On the Formation of Lower Cretaceous Potassium Salts and Tachhydrite in the Sergipe Basin (Brazil) with Some Remarks on Similar Occurrences in West Africa (Gabon, Angola etc.)

Summary

Within an area of about 30 × 40 km² of the Sergipe state in the north-eastern coastal region of Brazil there are three main oil field anticlines (Carmópolis, Siririzinho and Riachuelo) with similar structures in the environment and off-shore continuations into the Atlantic Ocean. Between these more or less dome-like structures we find three salina sub-basins with potash salts, the most important of them being the basin of Taquari-Vassouras and the basin of Santa Rosa de Lima.

Figures 1 and 2 show the general geological conditions with the Lower Cretaceous (Aptian) sequence of the Muribeca Saline Formation especially in the Ibura Series, followed by the Albian Riachuelo Formation, the Cotinguiba Formation of Turonian and Santonian and the Piaçabuçu Lower Tertiary Formation. The sedimentation during the last 1000 my — and already in earlier Pre-Cambrian times - has often been interrupted because the principal tendency of the Brazilian Shield was directed upwards and resulted in uplifting processes of many km. During these dominant movements, tensional stress tectonics created several developments of horst/graben systems and also of some larger epicontinental sea basins with salinar facies, for instance even during the Pennsylvanian (Central Amazonas Basin) and also during the Permian in North-East Brazil.

Stronger dilatational movements with Continent drifting processes have proceeded since Jurassic and especially during Lower Cretaceous, forming mainly the salinar basins of Sergipe and Alagoas north of the San Francisco river. Within the Aptian Muribeca Formation, the principal salt sequences have been deposited in the Ibura Series, and these were so uniform that the sections of dozens of boreholes on the slopes of the three oil field domes could easily be parallelled (cf. scheme of Fig. 3). Therefore, there can be no doubt that the primary sequence of salinar crystallization products with 18 Markers of mainly foliated shales and bituminous clays and anhydritic beds have been homogenous in the whole basin of Sergipe.

At present, however, we find a systematic lateral zoning. Above the three oil field anticilines of Carmópolis, Siririzinho and Riachuelo, all the highly soluble chloridic salts have been leached away completely. Near the tops of the slopes we then find only rock salt, followed downwards by intercalations of sylvinitic layers, whereas the deeper regions of the sub-basins generally show predominantly carnallitic and tachhydritic salt masses. Similar zonal arrangements around oil fields, and dome-like structures are also found on the eastern side of the Atlantic Ocean in Central Africa from Gabon in the north to Angola in the south.

All observations and arguments suggest that the principal cause of the recent lateral zoning of the different salt formation types cannot be primary, but has been produced by secondary leaching and replacement processes, mainly by descending atmospheric waters, but also by slow percolation and impregnation of deeper ground waters and brines. These solutions were wandering mainly along and between impermable foliated shales and/or bituminous clays, but they also penetrated the salt formations locally across the bedding planes by fissure and fault zones. Thus, collapse structures are often found. “Oil field waters” of the underlying Carmopolis Series may also have ascended from below and may have also contributed their share to more or less local “salt horses” (Vertaubungen). Reaction processes between such oil field waters, anhydrite and carnallite decomposing brines which have also enriched carnallitites into sylvinites often with contents of approximately 25% K₂O are responsible also for the formation of great masses of tachhydrite.

In the two main sub-basins of Taquari — Vassouras and Santa Rosa de Lima the thorough investigations of the Compania de Pesquisa de Recursos Minerais (CPRM) and the Departemento Nacional da Produçāo Mineral (DNPM), together with special Potash Projects of PETROBRÁS and supported by dozens of systematically arranged boreholes, have proved the existence of two principal sylvinite areas along the slopes of the main oil field anticlines. The mineable quantities of sylvinite in the layers of more than 3 m thickness — in some areas more than 6–9 m of rich sylvinite — have been calculated to be 525 × 10⁶ t, containing an average of 20–25% K₂O. These quantities and qualities would be sufficient for Brazil’s steadily increasing home consumption for many decades, with a yearly production of 500,000 t or even 1 million t of potash fertilizers with 60% K₂O.