Browsing by Author "Compton, J S"

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    Depositional environments of the lower Permian Dwyka diamictite and Prince Albert shale inferred from the geochemistry of early diagenetic concretions, southwest Karoo Basin, South Africa.
    (Elsevier, 2007) Herbert, C T; Compton, J S
    The upper Dwyka and lower Ecca Groups in the Karoo Basin of South Africa document the climatic and palaeoenvironmental changes associated with the final Permo-Carboniferous deglaciation of the Gondwana supercontinent. The depositional environments of these groups have, until recently, been interpreted on the basis of sedimentological and palaeontological evidence. Here we use the geochemistry of early diagenetic concretions – septarian calcite concretions from the upper Dwyka Group and phosphatic chert concretions and beds from the lower Ecca Group – to infer the depositional environment of these rocks in the southwestern Karoo Basin. δ18O values (7.8 to 8.9‰ SMOW) suggest that the calcite concretions precipitated from a mixture of meteoric and glacial melt waters rather than Permian seawater. δ 13C values (−15 to −3‰ PDB) indicate that the carbon was derived from a mixture of craton-derived calcareous material and organic matter, bacterially degraded in the lower sulphatereduction to upper methanogenesis zones during early burial diagenesis. The rare-earth element (REE) patterns, Sr concentrations and 87Sr/86Sr ratios (0.716–0.737) significantly greater than Permian seawater (0.708), together also support the interpretation that calcite and phosphatic concretions formed in glacial, fresh water sediments.
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    Reply to discussion by D. I. Cole
    (Geological Society of South Africa, 2005) Compton, J S; Franceschini, G
    We strongly disagree with D.I. Cole’s contention that our use of strontium isotopes has not lead to any meaningful improvement in the chronostratigraphy of the Prospect Hill Formation exposed in the Tabakbaai Quarry (Franceschini and Compton, 2004). Interpretation of the complex stratigraphy of the West Coast has suffered from a lack of chronostratigraphy. The integration of radiocarbon, uranium disequilibrium and luminescent dating has greatly improved our understanding of upper Pleistocene and Holocene deposits of the West Coast (Roberts and Berger, 1997; Felix-Henningsen et al., 2003; Compton and Franceschini, 2005). The challenge of the beach and aeolian deposits of the Prospect Hill Formation at Tabakbaai Quarry is that they are too old to date using these methods and so we applied the Sr isotope dating method because it has proved to be extremely useful in sorting out the complex late Cenozoic depositional history of the outer shelf offshore of Cape Columbine (Compton et al., 2004; Wigley, 2005). Unfortunately the resolution of the Sr-derived ages we obtained is relatively poor mostly because the marine Sr isotope curve flattens out in the late Miocene (Farrell et al., 1995)