From the top (younger strata) to the bottom (older strata), the following lithological
units are differentiated:
Figure 1 Sintoukola Stratigraphic Column
The evaporite sequence, depending on location and local development, contains up to 11 thin to thick depositional cycles. Within the project area, seven cycles are identified and differentiated. The Sintoukola sylvinite zones are found in cycle 7 and lie at an average depth of 261 metres below surface. Each of these cycles consists of several alternations of the sequence rock salt, carnallitite and sometimes with layers rich in bischofite and/or tachyhydrite.
The basic salt cycle of the Congo Coastal Basin is made up from bottom to top of:
1. a thin black shale;
2. a layer of halite;
3. a mixture of halite and carnallite; and
4. sometimes with an end member of bischofite or tachyhydrite.
The cycle structure is very clearly expressed on the down-hole logs. The halite beds can be identified from the logs by their low gamma-ray response and low neutron porosity. The carnallite beds are characterized by relatively high radioactivity, very high neutron porosity reflecting the high percentage of molecular water and a lower density.
Figure 2 – Sintoukola – Kola Cross-Section
The thickness of the evaporite sequence unconformably overlies reduced facies sandstones of the Cocobeach Formation. This contact marks the transition from a rift environment to a marine hypersaline environment. The Cocobeach Formation consist of a series of grey-green sandstone with interbedded pyrite bearing green claystone probably deposited under continental conditions in fluvial and lacustrine environments. This unit tends to form the footwall rock in the project area.
The unit is reported to measure more than a thousand metres locally. It rests unconformably on top of the crystalline basement.
- The project area is overlain by a Pliocene to Pleistocene layer of up to 50 m of unconsolidated and alternating clay and sand beds. The clay and sand units are variably coated by iron oxides. If the phosphate horizons are missing in the underlying weathered sandstones, it is not always possible to distinguish between the clays and sands and the underlying ferruginous sandstones.
- The clays and sand overlie deeply weathered and partly unconsolidated medium to coarse-grained ferruginous sandstone. The ferruginous sandstone formation consists of layers of relatively coarse grained sandstone locally with iron concretions and separated by layers of plastic red clay strata. Occasionally, thin discontinuous lenses of gravel are observed. In the top of this series, phosphate horizons can be present. The unconformable contact of this series to the overlying clay and sand suggests that the region has been tectonically active and that locally the phosphate horizons have been removed through erosion.
- Below the sandstones occurs an interbedded unit of mudstone and dolomitic siltstone referred to as the grey-blue claystone formation. It consists of grey-blue to grey-green claystone to siltstone and fine-grained sandstone with dolomitic cement and some layers/lenses of limestone. The contact to the overlying Iron Bearing Sandstone is an unconformity.
- The dolomitic limestone rests on a (5 to 16 m) thick anhydrite unit (Anhydrite Formation) which marks the top of a thick evaporitic sequence. The Anhydrite Formation consists of alternating anhydrite and marl of up to 16 m thickness. The anhydrite is variable in appearance varying from nodular, 41 brecciated, or laminated to massive. The marls are blue-grey to black and are locally very clay rich. The anhydrite member forms a gradational transition to the underlying evaporite sequence.
Figure 1 Sintoukola Stratigraphic Column
The evaporite sequence, depending on location and local development, contains up to 11 thin to thick depositional cycles. Within the project area, seven cycles are identified and differentiated. The Sintoukola sylvinite zones are found in cycle 7 and lie at an average depth of 261 metres below surface. Each of these cycles consists of several alternations of the sequence rock salt, carnallitite and sometimes with layers rich in bischofite and/or tachyhydrite.
The basic salt cycle of the Congo Coastal Basin is made up from bottom to top of:
1. a thin black shale;
2. a layer of halite;
3. a mixture of halite and carnallite; and
4. sometimes with an end member of bischofite or tachyhydrite.
The cycle structure is very clearly expressed on the down-hole logs. The halite beds can be identified from the logs by their low gamma-ray response and low neutron porosity. The carnallite beds are characterized by relatively high radioactivity, very high neutron porosity reflecting the high percentage of molecular water and a lower density.
Figure 2 – Sintoukola – Kola Cross-Section
Internal sequential cyclicity of the Evaporite Formation (modelled on drill hole EK_01 at the Kola deposit)
The entire evaporite sequence measures on average 600 metres thick with a range from 300 to 900 m. It is thinnest near the crystalline basement in the east and thickens to the west. Contour lines of the depth to the top of the evaporite sequence, based on regional onshore and offshore drill holes, indicate a relative stable and gradually deepening of the formation towards the off shore continental margin.The thickness of the evaporite sequence unconformably overlies reduced facies sandstones of the Cocobeach Formation. This contact marks the transition from a rift environment to a marine hypersaline environment. The Cocobeach Formation consist of a series of grey-green sandstone with interbedded pyrite bearing green claystone probably deposited under continental conditions in fluvial and lacustrine environments. This unit tends to form the footwall rock in the project area.
The unit is reported to measure more than a thousand metres locally. It rests unconformably on top of the crystalline basement.
