Bromide Formation  The Bromide Formation is a geological formation in Oklahoma, USA. It is well known for its diverse echinoderm and trilobite fossil fauna. The Bromide Formation crops out in the Arbuckle and Wichita Mountains and in the Criner Hills of Southern Oklahoma. It appears at the surface in particular within Carter, Johnston, Murray and Pontotoc counties (34.0° N, 97.0° W). The Bromide Formation is the uppermost part of the Simpson Group, and originates from the Upper Ordovician (early Sandbian). This mostly carbonate succession is divided into the Mountain Lake and overlying Pooleville members. Although it primarily consists of limestone, limestone interbedded with shale, and sandstone, also occur. The Bromide Formation is a shallow water marine sediment. Much of the Mountain Lake Member comprises meter-scale, deep ramp cycles that overlie a lowstand systems tract of sandstones and sandy crinoidal grainstones. Cycle tops are starved surfaces with irregular, mineralized hardgrounds. The Pooleville Member consists of an early highstand interval of shallow subtidal carbonates and late highstand peritidal carbonates (Corbin Ranch Submember). Down-ramp, the Pooleville is represented largely by centimeter-thick shales and interbedded lime mudstones. The Bromide Formation has been a source of oil and gas, with exploration slightly north of the area where the formation is exposed. The Bromide Formation was deposited in a shallow, storm-dominated epeiric sea that extended over part of the Laurentia continent, in what is today Southern Oklahoma. The sea spread into an area that sunk into a rift, that ultimately did not endure, a so-called aulacogen. Lying approximately at 30° Southern latitude, a low-land desert bordered much of the shallow sea from where well-rounded quartz sand blew in. This now represents the sandstone at the base of the Bromide Formation. Eventually, sea level rise caused by subsidence drowned the borderlands cutting off the supply of sand, and now the shales and limestones of the Middle Bromide (upper Mountain Lake Member) accumulated on a broad ramp. Gradually – primarily echinoderm – skeletons build up a carbonate shelf. Further eustatic sea level rise (transgression) cut off the supply of virtually all sediments from land, and remains of carbonate-producing organisms began filling the basin.