The pendent calcretes in semi-arid climates: An example located near Taforalt, NW Morocco

• Published in: Catena (Giessen) 1989, Vol.16 (3), p.237-249
• Main Authors: Ducloux, J., Laouina, A.
• Format: Article
• Language: English
• Subjects: Bgi / Prodig; Earth sciences; Earth, ocean, space; Exact sciences and technology; Geomorphology; Mineralogy; Mineralogy: general, methodology, regional studies; Petrology of sedimentary rocks except quaternary rocks; Physical geography; Sedimentary rocks; Weathering
• ISSN: 0341-8162; 1872-6887
• DOI: 10.1016/0341-8162(89)90011-8

In temperate climates, supergene carbonates often form “beards” or “pendents” on pebbles. Calcite crystallizations are known to develop according to a centripetal sequence, from the void to the pebble and correspond to a cemented calcrete. Under the semi-arid climate of NE Morocco, similar calcretes develop below pebbles. However, the concentration processes are different. Indeed, the first acicular low-Mg calcite crystals appear at the point of contact with the lower face of the dolomitic pebbles, and are mixed with discoi¨dal dolomite crystals. Towards the outside, composite needles, then lublinite and rhombohedrons are formed by successive crystallizations. In the first formed needles, the Mg-rate is subordinate to the rock composition (calcite or dolomite). Then it becomes approximately constant, except when palygorskite is noted. The calcrete would develop centrifugally: its carbonates would not come from external sources but would result from the pebble's weathering. This process could be connected with calcareous epigenetic weathering. Sous climat tempe´re´, les carbonates superge`nes s'accumulenta`la face infe´rieure des galets des formations graveleuses selon une se´quence mine´ralogique de la calcitea`différenciation centripe`te. Ces calcins sonta`rattacher aux encrouˆtements de cimentation. Sous le climat semi-aride du Maroc nord-oriental, et notamment pre`s de Taforalt, des calcins se de´veloppent de la meˆme manie`rea`la face infe´rieure de galets de calcaires dolomitiques. Cependant, les processus de concentration sont diffe´rents. En effet, les premiers cristaux de calcite, de forme aciculaire eta`faible teneur en magne´sium, se de´veloppent au contact du galet, mélésa`des cristaux discoi¨daux re´siduels de dolomie. Vers l'exte´rieur, des aiguilles compose´es, puis de la lublinite et des rhomboe`dres cristallisent. Dans les premiers cristaux, le taux de Mg is de´pendant de la composition de la roche (calcitique ou dolomitique), puis il devienta`peu pre`s constant, sauf lorsque apparaiˆt de la palygorskite. Ce calcin se de´velopperait donc selon une se´quence centrifuge; ses carbonates ne proviendraient pas de l'exte´rieur, mais re´sulteraient de l'alte´ration du galet suivie de recristallisations de calcite peu magne´sienne. Il seraita`rattacher aux encrouˆtementsépigéniques.