L. Buatois1, R. Netto2, and G. Mangano1
1Conicet-Insugeo, Casilla de correo 1 (CC), 4000 San Miguel de Tucumán, Argentina,
2Unisinos, Núcleo de Estudos Sedimentológicos e Estratigráficos, Av. Unisinos 950, 93022-000 São Leopoldo RS, Brasil. [email protected]
ABSTRACT: Application of Ichnologic Studies to Paleoenvironmental and Sequence-Stratigraphic Analyses of Permian Marginalto Shallow-Marine Coal-Bearing Successions of the Paraná Basin, Brazil
The Lower Permian Rio Bonito and Palermo formations represent part of the infill of the Paraná Basin, southern Brazil. Integrated analysis of cores, outcrops and well logs from coal fields in Rio Grande do Sul allows sedimentologic, ichnologic, and sequence-stratigraphic characterization of these successions. The Río Bonito Formation has been usually interpreted as fluvio-deltaic. However, the transgressive nature of the succession, the vertical increase in ichnodiversity and degree of bioturbation, and the vertical passage from brackish ichnofaunas to fully marine assemblages argue against a prograding delta and suggest deposition in fluvio-estuarine settings. Lowstand fluvial deposits are unbioturbated. Estuarine deposits of the transgressive systems tract comprise tidal channel, point bar, coal-bearing marsh, and estuary mouth facies. Estuarine ichnofaunas are characterized by simple tiering structures, low degree of bioturbation, low diversity, and dominance of simple burrows produced by trophic generalists. The Palermo Formation represents transgressive deposition in open-marine environments. A wave ravinement surface separates the underlying marginal-marine deposits from the overlying open-marine facies. Openmarine strata form regional parasequences. High-energy, storm-dominated, lower/middle shoreface sandstones, laterally grading into moderate-energy shoreface deposits, occur towards the base of the open-marine succession. High-energy shoreface deposits typically lack bioturbation, but deep burrows may be present locally. Moderateenergy shoreface deposits show alternation of opportunistic and climax suites. Offshore-transition to upper- and lower-offshore deposits are punctuated by transgressive surfaces of erosion demarcated by Glossifungites suites. Upper-offshore to offshore-transition softground assemblages are the most diverse. Degree of bioturbation is high and tiering structure is relatively complex. Lower-offshore mudstones are totally bioturbated.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado