Green River Formation
About the Green River Formation
The Green River Formation is a renowned, widespread geological unit extending over an area of more than 65,000 km2 in parts of modern-day Colorado, Wyoming, and Utah (Grande, 1984). It is especially celebrated for its abundant, exquisite fossils, which include a diverse array of vertebrates, invertebrates, and plants. Among its most notable finds are mammals, flamingo-like birds, amphibians, reptiles, and over 17 genera of fish, offering crucial insights into past biodiversity and processes leading to exceptional fossil preservation.
Fig. 1: Cliff-forming layers of the Green River Formation exposed at Fossil Butte National Monument, near Kemmerer, WY. Picture taken looking north from the Green River Stone Quarry.
The Green River Formation is thought to have accumulated in a major system of lakes (generally known as Lake Gosiute, Lake Uinta, and Fossil Lake) developed in adjacent intermontane basins (Smith et al., 2008). Throughout its deposition, the number of individual lakes, their size, depth, and predominant conditions (e.g., hypersaline vs. freshwater) varied. As a result, the Green River Formation shows important vertical and lateral differences in the composition of its layers (e.g., carbonates, evaporites, oil shales, mudstones, and sandstone) and in its total thickness (average of 600 m, but up to 2 km in the Uinta basin) (Bohacs et al., 2007; Grande, 1984; Keighley et al., 2003; Pietras & Carroll, 2006; Smith et al., 2008). Volcanic activity co-occurred with deposition of the Green River Formation. Its sediments include ash and interfinger with other volcanic-derived material (Smith et al., 2008). Volcanic eruptions have even been suggested as a possible cause for some fish mass-mortality layers found in Green River deposits (Grande, 2013; pp. 352-357).
The Green River Formation is lower to middle Eocene, in the Cenozoic section of the geologic column. The macroscale stratigraphy of the Western United States shows a shift from Mesozoic marine-influenced deposits, accumulating over subdued topography, to continental-dominated Cenozoic deposition (Feth, 1964). Land-locked lacustrine basins, like the Green River system of lakes, become characteristic of this region, even into the Pleistocene (e.g., Lake Bonneville) and Recent (e.g., Great Salt Lake).
Most examples of impressive preservation in Green River fossils are found in specific localities of Wyoming. For example, most of the fossil fish displayed in public and private collections come from three distinct beds in the smaller Fossil Lake basin (Grande, 2013; Nudds & Selden, 2008). However, there have been some fascinating fossils found in other areas of the Green River Formation (e.g. Fig. 2, Krell & Vitali, 2021).
The exceptional diversity of fossilized organisms within the Green River Formation opens a window into the reconstruction of past ecosystems. The outstanding preservation with articulation of delicate structures indicates that many of these organisms were not transported far from their original habitats before fossilization, and points to rapid fossilization processes that inhibited decomposition.
Rapid burial may have been an important factor to slow microbial decay while promoting the precipitation of minerals that encased the organic material (Elson et al., 2024). Experimental taphonomy studies using Carassius auratus (goldfish) indicate that decay in quiet-water environments alone is not enough to explain the consistent preservation seen, for example, in Green River fish. In the experiments, soft tissues decomposed rapidly, and skeletal disarticulation was much greater than what is typically seen in Green River fossils (Hellawell & Orr, 2012).
Fig. 2: Pulchritudo attenboroughi is a vividly preserved beetle that was found in layers of the Green River Formation in north-western Colorado. The astonishing fidelity of detail and pigmentation of this frog-legged leaf beetle fossil led researchers to declare it as “hitherto the most perfectly preserved pigment-based colouration known in fossil beetles” (Krell & Vitali, 2021).
Image credit: Wicker, Richard M., 2020. Pulchritudo attenboroughi. Denver Museum of Nature & Science. IV.EPI.8393.D.
References:
Bohacs, K.M., Grabowski Jr, G. and Carroll, A.R., 2007. Lithofacies architecture and variations in expression of sequence stratigraphy within representative intervals of the Green River Formation, Greater Green River Basin, Wyoming and Colorado. The Mountain Geologist, 44(2), pp. 39-60.
Buchheim, H.P. and Biaggi, R., 1988. Laminae counts within a synchronous oil shale unit: a challenge to the “varve” concept. Geological Society of America Abstracts with Programs, 20(7):A317.
Elson, A.L., Schwark, L., Whiteside, J.H., Hopper, P., Poropat, S.F., Holman, A.I. and Grice, K., 2024. A paleoenvironmental and ecological analysis of biomarkers from the Eocene Fossil Basin, Green River Formation, USA. Organic Geochemistry, 195, p. 104830. doi:10.1016/j.orggeochem.2024.104830
Feth, J.H., 1964. Review and annotated bibliography of ancient lake deposits (Precambrian to Pleistocene) in the Western States. Washington, U.S. Govt. Print. Office: U.S. Geological Survey, Bulletin, 1080, 119 pp., 4 maps. doi:10.3133/b1080.
Grande, L., 1984. Paleontology of the Green River Formation, with a review of the fish fauna. Geological Survey of Wyoming, Bulletin 63, p. 333. (1st ed. 1980.)
Grande, L., 2013. The lost world of Fossil Lake: Snapshots from deep time. University of Chicago Press. p. 425.
Hellawell, J. and Orr, P.J., 2012. Deciphering taphonomic processes in the Eocene Green River formation of Wyoming. Palaeobiodiversity and Palaeoenvironments, 92, pp. 353-365. doi:10.1007/s12549-012-0092-6.
