The Neoproterozoic São Francisco Basin, central Brazil: tectonic evolution, paleoclimate, and mineral systems

The São Francisco Basin (SFB; ca. 950-550 Ma), central Brazil, hosts sediments deposited during most of the Neoproterozoic Era and so is a critical repository of information concerning the evolution of the late pre-Cambrian world. Detailed study and revision of existing stratigraphy reveals that deposition occurred during separate stages of a complete Wilson Cycle, three Neoproterozoic glaciations, and global changes in the biogeochemical cycling of nutrient elements. The basal Macaúbas Group (ca. 950-650 Ma), composed of rift-related sediments, accumulated during breakup of the supercontinent Rodinia. Deposition was punctuated by globally correlative Sturtian glacial deposits (ca. 717-660 Ma). Copious post-glacial organic matter and pyrite-rich siltstones formed during the subsequent passive margin stage. The overlying Bambuí Group is a Cryogenian-Ediacaran mixed biochemical-siliciclastic succession (ca. 650-550 Ma) composed of six stratigraphic sequences. Sequence 1 contains Marinoan glacial diamictites and cap carbonate (ca. 635 Ma) with distinctive barite and aragonite crystal fans. Sequence 2, composed of neritic limestone, siltstone, and phosphorite, is unconformably overlain by dolostone and stromatolitic reefs of Sequence 3. A karst unconformity with sulphide deposits caps Sequence 3. Organic-rich, pyritiferous, deep-water, marine siltstones of Sequence 4 are overlain by shallow water carbonates of Sequence 5 that accumulated as sea-level fell during the Gaskiers Glaciation (ca. 579 Ma). Post-glacial glauconitic siltstones form the lower portion of Sequence 6 and are evidence of marine transgression during the peak of oceanic ventilation. Greywackes and conglomerates in upper Sequence 6 record basin closure by ca. 550 Ma. The organic-rich, pyritiferous, deep-water siltstones are interpreted as the product of combined glacial grinding on land during the Cryogenian glaciations and subsequent eolian, oceanward, transport of silt and clay particles. Copious eolian dust is interpreted to have fertilized the surface ocean to stimulate primary productivity, increase photosynthetic oxygen, and contribute to atmospheric oxygenation. Combined analysis of modern analogs, paleogeographic reconstructions, global 87Sr/86Sr values, chemistry of Ediacaran zircons, and clay mineralogy support this interpretation. The revised stratigraphy of the SFB consolidates its correlation with paleoclimatic events worldwide. Finally, an eolian origin of organic-rich, pyritiferous siltstone provides a novel control reductants for sediment-hosted sulfide mineral deposits.

Department dc:contributor.department

Geological Sciences and Geological Engineering

Year dc:date.issued

2024

Author dc:creator

• da Silva, Leandro Guimarães

Advisors dc:contributor.supervisor

• Pufahl, Peir
• James, Noel

• São Francisco Basin
• Neoproterozoic Earth evolution
• Interglacial aeolian dust
• Atmospheric oxygenation
• Ocean fertilization
• Mineral systems
• Redox traps

Statement dc:rights

• Attribution-NonCommercial-NoDerivatives 4.0 International

Licence dc:rights.uri

• http://creativecommons.org/licenses/by-nc-nd/4.0/

Language dc:language.iso

eng

Handle dc:identifier.uri

https://hdl.handle.net/1974/33602

OAI identifier oai:identifier

oai:queensu.scholaris.ca:1974/33602