Robert A.L. Wray

A global review of solutional weathering forms on quartz sandstones

Abstract Solutional landforms in limestone have been described for over a hundred years, but landforms of similar morphology on highly siliceous sandstones and quartzites have also been identified in a wide variety of environments and generally termed pseudokarst. These include large bedrock pinnacles and towers, caves, corridors, grikes, solution basins and runnels, and even silica speleothems. Quartzites and quartz sandstones have been held to be amongst the most chemically resistant of rocks, but the similarity, both in morphology and genetic process of many landforms developed from them to features of known solutional origin on limestone, has prompted some authors to refer to these quartzose landforms as true karst. The most detailed studies of quartzose karst landforms have been in present-day tropical regions, or areas believed to have been tropical in the geologically recent past. This concentration of research in hot-wet areas, allied with the long held assertion of the insolubility of silica, especially quartz, has led to a belief that tropical climatic conditions are necessary for karstic solution of these rocks. However, the existence of quartzose karst landforms in temperate and even sub-polar latitudes, especially where there is no evidence of prior tropical conditions, suggests that the requirement of tropical weathering is no longer tenable. The reports of these quartzose solutional landforms are widely scattered through the geomorphological and geological literature, but a comprehensive world-wide review of the range of solutional landforms on quartzose rocks has not previously been published. Because of the increasing awareness in this karst type such a summary is sorely overdue.

Geomorphology of the Namoi alluvial plain, northwestern New South Wales

The Quaternary history of the extensive alluvial plains of the northern part of the Darling River Basin has received little attention, and has generally been assumed to be an analogue of the very detailed history compiled for the Riverine Plain of southeastern Australia. Our study of the Namoi valley, which is a tributary to the upper Darling, shows that this assumption is unfounded. Thermoluminescence dating demonstrates that the oldest palaeochannels of the Namoi River correspond only to the youngest palaeochannels on the Riverine Plain. Unlike the streams on the Riverine Plain, the Namoi River has moved progressively away from its buried Tertiary palaeovalley, probably due to declining sediment input from its southern tributaries. In contrast to the streams of the Riverine Plain, the dimensions of the Namoi palaeochannels are indicative of substantially greater discharges until the mid‐Holocene. There is also evidence of significant aeolian input throughout the Late Quaternary. The water resources of this increasingly important irrigated region seem to be considerably constrained by the Quaternary heritage.

Alunite formation within silica stalactites from the Sydney Region, South-eastern Australia

2+ -oxides, or the biochemical activities of bacteria or other micro-organisms, also provide conceivable pathways for the generation of pH sufficiently low to contribute to alunite formation. The occurrence of alunite in these silica stalactites, whilst unusual, is consistent with the normal silica stalactite-forming process in this region, and in accord with observations of the authigenic formation of alunite and groundwater opal in weathering profiles elsewhere.

The Gran Sabana: The World’s Finest Quartzite Karst?

The Gran Sabana in tropical Venezuela, with its huge mist shrouded, mesa or tepui that tower over 1,000 m above the lowlands, undoubtedly form one of the world’s more spectacular landscapes. These vertically walled quartzite table mountains are composed of Pre-Cambrian Roraima Group quartzites and sandstones, and crest a series of regional planation surfaces. Because of their isolation the summits are often biologically unique. Extremely long periods of geologic stability and weathering have produced a vast range of karstic forms in the quartzites, such as caves, towers, grikes, and smaller karren. Quartzite is not a rock usually associated with karst forms, but here they are expressed better than anywhere else in the world. Sections of the Gran Sabana and its tepui are listed as World Heritage, and its remoteness and often-poor accessibility possibly make it the most poorly explored of the great geomorphological landscapes.

Palaeochannels of the Namoi River Floodplain, New South Wales, Australia: the use of multispectral Landsat imagery to highlight a Late Quaternary change in fluvial regime

Abstract Abandoned channels of ancient rivers are clearly visible on aerial photographs of the Namoi River floodplain in northern New South Wales, Australia. However, the spatial distribution of these palaeochannel systems over many tens of kilometres makes their broad-scale mapping with air photos difficult. Satellite imagery has previously been used to a limited degree in the identification of palaeochannels in many areas, but typically only in single image bands. This study finds that combining and enhancing multiple bands of Landsat 7 ETM+ data improves subtle differences in the spectral reflection of soil moisture and vegetation, aiding palaeochannel identification. These results are much better than using single band data alone in this important agricultural area, one suffering strong water and soil resource pressures. There are several generations of palaeochannels visible on this floodplain, and analysis shows they were very different from the modern rivers; they had greater discharges, longer meander wavelengths and flowed along different courses to the modern rivers. This study supports other recent investigations of the geomorphology and chronology of the Namoi River floodplain that showed that these changes only occurred during the Late Quaternary. These historical changes were not fully recognised until recently, and are different from what was assumed from studies of other riverine plains in south-eastern Australia.

