Swan Lake

Preliminary Paleoecolocic significance of the Swan Lake Quarries a newly discovered Orellan plant, invertebrate, and vertebrate bearing lake deposit from the White River Formation, Converse County, Wyoming

The Swan Lake plant locality was found in Orellan age lake deposits within the predominantly fluvial mudstones and sandstones of the richly fossiliferous White River Formation near Orin, Wyoming during 1999. The lake deposit is recognized by a resistant white massive to flaggy freshwater limestone that caps numerous hilltops over a 2 kilometer square area. The lake represents an impoundment of the ancestral North Platte River along a northeast trending monocline and fault zone within the White River Formation active during the Oligocene. Four meters of interbedded freshwater limestones, shales, bentonites, and mudstones comprise sediments within the lake sequence. Fossils include abundant carbonaceous limstones and shales containing prolific: leaf, stem, roots, seeds, pollen, and plant material. Millions of gastropods, arthropods and pelecypods occur within the 18 separate thin limestone lenses. Vertebrate mammals, fish, and birds have also been found within mudstones and limestones in the two separate quarry sites. The lake deposits lie 15 meters stratigraphically above a regionally recognized white ash marker bed radiometrically dated at 32.8 Ma at the Chadronian/Orellan boundary. The acidic reducing nature of the lake sediments has finely preserved carbonaceous plant material in an otherwise alkaline and oxidizing environment typical of the White River Formation. Although several excellent late Eocene Chadronian Floras (i.e. Florissant Colorado) have been well described, this deposit represents the first good Oligocene flora from the White River Formation in the central region of the North America. Local stratigraphy and interbedded volcanic ash beds allow a simple lateral correlation of the Swan Lake deposits to one of the richest mammal and reptile bearing fossil localities in North America with tens of thousands of recorded specimens. Through comparative phytolith and pollen studies from the teeth and stomach contents of laterally equivalent excellent mammal skeletons the precise diet of the herbivores will hopefully be ascertained and tied to specific plants in the Swan Lake Quarries/Oligocene boundary will hopefully be refined on the basis of this new locality within the classic Oligocene White River Formation of central North America.

Lake Deposits




Plant Material



Vertebrate Mammals



Quarry Sites