--> ABSTRACT: Stratigraphy and Sedimentology of Tropical Lake Bosumtwi, an Analogue Basin for Lacustrine Source Rock Deposition, by Keely M. Brooks, C. A. Scholz, G. Ellis, K. E. Lezzar, J. King, and J. Peck; #90906(2001)

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Keely M. Brooks1, C. A. Scholz1, G. Ellis2, K. E. Lezzar1, J. King3, J. Peck4

(1) Syracuse University, Syracuse, NY
(2) University of Miami, Miami, FL
(3) University of Rhode Island, Narragansett, RI
(4) University of Akron, Akron, OH

ABSTRACT: Stratigraphy and Sedimentology of Tropical Lake Bosumtwi, an Analogue Basin for Lacustrine Source Rock Deposition

Lake Bosumtwi, an 8km diameter crater lake located in tropical West Africa, is a useful modern analogue for understanding processes of lacustrine source rock deposition in a permanently stratified lake basin. Analysis of surface sediment samples, Kullenberg piston cores, high-resolution single- and medium-resolution multi-channel seismic data suggest dramatic changes in lake level over the past several hundred kyr. In this system-wide approach, surface sediments constrain downcore results, which in turn help calibrate the seismic reflection data. The modern lake bottom sediments are composed mainly of silt size particles with finer grain sizes (8-10æm) dominating the deeper basin (40-70m). Bulk densities decrease towards the deeper basin, while sediment surface area and total organic carbon (TOC) increases (max~12.5%). A 10m piston core contains continuously laminated (<1mm), very fine sediments (4-10æm). Downcore variations in TOC (7-39%) suggest large changes in lake productivity over recent geologic time.

The high-resolution single channel seismic data (4-10kHz) reveal a highly continuous acoustic stratigraphy in the basin center, whereas an extensive gas curtain masks underlying reflectors on the basin margin. Waterbottom reflections show several breaks in slope which may represent ancient lake level terraces draped by Late Pleistocene and Holocene sediments. Three erosional truncation surfaces, interpreted to represent desiccation events, are apparent between 30 and 40m in the high-resolution seismic data, and similar surfaces are observed in the medium-resolution (25-250kHz) airgun seismic data. Initial results suggest that lake level changes, induced by subtle climate shifts, may have a profound impact on source rock richness in tropical lake basins.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado