The character and interpretation of some pediment mantles

Abstract

Some pediment mantles provide crucial evidence concerning the processes responsible for the shaping of the cut bedrock surfaces, or pediments, upon which they rest. On the western side of the Flinders Ranges, the mantles on the dissected Brachina pediments consist of 2–3 m of limestone and quartzite boulders and cobbles set in a silt matrix. The cobbles are derived from the uplands to the east and have been transported by rivers. Comparisons with contemporary stream channels suggest deposition by divaricating streams emerging from the uplands on to the desert plains. No palaeosol intervenes between the mantle above and the cut bedrock surface below, and the latter is ascribed to planation by laterally migrating distributary streams. Similar deposits occur, and similar interpretations can be applied, to several other dissected pediments located at the upland margins, and to some within the Ranges. But the most prominent old valley floors, or pediment remnants, in intermontane situations are capped and preserved by coarse angular blocks of quartzite. Many of the remnants are quite steeply inclined (up to 7°) compared to the 3°–4° that is normal for the pediments proper. The cappings are interpreted as slump deposits laid down either on the toes of slopes or on the heads of pediments; they are related to scarps that have since retreated (partly through slumping). But on granite pediments (e.g. on Eyre Peninsula, in northern Queensland; and on dacite and rhyolite in the Gawler Ranges) the fine textured mantles are thin, and mineralogically indicate nothing but derivation from the underlying bedrock; there is no unequivocal evidence of downslope transport. Yet, after rains, there is evidence of rillwash and sheetwash; surely these pediment surfaces are surfaces of transportation like those of the Flinders Ranges. The crucial upper layer of sediment that may provide clues to the processes active appears to have been eroded away, possibly as a consequence of European settlement and land use.