Identification of sedimentary rocks

Instructions/suggestions: Flowcharts (or in biology - keys) are often used to capture a thought or other process. Identifying rocks often consists of asking a series of questions, the answers of which narrow down to an appropriate identification. This document is a linked series of questions that helps you to identify sedimentary rocks. By choosing the right answer from the possibilities provided (highlighted in blue) the web browser sends you to the next appropriate question or to a possible rock identification. By using this 'web flow chart' below you will begin to learn the process behind identifying sedimentary rocks. If you print these pages off and then draw arrows from one link to another you can visualize the structure of this flowchart in a more traditional manner. It will help to narrow the window when using this flowchart so that it shows only 5-6 lines of text (then it is clear where the browser is sending you).


Questions

Is your sample composed primarily of silicate clasts or nonsilicate material?

If silicate material, is it poorly sorted with matrix support, or well sorted with framework texture?

If poorly sorted, are clasts distinctly angular or rounded?

If it is well sorted with framework texture, then is the average grain size diameter a) coarser than several mms, b) less than several mms but clearly visible, c) not visible but gritty to touch, or d) submicroscopic and smooth to the touch.

If it is non-silicate material does it: 1) react readily with dilute HCl acid, 2) react with dilute HCl acid when rock is scratched or powdered, 3) salty to taste, or 4) is softer than a fingernail.


List of some sedimentary rock types.


diamictite: mostly silicate clasts, poorly sorted but somewhat rounded clasts, with matrix support.


breccia: distinctly angular clasts, often poorly sorted.


conglomerate: mostly silicate clasts, well sorted and rounded, framework texture, large clasts.


sandstone: mostly silicate clasts, well sorted, famework texture, medium sized clasts. What is the predominant composition of grains: a) feldspar, b) quartz, c) mixed with rock fragments, or d) calcite.


arkose: sandstone with signficant amount of feldspar grains, often reddish colored.


quartz arenite: sandstone composed of almost all quartz grains.


lithic arenite: sandstone composed of a mix of grain types, includind significant rock fragments.


calcarenite: sandstone composed mainly of calcite grains (note non-silicate).


Siltstone: mostly silicate clasts, well to moderately sorted, framework texture, small clasts.


mudstones: mostly silicates (clays), sub-microscopic.


limestone: carbonate rock that is relatively soluble. What is the texture? 1) Individual grains (and cleavage planes) not visible to the eye, often massive texture, 2) Crystalline and coarse grained texture, 3) Clastic texture with distinct rounded grains, 4) Clastic texture with almost only shell fragments, 5) clastic texture with distinct angular shapes.


dolomite: carbonate rock that is weakly soluble.


halite: salty taste (since it is salt), cubic cleavage.


gypsum: softer than a fingernail, one good and two less well developed cleavages.


micrite or carbonate mudstone: very fine grained limestone, often with subconchoidal fracture.


oolitic limestone: limestone composed of spherical grains each with individual laminations of growth.


sparite: coarse grained limestone with crystalline texture. Cleavage faces readily evident.


coquina: limestone made up of an aggregate of large identifiable shell fragments.


carbonate breccia: composed of angular fragments of carbonate rocks.


Return to lecture outline for sedimentary rocks.