BRIDGING THE GEOMORPHOLOGICAL DIVIDE: THE ROLE OF CONTRASTING DENUDATIONAL MECHANISMS IN LANDSCAPE EVOLUTION

A distinct polarisation has long existed between karst and non-karst geomorphological research. Although it is universally accepted that fundamental differences exist between denudational mechanisms for carbonate rocks and those for siliciclastics, the precise nature of these differences and the way they influence landscape development on folded, mixed-lithology outcrops is seldom, if ever, addressed.

Rates of mechanical erosion by water are constrained by the physics of particle entrainment and transport. Particles have minimum entrainment velocity thresholds, below which they remains stationary. Furthermore, particle entrainment may be inhibited by vegetation until much higher flow velocities breach this organic barrier between sediment and flowing water. Consequently, denudation of siliciclastic terrains is favoured by sparse vegetation cover and sporadic heavy rain, as in semi-arid regions. However, carbonate rocks are susceptible to mechanical erosion and, more significantly, to dissolution by water. Dissolution is not governed by any absolute velocity threshold and may be effected even by static water. Hence vegetation does not prevent denudation of limestone beneath. Furthermore, vegetation actually enhances dissolution rates by raising soil-water [CO2]. Consequently, carbonate dissolution is favoured by humid climates and abundant vegetation cover, exactly the converse of the conditions favouring mechanical erosion.

Clearly the development of differential relief on folded carbonate and siliciclastic outcrops is a function of precipitation and vegetation cover. Semi-arid climates create landscapes almost the exact inverse of those developed in humid regions. Furthermore, modeling and consideration of real examples indicates that unroofing of thick carbonates within folded clastic sequences, or vice versa, leads to topographic inversion and profound changes in drainage patterns through time.

Appreciating the contrasting roles of these fundamentally different denudational regimes is crucial to understanding the evolution of folded, mixed-lithology landscapes around the world. In particular it demonstrates that there is much to be learnt from looking again at the basic principles of weathering and erosion.