INVESTIGADORES
ORFEO Oscar
congresos y reuniones científicas
Título:
The origin and sedimentology of large sandy mid-channel bars
Autor/es:
REESINK, A.; ASHWORTH, P.; AMSLER, M.; BEST, J.; HARDY, R,; LANE, S.; NICHOLAS, A.; ORFEO, O.; PARSONS, D.; SMITH G, SAMBROOK; SANDBACH, S.; SZUPIANY, R,
Lugar:
London, UK
Reunión:
Conferencia; Sediment Body Geometry and Heterogeneity: Analogue Studies for Modelling the Subsurface' Conference; 2009
Institución organizadora:
The Geological Society, London, UK
Resumen:
Unit-bar and dune sets are sub-seismic scale structures that strongly influence alluvial reservoir heterogeneity. Parameterization of the geometry and spatial distribution of these structures is often based on measurements from 2D reservoir outcrop analogues and deposits from small (< km wide, < 5 m deep) rivers. This paper describes a new alluvial architecture dataset being assembled for mid-channel bars from the World’s 6th largest river, the sandy braided Paraná River, Argentina. Data on the architecture and internal structure of 10 mid-channel bars have been collected using ~40 km of Ground Penetrating Radar and 28, 4m-deep suction cores. Mid-channel bars are characterized by stacking of 2-7 unit bars with thicknesses up to 10 m and lateral extents in the order of 102-103 m. Mid-channel bars develop from the stalling of a large unit-bar, followed by lateral accretion and the development of downstream wings/tails. Bar growth is primarily associated with the stacking of dunes and small-scale bedforms. The downstream bar tail commonly contains large-scale angle-of-repose sets that enclose a bar-top hollow in which finer grained sediment can accumulate. The overall mid-channel bar morphology thus controls the spatial distribution of large-scale angle-of-repose sets and fine-grained sediment. The internal structure of the unit-bars that make up the mid-channel bars comprise fine-grained bottomsets, coarse-grained foresets, and coarse-grained topsets. Comparison of cores and GPR suggests that unit-bar foresets are dominantly composed of down-climbing dune sets. These inclined dune sets are 2-2.5 times thicker than the near-horizontal dune sets that make up the unit-bar topsets. Thus, large-scale alluvial morphology forms a dominant control on the geometry and preservation of dune sets. 2-103 m. Mid-channel bars develop from the stalling of a large unit-bar, followed by lateral accretion and the development of downstream wings/tails. Bar growth is primarily associated with the stacking of dunes and small-scale bedforms. The downstream bar tail commonly contains large-scale angle-of-repose sets that enclose a bar-top hollow in which finer grained sediment can accumulate. The overall mid-channel bar morphology thus controls the spatial distribution of large-scale angle-of-repose sets and fine-grained sediment. The internal structure of the unit-bars that make up the mid-channel bars comprise fine-grained bottomsets, coarse-grained foresets, and coarse-grained topsets. Comparison of cores and GPR suggests that unit-bar foresets are dominantly composed of down-climbing dune sets. These inclined dune sets are 2-2.5 times thicker than the near-horizontal dune sets that make up the unit-bar topsets. Thus, large-scale alluvial morphology forms a dominant control on the geometry and preservation of dune sets.