Dust storms and their geomorphological implications

Textural and permeability characteristics of faulted, high porosity sandstones

Silcrete as a palaeoclimatic indicator: evidence from southern Africa

Summary Entrainment, transport and deposition of dust in and around deserts has major geological implications. A consideration of the mechanics of dust transport and deposition shows that ‘desert loess’ can only accumulate in vegetated desert fringe areas which are relatively close to a major source of dust. Dust deposition can also contribute to the stabilization of active sand dunes, and infiltrated dust plays an important role in the early diagenesis of a wide range of desert sediments.

The mechanics and geological implications of dust transport and deposition in deserts with particular reference to loess formation and dune sand diagenesis in the northern Negev, Israel

The Rotliegend sandstones of northern West Germany are characterised by severe diagenesis with growth (in approximate time sequence) of: (1) iron oxide, (2) tourmaline, (3) authigenic feldspar (mainly albite), (4) illite, (5) quartz, (6) chlorite, (7) kaolinite, (8) anhydrite and carbonate. The illite forms (a) tangential rims, (b) radial rims, (c) mesh-work, (d) dense homogeneous masses. The chemical conditions responsible for this diagenesis involved alkaline pore fluids with high K+ and Na+. These conditions were initiated very early, sometimes only a few centimetres below the surface. The diagenetic materials came from detrital volcanic grains, and the key to extent of diagenesis may be in the amoung of dissolution of these grains.

Possible causes of Rotliegend sandstone diagenesis in northern West Germany

The Penrith Sandstone (Lower Permian) of north west England is a continental red bed sandstone, deposited as barchan sand dunes in a hot, arid desert environment. The rock displays a high degree of mineralogical and textural maturity, being a coarse, well sorted orthoquartzite, composed of rounded unstrained, strained and polycrystalline quartz grains, with subordinate feldspar and rock fragments. Distinct petrographic features of the detrital grains suggest derivation from the Millstone Grit (Upper Carboniferous) sandstones of Yorkshire, and perhaps County Durham. In the southern part of the area non-silicified sandstones are interbedded with calcite cemented alluvial fan breccias (brockrams). This contrasts markedly with northern areas where brockrams are absent, in that the sandstones are tightly cemented with secondary quartz, occurring as overgrowths of optically continuous, bipyramidal quartz crystals around detrital grains. Many varieties of overgrowth are present, their structure being pre-determined by the internal structure of the detrital grains. Thus, unicrystaline unstrained and strained, and polycrystalline overgrowths can be recognised. Overgrowths of secondary feldspar occur occasionally on detrital orthoclase and microcline. It is suggested that the source of the secondary quartz is due to the solution of the siliceous dusts produced during aeolian abrasion of the quartz grains. Alkaline desert groundwaters are considered to be responsible for taking the siliceous dust into solution, which, upon evaporation, precipitated the silica as optically continuous quartz overgrowths around the detrital grains. The inverse relationship between silicification of the Penrith Sandstones and brockram distribution is attributed to differences in compositon of the groundwaters in the two facies.

Brian Waugh

Papers

Petrology, provenance and silica diagenesis of the Penrith sandstone (lower Permian) of northwest England