The chemical determination of 3N HCI-soluble Mn and Fe was carried out on 90 samples taken from the carbonate-argillaceous formations of the Jurassic Bihendula sequence (northern Somalia). For purposes of comparison, 1 N CH <sub>3</sub> COOH-solublc Mn and Fe and whole rock Mn and Fe were also determined on some samples. Relationships between Mn and Fe contents, mineralogical composition, weathering on the source area, depositional environments of the formations analyzed, and diagenetic processes have led to the following conclusions: 1) Mn is associated mainly with the carbonate fraction, whereas Fe is essentially associated with the clay one. 2) There are no clear relationships between Mn content and depositional environments, despite the fact that Mn is associated with the carbonate fraction. In fact, Mn contents differ sharply in carbonate sediments deposited in similar environments. 3) Mn and Fe contents are related to the intensity and the type of weathering, chemical or physical, on the source area. Mn tends to be enriched in carbonate sediments deposited during periods of prevalently chemical weathering; Fe, on the other hand, tends to be enriched in detrital, clay sediments that are deposited during periods of prevalent erosion. 4) Dolomitization tends to produce an increase in the contents of Mn and Fe.

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