## Results
### Muzongo Survey
#### 3.4.3 Outcrop Analysis  
In order to obtain more information on the sedimentology of the area, two large outcrops exposed by the erosion of a meandering river were analysed near Chiluwe village, in the northeast of the zone that is indicated as Tertiary-Quaternary superficial deposits by [PERSON] and [PERSON] (1966) (Figures 2b and 10a).  
Outcrop 1 consists of debris-flow deposits separated by clayey layers (Figure 10b). The debrites are massive, with continuous thicknesses of up to 10 m, and comprise a matrix-supported structureless breccia of angular, pebble- to boulder-size gneiss elasts in a coarse to very coarse angular sandstone matrix of the same mineralogical composition as the gneiss elasts (quartz, felspars and hornblende). The clayey layers are thin (cm scale) and consist of grey-blue claystones containing stromatolites and accumulations of bivalve shell clasts. The clayey layers separate successive debris-flow units that display erosive bases. The deposits were interpreted as being those of alluvial fans that developed at the foot of gneiss hills and that were, at least partially, immersed in a lake.  
Outcrop 2 consists of a succession upwards of thinly bedded deposits, channel-fill deposits and debris-flow deposits (Figure 10c). The thinly bedded deposits are found at the base of the outcrop over 5 m and comprise thin (cm scale) alternating beds of medium sandstones and mudstones. The thinly bedded deposits were found slumped at places where they display convolutes. The channel-fill deposits were found above the thinly bedded deposits, are over 5 m and are made of medium to very coarse angular sandstones. The channel-fill deposits are organised as small individual channel-fill units (maximum 1 m thickness) with erosive bases; together, these units form a channel-fill body. The debris-flow deposits are found on top of the channel-fill deposits, are over 1 m and display the same textures and mineralogy as the debris-flow deposits observed on Outcrop 1. The deposits of Outcrop 2 were interpreted as being those of three sedimentary environments: (i) the thinly bedded deposits being those of lacustrine turbidite fans; (ii) the channel-fill deposits being those of lacustrine deltas; and (iii) the debris-flow deposits being those of alluvial fans that developed at the foot of gneiss hills and that were, at least partially, immersed in a lake.  
Figure 10: Chiluwe control analyses: (**a**) general view of outcrops, (**b**) Outcrop 1 analysis and (**c**) Outcrop 2 analysis. The locations of letter-labelled photo are marked on the geological schematic of the main outcrop photo.  
An association of sedimentary environments was proposed that explains the observations made on the Chuluwe outcrops (Supplementary Materials Figure S6). This association comprises alluvial fans, deltas, turbidite fans and a basin floor that developed in this order from gneiss hills (the source of the sediments) into a paleo-lake. Progradation and retrogradation events are generally quick in lacustrine environments, since lake levels are usually unstable on sedimentary timescales. The environmental succession turbidite fan/delta/alluvial fan identified upwards on Outcrop 2 was interpreted as indicating an evolution with time towards more proximal environments, hence, a progradation.  
In the light of these sedimentological findings, the interpretation of the Muzongo subsurface resistivity cross-section was revised (Figure 9b): (i) The channel-fill deposits were interpreted as deltaic distributary channel-fill deposits made of sandstones; (ii) the clay-rich deposits surrounding the channel-fill bodies were divided between inter-distributary and/or delta-front deposits made of clayey siltstones (more resistive and in contact with the channel-fill bodies) and prodelta deposits made of silty claystones (more conductive and further away from the channel-fill bodies); and (iii) the deltaic environment was assumed to have developed within a lake.