Tortoises and hares: dissolution, erosion and isostasy in landscape evolution

Abstract

Denudation mechanisms differ fundamentally between limestone and silicate rock types, which are subject to very different rate thresholds and enhancers/inhibitors. Silicates are removed largely by erosion, the mechanical entrainment and transport of particles. This is a relatively high energy, and highly episodic, process which occurs only when a minimum threshold flow velocity is exceeded; it is inhibited by vegetation cover and favoured by strongly seasonal runoff. Limestone is removed largely by chemical dissolution at a rate directly proportional to runoff. Dissolution is a relatively low energy process that can occur at any flow velocity or in static water; in general it is enhanced by vegetation cover and non-seasonality of runoff. These contrasting factors in the denudation of silicates versus limestone can produce strikingly uneven rates of surface lowering across a landscape, sometimes akin to the well known 'tortoise and hare race', where the slow and steady denudation of limestones may in the long term exceed the sometimes rapid, but often localized and episodic, erosion of silicates. Prolonged exposure of limestone to a humid temperate climate in a tectonically stable environment produces lowrelief corrosion plains in which limestone uplands are anomalous and, in most instances, due to recent unroofing from beneath a siliciclastic cover. In a highly seasonal or semi-arid climate almost the exact inverse may develop, with 'flashy' runoff and sparse vegetation favouring erosion rather than dissolution. Even under a constant humid climate progressive unroofing of a thick limestone unit within folded siliciclastics may lead to a topographic inversion over time, with the limestone outcrop always forming a topographic low flanked by siliciclastic uplands. Valleys will be initiated on anticlinal crests, where the limestone is first unroofed, but progressive lowering of the limestone causes these valleys to migrate to their final position in the synclinal troughs. In humid climates isostatic compensation in response to slow, but continuous, denudation of extensive limestone outcrops may be a significant factor in the development of relief on adjacent, more slowly eroding, silicate outcrops.

Review of tufa deposition and palaeohydrological conditions in the White Peak, Derbyshire, UK: implications for Quaternary landscape evolution

“…The evidence points to relatively recent unroofing of the eastern side of the White Peak. This helps to explain the lower topographical contrast between the limestones and the surrounding Namurian siliciclastic rocks than might be anticipated if continuous dissolution and consequential lowering of limestone left siliciclastic rocks as ridges surrounding the limestone, as is characteristic of more mature karst landscapes (Simms, 2004). 2.…”

Speleothem U-series constraints on scarp retreat rates and landscape evolution: an example from the Severn valley and Cotswold Hills gull-caves, UK

“…Concomitant hillslope processes, 350 landsliding and incision by tributary valleys cutting into the upstanding resistant rock-mass would 351 serve to modify the scarp-edge, creating the indented feature we see today. This scenario also 352 explains how the Cotswold Hills are able to maintain their elevation despite limestone denudation 353 rates (predicted from solute concentrations at springs; Goudie, 1990) suggesting that such limestone 354 scarps could not persist for more than one or two million years (Simms, 2004 In this model, the distance the scarp has retreated is predicted to be much less than the valley width 364 ( interpolation between the nodes along the drawn sections and the limits of the units, smoothed and 375 contoured ( Figure 11). Similarly, the maximum topographic 'summit' surface which approximates to 376 the sub-Cretaceous erosion surface was derived from analysis of the 5 m NEXTMap TM Britain DTM.…”

“…This denudational 345 lowering is likely to be most effective on the less indurated Triassic and Jurassic mudrocks which are 346 particularly susceptible to periglacial effects (Simms, 2004;Murton and Belshaw, 2011), especially 347 during the cold conditions predominant during the Pleistocene. Superimposition of the drainage 348 pattern of the former Cretaceous cover would have also played a role in shaping the relief, possibly 349 helping to create some of the major wind gaps and vales.…”

Groundwater flood hazards and mechanisms in lowland karst terrains

“…Limestone accounts for >40% (30 000 km 2 ) of the surface or near-surface outcrop in the Republic of Ireland, making it the most prevalent bedrock type and primary regionally important aquifer lithology in the country (Simms 2004;Drew 2008). The main Irish limestones were formed during the Early Carboniferous or Dinantian (Drew et al 1996) when a marine transgression in the Tournaisian period inundated much of the Old Red Sandstone continent and provided the depositional environment necessary for limestone formation (Guion et al 2000;Sevastopulo & Wyse Jackson 2009).…”