ENIGMATIC CONTINENTAL BURROWS FROM THE EARLY TRIASSIC TRANSITION OF THE KATBERG AND BURGERSDORP FORMATIONS IN THE MAIN KAROO BASIN, SOUTH AFRICA

摘要

An Early Triassic continental ichnofossil assemblage dominated by similar to 4 cm diameter burrow casts was discovered in the transitional zone of the Katberg and Burgersdorp formations in the SE main Karoo Basin (Eastern Cape, South Africa). Analyses of the burrow architecture and associated sedimentary facies aim to identify the possible trace makers, their behavior, and the local depositional conditions in the Olenekian (Early Triassic) in this part of Gondwana. The burrows were excavated into a floodplain where alternating periods of flooding, drying, and pedogenesis occurred, a set of conditions often mentioned for the Lower and Middle Triassic continental deposits in southern Gondwana. However, the burrow architecture and superficial morphology are distinct from previously reported older and younger burrows in this part of Pangea. These burrows are simple vertical shafts that are occasionally J-shaped with a circular to slightly elliptical cross-section (average aspect ratio 1.2), similar to 4 cm in diameter, 9-76 cm in length (average similar to 26 cm) and have subvertical axes with average vertical inclinations of approximately 30 degrees. The passively filled burrow casts lack surficial morphology, lining, or branching and have semi-rounded and unenlarged burrow terminations. Potential burrow-makers, ranging from crayfish to lungfish and tetrapods, have been considered, but a definitive producer for these probably dwelling, preying and/or sheltering structures remains elusive. Finally, the results reinforce that burrowing into semi-arid floodplains was a favored behavior among certain organisms in southern Gondwana in the aftermath of the largest biological crisis in the Earth's history. The study also highlights stratigraphic changes in the style of burrow architecture.