This has been kicking around the edge of consciousness for a while and may make a useful debate point
Basically boron, which is present in seawater, at approximately 4.5 mg/l for normal modern seawaters, get incorporated in to the mineral lattice of illite clay growing in sediments. The amount of boron is proportional to the salinity of the seawater and there exists a relationship between the concentration of boron and the amount incorporated in to illite clays. It is therefore possible to deduce with reasonable certainty, though any technique such as this is open to complications, what the salinity of the water body any particular sediment was deposited in. Typically works best for those rock with plenty of illite ie shales.
Creationists might care to posit a model for Flood Water geochemistry that explains how rocks can variable develop different salinity signatures from the same Flood Waters?
The reference is old, but gives the basics
http://www.clays.org/journal/archive/vo ... -1-203.pdf