The sedimentology and alluvial architecture of a large braid bar, Río Paraná, Argentina

SMITH G, SAMBROOK; ASHWORTH P,; BEST, J; LUNT I,; ORFEO, O.; PARSONS D,

JOURNAL OF SEDIMENTARY RESEARCH

Society for Sedimentary Geology

Lugar: Tulsa; Año: 2009 p. 629 - 642

1073-130X

Despite the importance of large rivers as conduits for the transfer of water and sediment to depositional basins, the alluvial architecture of their deposits is poorly understood. This paper presents ~ 30 km of Ground Penetrating Radar (GPR) data from a mid-channel bar in the 6th largest river in the world, the Río Paraná, Argentina. GPR profiles (up to 12 m below the bar surface) were collected from a 3 km long x 1 km wide sandy braid bar on a grid with 200 to 400 m spacing. Two facies dominate the sedimentary architecture. The principal facies (~83%) is trough and planar cross-strata related to the migration of dunes, with the thickness of the cross-strata decreasing towards the bar surface. The second significant facies (~15%) is high-angle (generally 10o-20o) strata that typically forms by accretion at the bar margins or bartail. Clay drapes (<2%) and cross-bar channels (<1%) comprise only a minor part of the deposits. The Río Paraná GPR surveys are compared with other GPR studies of sandy braid bars of different sizes from the South Saskatchewan, Wisconsin and Jamuna Rivers. The dominance of dune deposits is ubiquitous to all the rivers and all possess a significant proportion of high-angle strata. There is thus a high degree of similarity between these different rivers that suggests a common facies model may be applicable to all sandy braided rivers. However, differences between the rivers deposits do exist; 1) the compound bar deposits of smaller rivers may contain greater proportions of cross-bar channel fills; 2) the thickness of preserved high-angle sets (associated with deposition at bar margins) is found to decrease with the age of the bar. The former observation suggests a potential role for discharge regime as a factor in controlling alluvial architecture through its impact on the frequency of sediment reworking. The latter observation suggests that the deposits of older bars may provide more useful geometrical analogues for interpreting ancient successions, rather than smaller transient or recent bar forms which have undergone only limited modification.