The Quebrada Arriba Formation consists of alternating beds of chamositic oolite, limestone, sandstone, and shale. The oolites consist of a framework of originally calcareous ooids, fossils, and intraclasts set in a calcareous cement, all virtually replaced by chamosite, siderite, hematite, pyrite, and silica. Textural

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analysis reveals the following paragenetic sequence: (1) calcareous fossils and ooids, (2) silica/pyrite, (3) chamosite, (4) siderite, and (5) hematite.

Petrographic evidence suggests a quiet water back-reef origin for the calcareous oolite. The first secondary minerals to form are pyrite and silica, the latter being mostly concentrated in foraminiferal tests. The mutually replacive relationship of silica and pyrite implies their cogenetic origin in a reducing barred environment. Abundant diagenetic chamosite formed next, replacing calcareous ooids and fossils in a still reducing but shallower environment. At this stage, dissolution of original carbonate sediments resulted in a high concentration of carbon dioxide in the basin facilitating precipitation of siderite. Hematite formed last in an oxidizing environment at the expense of earlier formed iron-bearing minerals. The abundance of pyrite/siderite and a corresponding scarcity of h matite in subsurface samples and the reverse relationship in outcrop samples imply oxidation of pyrite/siderite under surface conditions to produce hematite. The source of iron for the ferriferous minerals could be lateritization of emergent source rocks during a regressive phase. Fluvial supply either as hydrosols, colloidal suspension or adsorbed particles on clays would have concentrated the iron in a barred environment. Shelf-margin barriers in the form of shoals and reefs (for example, the El Guamo and Berlin limestones) prevented dilution and loss of the iron-bearing solution which on reaching sufficient concentration started precipitating different minerals under different Eh-pH conditions.

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