Philippe Rollet ’20, Kenosha, Wisconsin

Major: Environmental Geology
Minor: French

This study uses petrography and fluid inclusion analysis to better constrain the depositional environment of Triassic halite samples from the Röt Basin, Netherlands. The studied halite comes from the Twente Rijn-382 core; one sample from 470.5 m (salt A) and another from 425.1 m (Salt D) depth. Previous studies conducting sulfur isotopic analysis indicate the halite was precipitated under marine conditions. However, the core was drilled on the edge of the Röt Basin where marginal marine conditions were likely, and subaerial exposure may have occurred.To better constrain the depositional environment, halite samples were cleaved into thick sections (a few mm thick) and polished with sandpaper; water was not used in order to avoid dissolution of samples. A petrographic microscope was used to look for features indicating shallow water, such as: chevron crystals, dissolution surfaces and pipes formed from influx of rain or normal marine water, and effervescent crusts and desiccation cracks, which show subaerial exposure. A Fluid Inclusion Stage was then used to discern pH by performing freezing/melting runs and comparing phase changes in fluid inclusions to previous studies.

This study found chevron crystals in both halite samples; this bottom-growth crystal form is indicative of shallow water depths (less than 60 cm). The freezing/melting runs showed features indicative of acidic parent brines as noted by previous studies, such as fuzzy borders, building block borders, and low first melt temperatures; however diagnostic features of acid brines like unidentifiable metastable solids and the duration of hydrohalite rims were not observed and/or require further study. These results suggest that the halite studied formed from evolved marine water in shallow water, restricted marginal marine (sabkha) settings.