Subsurface waters of ancient basins are thought to be remnants of sea water entrapped with the sediments at the time of their deposition. The post-depositional alteration of these waters is investigated in order to evaluate the possibility of using their chemical composition as an indicator of ancient sea water chemistry.

A review of literature on subsurface water chemistry indicates that the reliability of the data is, in general, poor. Using the best analyses available, the concentrations of Cl, K, Ca, Mg, Sr, Br, and I in waters from rocks ranging in age from Pliocene to Ordovician are compared. There appear to be no significant compositional trends with time. The post-depositional processes altering the water chemistry are discussed. It is concluded that the magnitude of the modifying processes are so great that it is unlikely that evidence on ancient sea-water chemistry can be obtained from the study of subsurface waters.