Layered mound, inverted channels and polygonal fractures from the Makgadikgadi pan (Botswana): Possible analogues for Martian aqueous morphologies Fulvio Franchi a, * , Ruaraidh MacKay b , Ame Thato Selepeng a , Roberto Barbieri b a Department of Earth and Environmental Science, Botswana International University of Science and Technology, Private Bag 16, Palapye, Botswana b Dipartimento di Scienze Biologiche, Geologiche e Ambientali, Universit a di Bologna, Via Zamboni 67, 40126, Bologna, Italy ARTICLE INFO Keywords: Salt pan Mounds Layered deposits Polygonal cracks Inverted channels Groundwater upwelling Mars analogues ABSTRACT Layered mounds and inverted channels with polygonal fractures from the Ntwetwe Pan in the Makgadikgadi Basin (central Botswana) have been herein investigated. These morphologies are from an evaporitic basin (the Makgadikgadi Basin) that is the remnant of an ancient Pleistocene lake and is currently part of the world’s largest evaporitic system. The mounds in the Ntwetwe Pan are characterized by a layered structure and low relief (max. 5 m above the pan floor) and can be in excess of 2 km wide. The mounds consist mainly of loose (non-lithified) sand and silt with high moisture contents, even during the dry season. Geophysical investigations have shown that groundwater processes, particularly those related to the capillary fringe that rises and conveys moisture through the mounds, are factors that make mound sediments resistant to wind erosion. The inverted channels, identified in the southern part of the Ntwetwe Pan, are characterized by gentle reliefs and depressions, which depend upon the distribution of calcretes and indurated sediments. Large scale (up to 100 m wide) polygonal fractures localized at the front of the channels, disappear at the transition with the present-day pan floor. We consider that these particular mounds, within the Ntwetwe Pan, are remnants of the strandline of the paleo- Makgadikgadi Lake, and that the inverted channels represent distributary channels of a relict fan delta, formed by an ephemeral river, most likely the paleo-Boteti River, during a Lake Paleo-Makgadikgadi highstand stage. We consider that large scale (up to 100 m wide) polygonal fractures, located on the channel-mouth lobes, represent large-scale desiccation cracks formed by rapid water evaporation from delta deposits. The results of this investigation highlight the importance of the paleo-drainage system and its interactions with the water table and wind-deflation as main geomorphological factors within salt pan environments. The mounds in the Makgadikgadi pans also show strong geomorphic similarities to spring mounds on the surface of Mars, localized in equatorial layered deposits (ELDs). These ELDs mounds are considered to result from cyclical groundwater upwelling, evaporation and wind deflation. The geological processes that resulted in the formation of mounds within the Makgadikgadi pans may, therefore, help to explain how similar layered deposits formed on Mars and confirm existing theories. 1. Introduction The Makgadikgadi Basin in central Botswana (Fig. 1A) is the relict of a mega-lake system (e.g., Thomas and Shaw, 1991; Burrough et al., 2009a; Podgorski et al., 2013; Riedel et al., 2014), known as Lake Paleo-Makgadikgadi (Grey and Cooke, 1977; Cooke, and Versteppen, 1984; Schmidt et al., 2017) and originated during the Pleistocene (Moore et al., 2012). Today the Makgadikgadi Basin consists of a system of ephemeral lakes (playa or pan) that covers an area of ca. 16.000 km 2 (Cooke and Verstappen, 1984). The Makgadikgadi Basin is subdivided into several pans, the largest of which are the Ntwetwe Pan in the west and the Sua Pan in the east (Fig. 1B). The north-western edge of Ntwetwe Pan is characterized by peculiar concentric layered mounds (see also Cooke, 1980; Burrough et al., 2009a, b; McFarlane * Corresponding author. E-mail address: franchif@biust.ac.bw (F. Franchi). Contents lists available at ScienceDirect Planetary and Space Science journal homepage: www.elsevier.com/locate/pss https://doi.org/10.1016/j.pss.2020.105048 Received 14 April 2020; Received in revised form 20 May 2020; Accepted 1 July 2020 Available online 26 July 2020 0032-0633/© 2020 Elsevier Ltd. All rights reserved. Planetary and Space Science 192 (2020) 105048