Thermoluminescence Dating of Alluvial Sequences in Coastal Valleys of Southern New South Wales: Problems and potential

Because of its potential for establishing chronologies far beyond the range of C14 thermoluminescence (TL) dating has made a significant contribution to the study of the Quaternary history of many Australian landscapes. But, as the reliability of the technique requires the removal by sunlight of any residual TL from quartz grains during transport, inadequate bleaching may yield ages for depositional events that are too old. Inadequate bleaching often can be detected by the shape of curves showing the ratio of natural TL vs laboratory-induced TL with increasing temperature. We use this technique here, together with C 14 dating and pedogenic evidence, to assess the reliability of TL determinations for alluvium in the valleys of the Clyde River and Termeil Creek. The Pleistocene TL ages from these valleys seem reliable, but Holocene dates do not. However, we demonstrate that, even where inadequate bleaching is demonstrable, TL analysis can still yield important insight to depositional processes.

6.36 Solutional Weathering and Karstic Landscapes on Quartz Sandstones and Quartzite

Landscapes on highly quartzose bedrock that exhibit almost identical scale and morphology to those on karstified limestones occur under a range of climates and on most continents. These include ruiniform towers, grikes, stone cities, caves, dolines, smaller surface karren, and silica speleothems. However, these rocks are much less soluble than most carbonates, and the weathering processes are quite different. However, because chemical solution is demonstratively a critical component in the genesis of these landforms, they may be regarded as karst. This chapter summarizes the processes of karstification in quartz sandstones and then reviews the incidence of these landforms around the world.

THE FORMATION AND SIGNIFICANCE OF CORALLINE SILICA SPELEOTHEMS IN THE SYDNEY BASIN, SOUTHEASTERN AUSTRALIA

The vast majority of speleothems are composed of calcium carbonate, but this paper focuses on a suite of less-commonly reported speleothems, those formed almost entirely of silica, and on a quartzose rather than carbonate base rock. On the quartz sandstones of the Sydney region of southeastern Australia, small branched coralline silica stalactites, up to 10 mm in diameter and 80 mm in length, frequently are found. These stalactites have not been described in detail before and are clear evidence of the solution of silica (including quartz) from within the sandstones, and its transport and redeposition at the rock surface. Detailed analysis using SEM, XRD, and thin-section techniques revealed layered composition of both amorphous opal-A and crypto-crystalline chalcedony. Calcite forms no part of these stalactites. The internal structure also shows that they do not form in a manner similar to calcite stalactites. While the growth rate of these silica stalactites remains unknown, it is believed to be quite sl...

Cainozoic heritage in the modern landscape near Bungonia, southern New South Wales

SUMMARY The regional topography of much of the New South Wales Southern Highlands was long attributed to Quaternary erosion, but it has been recently demonstrated that the region was, in fact, well developed by the Eocene. The hillsides at Inverary Park are mantled by the Late‐Eocene Reevesdale Basalt, thus indicating the extent of landscape development by the mid‐Tertiary. Pre‐, intra‐, and post‐basaltic alluvial and lacustrine sediments are compared with the current extent of the basalt, clearly demonstrating the age of this landscape and the paucity of modification since the Late Eocene. Stratigraphic techniques, U/Th, TL and 14C dating aid reconstruction of much of the Cainozoic history of Inverary Park. Major changes in sediment type and source have long been common, but Late Quaternary changes do not correlate well with similar observations elsewhere, suggesting that changes in hydrologic regime, notably climate change, are not necessarily responsible for changes in the stratigraphic record. This la...

Block gliding and rheological deformation in the Southern Sydney basin

SUMMARY Block gliding caused by low frictional resistance or by the deformation of plastic substrates has been well documented from many parts of the world, but neither of these mechanisms explains the widespread gliding of sandstone blocks away from cliffs in the southern Sydney Basin of south‐eastern Australia. The movement of large blocks over declivities from near zero to a maximum of 5°, high frictional resistance and lack of high porewater pressures rule out a simple sliding mechanism and it is unlikely that slender towers of sandstone could have survived seismic vibration sufficient to overcome frictional resistance to gliding. Highly preferential dip control of the direction of gliding and of the development of benches over which the blocks move, together with mounds, similar to pressure ridges, on the benches, indicate that the block gliding is due to the rheological deformation of the underlying rock, even though it is a sandy siltstone lacking readily deformable beds. Estimated rates of deforma...