Abstract

Ancient marine evaporites are commonly far more extensive than any known modern evaporite system. This is true not only for basin-central saline giants, but also for evaporites that occur in basin-margin settings, and the origin of these basin-margin systems is unclear. Extensive basin-margin evaporites of the Middle Jurassic Gypsum Spring Formation are well exposed along the eastern flank of the Bighorn Basin in Wyoming, USA, and a facies and sequence stratigraphic analysis was undertaken to understand the origin of these evaporites and associated deposits. The Gypsum Spring Formation was deposited on a northwestward-dipping mixed evaporate–carbonate–siliciclastic ramp, with facies and depositional environments that include distal ooid shoal grainstone, open-marine shallow-subtidal peloidal–skeletal wackestone to packstone, restricted shallow subtidal laminated lime mudstone, peritidal dolostone and microbialite, as well as red mudstone and gypsum deposited on mud flats, sabkhas, and salinas. Three depositional sequences are present, with the J-1, J-1a, and J-1b sequence boundaries characterized locally by chert-pebble lags, silicified horizons, and iron-rich paleosols. Evaporites accumulated in a basin-margin position, with 1–6 m beds of gypsum forming in salinas and sabkhas, and nodular anhydrite and gypsum forming in red mudstones deposited on sabkhas and mud flats. The limited lateral extent of salina-type gypsum beds and the variable number of these beds among the exposures indicates substantial autocyclic control on the formation of salinas. Salinas are well developed in the TST of the J-1 sequence, and sabkhas and mud flats are well developed in the J-1 HST, the J-1a HST, and the J-1b TST. The great lateral extent of the Gypsum Spring evaporites reflects the original expanse of the depositional environment in which they formed, and is not the result of diachroneity of a narrow evaporite-precipitating belt, such as the modern-day coastal sabkhas of the Trucial coast. The great lateral extent of Gypsum Spring evaporites suggests that they were precipitated mainly from continental brines on an expansive desert mudflat.