期刊
BRAZILIAN JOURNAL OF GEOLOGY
卷 46, 期 3, 页码 377-393出版社
SOC BRASILEIRA GEOLOGIA
DOI: 10.1590/2317-4889201620160001
关键词
black shales; paleoredox proxies; Irati Formation; Permian
资金
- CNPq (Brazilian Research Council) [200453/97-1]
- Sigma Xi
- Department of Geophysical Sciences
- Paleogeographic Atlas Project
Paleoredox indicators were used to assess the depositional environment of the Permian Irati Formation, and discuss the controls on the accumulation of organic-rich sediments. Geochemical data were measured for 43 drill-core samples in southern Parana Basin. We infer that redox boundary was located at the sediment-water interface during the accumulation of these sediments. Trace-element data demonstrate that Irati shales were similar to average shales, except during deposition of two organic-rich beds in which U and Mo are enriched, suggesting fully anoxic conditions. The variable degree of oxygenation of bottom waters is also attested by DOPT, Fe-T/Al, and sedimentological indicators of episodic ventilation. DOPT and Fe and S correlation points to an iron-limited environment, where the main sulfur sink is organic matter rather than pyrite. Cu and Cu/Mo ratio suggests bottom-water anoxia driven by increased organic flux. The Irati Formation was probably deposited in a salinity-stratified body of water, with positive water balance. The main control on the accumulation of the organic-rich sediments was the high organic carbon flux resultant from high primary productivity in surface waters. Anoxia arose as a consequence of increased productivity, not as the primary driver in the formation of these organic-rich rocks.
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