Karoo lava-fed deltas and a petrified forest from the Lower Jurassic of southern Gondwana

Abstract

Palaeoenvironmental changes during continental flood basalt volcanism in large igneous provinces are increasingly linked to global environmental perturbations. Whilst the geology of the Karoo-Ferrar continental flood basalts, including their eruption history, mode of emplacement, timing, geochemistry, palaeomagnetism, has been described, and the causal link of this massive volcanic event to global climate trends and the mass extinction as well as oceanic anoxia near the Pliensbachian–Toarcian boundary is established, little is known about the dynamics of the southern African palaeoenvironment and palaeoclimate at that time. This study is focused on a field investigation of the uppermost Karoo Supergroup, which accumulated in the changing Early Jurassic palaeo-landscapes of southern Gondwana. Using well-exposed successions in southwestern Lesotho, we document evidence of a humid phase at the onset of Karoo volcanism in the central part of the main Karoo Basin. The studied interval encompasses the conformable contact of the Sinemurian–Pliensbachian Clarens Formation and the Pliensbachian–Toarcian flood basalts in the lower Drakensberg Group. Facies changes in the succession provide evidence for shifts in the ancient landscape and in the associated subaerial and subaqueous conditions. Specifically, the facies suggest a climatic change from a dry desert setting with large, down-wind migrating sand dunes to a wetter desert ecosystem in the Late Pliensbachian. The latter sustained a fairly diverse biota of land-dwelling organisms, which included herbaceous and wooded areas where the diametre of gymnospermous tree trunks was in excess of 50 cm. The earliest flood basalts comprise massive basalt lava flows and up to ~10-m-thick foreset-bedded pillow lavas. These show that during the initial outpouring of the Karoo continental flood basalts, the land surface was locally covered by streams and lakes into which the earliest basaltic lavas flowed forming lava-fed deltas in the Late Pliensbachian.