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Soft-sediment deformation at the base of the Neoproterozoic Puga cap carbonate (southwestern Amazon craton, Brazil): Confirmation of rapid icehouse to greenhouse transition in snowball Earth

¹ Departamento de Geociências, Universidade Federal do Amazonas, Av. Gal. Rodrigo O. J. Ramos 3000, Manaus, AM 69.077-000, Brazil, and Programa de Pós-Graduação em Geologia Sedimentar, Instituto de Geociências, Universidade de São Paulo, Rua do Lago, 562, São Paulo, SP 05508-080, Brazil
² Departamento de Geologia Sedimentar e Ambiental, Instituto de Geociências, Universidade de São Paulo, São Paulo, SP 05508-080, Brazil
³ Núcleo de Estudos de Granitos–Laboratório de Isótopos Estáveis, Departamento de Geologia, Universidade Federal de Pernambuco, Recife, PE 50732-970, Brazil
⁴ Laboratório de Geologia Isotópica-Pará-Isso, Centro de Geociências, Universidade Federal do Pará, Belém, PA 66.075-900, Brazil
⁵ Departamento de Geologia Sedimentar e Ambiental, Instituto de Geociências, Universidade de São Paulo, São Paulo, SP 05508-080, Brazil

Stratigraphic and isotopic data identify the lower 45 m of the Araras Group, on the southwest margin of the Amazon craton, as a Neoproterozoic platform cap carbonate deposited below wave base upon Varanger glacial diamictites of the Puga Formation. The basal beds consist of moderately deep water pinkish dolomudstone with stratiform to wavy fenestral microbialites locally cut by tube-like structures and fenestral nonmicrobial planar laminites with tepee-like features. Above the basal carbonates are deep-water bituminous lime mudstones with alternating thin calcite crusts and lime mudstone laminae commonly disrupted by calcite crystal fans (pseudomorphs after aragonite). The basal contact of the Puga cap exhibits soft-sediment deformational structures (principally load casts) that are here attributed to rebound-induced seismicity acting upon both recently deposited carbonate sediments and underlying unconsolidated diamictite. These features constitute the first clearly recognized sedimentological evidence for the rapid change from icehouse to greenhouse conditions as postulated in the snowball Earth model of Neoproterozoic glaciation.

Key Words: Neoproterozoic • Amazon craton • carbonate rocks • glaciation • isotopes • deformation

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