Lowland river responses to intraplate tectonism and climate forcing quantified with luminescence and cosmogenic ¹⁰Be

Abstract

Intraplate tectonism has produced large-scale folding that steers regional drainage systems, such as the 1600 km-long Cooper Ck, en route to Australia's continental depocentre at Lake Eyre. We apply cosmogenic ¹⁰Be exposure dating in bedrock, and luminescence dating in sediment, to quantify the erosional and depositional response of Cooper Ck where it incises the rising Innamincka Dome. The detachment of bedrock joint-blocks during extreme floods governs the minimum rate of incision (17.4±6.5 mm/ky) estimated using a numerical model of episodic erosion calibrated with our ¹⁰Be measurements. The last big-flood phase occurred no earlier than ∼112–121 ka. Upstream of the Innamincka Dome long-term rates of alluvial deposition, partly reflecting synclinal-basin subsidence, are estimated from 47 luminescence dates in sediments accumulated since ∼270 ka. Sequestration of sediment in subsiding basins such as these may account for the lack of Quaternary accumulation in Lake Eyre, and moreover suggests that notions of a single primary depocentre at base-level may poorly represent lowland, arid-zone rivers. Over the period ∼75–55 ka Cooper Ck changed from a bedload-dominant, laterally-active meandering river to a muddy anabranching channel network up to 60 km wide. We propose that this shift in river pattern was a product of base-level rise linked with the slowly deforming syncline–anticline structure, coupled with a climate-forced reduction in discharge. The uniform valley slope along this subsiding alluvial and rising bedrock system represents an adjustment between the relative rates of deformation and the ability of greatly enhanced flows at times during the Quaternary to incise the rising anticline. Hence, tectonic and climate controls are balanced in the long term.

Introduction

River response to tectonic deformation determines local relief and the supply of sediment to basins. Surface uplift may steepen rivers, increasing their erosional capacity to incise bedrock and transport sediment, but the converse also occurs, for instance, where rising transverse structures cause deposition and possible river diversion (Schumm et al., 2000). In the case of low relief landscapes, the sensitivity to small changes in slope means that anomalous river patterns may be the first clue to tectonic activity (e.g. Nanson, 1980). Intraplate tectonism has notably perturbed sections of large lowland rivers such as the Amazon and Mississippi chiefly because such rivers flow across very low gradients (Adams, 1980, Holbrook and Schumm, 1999). Whether a river is diverted or maintains course by incising in pace with uplift is held to be a function of the surface uplift rate, sediment flux, and stream power relative to critical thresholds of erosion (Schumm et al., 2000), though the role of the latter has been questioned (Humphrey and Konrad, 2000). Stream power fluctuations in large, low-gradient rivers are primarily a function of flood magnitude, yet episodic bedrock erosion via extreme floods has barely been examined in large lowland rivers. Much of what is known of how such rivers respond to transverse uplift derives from simplified scenarios explored via physical and numerical modelling (e.g., Humphrey and Konrad, 2000, Molnar et al., 2006), with flume experiments, in particular, contributing major insights to the effects of transverse uplift, such as changes in sinuosity and planform style (Ouchi, 1985, Schumm et al., 1987). Although such observations have been corroborated qualitatively in natural rivers (e.g. Nanson, 1980, Holbrook and Schumm, 1999), there is rarely sufficient constraint on the magnitude of deformation and the associated river responses to fully evaluate a natural experiment over 10⁵–10⁶ y timescales.

A transverse structure rising across the path of an unconfined river is generally predicted to cause deposition upstream of the uplift axis at the same time as erosion downstream (Ouchi, 1985, Humphrey and Konrad, 2000). To test this idea, we examine both modes of fluvial response along a large, lowland river in east-central Australia, Cooper Ck, where it crosses the anticline known as Innamincka Dome (Fig. 1). The rate of alluvial deposition is quantified with luminescence dating, and bedrock incision with in situ cosmogenic ¹⁰Be. Based on analyses of (i) river profile and planform, (ii) spatial patterns of short and long-term deposition rates, and (iii) rates of bedrock channel incision, we show that climate-driven flooding episodes over the last glacial cycle play a key role in how rivers adjust to intraplate tectonism, whilst sediment load appears secondary.