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Appendix PEP Field Trip Guide: the “Late Cretaceous Tour”

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Appendix PEP Field Trip Guide: the “Late Cretaceous Tour” Appendix PEP Field Trip Guide: the “Late Cretaceous Tour” The “Late Cretaceous Tour’ was devised to help participants develop their actualistic thinking skills and gain perspective on geologic time. The tour was conducted in the vicinity of Fort Peck, Montana, primarily along State Highway 24 on the east side of Fort Peck Reservoir. The tour included four stops, each exemplifying a unique geologic setting and formation. The tour proceeded from older to younger strata, which was intended to help participants appreciate the extreme environmental changes that occurred in the region during the Late Cretaceous time period. At each stop, instructors asked participants to spend some time making observations, and then try to determine the depositional environment based on those observations. The following is a description of each stop on the tour and participant reactions. Late Cretaceous environmental sequence in the Fort Peck region and resulting geologic formations. 1 Stop 1: Bearpaw Shale The Bearpaw Shale was deposited during the final phase of the Western Interior Seaway and represents a shallow marine environment. It is characterized by tan to dark gray shale and unique “nodules” that are abundant throughout the formation (Stanton and Hatcher, 1905; Feldmann et al., 1977). The nodules are oval and can reach over a meter in diameter. They consist of an organic dark brown/grey outer rind and often contain a fossil nucleus. Fossils of crustaceans (lobster tails and claws), Cephalopods (ammonites, nautiloids), and bivalves (oysters, clams) are commonly found within nodules. An ammonite found within a nodule of the Bearpaw Shale. If the participants were unable to recognize this formation based on their initial observations, instructors asked questions designed to lead them to the correct answer, “What fossils, if any, do you see?” “Do you think these fossils lived in the water or on land?” “What type of modern environment do these rocks look like?” “What environmental processes would cause the rocks to look like this?” Participants were successful in identifying the depositional environment as marine and that it represented the Western Interior Seaway. They were given time to discuss how the various organisms lived, what they ate, and how they were finally preserved, until the instructors felt that the basic concepts had been grasped. 2 Stop 2: Fox Hills Sandstone The Fox Hills Sandstone (Meek and Hayden, 1861) directly overlies the Bearpaw Shale and represents the last regression of the Western Interior Seaway. The rocks consist of a light brown, well sorted, friable, very fine-grained sandstone with sparsely inter-bedded, thin, gray mudstone. Trace fossils representing burrows are common in the deposit. The formation is interpreted to represent a shore face and foreshore beach environment. Bedded sandstones and mudstones of the Fox Hills Formation. This stop was difficult for many of the students to interpret, perhaps because most of them had never seen a beach except through the media, and consequently, had difficulty envisioning one. To address this dilemma, the instructors asked questions aimed at drawing on their deductive reasoning skills. They were asked, “If we were in the ocean at the previous stop, where would we be now at this higher elevation?” After the questioning, the students were able to see how the fine laminations and yellow/orange coloration of the rocks could represent sediments from a beach environment. The students were thrilled by the revelation and turned the questions around on the instructors, “Why is the beach hard? Why are there no fossils here like you find in the marine sediments?” After such questions were answered, the groups continued exploring the site, paying close attention to sedimentary rock characteristics that were the result of deposition (cross-bedding, grain size), weathering (sculpting and channels), and the ancient biota (trace fossils). Although the preservation of a beach environment in rock was initially difficult for many of the students to grasp, by the end of the visit, most appeared to understand what they were seeing. 3 Stop 3: Hell Creek Formation This site represents a floodplain environment of the Hell Creek Formation (Brown, 1907; Hartman et al., 2002). The site was chosen for its exceptional vertebrate fossil preservation, which resulted from continual wetting and drying, typical of swampy floodplain environments. Triceratops and other dinosaur bone fragments, crocodile dermal plates and turtle scoots, petrified wood, and other plants fossils are abundant throughout the site. The fossil containing sediment is grayish brown muddy shale with a granular surface texture. This texture results from repeated wetting and drying and consequent volume changes, and is called “popcorn weathering” (Hartman et al., 2002). Dinosaur fossils on a popcorn weathered surface. At this stop, the abundance of fossil material strewn across the ground appeared to distract many students from making necessary scientific observations. Conveniently, bleached cattle bones were found among fossil bones, providing the basis for interesting discussions on fossil preservation. This discussion included the density, color, and hardness of mineral infilling of the bone and how to distinguish fossil bones from modern mammalian counterparts. Examples were passed around and participants were asked to infer whether they were fossils or modern bones. After everyone had voiced their opinion, the instructors described fossil preservation processes. Other topics of discussion included differential preservation, biases of taphonomy (the process of burial and change), in situ vs. “float” fossils (those that had been transported from their original place of deposition), and finally, the depositional processes involved. 4 Stop 4: K-Pg Boundary This exposure contains a light-gray and black coal and ash layer, which clearly marks the K-Pg (formerly called “K-T”) boundary, or the end of the Cretaceous period. This boundary coincides with a massive extinction event, in which over 70 percent of existing species perished, including non-avian dinosaurs and many sea creatures. The K-Pg extinction event is one of the best studied of the five great mass extinctions that have occurred in Earth’s history (Jablonski and Raup, 1995), and is visible at various locations in eastern Montana (Hartman et al., 2002). From the site’s vantage point, the dark boundary layer can be seen on adjacent exposures across the distant landscape. Participants examine the K-Pg boundary. Because this was the final stop on the tour, participants were asked to give an oral summary of what they had learned on the tour, including the depositional environments and formations represented at each of the previous three stops, as well as their evidence and reasoning that led them to their conclusions. Following the summary, the instructors asked questions such as, “What does this dark band consist of? What is the extent of this layer across the landscape? Does it contain any fossils? How could a layer of coal and ash have been deposited here? Given what you know about the Cretaceous Period, what major event could this layer represent?” The questions continued in this manner until the students understood that they were looking at a sedimentary layer deposited during a large-scale catastrophic event, which caused the extinction of most dinosaurs and marked the end of the Cretaceous period. 5 References Brown, B., 1907, The Hell Creek beds of the Upper Cretaceous of Montana: American Museum Natural History Bulletin, v. 23, p. 823–845. Feldmann, R.M., Bishop, G.A. and Kammer, T.W., 1977, Macrurous decapods from the Bearpaw Shale (Cretaceous: Campanian) of northeastern Montana: Journal of Paleontology, v. 51, p. 1161-1180. Hartman, J.H., Johnson, K.R., and Nichols, D.J., (eds.), 2002, The Hell Creek Formation of the northern Great Plains: An integrated continental record of the end of the Cretaceous, Geological Society of America Special Paper 361. Jablonski, D., and Raup, D.M., 1995, Selectivity of end-Cretaceous marine bivalve extinctions: Science, v. 268, p. 389-391. Meek, F.B., and Hayden, F.V., 1861, Descriptions of new lower Silurian, (Primordial), Jurassic, Cretaceous and Tertiary fossils collected in Nebraska territory: with some remarks on the rocks from which they were obtained: Proceedings of the Academy of Natural Sciences, Philadelphia, p. 415–447. Stanton, T.W., and J.B. Hatcher, 1905, Geology and paleontology of the Judith River Beds: U.S. Geol. Survev. Bull., v. 257, p. 174 6 .
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