Robrob
Philosopher
- Joined
- Aug 29, 2011
- Messages
- 5,497
It don't usually think of bioturbation as activity by humans. But earlier when I said "undisturbed beds", that was because sometimes things like bioturbation disturb the beds. A worm may burrow into soft sediments, then younger sediments fill the burrow, or a tree can grow, sometimes into near solid rock, leave a "root hole" which then gets filled with younger sediment and reburied. Also, ancient potholes can leave pits that can be filled with younger sediment. (This is NOT bioturbation*.) But these are all "unconformities" and their presence means that the rock is not undisturbed.
*Note: animal self-gratification is also NOT bioturbation.
Not to mention the frequent phenomenon of petrified tree stumps projecting vertically up several layers (and thousands of years. Simply erosion of softer sands around the petrified stump then tides depositing new layers.