RECONSTRUCTING CLIMATIC SHIFTS DURING POTASH DEPOSITION USING CLAY MINERALS IN THE PRAIRIE EVAPORITE OF WESTERN CANADA

The Devonian Elk Point Basin is one of the most aerially extensive evaporite basins of the world. Due to a basin-mouth barrier reef complex and intrabasin topographic highs, aerially extensive evaporites repeatedly developed as part of three brining cycles. The first and most prominent is the Givetian Prairie Evaporite, best known for the economically important potash giant contained within four potash members in Saskatchewan. Preserved across ca. 188,000 km², its mines are responsible for one-third of global potash production. To aid the reconstruction of the oceanographic and climatic controls on deposition, this study focused on insoluble minerals within the potash. Insolubles are found interstitially and in regionally extensive decimeter-scale beds that are more abundant in younger potash members. XRD analysis of over 200 samples from across the basin indicates that the clay assemblage within the insolubles varies systematically. All of the potash members contain illite, illite-smectite, and chlorite-smectite. However, an odinite-like phase was also found ubiquitously in the youngest potash members but is absent in the oldest members and the terrestrial-paralic mudstone that overlies the Prairie Evaporite. This is the first identification of a 7Å Mg-Fe-rich odinite-like mineral in the Prairie Evaporite. In modern environments, odinite forms through authigenic precipitation from terrigenous Fe-rich substrate in tropical near-shore settings. Its preservation in the Prairie Evaporite marks a rare example of ancient odinite that has not been affected by burial choritization. Its variability in the succession is interpreted to record a shift from more arid conditions during the oldest potash members to more humid conditions during accumulation of the youngest. The wetter climate intensified weathering in the surrounding desert, causing intermittent interruptions in potash deposition and triggering odinite authigenesis from terrestrial Fe supplied through riverine input. The climatic shifts manifest as temporal trends in insoluble composition and abundance through the upper Prairie Evaporite. These environmental changes ultimately contributed to the cessation of potash deposition, and the absence of potash in the two younger evaporitic cycles of the basin, providing new constraints on the accumulation of this distinctive evaporite deposit.