GEOLOGY
During the Precambrian times, the western edge of North America ran roughly north/south through what is now western Wyoming. From Precambrian through the Cretaceous the ocean basin to the west accumulated a large amount of sediment in a geosyncline from the continental land mass.
At the end of the Cretaceous period into the Paleocene, a mountain building episode known as the Laramide orogeny occurred. The Rocky Mountains were faulted up, largely along preexisting fault lines. The sediments which had been filling the geosyncline along the western edge of the continent were thrust over each other along low angle faults all along the western edge of the continental land mass. These thrust plates are called the overthrust belt.
North of Kemmerer, Wyoming the overthrust belt forms the Wyoming and Salt Ranges. The formations exposed in these mountains range from the Mississippian through Cretaceous ages. A basin between the ridges of two thrust plates filled with sediments just west of Kemmerer. The Hillard formation is of Paleocene age. Above that are Eocene age stream bed deposits of the Wasatch formation that are reddish brown and purplish. Above these are the whitish lake bed deposits of the Green River Formation. More reddish stream bed deposits of the Wasatch formation overlie these lake bed deposits. The basins here and in the rest of Wyoming continued to fill during the Oligocene. About 15 million years ago the entire western part of North America was pushed up to current elevations. Erosion has been carving the landscape since then.
Green River Formation
The Green River Formation is composed of freshwater lake bed deposits of Eocene age. Parts of the Green River Formation range in age from 55 million years old to 37 million years old. The Green River Formation west of Kemmerer, Wyoming is roughly 50 million years old. The sediments of the Green River Formation are shale, sandstone, marlstone, and limestone with some volcanic ash tuffs. Fossils can be in any of the sediments but fossil fish are most abundant and best preserved in the thinly bedded marlstone that are predominately calcium carbonate. The source of the calcium carbonate is skeletal remains of calcareous algae.
The fossil fish are very abundant. The number of fish and the thin bedding go together. The thin uniform bedding indicates that sedimentation was rapid. It also shows there was no oxygen in the water just above the lakebed. No scavengers have disturbed the bedding. When fish died and went to the lake bottom they were covered before they decomposed. But disarticulated specimens are often found. They are seldom offered for sale because most customers prefer well articulated fish.
Each quarry has slightly different productive zones. I have a little fish layer where the fish tend to split out. Beneath this is the gastropod layer that is full of snails but where also good quality fish are found. The upper gastropod zone is lighter colored than the lower gastropod zone. The mini fish layer is about 4 feet beneath the gastropod layer. About 30 feet beneath this is the 18 inch layer. The 18 inch layer has 10 inches of white and tan rock on top of about 10 inches of grey and dark tan rock. The grey rock has long been called bottom capping. The fish in the 18 inch layer generally are covered with matrix when found. They are cut out in the quarry and taken back to the workshop to prepare. Some 6 feet beneath this are the split fish layers, a chalky whitish zone. Very few of the fish actually split out clean and most need to be prepared.
Knightia
The most common fish found in the Fossil Lake sediments is the Knightia that averages about 3.5 inches long. Occasional specimens from the 18 inch layer are 7 inches in length. There are two species of Knightia, the Knightia eocena and the Knightia alta. The Knightia eocena is more abundant in Fossil Lake. These are herring relatives and is similar to the present day Alewife that lives in Lake Michigan and Lake Huron. Alewives die en mass during the summer and appear to be very sensitive to increases in temperature. We find mortality layers of Knightia which may represent a similar summer die off.
Diplomystus
The Diplomystus dentatus is another herring relative. This primitive fish is in the herring family but it is not closely related to the Knightia. The earliest known Diplomystus are from the lower Cretaceous from sites in Japan and Africa. They are fairly common in Fossil Lake and range in size from juveniles to 26 inches. The Diplomystus has the body form of a surface feeder. Its fine bone structure makes it a very attractive specimen.
Mioplosus
The Mioplosus labracoides is fairly common in Fossil Lake. They range in size from just under an inch to 20 inches in length, although I rarely see one over 16 inches long. The Mioplosus is a perch, fish that are very active predators. Occasionally a Mioplosus is found with another fish in its mouth.
Priscacara
Priscacara liops and Priscacara serrata are fairly common in Fossil Lake as well. The Priscacara liops are found in mortality zones and its average size is a little under 5 inches long. The Priscacara serrata has a bit larger bones than the Priscacara liops. The Priscacara serrata can be up to 15 inches long. But Priscacara serrata from the 18 inch layer seldom exceed 10 inches long. The Priscacara has dorsal spines that are similar to the modern day Bluegill.
Phareodus
Phareodus encaustus and Phareodus testis are not as common as the species previously mentioned. These fish are fairly large predators that would have been up the food chain . Large predators are always less abundant. The Phareodus are primitive looking fish with large scales and a turned up mouth. They are members of the Osteoglosserids, "bony tongued" fish. Current day Osteoglosserids are found in tropical waters of South America, Africa , Asia and Australia. One species, the Arawana that is native to the Amazon basin, is sold in pet stores for home aquariums. The Phareodus testis has a body that appears to be round. The Phareodus encaustus has a body that is a bit more narrow for its length. The Phareodus encaustus can be quite large, up to 32 inches long, while the Phareodus testis is rarely over 16 inches in length. .
Notogoneus
The Notogoneus is a member of the Gonorynchidae family of fish. Current living Gonorynchidae are found in Southeastern Asia. One species, called the beaked salmon, lives on sandy bottoms near shorelines. This is a nocturnal fish that feeds on invertebrates at night and burrows into sand and mud during the day. The Notogoneus of the Green River Formation are rarely under 10 inches long, and the largest ones are about 26 inches long. Since modern day beaked salmon live on muddy bottoms, there were probably large areas on the margins of Fossil Lake where Notogoneus and all other species lived, fed, and reproduced. The rock towards the margins of the basin are bioturbated, chewed up. Notogoneus are almost all from the 18 inch layer that is only found in the center of the lake.
Other Fossils
Other fish found in the strata of what was once Fossil Lake include Paddlefish, Gar, Eohoidon, Amia, Amphiplaga, Asineops, and Sting Rays. In 2004 I found a Gar that measured 64 inches long.
Some large vertebrate fossils show up each year among all the quarries. Aquatic vertebrates such as turtles and crocodiles are seen most often. Birds, bats, otters, primates, and a horse have also been found.
A large amount of aquatic plant life is often encountered while digging. Much of this could be referred to as pond weeds. But occasionally leaves from terrestrial plants are found. The most magnificent of these are palm fronds. Small flowers are also found that may be from lily pads.
Among the fossil insects found, the most common is Plecia pealei, the march fly. Water striders are fairly common; dragonflies are uncommon. In 25 years of digging I have found only 2 moths.
As I mentioned earlier, gastropods are abundant in the gastropod layer and are occasionally found in other layers.
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