--> The Role of Discharge Variability in the Formation and Preservation of Alluvial Sediment Bodies

AAPG ACE 2018

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The Role of Discharge Variability in the Formation and Preservation of Alluvial Sediment Bodies

Abstract

Extant, planform-based facies models for alluvial deposits are not fully fit for purpose, because there is little in the alluvial rock record that is distinctive of any particular planform, and because individual rivers show variable planform in both time and space. Accordingly, existing facies models have limited predictive capability. In this paper, we explore the role of inter-annual peak discharge variability as a possible control on alluvial architecture. A dataset from a suite of modern rivers, for which both a long-term gauging record is available and for which subsurface data on the alluvial record has been published from ground-penetrating radar and other methods, is analysed. The rivers are categorized according to their variance in peak discharge or the coefficient of variation (CVQp = standard deviation of the peak flood discharge over the mean peak flood discharge). This parameter ranges over the rivers studied between 0.18 and 1.22, allowing classification of rivers examined as one of very low (< 0.20), low (0.20-0.40), moderate (0.40-0.60), high (0.60-0.90), or very high (> 0.90). Deposits of rivers with very low and low discharge variability are dominated by cross-bedding on various scales and show ample preservation of macroform bedding structure, allowing the interpretation of bar construction processes. Rivers with moderate values preserve mostly cross-bedding, but records of macroform processes are in places muted and considerably modified by later reworking. Rivers with high and very high values of discharge variability show a wide range of bedding structures including common critical and supercritical flow structures, a general lack of macroform structure, abundant in situ trees and transported large, woody debris, and pedogenically modified mud partings. Such a facies assemblage is distinctively different from the conventional fluvial style recorded in published facies models, but is widely developed both in modern and ancient alluvial deposits. We propose that discharge variability (or “flashiness”) may be a more reliable basis for classifying the alluvial rock record than planform, and we provide some examples of three classes of alluvial sediment bodies (representing low, intermediate, and high/very high discharge variability) from the rock record.