Keighley, D., Flint, S., Howell, J. and Moscariello, A., 2003. Sequence stratigraphy in lacustrine basins: a model for part of the Green River Formation (Eocene), southwest Uinta Basin, Utah, USA. Journal of Sedimentary Research, 73(6), pp. 987-1006. doi:10.1306/050103730987.
Krell, F.T. and Vitali, F., 2021. Attenborough's beauty: exceptional pattern preservation in a frog‐legged leaf beetle from the Eocene Green River Formation, Colorado (Coleoptera, Chrysomelidae, Sagrinae). Papers in Palaeontology, 7(4), pp. 2101-2112. doi:10.1002/spp2.1398.
Nudds, J.R. and Selden, P.A., 2008. Fossil Ecosystems of North America: A guide to the sites and their extraordinary biotas. Chicago, Illinois: University of Chicago Press. p. 189.
Pietras, J.T. and Carroll, A.R., 2006. High-resolution stratigraphy of an underfilled lake basin: Wilkins Peak member, Eocene Green River Formation, Wyoming, USA. Journal of Sedimentary Research, 76(11), pp. 1197-1214. doi:10.2110/jsr.2006.096.
Smith, M.E., Carroll, A.R. and Singer, B.S., 2008. Synoptic reconstruction of a major ancient lake system: Eocene Green River Formation, western United States. Geological Society of America Bulletin, 120(1-2), pp. 54-84. doi:10.1130/B26073.1.
Green River Specimens at GRI
Knightia eocaena is the most common fish from the Green River Formation, and is the state fossil of Wyoming.
Phareodus was a predatory fish in the same family as the living arawana of South America.
Mioplosus labracoides fossils have been found with other fish, such as Knightia, lodged in their throats, hinting at rapid preservation.
This paddlefish had electrosensors tuned to detect the movements of its prey.
This fish had grinding tooth plates covered with molar-like teeth, probably for crushing snails and crustaceans.
There are at least 15 genera of damselflies reported from the Green River Formation.
Damselflies appear to be more common and diverse than dragonflies among the comparatively rare Odonata that have been identified from the Green River Formation.
Preservation of soft-bodied larvae exemplifies the unique taphonomic conditions documented by fossils of the Green River Formation.
These clam shrimp were the first to be described from the Paleogene of North America.
This leaf was shed by a tree with close affinity to the living sycamore trees and is a good example of morphological stasis.
Fragmentary remains of the mammal Hyopsodus are common in Eocene strata of North America, but this specimen (replica) is exceptional in being articulated and fairly complete.
This specimen (replica) is the holotype of a species that exemplifies the abrupt appearance of bats in the fossil record. This microbat had the ability to echolocate, like many species living today.
This isolated tooth was likely shed by a crocodilian species, at least two of which are well known from skeletal remains in the Green River Formation.
These fossilized tracks from the Uinta Basin of Utah were made by an unidentified shorebird.
Fish fecal excretions are abundant in Green River deposits. These three-dimensional trace fossils are an example of outstanding preservation of originally soft organic matter.
Thin, alternating dark and light laminae in sediments of the Green River Fm. were originally described as annual varves. However, this interpretation has been challenged.
Dr. Paul Buchheim can be credited with critically assessing the varve interpretation (annual deposition of two thin layers) for laminae in the Green River Formation, especially from deposits of the Fossil Lake basin (Buchheim, 1994; Buchheim & Biaggi, 1988; Grande & Buchheim, 1994).
Part of his work on the subject was funded by the Geoscience Research Institute (see acknowledgements section of Buchheim, 1994).
References:
Buchheim, H.P., 1994. Paleoenvironments, lithofacies and varves of the Fossil Butte Member of the Eocene Green River Formation, southwestern Wyoming. Contributions to Geology, University of Wyoming, 30(1), pp.3-14.
Buchheim, H.P. and Biaggi, R., 1988. Laminae counts within a synchronous oil shale unit: a challenge to the “varve” concept. Geological Society of America Abstracts with Programs, 20(7):A317.
Grande, L. and Buchheim, H.P., 1994. Paleontological and sedimentological variation in early Eocene Fossil Lake. Rocky Mountain Geology, 30(1), pp.33-56.
Exploring Further
Fine lamination in micritic carbonates of the Fossil Butte Member of the Green River Formation (Green River Stone Quarry, near Kemmerer, WY). Pencil for scale is 15 cm long, with tip resting on an ash bed.
Whitmore (2006) and Whitmore & Wise (2008) have provided general overviews of the Green River Formation from a creationist perspective, suggesting it represents an early post-flood depositional system:
Whitmore, J.H., 2006. The Green River Formation: a large post-Flood lake system. Journal of Creation, 20(1), pp. 55-63. Accessed: March 2025. Available at: https://creation.com/the-green-river-formation-within-a-biblical-geological-framework.
Whitmore, J.H. and Wise, K.P., 2008. Rapid and early post-Flood mammalian diversification evidenced in the Green River Formation. In Snelling, A.A., (ed.), Proceedings of the International Conference on Creationism, 6(1). Pittsburgh, PA: Creation Science Fellowship, and Dallas, TX: Institute for Creation Research. pp. 449-457. Accessed: March 2025. Available at: https://digitalcommons.cedarville.edu/icc_proceedings/vol6/iss1/36.