Lake dynamics in an Early Jurassic desert: Evidence from the Clarens Formation in southern Africa

Abstract

Interdune deposits are sensitive to climatic fluctuations and ancient interdune lacustrine systems can reveal the drivers of erg dynamics through time and space. This southern African study details the dynamics of interdune deposition in a vast ancient desert system that was active over Pangaea in the Middle Mesozoic and formed part of one of the largest known sand seas in Earth's history. Focusing on the Sinemurian to Pliensbachian Clarens Formation in the main Karoo Basin (south-west Gondwana), three facies associations were identified that formed in meso-scale and macro-scale lakes amongst aeolian dunes and sand sheets of the Clarens erg system. The northern facies association is restricted to interdune hollows within an actively climbing dune system. High-sediment availability and high-wind speeds limited the space available for these meso-scale lakes to expand. Here, migrating dunes outpaced the water table rise promoting the formation of discontinuous lenses of lake deposits, as is typical in erg centres. The southern facies associations developed both in spatially isolated meso-scale lakes, where flooding was controlled by groundwater inundation (water table rise), as well as in macro-scale lakes, where flooding was related to groundwater inundation and floods (surface runoff) that induced terminal splays. Relative to the north, the southern facies associations, with their more significant thickness and lateral extent, show that, here, the basin was characterized by a higher rate of relative subsidence. Moreover, the southern lake systems were established along the erg's margin, where the interdunes expanded during pluvial phases. Ultimately, the facies associations in the Clarens Formation show that the widespread arid conditions in the Early Jurassic of south-west Gondwana were subjected to prolonged and repeated pluvial phases in an otherwise harsh environment. Given the prominence of Jurassic aeolianites across Pangea, this study provides a southern African perspective on global palaeoerg dynamics and Early Jurassic climatic shifts.

Loess or lake sediments: Resolving the origin of massive beds in the Lower Jurassic of southern Africa

“…Given the prevalent west-to-east palaeo-wind regime during the deposition of the Clarens Formation (Beukes, 1969(Beukes, , 1970Bordy & Head, 2018;Eriksson, 1981Eriksson, , 1986Head, 2022), the interpreted loessites were located down-wind of a vast, mobile erg system that was active in SW Gondwana in the Early Jurassic. Similar observations of loess located down-wind of erg systems are that of the Negev loess that is located down-wind from the Sinai-Negev erg system, and the Chinese Loess Plateau where loess deposits are also interbedded with aeolian sand along the erg margin (Crouvi et al, 2010;Muhs et al, 2003).…”

“…This paper was generated from the doctoral thesis by Howard V. Head (Head, 2022). Emese M. Bordy conceptualized the research topic and was responsible for the research budget.…”

“…These quartz‐cemented, cream‐, pink‐, light green to buff‐coloured siltstones are often sandy (very fine sand), and in parts, may be silty sandstones. The interpreted aeolian depositional environment has been associated with a strong palaeo‐wind regime, as indicated by the regional measurements of eastward‐dipping foresets in large‐scale cross‐bedded sandstones (Beukes, 1969, 1970; Bordy, 2008; Bordy et al, 2009; Bordy & Catuneanu, 2002; Bordy & Head, 2018; du Toit, 1904, 1918; Eriksson, 1981, 1986; Haughton, 1924; Head & Bordy, 2022; Holzförster, 2007; Johnson, 1976; Robinson et al, 1969; Stockley, 1947). In addition, the massive sandstones have been linked to dust deposition, mass movement processes related to high‐magnitude precipitation or the destruction of primary internal structures by water percolation (Beukes, 1969, 1970; Eriksson, 1986; Johnson, 1976).…”

“…In addition, the massive sandstones have been linked to dust deposition, mass movement processes related to high‐magnitude precipitation or the destruction of primary internal structures by water percolation (Beukes, 1969, 1970; Eriksson, 1986; Johnson, 1976). Lenticular sandstones and mudstones in the lower and upper parts of the succession, combined with the stratigraphic distribution of the massive sandstones versus large‐scale sandstones prompted the classification of a threefold zonation in the Clarens Formation that may reflect climatic megacycles (Beukes, 1969, 1970; Bordy & Head, 2018; Head, 2022; Head & Bordy, 2022).…”

14th Symposium on Mesozoic Terrestrial Ecosystems and Biota

“…The Lower Jurassic of southern Africa represents the final events in the depositional history of the iconic main Karoo Basin of southern Pangea/Gondwana (Figure 1). This highly fossiliferous succession records evidence for extensive alluvial floodplains with calcic vertisols, ephemeral rivers, streams, and lakes in a semi-arid setting that eventually gave way to a vast sand sea before the return of more humid conditions at the end-Pliensbachian (Figure 1; Bordy et al, 2020;Head & Bordy, 2022). The upper Karoo is key to refining palaeo-ecological reconstructions not only during the biotic recovery after the end-Triassic extinction, but also prior to the extrusion of continental flood basalts that led to the end-Pliensbachian extinction.…”