Jones, Merren1, Phil Ashworth2, Jim Best3, James
(1) University of Manchester, Manchester, United Kingdom
(2) University of Brighton, United Kingdom
(3) University of Leeds, United Kingdom
ABSTRACT: The Influence of Sediment Supply on Braided Alluvial Architecture: An Integrated Field and Experimental Study
An integrated field and experimental modeling study was used to examine how sediment supply controls the architecture of coarse-braided alluvium. The late Quaternary evolution of the Canterbury Plains, New Zealand, is punctuated by several glacial and interglacial cycles that are associated with fluctuations in sediment supply. Continuous exposure of up to 25 m of coarse, braided-river deposits spans the most recent glacial-interglacial transition (18ka). The architectural geometry, net:gross and grain size was quantified at 15 sites across the 70-km wide Plains to quantify the influence on the stratigraphy of the change in sediment supply associated with the transition from glacial to fluvial conditions. To further investigate this relationship four physical-modeling experiments were undertaken based on boundary conditions scaled from the field prototype. The 1:50 model was run in a 3.5x5.5 m experimental basin, under constant discharge, with a bimodal grain size distribution derived from coastal exposures. Sediment was supplied to the system at three different feed rates to produce accumulation rates of 0.25, 1 and 2 mm hr--1. Architectural geometries, vertical proportion curves and grain size were collected from sections cut at 10-cm intervals down the basin, perpendicular to the flow direction. Both the field and experimental results suggest that a variation in the rate of sediment supply has no effect on the architectural geometry of the preserved stratigraphy. An increase in the rate of sediment supply did however result in a change in both the preservation potential of the depositional elements and in the heterogeneity of the deposits.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.