Subaqueous Sandstones Deposited by Meltwater-Fed Density Flows in a Late Paleozoic Glaciomarine Succession, Eastern Paraná Basin, Brazil
Fernando Farias Vesely
Petrobras/E and P, Rio de Janeiro, Brazil
The Itararé Group records the Late Paleozoic glaciation in the Paraná Basin of southern Brazil. In spite of strong glacial influence on sedimentation, the unit encloses thick sandstone successions in which prospective hydrocarbon reservoirs and aquifers take place. The origin of these sandstones has been the object of discussion because facies associations show characteristics of both “fluvial” and gravitational processes.
Previous models of turbidite deposition in the Itararé Group have proposed that turbidity currents were trigged by slide-induced processes (resedimentation). Data analyzed here suggests that the thick sandstone successions were deposited subaqueously in proglacial fans due to meltwater-derived highly concentrated jet flows that evolved to hyperpycnal turbidity currents. In order to discuss this depositional model, sedimentary facies and stacking patterns were analyzed in several outcrop sections exposed in the eastern basin margin.
Sandstones and conglomerates may occur as thick successions made up of amalgamated bodies as well as beds and bedsets interbedded with fine-grained facies. Detailed facies analysis allows the identification of several facies types ranging from boulder-rich conglomerates to fine-grained sandstones. Polymictic chaotic (boulder-rich), massive and graded conglomerates (facies Gc, Gm and Gg) occur as erosive-based bodies, commonly filling scours or small-scale channels. These are interpreted as hyperconcentrated flow deposits formed due to high energy meltwater jet efflux in proximal subaqueous outwash setting. Graded conglomerates may have a poorly developed sigmoidal geometry, being interpreted as a proximal mouthbar-like deposit. Locally, subangular clasts, often faceted and striated, testify glacial heritage. A-axis clast imbrication may occur, which are typical of density flows were clasts are transported in suspension due to grain-by-grain interaction.
Planar and trough cross-bedded coarse sandstones and conglomerates (facies Sp/St and Gp/Gt) occur as stacked sets and cosets showing lenticular, wedge-like or channelized geometries. These facies were deposited as bed-load dominated currents in a subaqueous meltwater delivered drainage system. Bedding patterns and architecture are similar to fluvial systems, but the close association with hyperconcentrated-flow sediments corroborates the subaqueous origin. Paleocurrent data indicate an average meltwater flow towards northwest, approximately parallel to the main ice flow direction in the eastern part of the basin.
Parallel-laminated and low-angle cross-bedded sandstones (facies Sh/Sl) are coarse to fine-grained and define laterally extensive amalgamated sheet-like bodies. These are interpreted as upper stage plane bed deposition probably from unconfined high energy flows. Facies Sl records the development of low-amplitude dunes in some flow stages, possible due to transition from upper to lower stage settings. Unstratified sandstones (facies Sm, Sg and Si) may occur in close association with facies Sh/Sl, and could be the result of deposition from a flow with higher fall-out rates. Thick beds of laminated and massive sandstones point out to relative flow steadiness, in order to promote continuous aggradation of the bed. Alternation between parallel and low angle bedding suggests fluctuations in flow velocity being strong evidence of waxing to waning flows. Purely waxing or waning flows are recorded by sharply bounded individual beds of inverse and normal graded sandstones respectively.
Medium to fine-grained sandstones showing large to small-scale climbing ripples (facies Sr) occur frequently in the Itararé Group, mainly in the upper portions of the sandstone successions described. Beds are few centimeters to one meter thick, eventually reaching five meters without any vertical grain-size change. Facies Sr formed as the result of traction plus fall-out from turbulent flows in which the stacking of ripples was possible due to long-lived bed aggradation. Vertical changes of the angle of climb and consequently type of lamination within an individual bed suggest fluctuating flow discharges. Rippled sandstones within turbidite beds are considered by several authors a typical feature of hyperpycnites.
The facies association described records deposition in a proglacial subaqueous setting during ice retreat. The linkage between hyperconcentrated gravelly density flows and strong traction currents is a common characteristic of subaqueous outwash fans, in which sedimentation is controlled by the discharge of meltwater directly into a water body. Fluctuations of meltwater discharge in the subaqueous outwash system resulted downflow in the alternation between waxing and waning flow beds verified in turbidite lobes.
The presented data suggests that the main trigger mechanism for turbidity currents in the Itararé Group was meltwater-fed jet flows. Resedimentation due to slide-induced processes have also been recorded, but these flows were not efficient enough to generate thick turbiditic beds. This interpretation can explain satisfactorily the high amount of sandstones commonly observed in glaciated basins dominated by temperate glaciomarine settings. Ancient deposits formed in this context provide a good opportunity to the study of flood-dominated depositional systems from an outcrop perspective. The Itararé Group can thus be considered a field analogue for hyperpycnal flow “turbidites” reported in offshore petroleum basins, including the eastern Brazilian margin.
AAPG Search and Discovery Article #90079©2008 AAPG Hedberg Conference, Ushuaia-Patagonia, Argentina