My first question is how do we know if the sediment is undisturbed. For example, lets say that there is a limestone layer that is full of fossils. A river cuts through this limestone layer, dissolving the carbonate rock but transporting the fossil. The fossil is then deposited in the river delta and becomes part of the delta silt. With this scenario, the rock encasing the old fossil dissolves (lack of rocks in the sediment) and the fossil is older than those below it in the delta record.
PS: playing devil's advocate for a change.
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