New Constraints from the Evolution of the Lake Malawi (Nyasa) Rift from Scientific Drilling

Christopher A. Scholz¹, Andrew S. Cohen², Thomas C. Johnson³, Erik T. Brown³, and John King^4
1Department of Earth Sciences, Syracuse University, Syracuse, NY
²Department of Geological Sciences, University of Arizona, Tucson, AZ
³Large Lakes Observatory and Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN
⁴Graduate School of Oceanography, University of Rhode Island, Narragansett, RI

In 2005 the Lake Malawi Scientific Drilling Project conducted a deep-water continuous coring program in the central and northern parts of the lake. This was the most ambitious large lake scientific drilling program undertaken to date, and required the fabrication of a dynamically-position drilling barge on the shores of Lake Malawi, in order to drill in water depths up to 600 m. More than 623 m of continuous core was recovered from 7 holes at two sites; maximum subbottom penetration at the deep Central Basin drill site was 380 m, representing more than a half million years of lacustrine sedimentation. Sample material recovered included a variety of laminated and homogenous algal-rich mud in zones of alternating dry/dense and soft/high-water content material; cemented siltstone; volcanic ash horizons; and fine-grained well-sorted sands at the base of holes at both sites. Prior to drilling, preliminary studies included detailed analyses of short cores and extensive seismic reflection datasets. The first results from drill core analyses indicate that on two occasions, between 135 and 75 kyr, water levels dropped to more than 550 m below modern levels. Lake level then increased and stabilized after 70 kyr, reaching modern levels sometime after 60 kyr. Down-core lithologic and geochemical data, correlated to nested seismic reflection site survey data sets, indicate marked variations in the organic-richness of basinal sediments; this variability is linked to climatically-forced precessional-scale lake level changes.

The resulting changes in lake hydrology and rift valley landscapes had a major impact on facies architecture in the basin. Although the project sampled only ~15% of the sedimentary section of the Lake Malawi Rift, the dating of the drill cores is providing new insights into the age of this part of the East African Rift System.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery