Lacustrine environment during Lower Beaufort (Upper Permian) Karoo deposition in Northern Malawi

Abstract

This paper provides the first detailed sedimentological facies analyses of a Lower Beaufort (Upper Permian) Gondwana sequence from Mt. Chombe are in northern Malawi. Approximately 300 m of undeformed non-marine mudstones, siltstones and sandstones are preserved in isolated grabens, exposed along the Mzuzu-Karonga Highway where it crosses the Lake Malawi escarpment. Five lithofacies are distinguished: (A) finely-laminated paper shales and muddy siltstones deposited in lakes with anoxic bottom waters, (B) massive, muddy siltstones and siltstones deposited in oxic lake waters, (C) thinly-bedded, horizontally stratified siltstones from episodic sheet flows into a lake, (D) cross-stratified and flaser-bedded siltstones and sandstones of higher energy lake environments, and (E) massive and cross-bedded, coarse-grained sandstones with small channels and intraformational pebble layers interpreted as marginal lacustrine deposits. New dates by pollen indicate an uppermost Permian interval. The whole sequence appears to have been deposited in a large shallow lake complex as none of the strata displays subaerial features such as desiccation cracks, pedogenesis, or rootlets. All facies are reworked as is also suggested by the lack of in-situ faunal or floral remains. Organic matter is mainly land-derived although some algal-dominated intervals occur. Clastic mineralogy is immature, uniform and dominated by feldspar and quartz. Clay minerals include smectite, kaolinite and illite. Carbonate contents greater than 5% and up to 40% are taken as an evidence of periods of authigenic precipitation in a freshwater lake setting. The Mt. Chombe sequence is lithologically similar, but differs in significant sedimentology aspects from equivalent sequences in the adjacent Luwanga and Ruhuhu Basins where sub-aerial deposition has been reported. Although all three basins are part of a stable, cratonic palaeoenvironment, the absence of the Lower Permian Upper Coal Measures from Karoo sequences in northern Malawi is taken as evidence of some syndepositional topographic relief in the region.