Characterisation of septarias in the Tatao-Ambondromamy region of Madagascar

Tatao area, localised at the northwest of Madagascar near by Ambondromamy is known for it's richness in septarian concretion deposits.Those septarian nodules are much admired by lithotherapy enthusiasts and collectors while geological concept about genesis of this sedimentary rocks is musenderstood.Analysing deposits in Tatao area helps to improve geodynamical knowledge of Mahajanga basin and can be used as a model of global warming effect and intense evaporation inside of lagoon.Field observation, thin sections were therefore carried out to try understanding the origin mechanisms likely to play a role in the phenomena that lead to the diversity of coloration, size, and types of septaria.The origin of the color diversification of calcites (yellow, brown, milky white, grey, black) can be interpreted as a different episode of crystallizations as well as different liquids origin.As for the size and shape of septaria, they are generally conditioned by the lithological nature, the conditions of sedimentary deposit and above all, by tectonics.