Background on Green River Formation

The Green River Formation is type of a formation known as a lagerstatten. According to Bottjer, a lagerstatten is a German word that means “lode places” or “motherlode.” It is a fossil locality highly remarkable for their diversity and quality of preservation, a spectacular rarity (1).

Here is just an example of the massive amounts of specimens!

Fossils (1, 3)

There are some 60 vertebrate taxa that have been described from the formation, as well as abundant invertebrates and plants.

Map of the Green River Formation (2)

The Green River Formation cuts through the states of Wyoming, Utah, and Colorado and was formed as part of an uplifting of the Rocky Mountains.

The lakes that make up this exquisite find are Fossil Lake, Lake Gosiute, and Lake Uinta. They are all varied in depth and expanse, but all provide a keen importance to the geological makeup of the area (3).

1. Bottjer 45
2. http://www.fossilmuseum.net/Fossil_Sites/GreenRiverSite.htm
3. http://www.fossilnews.com/2000/grnrv/grnrv.html
4. http://www.ucmp.berkeley.edu/tertiary/eoc/greenriver.html

Lakes

Basins of the Green River Formation (Stanley)

The lakes of the Green River Formation fluctuated in volume and areal extent according to climatic and tectonic variation. This variation in climate and tectonics resulted in significant interfingering of sediments along lake margins (1). It is this reason that we see how differently each lake’s formation developed. It was the localized climate, specific tectonic activities, as well as upstream hydrology that caused the sediments to layer differently between each basin. The location of each basin differentiated the depositional sequences based on the margins created by uplift (3).

Lakes of the Greev River Formation (Stanley)

Fossil Lake’s importance came from its depth. It was one of the smaller lakes surface wise, but amounted to have to largest number of fish specimens out of the rest of the lakes (2). This preservation was most likely because of the lake’s depth and abundant amount of fresh water from high precipitation (4). This allowed a more livable condition for animals than a saline environment.

Lake Gosiute, produced a smaller amount of fish findings, but was composed of a series of smaller lakes within the Green River Basin (1). The thing that made this lake interesting is how the preservation differed than the other lakes. Lake Gosiute is composed of a combination of shale and fossils, which came about by a constantly changing water supply. It was this water supply that caused the basin to be over filled and under filled regularly, primarily due to deformation at the margins of the hydrologic basin and intrabasinal sills (3).

The last of the three lakes was Lake Uinta. The lake existed the longest, in the amount of 17 million years. It was the total opposite of Fossil Lake as its key feature was its great span (2). The lake had the largest surface area, but the least depth. However, the sediments that formed here are one of the thickest lake deposits found at around 7000 feet via a river system that replaced the sinking Uinta basin (5).

1. Leggitt & Biaggi & Buchheim, 2007
2. http://www.fossilnews.com/2000/grnrv/grnrv.html
3. http://gsa.confex.com/gsa/2002AM/finalprogram/abstract_41772.htm
4. http://gsa.confex.com/gsa/2001CD/finalprogram/abstract_4180.htm
5. http://www.ugs.state.ut.us/utahgeo/geo/geohist.htm

Sediments

The composition of the lakes was formed from very fine sediments that preserved a subtropical environment.

1. Oolitic grindstones (grains of multiple coatings of carbonate, usually calcite and/or aragonite, that precipitated on a nucleus; they are round and smooth, and form in shallow water depositional environments)
2. Packstones (porous sedimentary rocks)
3. Oil shale
4. Mudstones (2)

Stratigraphy of the Green River Basin during the Eocene (1)

The formation itself and the sedimentary layers are a consequence of the tectonic uplift of the Rocky Mountains. The surrounding lakes varied in depth, which depended on the subsequent amount of water that drained into the basins, as part of constant isostatic regression and transgression sequences (3).

The stratigraphy of the Green River Basin tells us about the makeup of the area. In the lower Eocene the Green River Basin was at first a fresh water lake and suddenly changed to a salt lake. And after changing to a salt lake it stayed a mudflat from the lower Eocene to the start of the middle Eocene. It then quickly reverted back to a salt lake and then to a fresh water lake relatively quickly as well.

This map tells us key details as far as the sedimentary composition and the reason why there were so many organisms present (1). The mudflat is a location in which salt water is flowing in and out of the area, and in turn leaves deposit of rich nutrients. These rich nutrients are the reasons for the lush environment, providing for so many plants and animals to survive.

But what was the cause of such a quick shift in saline levels? It is thought to have been caused by the constant tectonic process that the Green River Formation underwent and can also be related to the Sevier and Laramide orogenic events, which led to thrust faulting and compression (4). Sevier was a subduction event and it was the source of the basins creation, and its consequence was the change in terrain. This was the driving force of the uplifting features, but it was the compressional collision of the continental to continental stress that changed water levels (salinity). These water levels, in rapid change, were the cause for the accumulation of carbonate and bicarbonate rich sediments. These sediments then evaporated to create temporary saline lakes. It was these events that left a unique environment, which we can see within the East Africa Rift Valley today.

Oil Shale

Estimated accounts of the oil resource in place within the Green River Formation range from 1.2 to 1.8 trillion barrels.

Piece of Oil Shale

The U.S. Department of the Interior, Bureau of Land Management of the state of Colorado in combination with the Department of Energy, suggests that there could be an estimated number of 800 billion barrels of recoverable oil! (5). However, not all the resources are able to be drawn from the site and much of it is trapped. Unfortunately, oil shale in a solid form and cannot be retrieved as easily as its liquid counterpart and requires strip mining. The strip mining process would require devastation (blowing up the land) amongst the area and would impede onto the competing markets of fossil sales and educational research.

1. http://www.geology.wisc.edu/~carroll/green_river.html
2. http://www.ucmp.berkeley.edu/tertiary/eoc/greenriver.html
3. http://www.fossilmuseum.net/Fossil_Sites/ GreenRiverSite.htm
4. http://gsa.confex.com/gsa/ 2002AM/finalprogram/abstract_44751.htm
5. http://ostseis.anl.gov/guide/oilshale/index.cfm

Fossils

One of the common fossil preservations at the Green River Formation, are the fish.

The most commonly attributed reason for such pristine preservation is because of two factors:

1. A cold period during the Eocene
2. The great depth of the lakes(1).

The cold would have caused dead fish to sink faster due to a less inflated swim bladder, as well as other organisms to the lake’s bottom. And in combination with animals sinking to the bottom quickly the great depths of the lakes prevented scavengers or other disturbances from following.

At lower depths the conditions were anoxic, or without oxygen, this allowed the sediments that were deposited at the bottom to preserve fossil remains perfectly! This was crucial for the quality; consequently, many fossils were found to be laid in laminated bed and recovered wholly (1).

One of the common fossil preservations at the Green River Formation, are the fish. The first collected fish fossil was found by Dr. John Evans in 1856 and was described by Dr. Joseph Leidy as the "Knightia eocaena.

98% of the collected species reside in a more common set of five fossils (2). However, the formation preserves over twenty different species of fish in total.

Fossil Fish Chart (3)

1. The more common fish are the Knightia, the first found fish and resembles a herring.
2. The second was Mioplosus, a perch-like fish, a voracious predator that was found to eat other fish.
3. There was also the Diplomystus, herring type surface feeder, similar to the Knightia.
4. Priscacara, crustacean eaters only found in the Eocene
5. Phareodous, which was the largest of the common fishes (2).

Fish fossil Knightia (2)

The Knightia was a schooling fish that ranged in sizes of 5 to 10 inches. They were at the bottom of the totem poll and were the dinner for many of other fish. They have been found in mass, which possibly indicates that they died from a single event. Some theories include stratified water turnovers or poisoning from blue-green algae blossoms (6)

Another fish found in the Eocene epoch is the Priscacara. Fossil records have indicated that this fish might have been a schooling fish, but its other attributes separated itself from the others. Priscacara, resembles a modern sunfish, but had thick dorsal and anal spines. These spines, perhaps, provided protection from being swallowed whole (7). The species were able to survive in saline environments better than the other surrounding species, but like the rest were more apt with fresh water (8). They were crustacean eaters only found in the Eocene and eventually went extinct (2). Some say that there is no modern relative today; however, some believe that there is a might be related to the modern-day Cichildae.

Cichildaes are found in the East African Rift Valley, and it is the valley that provides for a good deal of tectonic activity. It is interesting to see the correlation between the similar saline levels and constant activity that makes the relation of the Cichildaes to the Priscacara plausible.

Other Fossils

Approximately 60 vertebrate taxa have been found at Green River, including fish, reptiles, birds and mammals.

Fossil Bird Gruiformes (1)

Bat Fossil of the first flying mammal found, Icaronycteris (3)

But one of the most impressive discoveries was the Icaronycteris fossil. It is the oldest known flying mammal fossil in existence (4).

The importance of this fossil extends to the specialization of the auditory region of its skull, which suggested that this bat could echolocate. Echolocation is the sensory system in animals that help interpret the direction and distance of objects (9). Consequently, it is this bat fossil that can help tell researchers about the evolutionary history of echolocation and the evolution of life in the early Tertiary.

Bony Plates from Borealosuchus similiar type of crocodylian (1)

Eleven species of reptiles have been found, including one species of snake.

Fossil Wasp Ichneumonidae (1)

Invertebrate fossils are abundant, with remnants of snails and insects being common. They are still of high importance as they help contribute to the identification to family and subfamily of the area. This is a significant finding as it can sometimes lead to gender, wing structure, and the color patterns (5).

Algae Sample (1)

The plant fossils, including many reeds, leaves and wood specimens, are also very prevalent at Green River.

Therefore, the climate in which the organisms lived differs somewhat from that of the present-day western United States. The fossils, especially plants, found at this site indicate that the climate was moist temperate or sub-tropical, with temperatures ranging from 15 to 20 degrees Celsius. (1)

1. http://www.ucmp.berkeley.edu/tertiary/eoc/greenriver.html
2. http://www.fossilnews.com/2000/grnrv/grnrv.html
3. http://www.fossilmall.com/Science/Sites/GreenRiver/GreenRiver.htm
4. http://www.fossilmuseum.net/Fossil_Sites/GreenRiverSite.htm
5. http://www.ucmp.berkeley.edu/tertiary/eoc/greenbugs.html
6. http://www.tethysfossils.com/fishfossils/Knightia-eocaena.htm
7. http://www.geoclassics.com/fish/priscacara.htm
8. http://www2.nature.nps.gov/geology/paleontology/pub/grd3_3/fobu4.htm
9. http://www.ucmp.berkeley.edu/mammal/eutheria/chirofr.html

Conclusion

The Green River Formation is important because it reveals crucial information about the environment that cannot be found elsewhere.

In Europe and Asia, Eocene fossil localities are incomplete or have been disturbed, so the fact that the Green River locality is so complete allows scientists to form a more comprehensive picture of the era.

It is here where we can find a more specific map of the Eocene and the transitions during the period itself. It is these features, such as the inhabitants, that can help pinpoint the existence of ample rainfall, perhaps seasonally, and that temperatures were relatively mild in North America.

Reference List