Abstract

The sedimentary rocks of the Karoo Supergroup in the Tuli Basin (South Africa) may be grouped into four stratigraphic units: the Basal, Middle and Upper units, and the Clarens Formation. This paper presents the findings of the sedimentological investigation of the fluvial clastic deposits of the Middle Unit. The lack of bio- and chronostratigraphic control hampers an unequivocal correlation of the Middle Unit with the formations of the main Karoo Basin. Existing constraints place the Middle Unit in a stratigraphic position older than the Elliot Formation and younger than the Ecca Group. Between these limits, the Middle Unit may correlate with either the Beaufort Group or the Molteno Formation of the main Karoo Basin (Table 11).

The Middle Unit may be subdivided into three parts, based on lithological differences. The lower part includes rudites and arenites (lithofacies Gh, Sp, Sh, and Sm); the middle part is dominated by siltstones and very fine sandstones (lithofacies Fl); and the upper part consists of mudstones (lithofacies Fsm). This succession accumulated in a perennial braided fluvial system, with well developed inter-channel flood plains. The observed lithofacies and architectural elements are partly coeval, with the overall upward-fining profile related to gradual denudation and peneplanation. Petrographic analyses indicate multiple source areas, as well as reworking of the underlying Basal Unit. The accumulation of the Middle Unit in the Tuli Basin corresponds to a distinct stage of tectonic development in the region, when the topographic gradient was consistently dipping from south-east to north-west. In contrast with this, the Basal and Upper units were accumulated on topographic slopes dipping from east-northeast to west-southwest, and from north-northwest to south-southeast respectively.

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