A central question for the huge Castile Formation evaporite system is whether its parent brine was mainly seawater or a strongly mixed solution involving continental groundwater.
More information on this topic is available in the Rock Salt Chemistry section.
About 10,000 km³ of Evaporite
USGS describes the Castile Formation as a major evaporite succession of roughly 10,000 km³ containing calcite, anhydrite and halite.
That scale makes the origin of the parent brine a major geological question.
65-Sample Isotope Test
Researchers analysed 87Sr/86Sr ratios in 65 samples of anhydrite, gypsum and calcite.
Fifty-nine samples matched the strontium-isotope composition of early Ochoan seawater.
Interpretation
The USGS study found no evidence for a major meteoric-groundwater contribution to the Castile brine.
This strengthened interpretation of the evaporites as dominantly marine-brine deposits.
Sources
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