Nature mineralogique et origine des glauconies du plateau continental du Gabon et du Congo

Authors

  • PIERRE GIRESSE,

    1. Laboratoire de Géologie, Centre d'Enseignement Supériur, B.P. 69, Brazzaville, Congo et Université de Paris VI, Groupe d'Étude des Bassins Sedimentaires, 11 Quai Saint Bernard, 75 Paris 5e
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  • GILLES S. ODIN

    1. Laboratoire de Géologie, Centre d'Enseignement Supériur, B.P. 69, Brazzaville, Congo et Université de Paris VI, Groupe d'Étude des Bassins Sedimentaires, 11 Quai Saint Bernard, 75 Paris 5e
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RÉSUMÉ

Sur le plateau continental du Congo et du Gabon, on constate la formation de goethite, de berthiérine et de glauconies (smectite et glauconite). La berthiérine caractérise un milieu deltaïque a sedimentation active; les glauconies se forment sur le plateau continental à plus grande profondeur (—50à—300m). Dans un milieu original granulaire (généralement des coprolites) tous les stades d'une succession minéralogique depuis la kaolinite jusqu'à un minéral voisin de la glauconite ont été décelés. Cette formation passe nécessairement par le stade smectite ferrifère. La formation préalable de granules crée un milieu semi-confiné remarquable qui est, ici, la condition primordiale des évolutions minérales.

ABSTRACT

The detailed analysis of pellets deposited on the continental shelf off Congo and Gabon (Gulf of Guinea) reveals the present formation of goethite and berthierine-granules, as well as that of glauconite.

The goethite is being formed in the most oxygenized zones, independently of the initial nature of the granular support and, sometimes, takes the form of pseudooolites. The berthierine is being formed in the Ogooue delta zone. To us the deltaic environment, rather than the temperature factor, seems determinant. Principally, this berthierine is found in the form of coprolites, of initial kaolinic composition. Incrustations of calcareous debris and fillings of animalculae tests are present. This genesis is recent (post-Flandrian) and is still continuing. The glauconite exists practically uniquely in the form of coprolites, from the medium-depth zones of the continental shelf (—50 m) down to the upper continental slope (—300 m). The sèdiment in which it was formed, and is probably still being formed, is older (ante-Flandrian). The actual rate of sedimention is null when glauconite is forming.

The glauconite formation may be broken down into several stages, which are found at different sampling depths and, sometimes, in the same samples. The pellets are formed first. These original pellets, of kaolinic nature, gradually become smectitic. The evolution continues, becoming a component with diffractometric behaviour identical with that of an interlayered material (illite-vermiculite) with the illite dominant. Correlatively, the K2O percentage increases from 2–3% in the smectites to 5–6% in the most evolved component. The mineralogical composition of these last pellets is identical with that of certain fossil glauconites, the evolution of which could be followed up to the best crystallized glauconite mineral. Thus, we were able to reconstitute the formation history of the glauconite from any mineralogical support up to the typical glauconite mineral (Table l).

Two conditions necessary for this formation are detected here: the time factor, as has often been pointed out, is important. However, it is, above all, the environment factor which has attracted our attention. The forming of pellets favours and determines the geo-chemical evolution through the creation of a semi-confined environment. Reactions take place there in a more concentrated environment than in the open sea, this environment is characterized by a large surface of reaction, since the granule is porous.

Sedimentologically, it is possible to distinguish two groups of glauconites: one, monomineral, corresponds to more or less crystallized glauconite mineral; the other, plurimineral, consists of various TOT minerals. Only the first has the significance generally attributed to all glauconites : open epicontinental marine environment, free of sedimentation for a long time.

The various cases of evolution encountered here are summarized below (Table 1).

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