OR/18/004 Overview of current geological mapping of Sierra Leone

Goodenough, K M, Jones, D, and Ford, J. 2018. Geological mapping of Sierra Leone: baseline assessment and next steps. British Geological Survey Open Report, OR/18/004.

The bedrock geology of the north-eastern half of Sierra Leone is dominated by Archaean basement, belonging to the Kenema-Man Archaean domain of the West African craton (Rollinson, 2016). Within this basement a number of distinct supracrustal belts have been mapped, as well as a series of late-Archaean granitoids (Rollinson, 2016). The western margin of the craton in Sierra Leone is marked by a Pan-African deformation zone known as the Rokelides (De Waele et al., 2015) and a strip of Palaeoproterozoic gneiss, the Kasila Group (Williams, 1988), which has been accreted to the craton. The craton margin is unconformably overlain by two metamorphosed volcanosedimentary successions, the Marampa and Rokel River groups. Mesozoic mafic intrusions, associated with the continental rifting and opening of the Atlantic, are present throughout much of Sierra Leone; the most significant of these, the Freetown Complex, underlies the capital city. The youngest lithostratigraphic group is the Bullom Group, comprising onshore Cenozoic sediments deposited along the coastal margin. Superficial geology includes river terraces and alluvium that are locally associated with artisanal diamond and gold mining, and a coastal belt comprising marine and estuarine sediments.

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Figure 1    Simplified geological map of Sierra Leone, derived from the national map (CGS, 2004), © National Minerals Agency. Major lithologies indicated in key. The width of the country is c.250 km, from the most easterly to most westerly point. Numbers indicate supracrustal belts, as in Archaean supracrustal belts.

Most work on natural resources in Sierra Leone depends upon the 1:250 000 national geological map of Sierra Leone, which was compiled in 2004 by the Council for Geosciences for the Geological Survey of Sierra Leone (CGS, 2004). This map is now often treated as being accurate, despite the fact that it has clear shortcomings. Production of the map appears to have involved collation of existing data, possibly with some interpretation of aerial photographs, but no ground-truthing or additional primary data collection. The main data sources (Figure 2) included previous BGS geological mapping of the northern third of the country, carried out during the 1970s (MacFarlane et al., 1981), and mapping carried out by the Geological Survey of Sierra Leone (GSSL) from the 1950s to the 1970s, only some of which has been published (Wilson and Marmo, 1958; Mackenzie, 1961; Marmo, 1962; Wilson, 1965; Andrews-Jones, 1966; Hall, 1968). The published maps included a significant amount of observational detail, some of which was transferred to the CGS (2004) map, although not all the detail was included. In contrast, there are other areas of the country in which the geology has never been mapped, and which lack any real information. The CGS (2004) map is thus very inconsistent in level of detail, and gives a false impression that some areas contain no geological interest, when in fact they simply lack appropriate mapping: the simplicity of the map in these regions is due to absence of evidence, rather than any lack of geological features. In general, there is potential for significant updating of the CGS (2004) map to include a consistent level of detail across the whole country, including a much more consistent classification of the rock units present within Sierra Leone.

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Figure 2    Existing geological mapping of Sierra Leone. Some of the maps shown as unpublished are no longer readily available.

The landscape of Sierra Leone has been extensively affected by tropical weathering, and so natural outcrops are largely confined to river valleys and coastal sections; however, numerous quarries and road cuts provide valuable exposures. Significant amounts of core have been collected during exploration for mineral resources, and where this core material can be studied it will provide valuable additional information for map revision.

In order to develop a modern geological map of Sierra Leone, several important aspects should be addressed. As well as the planned airborne geophysical survey, all available remote sensing data should be gathered. Some remotely sensed data are widely available, for example satellite imagery from USGS Earth Explorer (www.earthexplorer.usgs.gov/) and digital elevation data from NASA (www2.jpl.nasa.gov/srtm/). All existing maps, including those in published papers and PhD theses (where available) should be scanned, georeferenced, and compiled into GIS. Existing published data and information on the geology of Sierra Leone are summarised in the sections below. The NMA already holds many of these maps (and associated bulletins and reports) but they have yet to be consistently georeferenced and stored in a common format. One key objective will be the set-up of a geological dictionary which should include the list of geological units to be mapped, with descriptions for each. Whilst undoubtedly changes and additions will need to be made during new mapping, it is important that any new mapping is done against a coherent list of unit names, and that appropriate data management procedures are followed. Similarly, a clear list of map symbols to be used should be developed, following international standards — a list of the symbols used by BGS will be provided to the NMA. New mapping should be collated in a GIS format to be stored in the GIMS.