Abstract
Turbidites have captioned the attention of sedimentologists during the last decades due their importance as hydrocarbon reservoirs. However, their relationship to delta systems still deserves further studies. This paper presents examples from a late deglacial to early post-glacial deltaic and turbidite strata exposed in the surroundings of Vidal Ramos (Santa Catarina State, Brazil), southern portion of the Parana Basin. There, the uppermost part of the Mafra Formation and the Rio do Sul Formation onlap the Proterozoic basement and comprises an up to 360 m thick package. It includes (base to top) black shales, mass transport deposits (MTD) and sandy turbidites (Mafra Formation) as well as thin bedded turbidites (tbt), including one interval of black shales and sandy turbidites, overlain by proximal delta front sandstones (Rio do Sul Formation). The analysis of the succession shows two more than 150 m thick coarsening-upwards deltaic successions composed of turbidite sand sheets at their base (prodelta), followed by partially collapsed thin bedded turbidites (delta slope wedge) and delta front sandstones. Both turbidite sand-sheets abruptly overlay black shale intervals related to maximum flooding surfaces and therefore record correlative conformities. A detailed stratigraphic section elaborated from the correlation of four logs (1/100) suggests that distal delta front sands includes both thin bedded turbidites and wave reworked sands whereas the proximal delta front was dominated by long-lived underflows (hyperpycnal flows). The succession suggests that the most expressive turbidite beds (base of the delta systems) have resulted from relative sea-level falls (early lowstand) whereas the thin-bedded turbidites were related to the development of the late lowstand wedge. Black shales represent the transgressive systems tract and HST were not deposited or preserved in the area. High sediment supply associated with lowstand tracts could explain the occasional (Vidal Ramos) to common occurrence of slope failures (slumps and diamictites) involving thin bedded turbidites and delta front sandstones. This situation is quite logical in terms of deglacial periods, and resulting high sediment supply, within a long-term icehouse context, with prevalence of lowstand to transgressive settings.
Highlights
Mass and sediment gravity flows are not restricted to a specific depositional depth as they occur from subaerial to deep-marine environments
The succession exposed around Vidal Ramos region (Fig. 2) is up to 360 m thick in its local depocentre and fills an irregular topography carved by glacial processes (Carvalho 2014) into Paleoproterozoic schists and marbles of the Brusque Metamorphic Complex (Fig. 3)
Black shale: A 20 m thick interval composed of black shale defines the base of the Paleozoic succession in the studied area
Summary
Mass and sediment gravity flows are not restricted to a specific depositional depth as they occur from subaerial to deep-marine environments. Facies tracts are a result of sediment supply, flow transformation and flow efficiency, basin configuration and confinement, climate and relative sea-level. It is known that subaqueous environments favour the maintenance of gravity flows due to the preservation of water in the dense, sediment plus water mixture, in opposition to subaerial environments, where ground infiltration is a restraining factor. High sediment supply coupled with steep slopes largely enhances the possibility of gravity flows development. Probably more important, relative base-level drops strongly enhance upstream erosion and resulting high sediment supply into the deeper parts of the basins
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