Tracking the Triassic-Jurassic boundary in the roof of Africa

MARSICANO, CLAUDIA; SMITH, ROGER; SIDOR, CHRISTIAN

Mendoza

Congreso; Gondwana Conference; 2005

The final stages of the infilling of the South African Karoo Basin are recorded by the sediments of the Molteno, Elliot, Clarens formations, which together with the overlying Drakensberg volcanics comprise the informally recognised ‘Stormberg Group’. In the highlands of Lesotho, this thick sequence of fluvial-to-aeolian dominated sedimentation overlies the Permo-Triassic Beaufort Group, and is conformably capped by the flood basalts of the Drakensberg Group. The basalts indicate the beginning of the break-up of Gondwana with radiometric dates of 193 Ma at the base followed by the main phase of eruption at around 179 Ma (Duncan et al., 1997), or from the Sinemurian to Toarcian (Gradstein et al., 2005). However, the lack of radiometrically datable material within underlying ‘Stormberg’ sediments has necessitated a reliance on biostratigraphic age control. In particular, the stratigraphic position of the Triassic-Jurassic (TJ) boundary in the Karoo succession has always been poorly constrained, and is currently considered to lie within the middle part of the Elliot Formation (Kitching and Raath, 1984; Olsen and Galton, 1984; Knoll, 2004). The Jurassic Upper Elliot and Clarens formations host one of the richest assemblages of early Mesozoic tetrapods, including dinosaurs, tritylodontids, early mammals and turtles. In addition to body fossils, tetrapod footprints are well represented and have played an important role in the placement of TJ boundary (Ellenberger,1972, 1974; Olsen and Galton, 1984; Knoll, 2004). We recently re-evaluated the stratigraphic position and sedimentological context of Ellenberger’s classic Moyeni locality in southern Lesotho, in addition to restudying its ichnocoenosis. Based on the results of recent geological work (Bordy et al., 2004) and our measured sections, we consider the Moyeni tracksite to be positioned close to the base of the upper Elliot Formation. The imprinted surfaces occur in the upper part of a 3 m-thick channel sandstone that is interdigitated with overbank mudrocks. We interpret this setting as the inner bank facies of an exhumed point bar within a meandering river channel. The Moyeni surface contains several distinct track-types, mainly represented by tridactyl footprints and trackways of bipeds. Detailed analysis of the northern end of the paleosurface, an area apparently not recorded by Ellenberger, has yielded unambiguous evidence for the presence of “chirotheroid”-type footprints. They consist of two sets of heteropodous manus and pes impressions, with the manus approximately half the size of the pes. The pes prints are pentadactyl, with an everted digit V, whereas those of the manus are probably tetradactyl. The discovery of “chirotheroid”-type footprints at Moyeni in levels of the Upper Elliot Formation is remarkable and biostratigraphically important because of their widespread use as Triassic “markers”, and their absence above the TJ boundary (e.g., Lockley and Meyer, 2000; Olsen et al., 2002). Two scenarios can explain our results. First, the stratigraphic range of “chirotheroid”-type footprints could be extended into the Jurassic. Second, the upper Elliot Formation could be older than currently considered. If true, the second hypothesis would have important implications for our understanding of the diversity of Late Triassic tetrapod faunas in Gondwana and the putative tetrapod continental extinction at the TJ boundary.