Erosional and destructional surfaces

Australia: The Land Where Time Began

Erosional and destructional surfaces²

Weathering and the processes of erosional activity at and near the surface of the land shape erosional surfaces, these plains being classified as subaerial or epigene, etch and exhumed (Twidale, 1985).

Rivers, glaciers, wind and waves shape epigene surfaces, of which rivers and wash are overwhelmingly the predominant factors involved. Over a period of time fluvial plains develop that have an age-range (Twidale, 1956; King, 1962). Therefore stream incision is caused by baselevel lowering or uplift of the land. Migration upstream of a rejuvenation head or nickpoint, that marks the upstream limit of the new stream profile, occurs, That may be marked by a waterfall or rapids, or possibly a sector of steeper gradient. Adjacent to the incised sector the slopes are graded to a new lower and local baselevel, that are also graded to form distinct valley-side facets. These valley-side facets extend upslope, just as the nickpoint migrates inland. Such a landscape is said to be revived. The new land surface that results, that is said to be diachronic, has an age-range because the migration of the stream and slope take time to develop. A plain produced by the wearing back of scarps (King, 1942; Twidale & Milnes, 1983) is older near the stream lines where the initial scarps were produced by incision, and the present scarp foot is the youngest, so that it develops over a time period: it, also, has an age-range and again is diachronic.

The initiation of etch surfaces is initiated at the base of the regolith below the surface of the land as some of the rain falling on the land percolates down into the rocks beneath the regolith, which results in the weathering of these rocks by the shallow groundwater. A mantle of disintegrated and altered rock is formed where the rate of erosion is exceeded by the rate of weathering. The junction between the base of the regolith and the intrinsically fresh bedrock, known as the weathering front (Mabbutt, 1961a), is often quite sharp, whatever the thickness of the regolith.

The regolith is usually friable, unlike the intrinsically fresh rock below that is still cohesive, though a duricrust may have formed elsewhere, the resistant horizon often has a clay, that is commonly kaolin, beneath it that is susceptible to attack. As the capping is often undermined such regoliths are eroded readily which results in its collapse. The result is that many uplifted and dissected plains have been stripped of the former cover of weathered rock and expose what had been the weathering front. The growth morphology of the original low relief feature is mimicked by that of such surfaces, though in some places the weathering front is irregular. In particular, the rock has been altered by water penetrating along joints which leaves rocks that are rounded masses of rock that is still fresh that is either projecting as incipient bornhardts or as kernels, or corestones that are isolated within the regolith. These remnants are exposed as landforms when the altered rock has been stripped away.

As well as these erosional forms that are obvious there are also structional surfaces or benches. Structional benches stand at various heights in the landscape and according to the author¹ their arrangement is not likely to have any chronological significance. A problem is posed by surfaces that are more extensive that are coincident with an outcrop that is resistant, the discontinuity such as between sandstone, that is flat-lying, above which is an unconformable argillaceous sequence may be exploited by weathering and erosion, which in detail transect bedding, cross-bedding, etc. Such a surface that is essentially structural may be coincident with an erosional plain that it is part of, and be of the same age, though it may stand higher. There is also the possibility it may be inherited from a surface that is much older.

Sediments or lavas may bury epigene or etch surfaces, the discontinuity between the older and later rock is called an unconformity where this has happened, which implies that there was a hiatus or time-gap as the older series of rocks were folded or weathered or eroded prior to the younger rocks being introduced. The rock type above the unconformity often contrasts with the rock type below it, and it is not uncommon for the older surface on which the marine beds or lavas or whatever rock type were laid down to be re-exposed as a resurrected or exhumed surface.

Change is not precluded by burial as when any earlier regolith is most likely stripped by being inundated by the sea, though when the covering material is of volcanic or wind-blown (aeolian) origin that preserve older regoliths and forms (e.g., Almeida, 1953; Molina Ballesteros et al.,1995; Jerram et al., 2000). There is also the possibility that weathering by moisture may occur at the unconformity. It is physically impossible for the buried rock surface to be lowered or eroded unless the bedrock is susceptible to solution, though bedrock surfaces may be weathered following burial.

Forms that are exhumed are younger than the youngest rocks that are cut across by the surface or unconformity, but older than the oldest rocks deposited on the unconformity. For such a surface the age range that is provided by the age of rocks in which the surface is cut, and that of the basal rocks covering the surface, and both surfaces may be dated physically and by stratigraphy. Exhumed surfaces, as with etch forms, can be regarded as having 2 ages, one that refers to the burial time and the other to the age when it was re-exposed. The exhumed form is said to be sub Y or pre Y, where Y is the age of the covering rock. The hiatus that is implied may be an extended period of time but the age of the exhumed surface can be taken as immediately preceding the cover. The age of the exposure of the unconformity is significant in the context of the persistence of the surface.

Sources & Further reading