Landscapes and Landforms of the Chobe Enclave, Northern Botswana

Abstract

The northern part of the Chobe Enclave (an administrative district of northern Botswana) is an agricultural area situated between relatively pristine national parks situated in the Middle Kalahari Basin. It belongs to the Linyanti-Chobe structural basin and constitutes a syntectonic depocenter formed within a large structural depression, known as the Okavango Graben, a tectonic structure of a likely trans-tensional nature. The landscape includes fossil landforms, such as sand dunes, pans, sand ridges, and carbonate islands resulting from palaeo-environmental and palaeo-drainage changes through the Quaternary and associated to (neo)tectonic processes. In addition to riverand wind-reworked Kalahari sands, the sediments include diatomites and carbonate deposits, forming inverted reliefs and originating from palustrine palaeo-environments. The Linyanti-Chobe basin is at the convergence of several ecoregions from tropical and subtropical grasslands to savannas and shrubland biomes. The hydrological cycle in the northern Chobe Enclave is governed by a complex interplay between the Okavango, Kwando, and Upper Zambezi drainage basins, which originate from tropical watersheds of the Angolan highlands. Finally, the widespread development of termite mounds impacts the diversity of soils and sediments of the northern Chobe Enclave, which is also reflected in the vegetation.

Geometry of sedimentary deposits and evolution of the landforms in the Chobe Enclave, Northern Botswana

“…This configuration triggered the formation of sharp linear eastern margin of the sand ridge, also representing a possible paleolake/marsh shore. Moreover, during this period, there is a prominent continued erosional phase, revealing the existence of carbonate islands emerging from the floodplain areas and creating a topographical inversion relief within the basin (Mokatse et al, 2022). Indeed, the presence of these palustrine carbonates refers to periods and/or areas of reduced clastic input and may be used as an indicator of the aggradation rate at work in the floodplain (Alonso-Zarza, 2003), as well as a change in the water chemistry.…”

“…Cenozoic deposition of Kalahari Group sediments in the Chobe Enclave resulted in the accumulation of alluvial, lacustrine, fluvial, and aeolian deposits (Huntsman-Mapila et al, 2005;Podgorski et al, 2013). Sand ridges are the dominant landforms in the area (Mokatse et al, 2022), and are considered to represent late Quaternary paleolacustrine shorelines formed under hydrological conditions different from the present-day (Grove, 1969;Cooke, 1980;Mallick et al, 1981;Shaw and Cooke, 1986;Shaw et al, , 1997Ringrose et al, 1999Ringrose et al, , 2005Ringrose et al, , 2009Burrough and Thomas, 2008;Burrough et al, 2009;Moore et al, 2012). These ridges have been mostly studied in order to understand how they might relate to paleohydrological and paleoclimatological processes (Fig.…”

“…The Chobe Enclave also includes carbonate deposits in an otherwise large siliceous-dominated basin. These deposits, which were described as palustrine limestones (Diaz et al, 2019), outcrop in the landscape as "islands" among the sands and are mostly observed in the central part of the Chobe Enclave, between the paleo-floodplain in the west and the large present-day floodplain in the east (Mokatse et al, 2022). These "islands" are also associated with diatomites in an unclear geometrical relationship, suggesting periods of wetter conditions and increased flows into the basin from the Chobe and Zambezi fluvial systems.…”

“…Indeed, these features likely correspond to silty fine sand bodies within the conductive silt-rich floodplain sediments. Most of the Chobe Enclave landscape is occupied by the floodplain undergoing aeolian and fluvial dynamics (Mokatse et al, 2022), and its extent limits are bounded by the Chobe and Linyanti faults. However, the sand ridge overlying the floodplain deposits displays quite astonishing features on its margins with the floodplain.…”

“…Indeed, the presence of these palustrine carbonates refers to periods and/or areas of reduced clastic input and may be used as an indicator of the aggradation rate at work in the floodplain (Alonso-Zarza, 2003), as well as a change in the water chemistry. The carbonate precipitation events in the Chobe Enclave occurred irregularly, since at least ~75 ka (Diaz et al, 2019;Mokatse et al, 2022); they witness changes and some disorganization in the hydrological network, probably associated with phases triggered by neotectonics, although some climate influence cannot be totally discarded. Nevertheless, it is certainly during this phase, between ~25 and ~6 ka, that the watergaps started to form.…”