| Summary: | Abstract Cation mole precents (XCa, XMg, and XNa) on clay minerals in a siliciclastic coastal aquifer calculated from groundwater compositions, can aid in distinguishing fluvial from marine sediments and locate the fluvial/marine transition zone. The clay‐cation compositions of two clay‐rich siliciclastic coastal aquifers calculated from the composition of the groundwater were compared to the location of fluvial, fluvial‐marine transition and marine environments identified by traditional core, well log and paleontological analyses. The traditionally defined fluvial depositional environment is distinguished by clays with low XNa < 8% (for 92% of the samples) and high XCa from 55% to 98% with the remainder XMg. In comparison, the traditionally defined marine depositional environment is distinguished by clays with higher XNa > 8.0% (for 86% of the samples) and lower XCa ranging from 5% to 39% with the remainder XMg. The traditionally defined fluvial/marine transition zone has values overlapping the fluvial and marine end members, with 59% with XNa < 8% (fluvial) and 41% with XNa > 8% (marine), and XCa ranging from 45% to 98% with the remainder XMg. Calculations support that the excess Na (XNa > 8%) on marine clays can be retained for 10s of millions of years despite flushing with meteoric water, because of the high clay content in marine sediments. The calculated cation composition of clays deposited from river water are XNa < 8%. In contrast XNa 8%–54% is possible for seawater equilibrated clays supporting that the values found in the different depositional zones are possible. Calculated cation compositions might supplement traditional stratigraphic interpretations in distinguishing fluvial/marine depositional environments.
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