Abstract: Viodo Carbonate (Toca): A Lacustrine Carbonate in the South Atlantic Rift (Congo Basin)

Nicholas B. Harris, Patrick Sorriaux, Frederic Walgenwitz, Gilles Sermondadaz

The Barremian lacustrine carbonates of the Toca Formation in Congo were deposited in the rift lake formed during the Aptian continental break-up and the opening of the South Atlantic.

In the Viodo Area, cores and cuttings permit analysis of the complex relationships between carbonate facies and coeval organic rich-shales of the Marnes de Pointe Noire Formation. Carbonate deposition was primarily controlled by water depth, initiated by falls in lake level. During the most pronounced lowstand, mollusc coquinas formed a reef-like margin on the platform edge. During lowstands of lesser magnitude, gastropods and oncolite shoals prograded to the plate form edge. Carbonates produced in shallow environments can be periodically resedimented as turbidites in deeper anoxic parts of the lake characterized by deposition of organic-rich shales.

The isotope data provide evidence of significant changes in the lake hydrology. Carbon isotope compositions are closely related to the depositional depth and reveal a strongly stratified water body, with relatively saline anoxic hypolimnion and a warm, oxygenated epilimnion. Dramatic variations of the water budget, probably related to climate changes, led to major lake level falls with the development of salt pans on the margins. Density-driven flows of supersaturated brines, formed during periods of intense evaporation, is likely to have caused gradual salinity increase of the hypolimnion, thus contributing to the isolation of deep waters. Dolomitization, sometimes from direct precipitation, seems to have been triggered by an increase of water alkalinity due to CO₂ production from the microbial decay of organic matter.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France