Clay-Cation Compositions from Groundwater Chemistry as an Indicator of Depositional Environments, Paleoclimates, and Paleo-Groundwater Flow

New methods to calculate the cation compositions of clays from equilibrated groundwater are presented and applied in three clay-rich coastal aquifers where significant chemical differences are shown to exist among clays based on their depositional environment and the climate at deposition. River-water analyses show that fluvial deposited sediments are represented by majority XNa≤ 8.0% and marine deposited sediments by majority XNa > 8.0%. In a natural system with fluvial, mixed, and marine environments identified by traditional core, log and paleontological analyses, each depositional environment has a distinguishing range of XNa, XCa and XMg consistent with these criteria. Differences in XNa are related to climate of deposition among clays deposited in a fluvial environment, where a larger portion of the fluvial clays deposited in a semi-arid paleoclimate have elevated XNa compared to those deposited in a humid subtropical paleoclimate. Perturbations in these trends were compared to elements in the groundwater not involved in ion exchange to identify areas of upwelling and halite dissolution. Mass-balance calculations show that the effect of cation exchange on each pore volume of fluid that flows in the aquifer is large, whereas the effect on the clay composition is small, so clays will not only retain the signature of their depositional setting, but changes in the clay exchange composition can be used to identify the aquifer’s past chemical evolution and historical-flow paths. These relationships might be used to aid stratigraphic interpretations based on well logs, sediment samples, and paleontology in distinguishing fluvial, marine, and mixed fluvial-marine depositional environments.