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Abstract

Deposits from the modern highstand lacustrine deltas in Lake Malawi offer an excellent opportunity to test models of deltaic sedimentation in a tectonically active setting and to examine variations in sand-body geometry along the axis of a large lake that has significant gradients in physical processes. In 1991 we initiated a coring project in five of the largest deltas in the lake to describe, for the first time, the shallow-water environments of deposition and processes of sedimentation. Percussion drill holes through the lower delta plain in the Linthipe and Dwangwa (shoaling margin) deltas revealed moderate to extreme lithologic variability with sand units up to 15 m thick. Deposited in an alluvial or shallow subaqueous deltaic setting, the sediments ranged from clay to gravel, dark green to brown to orange in color, and contained sections with significant amounts of organic material. Units of gravel up to 1 cm in diameter were recovered, and 60 to 70 percent of the recovered sediments consisted of at least 75 percent sand and gravel. Silts and clays occurred in units up to about 1 m thick, usually in the middle section of each sequence. Although the age, and thus the sediment accumulation rate, is unknown, it is likely that the cored sections (20-26 m deep) represent a few hundred to a few thousand years of accumulation.

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