A lithofacies terrain model for the Blantyre Region: Implications for the interpretation of palaeosavanna depositional systems and for environmental geology and economic geology in southern Malawi

Abstract

The Blantyre City Area is part of the African savanna in southern Malawi. Sedimentological, geomorphological, chemical and mineralogical studies were conducted to create a lithofacies terrain model. The project involves mapping, cross-sectioning, grain size, heavy mineral analysis, XRD and the study of sedimentary textures under the petrographic microscope. These classical techniques were combined with GIS-based field and office works. The combined efforts led to 2-D maps and 3-D block diagrams that illustrate the geomorphological and sedimentological evolution of the landscape in southern Malawi during the late Mesozoic and Cenozoic. The results obtained through integrated geomorphological–sedimentological studies form the basis for land management (planning of residential areas, waste disposal sites, assessment of bearing capacity of rocks), geohazard prediction (delineation of high risk zones in terms of mass flow and inundation) and the evaluation of high-place (ceramic raw materials) and high-unit value (placers of precious metals and gemstones) mineral commodities in the study area. The study addresses regional and general aspects alike. In regional terms, the study aimed at unraveling the evolution of landforms at the southern end of the East African Rift System during the most recent parts of the geological past. Four stages of peneplanation were established in the working area. Planation was active from the Cretaceous to the Quaternary (stage I: early to mid-Cretaceous, stage II: early Tertiary, stage III: early to mid-Tertiary, stage IV: mid- to late Tertiary). During the most recent parts of the Quaternary, strong fluvial incision was triggered by the base-level lowering of the Shire River. Geomorphological alteration of the landscape goes along with a phyllosilicate–sesquioxide transformation from minerals indicative of more acidic meteoric fluids (e.g., gibbsite, kaolinite) to those typical of more alkaline conditions (e.g. smectite, vermiculite, hydrobiotite). In addition, the investigation is to provide some characteristic features suitable for the recognition and interpretation of terrestrial environments resembling the present-day savanna in the ancient sedimentary record: (1) Conspicuous bimodality in the grain-size distribution. (2) High degree of gravel roundness. (3) Boulder fields (basal conglomerates) with fitting breccias and no rotation of structural elements. (4) Poor to very poor sorting of siliciclastics. (5) Stratification is rare; if present only in sandy beds with antidune and tabular cross-bedding at low angle prevailing over cross-bedding at high angle. (6) Grading rare; in arenaceous deposits poorly developed FU sequences, in rudites poorly developed CU sequences, locally with surface armoring. (7) Directional sedimentary structures in coarse-grained deposits are unimodal. Fabric types: a(t)b(i) and a(p)b(i); shear planes indicative of slide may be present. (8) Ribbon-shaped architectural elements prevail over channel-like types. Bounding surfaces/unconformities are uneven to wavy and more widespread than scour-and-pool structures. (9) Fine-grained carbonaceous interbeds are restricted to shallow depressions. (10) Etch marks such as tafonis, flutes, honeycomb textures are common. (11) Placer deposits of metallic and non-metallic mineral commodities are abundant. (12) Alucretes and ferricretes of bog iron (ferrihydrite > goethite) and lateritic (goethite > ferrihydrite) types are common, calcitic freshwater limestones and gypcretes are scarce. (13) Kandite-group minerals “in” (relic), smectite-group minerals “in”, vermiculite “in” (recent), mica-group minerals “out”.