An Atlas of Pressure Dissolution Features

L. Bruce Railsback
Department of Geology, University of Georgia

 

INTRODUCTION

BASIC CONCEPTS AND FEATURES

     Pressure dissolution is the petrologic process wherein minerals dissolve as the result of pressure applied externally to them. This may happen because minerals under pressure (and thus undergoing elastic or temporary strain) are more soluble than unstressed minerals, and/or because deformation (permanent strain) due to stress leaves minerals more soluble. Because pressure dissolution leads to a reduction of volume of the rock in which it occurs, it is also called "chemical compaction".

     At the microscopic scale, pressure dissolution occurs at contacts between grains, because stress is localized at grain contacts if surrounding pore volumes support no load. The result is intergranular compaction, which is most strikingly visible as sutured intergranular contacts. Flattened contacts and concavo-convex contacts may also be the result of pressure dissolution, although in some cases they may instead be the result of mehanical deformation and thus be attributed to mechanical (rather than chemical) compaction. If no intergranular compaction has occurred, most intergranular contacts are tangential, although in limestones the diverse shapes of carbonate grains may allow some primary or unmodified flat or even concavo-convex contacts merely as the result of coincidental grain shapes.

 

 
 

 
Figure I-1. Tangential (A), flattened (B), concavo-convex (C) and sutured (D) intergranular contacts
as seen in thin section. Rectangular backgrounds are same size for all four panels; note reduction
of sediment volume.

     Extensive intergranular pressure dissolution reduces intergranular porosity as grains move together. Intergranular pressure dissolution also generates dissolved solids that may be precipitated in nearby pores, occluding porosity abd eventually precluding further intergranular pressure dissolution. Intergranular pressure dissolution may also stop as the areas of grain contacts expand to provide surfaces sufficient to support the force previously focused on smaller areas as greater stress. In that case, remaining intergranular pore space may be filled by cements precipitating by advexting fluids. If intergranular pressure dissolution proceeds to destroy all pore space with no cementation, so that the rock consists of sutured grains with no intergranular space at all, the resulting texture is called "fitted fabric".

     At the macroscopic scale, pressure dissolution between bodies of rock larger than individual grains leads to the development of dissolution seams and stylollites. Dissolution seams are subplanar non-serrate surfaces between two such rock bodies, whereas stylolites are serrate surfaces resulting from mutual interpenetration of two rock bodies, commonly as interdigitate columns. Because formation of dissolution seams and stylolites requires dissolution of bodies of rocks, rather than of individual grains, and requires coherent motion of those bodies toward the seam or stylolite to destroy any resulting pore space to and sustain pressure, lithification of the rock prior to development of seams or stylolites is required. Such lithification may occur through cementation or intergranular compaction.

 

 
 
Fiure I-2. Two-dimensional cross-sectonal views of a dissolution seam (A), stylolite (B),
highly serrate stylolite (C) and deformed stylolite (D). A few grains are shown
schematically to emphasize the change in scale from the previous figure.

     The sketches of dissolution seams and stylolites above are two-diimensional profiles perpendicular to the seams or stylolites. Seams and stylolites are, or course, three dimensional features, as the sketch below shows. Such a sketch emphasizes that any one two-dimensional profile or cross-section of a stylolite is only one among an infinite number of unique expressions of the morphology of that stylolite.

 

 
 
Figure I-3. Three-dimensional block diagram of a layered rock with a horizontal stylolite. The
front half of the upper rock mass has been removed to show the columns of the lower block.
The side at right shows the two-dimensional profile of the stylolite, analogous to Part B of the
illustration above. This illustration was inspired by the work of John V. Smith (2000).

    

TERMINOLOGICAL NOTES

     This document uses the expression "pressure dissolution", whereas "pressure solution" has traditionally been used more widely. Bathurst (1987) pointed out that, because the critical process is dissolution, "pressure dissolution" is the more reasonable term. This document therefore follow's Bathurst's usage.

     The document follows typical modern American usage in using "stylolite" to refer to a surface defined by interdigitate columns or projections. However, some older literature (eg., Stockdale, 1922) and more recent Eurasian literature (e.g., Hancock and Atiya, 1979) use "stylolite" in reference to the columns and projections themselves. Thus, in current American usage and increasingly in world-wide usage, Figure I-2B shows one horizontal stylolite, whereas older or Eurasian literature might say that the same figure shows at least four vertical stylolites. This is a non-trivial issue, because stylolites in the modern American sense can be horizontal or vertical, where the horizontal ones commonly result from loading of overburden and the vertical ones result from tectonic compression. On the other hand, in older or Eurasian usage, tectonic stylolites are horizontal, and ones caused by overburden are vertical.     

    

   
   
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