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Lower Karoo Supergroup, Central Botswana The nonglacial diamictite of Toutswemogala Hill (lower Karoo Supergroup, central Botswana): Implications on the extent of the late Paleozoic ice age in the Kalahari–Karoo Basin Pierre Dietrich, Fulvio Franchi, Larona Setlhabi, Rose Prevec, Marion Bamford To cite this version: Pierre Dietrich, Fulvio Franchi, Larona Setlhabi, Rose Prevec, Marion Bamford. The nonglacial diamictite of Toutswemogala Hill (lower Karoo Supergroup, central Botswana): Implications on the extent of the late Paleozoic ice age in the Kalahari–Karoo Basin. Journal of Sedimentary Research, Society for Sedimentary Geology, 2019, 89 (10), pp.875-889. 10.2110/jsr.2019.48. hal-02363921 HAL Id: hal-02363921 https://hal.archives-ouvertes.fr/hal-02363921 Submitted on 14 Nov 2019 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Journal of Sedimentary Research, 2019, v. 89, 1–15 Research Article DOI: http://dx.doi.org/10.2110/jsr.2019.48 THE NONGLACIAL DIAMICTITE OF TOUTSWEMOGALA HILL (LOWER KAROO SUPERGROUP, CENTRAL BOTSWANA): IMPLICATIONS ON THE EXTENT OF THE LATE PALEOZOIC ICE AGE IN THE KALAHARI–KAROO BASIN 1,2 3 3 4,5 6 PIERRE DIETRICH, FULVIO FRANCHI, LARONA SETLHABI, ROSE PREVEC, AND MARION BAMFORD 1Department of Geology, P.O. Box 524, Auckland Park Kingsway Campus, University of Johannesburg, Johannesburg 2006, South Africa 2Geosciences-Rennes,´ UMR6118, Universite´ de Rennes 1, 35042 Rennes Cedex, France 3Department of Earth and Environmental Science, Botswana International University of Science & Technology (BIUST), Private Bag 16, Palapye, Botswana 4Department of Earth Sciences, Albany Museum, Somerset Street, Makhanda 6139, South Africa 5Botany Department, Rhodes University, P.O. Box 94, Makhanda 6140, South Africa 6Evolutionary Studies Institute, 1 Jan Smuts Avenue, Braamfontein, University of the Witwatersrand, Johannesburg 2000, South Africa [email protected] ABSTRACT: Along the easternmost edge of the Karoo–Kalahari Basin (KKB) of Botswana, the Toutswemogala Hill succession exposes a 30–50-m-thick suite of siliciclastic deposits interpreted by some as glaciogenic in origin tied to the Permo-Carboniferous Late Paleozoic Ice Age (LPIA). Six facies associations (FA) were recognized in this succession, resting unconformably on a highly uneven Archean gneissic basement, and consisting from base to top of: 1) clast- supported breccia made up of angular cobbles and boulders ubiquitously derived from the underlying basement, 2) well-bedded siltstones sealing or locally interdigitated with the underlying breccia, and bearing abundant remnants of Glossopteris sp. leaves, 3) a chaotic to faintly laminated matrix-supported diamictite bearing angular and subrounded clasts and tree logs attributed to the genus Megaporoxylon, 4) cross-bedded conglomerate bearing well-rounded quartz and clasts, 5) planar-laminated to ripple-laminated, poorly sorted, muddy sandstones showcasing dispersed mud chips that grade upward into 6) poorly sorted, cross-bedded coarse-grained sandstones displaying convolute beds and abundant imprints of unidentifiable tree logs. No evidence of glaciogenic processes have been found in this succession, in the form of either pavement or clasts striations. The breccia and diamictite are interpreted as scree and mass-flow deposits, respectively. Along with the age of the deposits, inferred from the plant debris (upper Carboniferous to lower Permian), the stratigraphic position of this sedimentary succession resting on the Archean basement suggests that it corresponds to the Dukwi Formation, a stratigraphic equivalent of the Dwyka Group in the Main Karoo Basin. This would explain the resemblance of the facies to those recovered at the base of the central Kalahari–Karoo Basin and in the neighboring Tuli, Ellisras, and Tshipise basins. The absence of diagnostic criteria for glacial processes in the studied succession raises the question of the extent, in both time and space, of the LPIA-related ice masses over southern Africa and particularly in southeastern Botswana. It is suggested here that during this glacial epoch, spatially restricted ice masses were confined in bedrock valleys (valley glaciers) in an uplifted setting otherwise characterized by non-glaciogenic processes, further strengthening the scenario of fragmented ice masses over southern Gondwana. INTRODUCTION interpretation of these deposits, diamictite may also result from resedimentation through non-glaciogenic processes, of initially glaciogenic Diamictite is defined as ‘‘a non-sorted terrigenous