Field School, Day 6 (7/11/03) - West Bijou Creek, private property
On Day #6, Stop #1, we ventured deep into West Bijou Creek to hunt for mammal bones in two known localities. These sites are very early Paleocene (Puercan) and mammals are very rare. So rare, in fact, that Kirk Johnson felt completely safe in offering steak dinners to anyone finding a mammal jaw! Steve Wallace of the Colorado Department of Transportation (CDOT) found ash beds to accurately date this area. Puercan mammals of PU1 are tiny, so we again hunted on hands and knees. By the time of PU2, they were much larger with bones similar to a lamb. However, most mammal fossils from the Puercan are bone fragments and teeth. Only one skull has been found in the Denver Basin. It is a stylinodontine taeniodont which was discovered by two people who had just stopped on the side of the road. The geologic name of the Stop #1 area is "Dawsom Arkose", not to be confused with the "Dawson Arkose". A permit is required to collect at these locations because the land is owned by CDOT. No personal collecting is allowed.
At Stop #2, we hiked to another leaf locality on private land known as "Cheryl's Festoon". We diagrammed the geology of the cut bank exposed in a gully. The layers dipped to the north with a variety of siltstones & mudstones, limonite concretions, ironstone nodules and a lignite bed. We could easily observe evidence of a fast moving river, a slow moving river, a pond or lake and a swamp - all exposed beautifully in approximately 4 vertical meters of this present-day meandering gully. Age estimates for this locality indicated that it is approximately 63 mya.
We excavated from above the gully, working down to the known leaf layers (see left). A common species here is Platanus reynoldsii. After becoming somewhat discouraged by the poor preservation of the leaves, we started to pack up. However, a few swings of the rock hammer into the pond/lake layer near the bottom of the outcrop yielded very well-preserved leaves. We again dropped our packs and went to work collecting nice specimens from this layer.
An interesting discussion took place regarding ancient leaf transportation. Stream channels often contain a much more diverse flora than lakes, ponds or swamp. The reason for this is transportation. Leaves float for a considerable distance in a river before becoming saturated and sinking to the bottom. So, streams are "sampling" leaves from miles upstream. Therefore, fossil leaves found in a stream/river channel will contain a diverse selection of species. On the other hand, lakes, ponds and swamps have much less transportation because the water motion is less. In a swamp of standing water, fossils recovered at any specific site will generally be from an ancient tree that live right there, with only minor transportation of leaves during flooding events.
At Stop #3, we broke up into 3 teams: 2 working gully outcrops for leaves; 1 hiking to badlands areas (see left) to prospect for vertebrate fossils and study the geology. One of the leaf teams discovered a fossil tree in-situ (fossilized in its upright, growing position).
On the geologic walk to the badlands, Bob Raynolds discussed his "Turtle Indicator". It turns out that turtles are very useful in understand the depositional environment in the Denver Basin. Basically the turtles chose to live in 3 types of ancient environments: lakes, rivers and land. DMNS works with other individuals who specialize in fossil turtles identification. This is often done using the patterns on the fossil turtle shells. The "Turtle Indicator" is fairly reliable - once you know the species, you have a good idea of the geologic setting and depositional environment. This is yet another example of the close ties between paleontology and geology. The disciplines have a much to share and a person having knowledge of both is very valuable.