Geologic Summary of the Bridger Formation


A geologic formation is a body of rock that consists of a certain rock type or combination of types derived from related depositional events and environments in both time and space. The Bridger Formation is an approximately 842 m thick sequence of green, tan, brown and bluish-gray mudstone and claystone beds, ribbon and sheet sandstones, resistant bench-forming limestones, and thin but widespread ash-fall tuff beds. The sediments from which these rocks were formed accumulated in the Green River Basin as the result of related depositional events during an approximately 3.5 million year long interval during the middle Eocene.

View looking northwest at exposures of the Bridger Formation (subdivisions lower and middle C) north of Sage Creek Mountain in Uinta County, Wyoming. The widespread bench at the horizon is supported by the Sage Creek limestone, which forms the base of Bridger Formation subdivision lower C.

Anterior view of a carapace of the fossil tortoise Hadrianus weathering out of a sandstone overhang at the base of a stream channel deposit in the Bridger Formation (subdivision upper C) on Black Mountain, Sweetwater County, Wyoming.

Bridger Formation claystone, mudstone and sandstone lithologies are composed mostly of fragments of volcanic debris that were washed into the basin in rivers and deposited in river channels and on floodplains. Source volcanoes were located in the Absaroka volcanic field in northwestern Wyoming and further to the west in what is now Idaho and possibly sources further to the west. Bridger Formation limestone deposits were formed largely as a bi-product of algal photosynthesis which took place in lakes and ponds, forming tiny grains of lime, or marl. These lakes and ponds expanded and retreated over time, washing over floodplains and mixing aquatic and terrestrial sediments. Bridger marlstone and limestone beds preserve the fossilized remains of many of the animals which lived in and around ancient Bridger lakes and ponds.

Ash-fall tuff deposits were formed by volcanic ash which was blown into the atmosphere by erupting volcanoes, and then settled onto the landscape as a thin blanket often hundreds of miles away from the source volcano. Ash-fall tuffs are particularly important rocks because they contain radioactive minerals that can be dated. The dating method measures the ratio of the parent isotope, 40Argon, to a daughter isotope, 39Argon. This ratio is then compared with a ratio from a sample of a known age, and an age for the sample of unknown age is then calculated.

View looking east from Sage Creek Mountain in Uinta County, Wyoming at colorful exposures of the Bridger Formation (subdivisions C and D) north of Cedar Mountain in Sweetwater County, Wyoming. Cedar Mountain, which forms the horizon, is capped by the northward-dipping Oligocene-age Bishop Conglomerate, which was derived from the Uinta Monutains to the south.

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