lithified sedimentary deposits (Caron et al. 2011; Le Heron et al. 2017; Kennedy et al. 2019). rock containing a wide range of particle sizes’’ (Bell et al. 2016). The Diamictites have been extensively recognized at the base of Karoo-age recognition and hence correct interpretation of diamictite in the geological basins (Carboniferous–Jurassic) across sub-Saharan Africa (Fig. 1; record has long been a source of controversy for sedimentologists (e.g., Visser 1994; Eyles and Januszack 2007; Soreghan et al. 2009; Le Heron et Haughton 1963; Isbell et al. 2012; L´opez-Gamundi and Buatois 2010; al. 2017; Vesely et al. 2018), yet is crucial for paleoenvironmental and Craddock et al. 2019). In southern Africa, these poorly sorted sediments climatic reconstructions. While a glaciogenic origin is commonly invoked are most frequently characterized as Permo-Carboniferous glaciogenic to interpret such facies (tillite, grounding-zone wedges, rain-out under an deposits tied to the Late Paleozoic Ice Age (LPIA), and are formalized as ice shelf), they may also constitute the product of a wide range of non- the Dwyka Group, forming the basal unit of the Karoo Supergroup (e.g., glaciogenic sedimentary processes such as subaqueous sediment gravity Visser 1989; Catuneanu et al. 2005; Herbert and Compton 2007; Isbell et flows and subaerial alluvial and colluvial fans or debris flows, among al. 2008a; Stollhofen et al. 2008). The correct interpretation of these others (e.g., Fedorchuk et al. 2018). To further complicate the widespread deposits is therefore crucial for the reconstruction in both time Published Online: Month 2019 Copyright Ó 2019, SEPM (Society for Sedimentary Geology) 1527-1404/19/089-000 Allen Press, Inc. 4 September 2019 3:21 pm Page 1 //titan/Production/s/sedp/live_jobs/sedp-89/sedp-89-09/sedp-89-09-01/layouts/sedp-89-09-01.3d RaNgE#?!1-15#?! 2 P. DIETRICH ET AL. JSR FIG. 1.—A) The Karoo-aged basins of southern Africa. The study site in the Kalahari–Karoo Basin (KKB) of Botswana is indicated. Modified after Catuneanu et al. (2005). B) Schematic geological map of the Toutswemogala Hill area (see Part A for location). Modified from Key and Golding (1994). Allen Press, Inc. 4 September 2019 3:21 pm Page 2 //titan/Production/s/sedp/live_jobs/sedp-89/sedp-89-09/sedp-89-09-01/layouts/sedp-89-09-01.3d RaNgE#?!1-15#?! JSR THE NONGLACIAL DIAMICTITE OF TOUTSWEMOGALA HILL (KAROO SUPERGROUP, BOTSWANA) 3 and space of the LPIA across southern Gondwana (e.g., Gonza´lez- 1991 for a review) and the siliciclastic deposits of the Proterozoic Palapye Bonorino and Eyles 1995; Visser 1997; Bordy and Catuneanu 2002; Group (Mapeo et al. 2004). The present research focuses on the Fielding et al. 2008; Dietrich and Hofmann, in press). stratigraphic succession of the Hill, comprising diamictites and breccias Diamictite deposits have been reported in southeast Botswana in the unconformably overlying gneissic basement rock and in turn overlain by Toutswemogala Hill area (Fig. 1). In his precursor book, Du Toit (1954) coarse siliciclastic deposits. recognized the lack of glaciogenic features in this diamictite, but in more recent times, these sediments have been interpreted as glacial deposits MATERIALS AND METHODS (tillite) tied to the LPIA (Modie 2002) and are portrayed as such in the most recent geological map of Botswana (Fig. 1B; Key and Golding 1994). Four high-resolution stratigraphic sections were measured at outcrops The paucity of thorough sedimentological analyses has allowed the along the Hill, across an interval of 850 m (Fig. 3). Facies analysis included perpetuation of the unsupported glaciogenic interpretation of the deposits, observations of grain size, sorting, sedimentary structures, fossil content, and university students are regularly taken to visit the site to appreciate the paleocurrents, geometries of the sedimentary bodies, and stratigraphic spectacular ‘‘tillite.’’ The aim of this paper is to present, for the first time, a relationships with underlying, overlying, and adjacent units. Samples for detailed sedimentological and petrographic characterization of the petrographic studies were collected and thin sections prepared at the sediments of the Toutswemogala Hill. The analysis of this new data set University of Johannesburg. Two thin sections of fossil wood were in the regional context of the Kalahari–Karoo Basin (KKB), sheds light on prepared and described at the University of the Witwatersrand. To obtain a the paleoenvironmental conditions at the time of deposition and has statistically sound
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