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ABSTRACT: Climate and Tectonic Controls on Lacustrine Source Rock Deposition: Examples from the Kwanza, Congo and Atlantic basins of West Africa

Harris, Nicholas B.1, Katherine H. Freeman1, Mario G.P. Brandão2, and Christopher J. Poulsen3
(1) The Pennsylvania State University, University Park, PA 
(2) Sonangol, Luanda, Angola 
(3) University of Chicago, Chicago, IL

The Cretaceous West African rift basins, formed during rifting of South America from Africa, spanned 10 degrees of paleolatitude. The northernmost basin, the Atlantic basin (Gabon), impinged on the InterTropical Convergence Zone, while the southern basins, the Congo basin (Congo and Angola) and Kwanza basin (Angola), were located in increasingly arid settings. The synrift section in these basins includes important lacustrine source rocks, locally of remarkable quality and thickness. We have assessed the quality and origin of these source rocks with multiple techniques. We relate their distribution and quality to rift evolution and paleoclimate.

In the Atlantic and Congo basins, a basal sandstone, deposited in fluvial to lake margin settings, is overlain by thick deep water lacustrine shales and sediment gravity-flow sandstones. Evidence of shallowing is present near the top of the shale, with local fluvial/deltaic deposits. The section is capped unconformably by a transgressive marine sand/shale unit. Synrift sediments in the Kwanza basin differ greatly. Red sandstones and shales are common throughout most of the section; deep water sedimentation is restricted and uncommon.

Total organic carbon (TOC) contents are relatively high in both the Congo and Atlantic basin, but generally lower in the Kwanza basin. We attribute the coarse grained character and lower %TOC in the Kwanza basin to the more arid climate. The resulting lower vegetation cover permitted transport of coarser sediment into the basin. The low precipitation rates also resulted in decreased rates of chemical weathering and nutrient supply to the rift lake.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia