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

Abstract

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. δ 18 O 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 13 C 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 87 Sr/ 86 Sr 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.

Aragonite Crystal Fans In Neoproterozoic Cap Carbonates: A Case Study From Brazil and Implications For the Post-Snowball Earth Coastal Environment

“…Their M (middle)-REE enrichment relative to their L-REE may result from a diagenetic effect (cf. Shields and Stille 2001;Herbert and Compton 2007). The Ce anomalies (Ce/Ce*) in the range 0.34-0.96 are typical of rather suboxic conditions, contrasting with the large negative Ce anomalies observed in present-day oxygenated seawater (Fig.…”

“…Provided that terrigenous contamination was absent (the opposite would be easily detectable by a flat REE pattern), REEs can be used as proxies for reconstructing seawater compositions, as demonstrated for recent and Jurassic cases, respectively, by Webb and Kamber (2000) and Olivier and Boyet (2006) and for Archean microbialites by Kamber and Webb (2001). Nonmarine carbonates can also be identified from their REE + Y distribution patterns (Herbert and Compton 2007). Precambrian carbonates with seawater-like REE + Y patterns often display no or a small negative Ce anomaly, regarded as an indicator of marine anoxia or dysoxia (Bau and Alexander 2006).…”

Application of Magnetic Geothermometers in Sedimentary Basins: An Example From the Western Karoo Basin, South Africa

“…During early basin development, sedimentary processes in the southwestern part of the basin were characterized by a deep marine environment, and a gradational contact exists between the marine till sheets of the Dwyka Group and the pelagic sediments of the lower Ecca Group (Herbert andCompton, 2007, Catunaneau et al, 2002). The Ecca Group in the northern section of the southwestern parts of the basin is divided into the Prince Albert, Whitehill, Tierberg and Waterford formations (Johnson et al, 2006).…”

Exhumed lateral margins and increasing flow confinement of a submarine landslide complex

“…Facies Association 1 (FA 1) is the Prince Albert Formation, which was deposited either in a marine basin as shelf deposits (Strydom, ; Buhmann et al ., ; Visser, , ), or in a freshwater lake environment (Herbert & Compton, ). Prince Albert Formation sediments accumulated from syn‐glacial to post‐glacial suspension fall‐out and flocculation of fines from large inflows of sediment‐laden water (Domack, ; Smith & Ashley, ), with some input by turbidity currents and mud flows of semi‐consolidated sediments (Tankard et al ., ; Visser, ).…”