Sedimentary Rocks

What the Syllabus says - and the details...

Identifying Sedimentary Rocks

You need to be able to identify the following sedimentary rocks; breccia, conglomerate, sandstone, shale, limestone using their diagnostic properties; colour; texture (grain size/sorting and shape), reaction with acid]...

You need to be able to identify the stated sedimentary rocks in hand specimen from their texture [use of: sediment size comparator to determine grain size; shape chart to determine grain shape; reaction with 0.5M hydrochloric acid].


You will also need to be able to recognise the following structures used to identify rocks: 
lamination (common in shales) - layered build up of sediment with frequent breaks of deposition...

bedding - layered build up of sediment with occasional breaks of deposition...

graded bedding - formed from a sudden in-rush of sediment in storm conditions. The lighter the particles, the longer hey take to settle, so the rocks build up in well sorted layers.

cross bedding - found in sandstones where the beds are build up at angles, the top of each bed is 'cut off' by the next layer. Desert sandstones can have cross bedding angles up to about 30 degrees, water bedded sandstone have angle of around 15 degrees.

ripple marks, which only develop in very shallow water deposits - normally mudstones, siltstones and sandstones...

desiccation cracks - also only found in very shallow water deposits, caused by contraction of the sediement into hexagonal patterns as it shrinks due to drying out.

and other diagnostic features - see your notes!

Sedimentary rock type is dependent upon the environment of deposition.

You need to be able to use the characteristics of sedimentary rocks, including their distinctive sedimentary textures and structures and their fossil content, as seen in hand specimens, rock exposures, diagrams/photographs to interpret their environments of deposition. The tables below tell you what to look out for.

Shallow marine: 

Deep marine: 

Land:

Working out environments by analysis of simple geological maps and cross sections.

This is done by looking at the rocks found together. 
For example; sandstones can be  made in all sorts of environments but by looking at what it is found with we can work out where it must have been made...
...if the rocks are sandstones and shales, these can both be deep water rocks so it must have been a deep water environment.
...if you get sandstones with conglomerates then you would say that would be a shallow water environment.
...sandstones with coal would imply a delta environment.
Essentially - to work out the environment, you need to decide which is the key indicator rock on the map or cross section.


Using Fossils to Work out Environments


You need to be able to distinguish between the following fossil groups on the basis of their morphology; trilobite (top), ammonite (second), coral (third and bottom pictures).

You need to know which fossils are indicators of which past environments:

reef-building corals [marine, shallow, warm], trilobite [marine], ammonite [marine], plants [land, indicating past climate], trace fossils [tracks indicating land, burrows indicating shallow water]

..and finally: Porosity and Permeability of Sedimentary Rocks

You need to know how to distinguish permeable from impermeable rocks by observing the effects of dropping water on specimens and seeing if it goes through (permeable) or won't go through (impermeable). Well compacted rocks, like shale, tend to be impermeable.


You also need to know how to distinguish porous from non-porous rocks by immersing them in water and seeing what happens. If air bubbles come out and the water level drops as the water goes into the rock then it's porous. If no bubbles emerge and the water level stays the same then the water isn't going into the rock - it isn't porous. Rocks made from cemented grains, like sandstone, tend to be porous.