Kenneth Pye

Petrographic Examination of Sedimentary Rocks in the Sem Using Backscattered Electron Detectors

ABSTRACT Most of the geological work in scanning electron microscopy has been undertaken using the secondary electron (SE) mode and, to a lesser extent, the cathodoluminescence (CL) mode, but in the last few years attention has been drawn to the advantages of imaging with backscattered electrons (BSE). The coefficient of electron backscattering from polished specimens is strongly dependent on average atomic number, allowing different mineral phases to be distinguished on the basis of differences in gray level. A great deal of useful information about the size, distribution, shape, orientation, and textural relationships of minerals in situ can be gained from BSE images. These data can be processed by various image-analysis systems which are now commercially available. This paper reviews the na ure and formation of BSE electrons, discusses some of the problems which limit the quality of BSE images of sedimentary rocks, and illustrates some applications of the technique.

Microfabric, mineralogy and early diagenetic history of the Whitby Mudstone Formation (Toarcian), Cleveland Basin, U.K.

The Whitby Mudstone Formation in northeast England contains three shale facies (normal, restricted and bituminous) which have previously been differentiated on the basis of sedimentary structures and faunal assemblages. This paper shows that the three facies can also be differentiated by mineralogical, geochemical and microtextural criteria determined using BSEM, EDXRA, XRD and XRF. The bituminous shale facies consists of finely laminated shales containing an early diagenetic mineral assemblage dominated by pyrite, calcite and dolomite. The weakly laminated restricted facies shales contain an early diagenetic assemblage dominated by pyrite. The normal facies shales are very weakly laminated and contain siderite in addition to pyrite. The geochemical conditions which left a dominant early diagenetic imprint on the three facies are interpreted to be carbon-rich sulphidic, carbon-poor sulphidic and very carbon-poor sulphidic respectively.

Formation of quartz silt during humid tropical weathering of dune sands

Abstract Quartz dune-sand grains in northeast Australia break down in situ under humid tropical weathering conditions to form substantial amounts of silt. Fragmentation appears to occur primarily due to silica solution along microfractures and dislocation structures in grains which have experienced tectonic deformation prior to deposition. The surface textures of quartz grains viewed with the SEM exhibit a close dependency on crystallographic properties. In the B and C horizons of some weathered late Pleistocene dunes silt forms up to 10% of the bulk sediment. Local remobilisation of such deposits by wind allows selective removal of the silt which is later deposited as a thin surficial sheet in more sheltered areas downwind. These observations provide further evidence that weathering processes provide a major mechanism of silt formation and are an important factor to be considered in relation to the origin of loess and siltstones.

Negatively skewed aeolian sands from a humid tropical coastal Dunefield, Northern Australia

Abstract This paper describes the nature and environmental significance of variations in textural parameters within a large coastal dunefield in tropical northern Australia. Grain-size parameters of modern beaches and foredunes are very similar but are distinct from those of parabolic dune and sandplain environments. On average, beach and foredune sands have a normal grain-size distribution, but parabolic dune sands are predominantly negatively skewed. Negative skewness is interpreted as primarily reflecting sorting of dune sediments by winnowing at dune crests and by avalanching on slip faces. A general relationship between mean size and skewness is supported, with finer sediments showing increased negative skewness. Positive skewness can therefore not be used as a diagnostic property of aeolian sediments.

Holocene development of an episodic transgressive dune barrier, Ramsay Bay, North Queensland, Australia

Abstract An investigation of the stratigraphy and sedimentological properties of a tombolo barrier at Ramsay Bay, North Queensland, has indicated that the barrier consists largely of aeolian sands which in places extend more than 30 m below present sea level. Two major episodes of dune activity are recognized on morphological and pedogenic criteria. Radiocarbon dating of organic muds associated with the barrier sands suggests the older dunes were partly drowned during the early Holocene marine transgression (9500-6000 C-14 yrs B.P.). The later generation of dunes formed in the late Holocene, probably within the last 900 C-14 yrs. The ultimate causes of the aeolian episodes are not certain, but appear to be related to periods of rapid shoreface erosion.

Backscattered Scanning Electron Microscopy and Image Analysis of Sediments and Sedimentary Rocks: Sediment grains, weathering, and early diagenetic phenomena

1. Introduction 2. The nature of backscattered scanning electron images 3. Sediment grains, weathering, and early diagenetic phenomena 4. Sandstones 5. Shales 6. Carbonates 7. Desert (rock) varnish 8. Glauconite 9. Image processing and image analysis.