Matthew R. Buoniconti¹, Christopher A. Scholz²

(1) University of Miami- RSMAS, Miami, FL
(2) Syracuse University, Syracuse, NY

ABSTRACT: The Role of Lake Level Dynamics in Lacustrine Stratal Architecture and Sequence Development: Evidence from The Songwe Delta, Lake Malawi, East Africa

During the past 200 ka, Lake Malawi has undergone a series of high-frequency, high-amplitude lake-level changes that are recorded in the lacustrine stratal architecture. Approximately 500 km of intermediate resolution (50-500 Hz) 2D seismic reflection data show that stratigraphic response to lake-level change is tightly coupled to evaporation/precipitation-driven modulation of fluvially-derived sediment input.

The Songwe River drains the northernmost part of the Malawi basin and enters the rift axially, intermediate to the basin-bounding border fault and the shoaling, flexural margin. Because of its medial position and moderate slope (~ 1o), the margin is ideally situated for the study of shore-to-basin depositional processes, lateral facies partitioning, and facies response to lake-level change.

Lake level in this setting is controlled by the delicate balance of evaporation and precipitation over the drainage basin. Hydrologic modeling indicates that lake-level changes on the order of 500 m in amplitude can occur in less than 5 ka. These changes are recorded across the Songwe delta by deposition of coherent, coarse-grained allochthonous sediments. During periods of falling lake level, increased evaporation and decreased precipitation lead to reduced runoff and storage of sediment in the river drainage. In intervals of balanced evaporation and precipitation and constant runoff, lake-level stillstands result, and a series of prograding deltas, which are delineated by lobate clinoform packages, develop. Sediments stored during falling lake level are liberated during lake-level rise and deposition of lobate turbidite fans occur. This turbidite deposition is in contrast to marine systems, which develop turbidites during sea-level fall.

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