In what way can the rate of deposition be used to estimate absolute age?
By using the theory of uniform process divide the total sedimentation by the present rate of deposition.
The assumption is that the present is the key to the past. This assumption is that the rates of deposition have been maintained constant throughout geological history.
The assumption of uniform process though fundamental to much of historical geology is likely a faulty assumption.
Look at the rejection of the Missoula Flood theory based on the wrong assumption of uniform process.