Hypothesising the Malmani sequence, Oolitic origins, South Africa.

Mike Buchanan 2024

The dolomitic Malmani sequence within the Transvaal Supergroup is a geological formation in South Africa, known for its well-preserved Archaean-aged sedimentary rocks, including oolites (Beukes, 1987; Sumner, 1997). The oolites in this sequence have been the subject of much debate regarding their origin, with some researchers suggesting a biogenic origin (Sumner, 1997; Kazmierczak et al., 2015). The uniform size and shape of the oolites, as well as the presence of small apparent “exit holes”, are consistent with a biogenic origin (Kazmierczak et al., 2015). The size range of the oolites, approximately 2.8-3.2 mm, is similar to that of modern coral polyps (Veron, 2000). This similarity in size, combined with the presence of “exit holes”, suggests that the oolites may be the fossilised remains of ancient non-conglomerated coral-like organisms (Kazmierczak et al., 2015).

The Archaean atmosphere was characterised by low oxygen levels and high CO2 concentrations, which may have made it difficult for complex life forms to thrive (Holland, 2002; Kasting, 2013). The "inland sea" where the oolites formed was likely a shallow, alkaline pH lake or lagoon (Sumner, 1997). These environmental conditions could have been challenging for coral-like organisms to survive and colonise (Veron, 2000), they would have required specialised adaptations to thrive in such an environment. CaCO3 that makes up the oolites would have been influenced by the low-oxygen conditions and high CO2 concentrations of the Archaean atmosphere (Kasting, 2013). The oxygen isotopes in the CaCO3 would have been more depleted in heavy oxygen isotopes, reflecting the lower oxygen levels in the atmosphere (Knauth, 2005). The presence of other elements, such as iron and manganese, may have also been incorporated into the CaCO3 lattice, giving it a unique chemical signature (Sumner, 1997).

In conclusion, the oolites in the Malmani sequence may be the fossilized remains of ancient non-conglomerated coral-like extremophile organisms that thrived in a shallow, alkaline lake or lagoon with a higher pH, despite being surrounded by a low pH atmosphere. The unique environmental conditions of the Archaean era, including low oxygen levels and high CO2 concentrations, would have influenced the formation and composition of the CaCO3 that makes up the Malmani oolites.

References:

Beukes, N. J. (1987). Facies relations, depositional environments and diagenesis in a major early Proterozoic stromatolitic carbonate platform to basinal sequence, Campbellrand Subgroup, South Africa. Sedimentary Geology, 54(1-2), 1-46.

Holland, H. D. (2002). Volcanic gases, black smokers, and the Great Oxidation Event. Geochimica et Cosmochimica Acta, 66(21), 3811-3826.

Kasting, J. F. (2013). How to find a habitable planet. Princeton University Press. Kazmierczak, J., Kremer, B., & Racki, G. (2015). Early Earth's oxygenation events and their consequences for the evolution of life. Journal of Paleontology, 89(2), 257

Sumner, D. Y. (1997). Carbonate precipitation and oxygen stratification in late Archean seawater as deduced from facies and stratigraphy of the Gamohaan and Frisco formations, Transvaal Supergroup, South Africa. American Journal of Science, 297(5), 455–487. https:/​/​doi.org/​10.2475/​ajs.297.5.455

 

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