The Application of Cathodoluminescence to Interpreting the Diagenesis of an Ancient Calcrete Profile
- V. Paul Wright3 and
- Maurice E. Tucker4
Published Online: 8 APR 2009
Copyright © 1991 The International Association of Sedimentologists
How to Cite
Solomon, S. T. and Walkden, G. M. (1991) The Application of Cathodoluminescence to Interpreting the Diagenesis of an Ancient Calcrete Profile, in Calcretes (eds V. P. Wright and M. E. Tucker), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304497.ch17
Postgraduate Research Institute of Sedimentology (PRIS), University of Reading, UK
Department of Geological Sciences, University of Durham, UK
- Published Online: 8 APR 2009
- Published Print: 13 JUN 1991
Print ISBN: 9780632031870
Online ISBN: 9781444304497
- application of cathodoluminescence;
- needle-fibre calcite, representing final stage of vadose cementation;
- non-marine micrite in calcrete profile, appearing needle-fibre calcite;
- non-marine micrite in the calcrete profile appears as needle-fibre calcite;
- micritic fabrics, characteristic of calcretes
A laterally extensive calcrete profile has been identified in the Late Asbian (Lower Carboniferous) shallow marine shelf limestones of the Llangollen area, North Wales. The upper surface of the profile is defined by a laterally discontinuous palaeokarstic surface and by laminated calcareous crusts which developed within the underlying limestone.
The profile contains a unique series of early pore-filling vadose cements which only occur down to 1 m below the palaeokarstic surface. Cathodoluminescence reveals that these cements pre-date the late pore-filling meteoric phreatic cements which occur throughout local Asbian lithologies. A spar cement stratigraphy has been established for the calcrete profile. Subaerial vadose cements comprise two generations of non-luminescent cement, followed by a brightly luminescent generation which occasionally shows an acicular habit. This needle-fibre calcite represents the final stage of vadose cementation. Precipitation of vadose cements was contemporary with subaerial alteration and micritization of the limestone.
Textures, visible only with cathodoluminescence, provide evidence of recurrent periods of fabric dissolution. The most extensive phase of dissolution occurred immediately after the precipitation of the non-luminescent subaerial vadose cements. Several different textures have been recorded, each reflecting the morphology of a partially dissolved substrate. Dissolution textures are generally confined to the walls of the larger pores and to early brecciation fractures. These probably acted as fluid pathways in the calcrete during early subaerial diagenesis.
Much of the non-marine micrite in the calcrete profile appears as needle-fibre calcite under cathodoluminescence. This acicular calcite was probably formed in response to localized supersaturation of meteoric pore fluids caused by periods of near-surface evaporation. Since needle-fibre luminescence is strongly variable, these ambient conditions are not believed to have directly controlled the activator ion concentrations of cementing pore waters. Needle-fibre calcite is considered to be a cement precipitate which has almost completely recrystallized to micrite, probably during the late stages of subaerial diagenesis. Two generations of subaerial micrite which define a ‘micrite stratigraphy’, have been distinguished under cathodoluminescence.
Reconstructing the diagenetic history of this ancient calcrete profile has revealed that subaerial alteration was multistaged, with many diagenetic processes acting simultaneously during a single phase of emergence.