Box canyon erosion along the Canterbury coast (New Zealand): A rapid and episodic process controlled by rainfall intensity and substrate variability

Abstract

<jats:p>Abstract. Box canyon formation has been associated to groundwater seepage in unconsolidated sand to gravel sized sediments. Our understanding of box canyon evolution mostly relies on experiments and numerical simulations, and these rarely take into consideration contrasts in lithology and permeability. In addition, process-based observations and detailed instrumental analyses are rare. As a result, we have a poor understanding of the temporal scale of box canyon formation and the influence of geological heterogeneity on their formation. We address these issues along the Canterbury coast of the South Island (New Zealand) by integrating field observations, optically stimulated luminescence dating, multi-temporal Unmanned Aerial Vehicle and satellite data, time-domain electromagnetic data, and slope stability and landscape evolution modelling. We show that box canyon formation is a key process shaping the sandy gravel cliffs of the Canterbury coastline. It is an episodic process associated to groundwater flow that occurs once every 227 days on average, when rainfall intensities exceed 40 mm per day. The majority of the box canyons in a study area SE of Ashburton has undergone erosion, predominantly by elongation, during the last 11 years, with the most recent episode occurring 3 years ago. The two largest box canyons have not been eroded in the last 2 ka, however. Canyons can form at rates of up to 30 m per day via two processes: the formation of alcoves and tunnels by groundwater seepage, followed by retrogressive slope failure due to undermining and a decrease in shear strength driven by excess pore pressure development. The location of box canyons is determined by the occurrence of hydraulically-conductive zones, such as relict braided river channels and possibly tunnels, and of sand lenses exposed across sandy gravel cliff. We also show that box canyon formation is best represented by a linear diffusive model and geometrical scaling. </jats:p>

Rectangular drainage pattern evolution controlled by pipe cave collapse along clastic dikes, the Dead Sea Basin, Israel

“…Importantly, while seepage erosion is commonly associated with continuous subsurface flow below the water table (Dunne, 1980, 1990; Laity & Malin, 1985; Lamb et al., 2006; Micallef et al., 2020; Pelletier & Baker, 2011; Schumm et al., 1995), in the Ami'az Plain the water table is deeper than the base of the tributaries and seepage erosion probably occurs during scarce and sufficiently large rainfall events. Indeed, several morphologies that are commonly associated with seepage erosion are widely seen in the central region of the Ami'az Plain, these include amphitheater canyon heads, steep canyon banks and an approximately constant canyon width (Laity & Malin, 1985; Micallef et al., 2020; Schumm et al., 1995; and references therein).…”

Rectangular drainage pattern evolution controlled by pipe cave collapse along clastic dikes, the Dead Sea Basin, Israel

“…The role of seepage erosion in carving deep canyons in soft lithologies has been demonstrated in experiments (e.g. Howard and McLane, 1988) and was invoked in various field settings (Higgins, 1982;Laity and Malin, 1985;Schumm et al, 1995;Micallef et al, 2020).…”

“…The Lisan formation, although formally a bedrock, represents a soft end-member. Importantly, while seepage erosion is commonly associated with continuous subsurface flow below the water table (Dunne, 1980;Dunne et al, 1990;Laity and Malin, 1985;Schumm et al, 1995;Lamb et al, 2006;Pelletier and Baker, 2011;Micallef et al, 2020), in Ami'az Plain, the water table is deeper than the base of the tributaries and seepage erosion probably occurs during scarce and large rainfall events. Indeed, several morphologies that are commonly associated with seepage erosion are widely seen in the central region of Ami'az Plain, these include amphitheater valley heads, steep channel banks, and an approximately constant valley width (Laity and Malin, 1985;Schumm et al, 1995;Micallef et al, 2020, and references therein).…”

“…Fractures could also act as preferential subsurface flow pathways, inducing erosion at the subsurface and prescribing the course of flowlines (Dunne, 1980;Dunne et al, 1990). More specifically, the collapse of pipe caves formed by internal erosion (Nieber et al, 2013) forms gullies along hillslopes (Parker and Higgins, 1990;Zhu, 2012;Bernatek-Jakiel and Wrońska-Wa lach, 2018), and seepage erosion gradually undermines channel heads and promotes headward retreat (Pillans, 1985;Schumm and Phillips, 1986;Schumm et al, 1995;Nieber et al, 2013;Micallef et al, 2020). While these processes can occur without any association with fractures, in fractured terrains, the tendency of fractures to focus subsurface flow accentuates their role in inducing subsurface erosion (Dunne, 1980;Dunne et al, 1990).…”

The Role of Fluid Seepage in the Erosion of Mesozoic Carbonate Escarpments

“…In subaerial settings, fluid seepage has been shown to unambiguously lead to the formation of box canyons only in unconsolidated sand to gravel sized sediments (Lapotre & Lamb, 2018;Lobkovsky et al, 2007;Micallef et al, 2021;Schumm et al, 1995). The efficacy of fluid seepage as an erosive agent in bedrock remains disputed, however, and there is no unambiguous example of a bedrock box canyon formed by seepage erosion (Lamb et al, 2006).…”