Life and land engulfed in the late Early Jurassic Karoo lavas of southern Gondwana

Abstract

The rock record from the late Early Jurassic in southern Africa encompasses the history of voluminous continental flood basalt outpourings associated with the magmatic events in the Karoo–Ferrar Large Igneous Province (LIP) in southern and eastern Gondwana. This multiphase magmatism produced one of Earth’s largest continental flood basalt successions volumetrically and is assumed to have been a main driving mechanism in late Early Jurassic global environmental perturbations, including mass extinctions and changes in climate. In southern Africa, these Lower Jurassic flood basalts are interbedded with fossiliferous sedimentary rocks, which in turn host the last signs of ‘Karoo life’ in the form of fossil plants, invertebrates and vertebrates, including the trackways of hopping mammals and the ultimate Karoo dinosaurs. The sedimentology and palaeontology of the interbeds archived depositional and biotic processes in running water as well as in and around shallow, up to ∼10 m deep freshwater lakes and ponds in the late Early Jurassic. This study explains how a complex freshwater palaeo-habitat prevailed – albeit temporarily – in this extremely stressful environment, which was unlike any modern volcanic system. The evidence collectively points to seasonally wet, warm temperate climatic conditions during the early phases of Karoo volcanism. Moreover, the evidence in the rocks also suggests that the dynamic volcanic conditions resulted in shifting habitats that likely facilitated the migration of the ultimate Karoo biota towards the north and west, away from the main Karoo land of fire, just before Gondwana started to disassemble. This refinement of the environmental dynamics in southern Gondwana presented herein lays the groundwork for future high-resolution volcanological, geochronological and chemostratigraphical studies aimed at the nuanced understanding of the global environmental effect of the Karoo–Ferrar LIP.

Sedimentation tempo in an Early Jurassic erg system: Refined chronostratigraphy and provenance of the Clarens Formation of southern Africa

“…The conformable upper contact with the Drakensberg Group is sharp due to the lithological contrast between the sedimentary and igneous rocks. Nonetheless, this complex interbedded relationship is also transitional and indicates that aeolian and volcanic processes coexisted in the Early Jurassic (Figure 1c; Bordy, Rampersadh, et al, 2020;Bordy et al, 2021Bordy et al, , 2022Moulin et al, 2017).…”

“…The diverse fossil assemblage of the Clarens Formation includes vertebrates (tetrapod bone fossils, fossil fish), invertebrates (crustaceans, insects), various ichnofossils of both vertebrates and invertebrates, plant impressions and petrified wood (see references in Bordy & Head, 2018;Abrahams et al, 2021;Bordy et al, 2021). It is therefore envisaged that the Clarens Formation hosted locally diverse palaeo-ecosystems throughout its deposition, which continued well beyond the initiation of the large-scale continental flood basalts in the earliest Toarcian (Bordy et al, 2021(Bordy et al, , 2022Duncan et al, 1997;Moulin et al, 2011Moulin et al, , 2017Muir et al, 2020;Svensen et al, 2012). This interplay between the aeolian system and ongoing volcanism is suggested to have started during a proposed early onset of the Karoo volcanism between 187 and 185 Ma (Bordy et al, 2021;Moulin et al, 2017).…”

“…Overlying and pervasively intruding the upper Karoo sedimentary sequence are the continental flood basalts and their subvolcanic counterparts of the Drakensberg Group, associated with igneous activities in the Karoo‐Ferrar Large Igneous Province (Figure 1b,c; Duncan et al., 1997; Moulin et al., 2017; Svensen et al., 2006). During the final stages of the MKB history, during the Pliensbachian–early Toarcian, sedimentary and igneous activities co‐existed in an increasingly extensional tectonic setting that ultimately led to the disintegration of Gondwana (Bordy, Rampersadh, et al., 2020; Bordy et al., 2021, 2022; Moulin et al., 2017; Muir et al., 2020). In post‐Karoo times, the fill of the MBK was extensively eroded.…”

“…Its Early Jurassic, specifically Pliensbachian, age determination is supported by biostratigraphy (Bordy, Abrahams, et al., 2020; Bordy & Head, 2018; du Toit, 1905; Haughton, 1924; Knoll, 2005; McPhee et al., 2017; Viglietti et al., 2020), a limited geochronological dataset from the Clarens Formation (Abrahams, 2020; Bordy, Abrahams, et al., 2020; Nxumalo, 2020; Rademan, 2018) and the radiometric dating of the continental flood basalts and associated subvolcanic complexes of the Drakensberg Group. The latter partly forms a conformable, ~2‐km‐thick lava pile over the Clarens Formation and partly intrudes the Karoo sedimentary units in the form of dolerites throughout southern Africa (Figure 1c; Bordy, Rampersadh, et al., 2020; Bordy et al., 2022; Duncan et al., 1997; Moulin et al., 2011; Moulin et al., 2017; Svensen et al., 2012).…”

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

“…The upper contact with the Drakensberg Group basalts is also conformable and sharp, occurring at the stratigraphic arrival of extrusive volcanic above the sandstones of the Clarens Formation. Field evidence further demonstrates that this contact is transitional and diachronous across the basin, suggesting that both the aeolian and volcanic systems were active simultaneously during the onset of flood basalts associated with the Karoo‐Ferrar Large Igneous Province (Bordy et al, 2020, 2021; Bordy et al, 2022; Moulin et al, 2017).…”