The Record of Redox Conditions in Deep-Water Rocks at the Neoproterozoic–Cambrian Boundary in Oman: A Case for Possible Anoxia

Stefan Schröder and John P. Grotzinger
Massachusetts Institute of Technology, Cambridge, MA

The Neoproterozoic-Cambrian boundary in Oman is defined by a negative d13C excursion and a U-Pb age of 542.0Ma in platform carbonates. These rocks correlate basin wards with two transgressive black shale units and organic-rich siliceous rocks ('silicilyte').

Silicilyte rocks are characterized by fine crinkly lamination, >80wt.% authigenic microcrystalline quartz, little detrital fraction, and disseminated organic material (average 7-8%). Thickness increases from the basin margins to the center; detrital input decreases in this direction. The silicilyte represents an unusual facies uncommon in Phanerozoic basins, but related to chert in Precambrian BIF.

Enriched d34S in platform evaporites and anoxic biomarker in the silicilyte suggest significant burial of reduced sulfur in a probably anoxic basin. To further test this hypothesis, we have studied redox-sensitive trace elements (TE) and rare-earth elements (REE).

Trace elements are enriched relative to their crustal concentrations. In silicilyte samples, the TE enrichment was decoupled from detrital input, suggesting authigenic concentration of TE under oxygen-deficient conditions. Trace elements and REE indicate higher detrital input in both shale units, which masked the authigenic signal. Limited correlation between TE and detrital fraction in shales can be related to coupling of some elements to the detrital fraction, oxic intervals with selective element enrichment, and/or rapid burial of TE, which prevented oxidation and depletion.

Detrital input and authigenic element enrichment are the main factors controlling the TE record. The Oman data provide further evidence for anoxia in deep basinal areas around the Neoproterozoic-Cambrian boundary, which could have been a worldwide event.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005