Not necessarily true for the shales. We did studies in a sedimentation
tank in the 1970's in which we deposited successive sand turbidites under a
water column saturated with mud. We obtained alternating layers of sand
and mud simultaneously under these circumstances. The mud is depositing
continuously, and the sand layers represent punctuations in this process.
We could easily deposit alternating sand and mud layers as fast as we
wished, all day long.