EMESE M. BORDY, P. JOHN HANCOX AND BRUCE S. RUBIDGE SOUTH AFRICAN JOURNAL OF GEOLOGY,2004,VOLUME 107 PAGE 587-602 587 Provenance study of the Late Triassic - Early Jurassic Elliot Formation, main Karoo Basin, South Africa Emese M. Bordy University of the Witwatersrand, School of Geosciences, Johannesburg, Private Bag 3, Wits 2050, South Africa. Corresponding address: Department of Geology, Rhodes University, Grahamstown, 6140, South Africa e-mail: e.bordy@ru.ac.za P. John Hancox and Bruce S. Rubidge University of the Witwatersrand, School of Geosciences, Johannesburg, Private Bag 3, Wits 2050, South Africa. e-mail: HancoxP@geosciences.wits.ac.za; rubidgeb@geosciences.wits.ac.za © 2004 Geological Society of South Africa ABSTRACT Based on regional palaeocurrent pattern analysis and petrographic studies, the Late Triassic-Early Jurassic Elliot Formation (Karoo Supergroup, South Africa) is divisible into a Lower and an Upper Elliot Formation. Laterally, along a north-south section, the lower unit displays both changes in the sediment supply pattern and petrological composition, whereas the upper unit lacks significant petrological variations. A break in sedimentation took place after the deposition of the Lower Elliot Formation, and a regional subaerial unconformity is detected between the Lower and Upper Elliot formations. This unconformity is emphasized by marked differences in the sedimentological and palaeontological composition of the deposits below and above the surface. Prevailing northerly sediment supply patterns of the Lower Elliot Formation are replaced by mainly easterly-northeasterly transportation directions in the Upper Elliot Formation, trends which were mentioned, but not accounted for by earlier researchers of the Late Triassic-Early Jurassic Elliot Formation. Simultaneously with the reorganization of the sediment supply patterns, the origin of the sandstones also shifted from recycled-orogen provenance, to a transitional continental source situated between craton interior and basement uplift provenances. This paper presents the contrast in palaeocurrent trends and sandstone composition across this surface, and reports, for the first time, the basin-wide presence of outsized quartzite pebbles and boulders, sourced from the Cape Fold Belt. Introduction The continental red beds of the Elliot Formation provide key information on the Late Triassic-Early Jurassic development of the Karoo foreland system which cosist of the main Karoo Basin and the Cape Fold Belt (CFB) (Figure 1A). The uppermost sedimentary fill, the informal “Stormberg Group” comprising of the Molteno, Elliot and Clarens formations, is considered to lack drastic lateral facies changes or thickness variations. According to Catuneanu et al. (1998) there are in fact only two major lateral changes along the south-north profile: a gradual grain-size reduction and an overall thickness decrease from south to north. The basin development model of Catuneanu et al. (1998) anticipates that the vertical grain-size distribution of Late Triassic-Early Middle Jurassic sedimentary fill shows three second-order coarsening-upward sequences: two of them are preserved in the Molteno Formation (Hancox and Rubidge, 1995; Catuneanu et al., 1998; Hancox, 1998a; b) and the third is the Elliot-Clarens formations (Catuneanu et al., 1998) (Figure 1B). The Upper Karoo sedimentary rocks are believed to originate from reworking of older Karoo Supergroup rocks, and Cape Fold Belt lithologies (Johnson, 1991; Catuneanu et al., 1998). This paper investigates the provenance of the Elliot Formation sedimentary strata by documenting the major sediment supply patterns, sediment provenance (sediment dispersal patterns and petrology combined), and lateral and vertical grain-size changes. Geological background The compressive Karoo foreland system (including the Cape Fold Belt and main Karoo Basin) developed in response to at least eight tectonic events related to the Late Palaeozoic-Early Mesozoic subduction of the palaeo-Pacific plate beneath the Gondwana plate (Catuneanu et al., 1998). The entire ”Stormberg Group” was deposited in a foresag setting formed in response to the final, first-order orogenic unloading of the system (Catuneanu et al., 1998; Hancox, 1998b) (Figure 1B). Two well-documented, smaller tectonic events (P7 final phase ~223 Ma; P8 final phase ~215±3 Ma) (Halbich et al., 1983; Gresse et al., 1992; Catuneanu et al., 1998) are known to have occurred during “Stormberg” deposition (Figure 1B). These two brief orogenic pulses, recorded by two subaerial unconformities (one within Molteno Formation and the other at the base of the Elliot Formation - Figure 1B) were generated as a result of forebulge uplift (Catuneanu et al., 1998). In other words, gradual steepening of the foreslope during the dominant, first-order orogenic unloading was interrupted at least two times, and resulted in the above- mentioned coarsening-upward sequences (two in the Molteno and and one in the Elliot-Clarens formations - Figure 1B).