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Abstract

The Lower Cretaceous Viodo Carbonate, a lacustrine carbonate in the Congo Basin, was deposited in the evolving South Atlantic rift basin between Africa and South America. Extensive cores and cuttings permit analysis of the complex relations between carbonate facies and the coeval organic-rich shale of the Marnes Noires Formation. Carbonate deposition was primarily controlled by water depth, initiated by falls in lake level, and terminated by rises. During the most pronounced low stand, mollusc coquinas formed a reef-like margin on the platform margin. During lowstands of lesser magnitude, gastropods and oncolite shoals prograded to the platform edge.

The Viodo Carbonate can be divided into three lithostratigraphic members designated the Lower Viodo Carbonate, the Mid-Viodo Shale, and the Upper Viodo Carbonate. Deposition of the Lower Viodo Carbonate began with a major fall in lake level; it consists of limestone and dolomite grainstone beds with characteristics of a Bouma sequence, deposited as a series of turbidite flows. Most clasts originated on a nearby shallow-water platform, but the presence of organic-rich interbeds between the grainstones and shale rip-ups implies anoxic, relatively deep water depositional conditions.

The Mid-Viodo Shale is largely calcareous shale with thin beds of carbonate grainstone, interpreted as distal turbidites. It represents submergence of the nearby shallow-water platform.

With a second fall in lake level, a shallow-water platform redeveloped basinward of the earlier platform. The Upper Viodo Carbonate includes both shallow-water platform and deep water basinal deposits. An inner platform section is divisible into a lower gastropod zone and an upper oncolite zone. Outer platform facies consist almost entirely of mollusc coquina. Coarse coquina conglomerate is the dominant basinal lithofacies adjacent to the platform, with subordinate oncolite and ostracod grainstones and dark, laminated, organic-rich calcareous shale. The conglomerates were deposited entirely in a relatively deep lake as subaqueous debris flows triggered by collapse of the bank margin. Shales accumulated under anoxic conditions during pauses in carbonate sedimentation. Above both platform and basinal facies, the section consists of shale with beds of carbonate conglomerate, reflecting resubmergence of the platform and displacement of the shoreline landward.

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