Field School, Day 2 (7/7/03) - West Bijou Creek at Soil Conservation District
Day #2 began with a morning lecture before heading to the field. It was emphasized the Western Interior uplift is not well understood. Generally, mountain uplifts occur along tectonic plate margins. However, in the case of the Rocky Mountains, these margins are well to the west along the coast of California. It's currently believed that a plate buckling or submerged plate uplifting provided the mountains we see today.
The regional area being studied on this Field School are all drainages of tributaries leading to the South Platte river. The Denver Basin is mostly flat, with rolling hills which makes it difficult to map the geology. It's mostly grass with less than 1% of exposed bedrock. The key to learning about its geology and fossils lies in finding good exposures in cut banks of rivers and tributaries; and road/building construction.
We visited the K-T boundary which, in this area, resulted in a 30% extinction of pollen producing plants. The boundary is 2 mm thick here. The sedimentary layer known as D1 consists of alternating sandstones; and yellow, white and green mudstones which represents the fossil soil horizon known as the Paleosol.
On Day #2, we did research and excavation in West Bijou Creek (a.k.a. "Grand Canyon of the Denver Basin") at the Plains Conservation Center. After the morning lecture (notes above), we traveled to the site where Rich Barclay posted area maps on the side of the van with magnets. Rich explained that the first uplift of the Rocky Mountains occurred about 68 mya. In research for his Masters Thesis, he took pollen samples from the base of West Bijou Creek which yielded a date of 68 mya. Then, he discovered fossil plants at the top of the escarpment (see notes below) that were Paleocene in age. This put the K-T boundary somewhere in the vertical area covered by the West Bijou Creek escarpment. During the discussion, we learned that the average rate of sediment deposit in the Denver Basin is roughly 100 meters per million years (or 1 meter/10,000 years).
At the first stop, we drew diagrams in our field books of one of the outcrops, named "Grace's Oven" which Rich has worked extensively. We collected a few fossil leaves of the Paleocene at the location. After lunch, we ventured further down in the formation to find Cretaceous dinosaurs and turtles. So, we had crossed the K-T boundary during our morning and afternoon excavations. The final stop was at a location know as the "phone booth" where Rich had discovered the precise location of the K-T boundary. It's a subtle outcrop of coals and ash, yet detailed research revealed may of the telltale signs of the K-T boundary: high levels of iridium, shocked-quartz, and the fern spike above the boundary.
An interesting phenomenon called "log concretions" (see image below) was pointed out by Kirk Johnson. These concretions are almost always pointed in a parallel direction to the paleocurrent of ancient rivers. It's believed that they are fossilized log concretions that lying in the river bed and parallel due to the flow of the river.
After lunch, we explored an area known for Cretaceous animals. At left, Kirk demonstrates an easy method of identifying whether something is bone of rock. Bone is porous and will stick to your tongue, while rock simply falls off. In fact, bone is so different than rock that we often found ourselves pulling the bone from our mouths in fear for our tongues! We stopped at this location because it was: (1) in Cretaceous soil; (2) it was almost flat with just a slight decline for runoff; (3) didn't have enough declination for dirt and tiny bone fragments to runoff. During a quick 20 minute discovery, we found fossils of alligator gar, crocodile teeth, turtle shells, a rib of the hadrosaur dinosaur , and misc. dinosaur parts from a duckbilled dinosaur. These were all found within 4.5 meter below the K-T boundary (or 40-50K years based on the average sedimentary deposit of the Denver Basin). It is interesting to note that these were all found very close to the K-T boundary. The dinosaur rib was found at 3.6 meters (or 30-40K years before K-T). This provides proof that the dinosaur did not gradually die off prior to the major asteroid impact at the K-T boundary. To the contrary, they continually provide evidence of their existence right up to the catostrophic impact.
During Rich Barclay's research, he finally discovered a site he termed the "big phone booth". At this site, he discovered all the tell-tale signs of the K-T boundary including: pollen dating, shocked quartz (see notes below), high levels of iridium, and fern spike which follows K-T. In this region, it is estimated that 50% of the species disappeared (24% regionally). The boundary is 3 cm thick here compare to 1 cm in the Raton basin. The boundary has a magnetic polarity of 29R. There are some concerns regarding dating at this site. For example, the tuft layers above have a contradictory date. But, quoting the explicative Bob Raynolds, "You're O.K. with it during the day, but at night you have these little creeping kittens of concern!" Until this saga is resolved, we live by the science as it is revealed...
It was an exciting day to visit such important sites that resulted from months of field research by Rich Barclay. To see images from Day 2, click on the link below.