Rare-earth elements and stable isotope geochemistry of early Cambrian chert-phosphorite assemblages from the Lower Tal formation of the Krol Belt (Lesser Himalaya, India)

The Neoproterozoic-Cambrian transitional sequence in the Mussoorie and Garhwal Hills of the Lesser Himalaya is represented by dolomites of the Upper Krol Formation and chert-phosphorite assemblages of the lowermost Tal Formation. These rocks provide valuable information regarding the assumed existence of a stratified ocean during the terminal Neoproterozoic and beginning of earliest Cambrian (Nemakit-Daldynian). The isotopic compositions of carbonate and organic carbon in the transition profile can be best interpreted as reflecting an oceanic anoxic event (OAE) prior to the phase of early Cambrian phosphate formation. The chert-phosphorite beds record negative Ce and positive Eu anomalies. While Ce is an important paleooceanic redox indicator, Eu is rather immobile under normal diagenetic conditions. The observed Eu anomaly is linked to the high Ba content in these phosphorites and is indicative of an anoxic (or sulfate reducing) diagenetic environment. The suggested stratification of the ocean during the Late Neoproterozoic and Nemakit-Daldynian times would have been Linked to the reduction of oxygen supply to the zone below the redoxcline, causing C-12-and P-rich organic matter previously produced in the photic zone to be trapped in large quantities within the deeper anoxic zone. This would, in turn, increase the C-13 content of the water in the photic zone. Moreover, in the deeper anoxic waters, reduction of Ce4+ to Ce3+ would result in an enrichment of dissolved Ce (as Ce3+). Upwelling of this anoxic bottom stratum would bring the C-12-and Ce-enriched waters back to the oxic marine realm. However, on entering the highly oxic productive zone, the excess Ce is expected to be removed from the water by oxidation to insoluble Ce4+ followed by precipitation or preferential scavenging. Due to these oxidation reactions, the waters of the shallow oxic zone would tend to become rapidly depleted in Ce. In other words, ocean mixing and upwelling during the early Cambrian were ultimately responsible for the transport of C-12, P and Ce to the oxic shelves. Sharp drops in both delta(13)C(carb) and delta(13)C(org) support the existence of oceanic anoxia followed by oceanic mixing at this juncture of Earth history. The large spread of the delta(34)S values of early diagenetic pyrites within the phosphorite bearing sequence in contrast to the limited spread of delta(13)C(carb) values suggests that phosphatisation took place under exclusively suboxic diagenetic non-sulfate reducing condition and pyritisation began only when phosphate layers entered the zone of sulfate reduction. (C) 1999 Elsevier Science B.V. All rights reserved.