No abstract

The demise of the Late Paleozoic Ice Age has been hypothesized as diachronous, occurring first in western South America and progressing eastward across Africa and culminating in Australia over an ∼60 m.y. period, suggesting tectonic forcing mechanisms that operate on time scales of 106 yr or longer. We test this diachronous deglaciation hypothesis for southwestern and south-central Gondwana with new single crystal U-Pb zircon chemical abrasion thermal ionizing mass spectrometry (CA-TIMS) ages from volcaniclastic deposits in the Paraná (Brazil) and Karoo (South Africa) Basins that span the terminal deglaciation through the early postglacial period. Intrabasinal stratigraphic correlations permitted by the new high-resolution radioisotope ages indicate that deglaciation across the S to SE Paraná Basin was synchronous, with glaciation constrained to the Carboniferous. Cross-basin correlation reveals two additional glacial-deglacial cycles in the Karoo Basin after the terminal deglaciation in the Paraná Basin. South African glaciations were penecontemporaneous (within U-Pb age uncertainties) with third-order sequence boundaries (i.e., inferred base-level falls) in the Paraná Basin. Synchroneity between early Permian glacial-deglacial events in southwestern to south-central Gondwana and pCO2 fluctuations suggest a primary CO2 control on ice thresholds. The occurrence of renewed glaciation in the Karoo Basin, after terminal deglaciation in the Paraná Basin, reflects the secondary influences of regional paleogeography, topography, and moisture sources.

The response of sediment routing to climatic changes across icehouse-to-greenhouse turnovers is not well documented in Earth’s pre-Cenozoic sedimentary record. Southwest Gondwana hosts one of the thickest and most laterally extensive records of Earth’s penultimate icehouse, the late Paleozoic ice age. We present the first high-resolution U-Pb zircon chemical abrasion−isotope dilution−thermal ionization mass spectrometry (CA-ID-TIMS) analysis of late Paleozoic ice age deposits in the Kalahari Basin of southern Africa, which, coupled with existing CA-ID-TIMS zircon records from the Paraná and Karoo Basins, we used to refine the late Paleozoic ice age glacial history of SW Gondwana. Key findings from this work suggest that subglacial evidence in the Kalahari region is restricted to the Carboniferous (older than 300 Ma), with glacially influenced deposits culminating in this region by the earliest Permian (296 Ma). The U-Pb detrital zircon geochronologic records from the Paraná Basin of South America, which was located downstream of the Kalahari Basin in the latest Carboniferous and Permian, indicate that large-scale changes in sediment supplied to the Paraná were contemporaneous with shifts in the SW Gondwana ice record. Gondwanan deglaciation events were associated with the delivery of far-field, African-sourced sediments into the Paraná Basin. In contrast, Gondwanan glacial periods were associated with the restriction of African-sourced sediments into the basin. We interpret the influx of far-field sediments into the Paraná Basin as an expansion of the catchment area for the Paraná Basin during the deglaciation events, which occurred in the latest Carboniferous (300−299 Ma), early Permian (296 Ma), and late early Permian (<284 Ma). The coupled ice and detrital zircon records for this region of Gondwana present opportunities to investigate climate feedbacks associated with changes in freshwater and nutrient delivery to late Paleozoic ocean basins across the turnover from icehouse to greenhouse conditions.

No abstract

A Glaciação Neopaleozoica e a transição completa para condições pós-glaciais são considerados eventos climáticos únicos na história geológica. O principal registro de tais eventos está nas bacias sedimentares gondwânicas, que sugerem uma glaciação dinâmica marcada por repetidos períodos glaciais e interglaciais. A Bacia do Paraná abriga o registro mais espesso e geograficamente extenso da glaciação, referido como Grupo Itararé. Este trabalho pretende analisar o registro da última influência das geleiras Neopaleozoicas no norte da Bacia do Paraná e a natureza do contato com a Formação Rio Bonito, pós-glacial. A região de Ibaiti, no nordeste do Paraná, foi escolhida como área de estudo por conter uma das maiores espessuras do intervalo na bacia e por necessitar de estudos de maior detalhe, capazes de refinar a estratigrafia e história deposicional. Para tal, foram empregados métodos sedimentologicos (levantamento de fácies, associações de fácies, perfis estratigráficos verticais) e mapeamento geologico. Ademais, foram traçadas correlações estratigráficas entre perfis de afloramento e perfis de raios gama de poços profundos selecionados. A Formação Taciba apresenta espessuras de cerca de 150 m e foi subdividida em três unidades genéticas mapeáveis: 1) Depósitos proglaciais, dispostos em um padrão de granodecrescência ascendente e que incluem conglomerados, arenitos, diamictitos e lamitos; 2) Depósitos deltaicos, compreendendo ritmitos, heterolitos e arenitos empilhados em um padrão de granocrescência ascendente; e 3) Depósitos glácio-marinhos ressedimentados, englobando diamictitos que definem um padrão granocrescente ascendente. Afinidade glacial nas unidades 1 e 3, e decréscimo/ausência de evidência glacial na Unidade 2 sugerem dois episódios distintos de avanço glacial, separados por uma fase interglacial. As tendências deposicionais definidas em superfície prolongam-se por centenas de quilômetros bacia adentro. A passagem glacial-pós-glacial no norte da bacia é abrupto, definido por uma superfície erosiva associada à formação de paleossolo e vales incisos no topo da Unidade 3.