ABSTRACT: Deltaic Sedimentation in Lake Malawi: Basin-Wide Variability and Models for Sand-Body Geometry

WELLS, JOHN T., University of North Carolina-CH, Morehead City, NC, and CHRISTOPHER A. SCHOLZ, THOMAS C. JOHNSON, and JAMES W. MCGILL, Duke University Marine Laboratory, Beaufort, NC

At least seven cuspate-to-lobate lacustrine deltas have formed along the margins of Lake Malawi in east Africa since water level reached its most recent high-stand elevation. These deltas provide a truly unique opportunity to study variations in sand body geometry in a tectonically controlled setting and to assess the effects of physical processes on strata formation within a single 550 km long by 50 km wide lake basin. Our initial synthesis is of data from five deltas located within different parts of the rifted half-graben framework, including the shoaling margin, axial margin, border fault margin, and accommodation zone settings. Split-spoon cores, vibracores, and aerial photographs reveal that (1) each delta consists of an extensive sand sheet, terminating subaerially as a beach, eposited during high discharge and reworked into bundles of beach ridges during periods of maximum wave energy at low discharge, (2) surface and subsurface sediments consist almost exclusively of angular sands and gravels ranging from Precambrian metamorphics to Recent siliciclastics, and include extensive heavy mineral deposits, (3) many "typical" low-energy deltaic environments are absent and fine-grained sediments appear to be accumulating subaerially only as channel-fill deposits, and (4) deltas switch depositional sites episodically at decade time scales, forming sandy 2-D meander belts and a 3-D geometry that may consist of overlapping, stacked deltaic sequences. Despite proximity to sediment sources and steep offshore slopes of 1-5 degrees, these deltas bear far more similarity to mixed-energy marine deltas than to either alluvial fans or braid deltas.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)