Sedimentary Structures
Sedimentary structures are features of or in beds, formed by physical, chemical and biological processes at or shortly after the time the sediment was laid down (deposited). They are useful large scale features (centimetres to metres) that can help us to understand about where the sediment was laid down.They can help answer questions such as:
- What was the agent carrying the sediment? Ice, water or wind?
- Did the water or wind move rapidly?
- Was the sediment dumped or did it lay down over longer periods of time?
- Were the sediment laid down in deep water or in shallow seas or in streams and rivers?
Because we can see the processes that form sedimentary structures in action in various places today we can imagine the same processes happening in sediment laid down in the past.
Sedimentary structures: Cross bedding
Cross-bedding is formed by dune migration. Sand bounces up the long side of the dune and then cascades down and deposits on the down current side of the dune.
Dunes are common in deserts, rivers and in the shallow seas.
- Cross-bedding image
- Beds marked in dark solid line, cross-bed marked in dashed line.
overturned cross-bedding
A variant on cross-bedding is overturned cross-bedding. This happens when the sand is saturated and sloppy and is caught up in a strong current so that it slides down the down current slope.
- Overtuned cross bedding image
- Thick Black line shows beds. Arrows point to over-tuned cross bed
Sedimentary structures: Ripples
Ripples form in the same way as dunes, but are at a smaller scale so millimetre to centimetre layers. If you go to the link on dune formation, you can imagine the same process creating ripples. In water, high ripples form from coarse sand in deep water and low ripples form from fine sand in shallow water. Ripples can also form by wind action.
Ripples form in deserts, rivers, on river flats, beaches, shallow seas and tidal flats
Sedimentary structures: Desiccation cracks (or mudcracks)
Removal of water by evaporation from waterlogged sediment forms desiccation cracks.
These indicate that the area experienced wetting and drying, so was above water level at the time these features were forming. They are common on lakes, tidal zones and river flats.
Sedimentary structures: Nodules
Nodules form in sediments saturated by groundwater after burial. They do not indicate where the sediment was laid down, but are a later feature present in many sedimentary rocks. Minerals that make up the nodules will depend on the sediment and the composition of the groundwater.
If you want to learn more:
- You are welcome to learn more this Sedimentary Structures site. It is useful in terms of sedimentary images.
- This Old Earth is an online resource that delves a little deeper into Sedimentary structures. Really useful for providing you with more context about sedimentary structures.
