A New Model of Oceanic Crustal Ages Derived From Magnetic Data: Implications for Source Rock Distribution Along the Transkei Margin of South Africa

Abstract

Approximately 70,000 km of high resolution aeromagnetic data were recently acquired along the Transkei margin. The sampling and spatial extent of these data present an opportunity to more confidently map magnetic field reversals than previously possible. Negative linear magnetic anomalies trend perpendicular to the margin and are notably present southwest of the previously interpreted M0r isochron. Initial simplistic forward magnetic models were built using homogeneous magnetic layer thickness, magnitude of magnetization, and spreading rates. These facilitated correlation of observed and calculated anomalies allowing for spatial registration of the model, calculation of estimated spreading rates, and integration with crustal thickness mapping from 2d seismic data. Crustal thickness variations are correlated to these estimated spreading rates and found to be in general alignment with global observations of slow-spreading crust. A final magnetic isochron model was built using the mapped crustal thicknesses and calculated, variable spreading rates from the initial anomaly interpretation. This iterative analysis provides a good fit between observed and calculated magnetic anomalies and results in a south-westward shift of M0r, which creates an important constraint for predicting source rock presence. Forward magnetic modelling shows a long wavelength magnetic low in central Transkei is directly related to an observed area of thin oceanic crust (3-4 km); Progressive southward downlap of sedimentary fill excludes the possibility of ridge jump and the highly rugose upper surface in this zone is characteristic of ultra-slow spreading. These observations constrain the age of crust within Cretaceous long magnetic normal, south of M0r. Plate modelling and DSDP wells from the southern Atlantic region indicate restriction and organic productivity in the Weddell Sea creating conditions for source rock deposition throughout the lower Cretaceous, with open marine conditions returning by the late Aptian. In contrast to previously published oceanic crustal ages this new model places pre-late Aptian crust along the entire Transkei margin with an associated potential for the presence of source rocks.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018