Palaeo-environmental and provenance reconstruction of the Lower Permian mudstones (lower Ecca Group) in the main Karoo Basin, South Africa

Geel, C.; Bordy, E. M. (in review) Palaeo-environmental and provenance reconstruction of the Lower Permian mudstones (lower Ecca Group) in the main Karoo Basin, South Africa

Assessing the composition and origin of the organic matter
rich mudstones, which dominate the early basin fill of the main Karoo Basin of
South Africa, are essential for reconstructing the palaeo-environment,
palaeo-climate and potential provenance areas in southern Gondwana during the
Early Permian. This study addresses these topics by investigating the
sedimentary geochemistry and petrography (e.g., major oxides, trace elements,
mineral compositions, sedimentary structures, textures) of the lower Ecca Group,
which was sampled in newly drilled boreholes. Our results show that the rate of
weathering decreased over time as the environment changed from an ice-house to a
more green-house setting (i.e., the Chemical Index Alteration is 72–78 in the
Prince Albert Formation and 64–66 in the Collingham Formation). Comparison of
the V/Cr, V(V+Ni) and Ni/Co ratios and Fe-S-TOC ternary plot show fluctuations
between anoxic and oxic conditions during deposition. Furthermore, there is
evidence, albeit somewhat equivocal, for changing palaeo-salinity levels, low
sedimentation-rates and high bio-productivity. Trace element Principle
Component Analysis, the Index of Compositional Variability and Discriminant
Function Analysis of the older mudstones (Prince Albert and Whitehill Formations)
imply different provenance regions with felsic to intermediate vs mafic
signatures in the western and eastern MKB, respectively. The youngest unit, the
Collingham Formation, has a more uniform composition across the region, and was
mostly sourced from the quartz-arenites and granites most likely associated
with the Cape Fold Belt.