Bromide Formation

Location

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).

Stratigraphy

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.

Economic use

The Bromide Formation has been a source of oil and gas, with exploration slightly north of the area where the formation is exposed.

Origin

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. This now forms the limestone of the upper Bromide (Pooleville Member). Finally, a drop in sea level (regression) exposed the entire platform, and became a broad, nearly featureless, hot, semi-arid sabkha.

Fossils

Fossils have been found in the Bromide Formation of green algae, sponges, corals, graptolites, lampshells, moss animals, trilobites, clam shrimps, molluscs, several groups of echinoderms, and teeth of jawless fish. The animals we know from the fossil record from many well-known locations (including the Bromide Formation) can be viewed at a website that several paleontological institutions contributed to.

green algae

sponges

corals

graptolites

lampshells

moss animals

trilobites

clam shrimps

molluscs

sea urchins and sand dollars

edrioasteroids

cystoids

sea lilies

teeth of jawless fish (conodonts)