Palaeoenvironmental change from the Hettangian to Toarcian in Moyeni (Quthing District), southwestern Lesotho

Haupt, T'Nielle

Palaeoenvironmental change from the Hettangian to Toarcian in Moyeni (Quthing District), southwestern Lesotho

Master Thesis

2018

Abstract

The upper Karoo Supergroup of southern Africa encompasses the Late Triassic-Early Jurassic geological evolution of southern Gondwana. The environment is known to have shifted from wet swamps to a semi-arid fluvio-lacustrine and then to an aeolian system, preceding the outpouring of the Karoo continental flood basalts. The study area at Moyeni in SW Lesotho, exposes the richly fossiliferous Lower Jurassic upper Elliot and Clarens Formations (Stormberg Group) as well as the Barkly East Formation (lowermost Drakensberg Group). With the aid of integrated geological methods (e.g., field mapping, sedimentology, ichnofacies analysis), this research aims to document Early Jurassic palaeoenvironmental and tectonic changes in this region. The upper Elliot Formation comprises very fine- to fine-grained sandstone units with either massive or with ripple cross-lamination, low-angle cross-bedding as well as sandy siltstone with in-situ carbonate nodules, desiccation cracks, bone fossils and a diverse ichnofauna (e.g., algal mats, vertebrate burrows, tracks of theropods, ornithischians, amphibians, crocodylomorphs). Sedimentological evidence indicates that in the Hettangian the area was prone to flash floods and drying in a low energy depositional system with small rivers and shallow lakes. The bulk of the Clarens Formation comprises very thick, tabular, fine- to medium-grained, massive to large-scale cross-bedded arenites. However, the lower part of the Clarens also preserves thin-bedded mudstones and theropod tracks. These suggest a change in the Sinemurian from a wet to a dry desert with large, down-wind and eastward migrating sand dunes. It also appears that in spite of climatic changes these Hettangian Sinemurian ecosystems sustained a diverse biota. Within the lowermost Drakensberg Group, interbedded with lava flows and pillow lavas, massive to cross-stratified sandstone units are common. These beds typically thin and fine upward and, at least locally, terminate in surfaces that are covered by symmetrical ripple marks. These suggest that by the Toarcian the land surface was again covered by temporary lakes and streams into which the lavas outpoured during the early stages of the Karoo volcanism. The study area is structurally deformed by ~ESE-WNW trending normal faults, some of which appear to have been active already during Clarens Formation times. This syn -sedimentary faulting is interpreted as part of the growing body of evidence for the changeover from a compressional to extensional tectonic regime during the Early Jurassic and may be the first signs of Gondwana break-up.

Keywords

Geology

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