other thematic maps.

All the geological maps are accompanied by a descriptive legend of the cartographic units and a list of geological symbols used (contacts, traces and point data) (Figure 2a). In the legend of the cartographic units the characteristics of each cartographic rocky volume are explained. In general, the legends are essentially descriptive, but they also give information about the genesis of each unit, its properties, its age, the formal names or other data which are considered to be relevant. The order of the cartographic units in the legend is chronological, the most modern at the top and the oldest at the bottom.

This systematization is evident for the sedimentary record, although it is a little more complex when including the igneous and metamorphic rocks. The legend of geological symbols is the list and the explanation of the graphic symbology of the contacts between the cartographic units and of other traces and point data.

The geological maps can incorporate other graphic elements that extend the information expressed in the map and complement the description of the cartographic units. In the case of the sedimentary record stratigraphic columns (figure 3), correlation panels and diagrams of stratigraphic relationships are included. To facilitate the visualization of the structure in depth the geological cross-sections (figure 4) or the block diagrams (figure 2 b) are included.

Figure 3: Stratigraphic columns with indications of the correlation between them.

The stratigraphic columns are a graphic representation of the vertical succession of the sedimentary and volcanic rocks just as they are deposited: the oldest in the lower part and the most modern in the upper part; the thickness of each unit is measured directly in the field and is represented at scale. In the stratigraphic column, the lithologic constitution of each unit is indicated through a specific symbology.

The outlines of stratigraphic relationships and the correlation panels conceptually synthesize the relative position of each cartographic unit with respect to the other units and also with regard to the geological structures. They provide an idea about the succession of geological events and the relationship between them, completing the information expressed in the legend of the cartographic units.

The geological cross-sections are the graphical representation, in a vertical plane, of the interpretation of the disposition of the materials in the subsurface (figure 4). The geological cross-sections are constructed through the projection in depth of the data obtained on the surface, essentially the position of the contacts, the measurements of the dip of the layers and the relationships between the different rocky volumes. They are a model of the real distribution of the rocks in depth, coherent with the rest of the information in the map. The interpretation in depth is more accurate if information is available about the subsurface, as for example, from boring.

In short, the information in the geological maps is structured to allow different levels of reading, from the most basic, in which the user identifies the lithology at a concrete point of the terrain (two dimensions, 2D), the forecast of what can be found in depth at a concrete point (three dimensions, 3D) until being able can deduce the succession of geological events over time (four dimensions: 3D + Time) of the region that the map covers, in other words, geological history.

Figure 4: Geological cross-section.