Africa Geological Atlas

METHODOLOGY

The research towards this atlas has been long and complex .The reasons for the lack of any previous attempts at mapping at this scale are many. Africa has no centre for regional geological study : attempts at matching geological maps over national boundaries show the general lack of coordination between countries in the past, though this is improving. Academic studies, are
often necessarily confined to outcrop geology. Much of the required subsurface data on which this atlas is compiled, and most of it on continental margins, comes therefore from petroleum drilling.  The frustrating issue here is that despite the huge amount of such data existing, very little has been placed in the public domain. Many African governments explicity ban the public release of offshore geological data by operators. The main author of this atlas is perhaps well placed to improve this situation, having worked in the African oil industry for nearly 30 years and having organised and attended several technical industry conferences, most noteably 20 years of the Petroleum Exploration Society of Great Britain/Houston Geological Society ‘Africa conference’. In addition, I have held numerous side discussions at these conferences with workers who have seen much more offshore data or have delved much deeper than I have into particular aspects and regions. From such verbal presentations and discussions, when combined with what has been published, consistent models can be developed and these form the cornerstone of the offshore interpretations shown on these maps. In many cases, the unpublished interpretations derived from operators have made the choice apparent between differing interpretations provided in the written literature. Each aspect of the maps will now be considered, though it should be noted that there are important interplays between these, e.g. a high sedimentation rate not only requires a large river catchment but also a wet climate and the development of steep slopes, i.e. topography (and of course an explanation for this) and so solutions can be a circular process.


Plate Tectonic Model

The prime requirement for this study is to establish relative plate positions over time as accurately as possible. The starting point for such reconstructions is the work of the co-author of this atlas, Colin Reeves www.reeves.nl/gondwana . Supporting material   can be accessed separately on  this website  The work has focused on matching conjugate ocean fracture zones (Reeves and de Wit, 2000), time-calibrated using the limited number of identified pre-83 Ma marine magnetic anomalies. The mid-ocean ridges themselves have also been modelled quantitatively. Margins for error are reduced overall by matching not only conjugate pairs of fracture zones at the ridges but also by working across all conjugate margins of Gondwana simultaneously and including the behaviour of the ridge triple junctions. Inconsistencies revealed through animating the resulting model are then eliminated iteratively to produce a credible dynamic model that honours first principles and as much of the oceanic data as possible. An amendment is made to the Africa-Falklands fit so as to the tie to the stratigraphic evidence presented under Jurassic Tectonic Maps. Over some margins, the fits are also relaxed slightly (circa 50km) as a bit of ‘artistic licence’ so as to prevent any overlap of tectonic lineaments or facies belts on conjugate margins. The African plate is kept fixed on these maps, with an expansion and clockwise rotation of circa 150km applied over the Early Cretaceous to account for the widespread extension at that time, plus some movements along other shear zones at different times. Surrounding continents have been located on the maps  by georeferencing to African shorelines and therefore may show warped geometries..   Reeves reconstructions do not extend to Tethys so the Arabian Plate model is after  Barrier et al 2016  , while  the relative positions of Iberia, Alcapeka and ‘Apulia’ are after Handy et al 2010,  van Hinsbergen et al, 2020 and Carminati et al, 2012. The Central Atlantic reconstructions also accommodate  the models of Teasdale, in Casson. PhD thesis .  The order of uncertainty in these  plate reconstructions in terms of decreasing confidence in fit and timing is : Red Sea, South Atlantic, Indian Ocean and Somali Basin, Central Atlantic, Western Mediterranean, Agulhas Margin/Falklands, and finally Eastern Mediterranean.

Intra-Plate Lineaments

Extensional, compressional and tectonics are portrayed on the maps in the time periods in which they are interpreted to be active. The first stage of the analysis was to compile an ArcGIS shape file of faults across Africa based on georeferenced tectonic elements maps. These faults were then assigned attributes as to when they were and were not thought to be active, facilitating the creating of a manipulatable dataset that could then display active faults at specific geological times.  In most cases, there is relatively good control on the timing of faults in the form of geological evidence such as bounding unconformities, stratigraphic and volcanic ages and stratigraphic growth on seismic  into faults.  In a supporting paper by the author (Macgregor (2014)) , several other lines of evidence for the timing of faults are discussed in more detail, as applied to the East African Rift System

Extensional faults are differentiated as when these are most active or remain active at a slower rate, e.g. the main syn-rift phase of rifts and periods in which periods in which post-rift relaxation or reactivation. The category of compressive features includes anticlines and reverse faults; the density of these is a guide to the intensity of these. Transform and shear faults are differentiated as to whether the movement at the time of the map concerned is sinistral or dextral. The extents of the large interpreted shear zones shown crossing the continent are often conjectural.

Paleotopography

The interpretation of topography decreases in confidence with increased age. For Neogene and Oligocene intervals, interpretations of the age of existing topography can be backtracked through time (e.g. Paul et al, 2014) using river profiles. Marked changes around the early Oligocene mark the effective limits of this technique and a step change in confidence. For Paleogene and Mesozoic intervals, AFTA evidence for rapid cooling, presumably related to uplift, becomes the primary technique. The literature for Africa has been scanned and arrows are added on the maps for when minor and major uplift is evidenced. Again, this technique has a lower stratigraphic limit around the middle of the Cretaceous, as apatite clocks were generally set after this time, and another step change down in confidence occurs. Other more indirect lines of evidence include sedimentation rates, predictions from tectonic models , vitrinite reflectance profiles, extrapolations of coastal monoclines and of erosion surfaces and the frequency of marine transgressions, which are taken to imply topography below 150m. The Present Day is used as an analogue, with carbonate deposition tied to low sediment supplies and hinterland topography, and high clastic sedimentation rates tied to large erosion prone slopes within their catchment areas . All paleotopography is relative with no elevation figures implied by the categories assigned.

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Paleodrainage

sThe assessment of paleodrainage relies on a circular process reaching agreement over a number of techniques, involving climatic and topographic interpretations amongst other. A starting point is the interpretation of Goudie (2005), though substantial modifications have been made. Modern rivers can be modelled from their profiles as far back as the Paleogene (Paul et al, 2014) . For older intervals, the key evidence used is sedimentary rates and volumes off the mouths of interpreted rivers.  A rough relationship between sedimentary rate, volume and the length of the river system (assuming a wet climate) was established by Nyberg et al, 2018. Offshore sedimentary rates calculated using the methodology presented in Macgregor (2012) are thus used to interpret relative sizes of catchment areas. Marker mineral work, including zircons,  is available only locally, including for the Nile (Fielding et al, 2018), over East Africa (Geotrack) and Equatorial Margin (Ye, 2020), the latter two being papers presented at PESGB Africa conferences. Work on paleodrainage is well established in the southern Africa literature, where it is often based on heavy mineral, gold and diamond distributions (e.g. Phillips et al 2018)   The absence of significant continental margin depocentres from the Jurassic downwards makes the interpretation of paleodrainage in such older intervals very uncertain.

Facies Distributions and Shorelines

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Paleoclimate

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Volcanism

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