Heritage Cases

The Jurassic-Cretaceous (J-K) boundary is the last of all Phanerozoic system boundaries to be resolved because the traditional northern hemisphere localities from which it is defined exhibit strong provincialism of fossil assemblages and lack absolute age constraints (Wimbledon et al., 2011, Gradstein et al., 2012). Marine faunal turnovers at the J-K boundary in these well-studied localities are spatially varied and protracted compared to more famous chronostratigraphic boundaries that co-inside with sudden, global mass extinction events (e.g., Cretaceous-Palaeogene and Permian-Triassic). To reach consensus on the J-K boundary, new precisely dated sites from the southern hemisphere are required to establish what environmental fluctuations occurred and their timings. One such advance recently came from the Neuquén Basin of Argentina, where fossiliferous deposits that contain datable volcanic interbeds span the boundary (Vennari et al., 2014). It is imperative that additional new localities are included to build a global palaeoenvironmental record of this time period, especially in areas where temporal calibration is possible. The poorly studied Jurassic to Lower Cretaceous Uitenhage Group of South Africa is uniquely suited to this purpose because it hosts a large diversity of fossil flora and fauna in continental and shallow marine environments and contains datable volcaniclastic interbeds (McLachlan & McMillan, 1976; Muir et al., 2017). Furthermore, during its deposition, a large meteorite impacted earth just 800 km away and given the size of the resultant crater at modern-day Morokweng, it probably perturbed local environments (Koeberl et al., 1997). This impact has been dated to co-inside with the J-K boundary and may even be linked to it. The Uitenhage Group is the closest palaeoenvironmental repository available to assess whether this impact had a significant effect on life and its relationship with the J-K boundary. By pinning down the boundary in the Uitenhage Group and extracting novel palaeoenvironmental information spanning it, this project contributes towards resolving the long-standing problem: What marks the end of the Jurassic, when did it occur, and what ushered in a new geological period—the Cretaceous? The aspect of the research that is relevant to this ECPHRA application is the marine fossil stable isotope analysis, which is a method I hope to employ to determine the palaeo-sea conditions. This research will help establish what the climate and sea-water conditions were in the Late Jurassic and Early Cretaceous during the initial formation of the African continent, when the sea first transgressed across southern Gondwana and wrapped around what is now the southern Cape of South Africa. The principal aims of this part of the project are: (A) Sample marine invertebrate fossils along a vertical stratigraphic profile in the Sundays River Formation at Coega Quarry (site 1) and from the marine Bethelsdorp Member of the Kirkwood Formation exposed at Jagtvlakte (site 2). (B) Analyse the shells for stable isotopes of carbon (C) and oxygen (O), to determine palaeo-temperature and identify shifts and excursions in the global carbon cycle. (Methods and motivation): These two sites expose Lower Cretaceous and Upper Jurassic fossiliferous strata and in-situ marine fossils. The isotopic composition of their shells likely record sea-water conditions (temperature/evaporative rates and global carbon cycling) during shell growth, when the marine organisms were alive. I plan to collect a grand total of 30 specimens of some of the most abundant taxa from Coega Quarry (Pistotrigonia kraussii, Transitrigonia herzogi & Steinmanella holubi e.g., Cooper, 2018), and oysters from Jagtvlakte (Amphidonte (Ceratostreon) see Almond, 2012). The samples will be brought back to the University of the Free State for processing, which involves cutting them in half to expose a cross section and using a microdrill to extract about 50 mg of powder from sections of the shells. These will then be analysed at the University of Cape Town stable isotope laboratory. The samples will be kept for storage at UFS upon completion of the analyses, in order to preserve the neatly sampled specimens.