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¡stone Lignite Carbonaceous Variegated shale beds

Figure 6.17 Occurrence of angiosperm pollen in samples from the Mud Buttes section (from Nichols and Johnson 2002). HC = Hell Creek Formation, FU = Ludlow Member of the Fort Union Formation; stars designate species usually restricted to Maastrichtian rocks. Reprinted by permission.

:other taxa

It is instructive to compare the pattern of fossil pollen occurrences at Mud Buttes (locality 5; Figure 6.17) with that at Pyramid Butte. At both localities, the precise position of the boundary is also known from the physical (geochemical and mineralogical) evidence as well as from the palynological extinction. The physical evidence of the K-T boundary at the Mud Buttes locality includes an iridium anomaly of 1.38 ppb, shocked quartz (Figure 6.18), and smectitic spherules altered from tektite glass (Figure 6.19). There is a very thin (ca. 1 cm) lignite bed just above the K-T boundary. The position of the boundary is marked by the disappearance (within 20 cm) of 14 K taxa and confirmed by the presence of impact debris. There is no evidence of a facies effect on the palynoflora at Mud Buttes, that is, there is no appreciable decline in the number or relative abundance of K taxa within the 50-cm interval of mudstone below the boundary (Figure 6.17). Note that only occurrences of angiosperm pollen are plotted in Figure 6.17. A single specimen of one K taxon was found about 2 cm above the K-T boundary; presumably, it is a reworked specimen. Figure 6.20 shows an abrupt but temporary increase in the relative abundance of fern spores just

Figure 6.18 Scanning-electron micrographs of etched grains of shocked quartz from the K-T boundary at Mud Buttes (from Nichols and Johnson 2002). Individual grains measure 110 to 190 mm. "b" is enlargement of part of "a". Reprinted by permission.

above the boundary - this is a fern-spore spike of the kind discussed in Section 5.3. The Mud Buttes locality is the most complete K-T boundary locality in the Williston Basin and the second best in North America. It has a strong (30%) palynologic extinction at the boundary and a fern-spore spike just above; a boundary claystone layer with associated iridium anomaly, shocked quartz, and spherules; plant megafossils from the richest known Cretaceous leaf quarry in the world (the single quarry yielded more than 85 species of leaves, Johnson 2002), as well as a series of Paleocene leaf quarries; vertebrate fossils including those used to argue for the highest stratigraphic occurrence of Cretaceous dinosaurs (Sheehan et al. 2000, Pearson et al. 2001); and paleomagnetic data placing it within subchron C29r (Hicks et al. 2002). The only feature this section lacks is a radiometrically dated horizon.

The pattern at Torosaurus Section (locality 17; Figure 6.21) is similar to that at the Mud Buttes locality, but there are minor differences in the stratigraphy of these localities. No lignite bed is present in close proximity to the K-T boundary, which is identified solely by the palynological extinction (no analyses were

Figure 6.19 Spherules of tektite origin from the boundary claystone layer at the Mud Buttes locality. a - small hand specimen; b - polished section.

conducted to detect an iridium anomaly or shocked quartz). Ten K taxa disappear within 50 cm below the K-T boundary, five of them within 5 cm. There is no evidence of a facies effect (the lithology is a homogeneous mudstone throughout the 3 m of the 25-m measured section shown in Figure 6.21), but the Signor-Lipps effect is expressed in the pattern of highest stratigraphic occurrences of the less common K taxa. This locality takes its name from the skeleton ofthe ceratopsian dinosaur Torosaurus latus that was excavated here by the Pioneer Trails Regional Museum of Bowman, North Dakota. We collected and analyzed palynological

K taxa K taxa Pteridophyte Gymnosperm

(number) (percent) spores (percent) pollen (percent)

K taxa K taxa Pteridophyte Gymnosperm

(number) (percent) spores (percent) pollen (percent)

Mudstone Lignite Carbonaceous Variegated shale beds

Figure 6.20 Numbers of K taxa (pollen restricted to Maastrichtian rocks) and relative abundances of K taxa, pteridophyte spores, and gymnosperm pollen in part of the Mud Buttes section (from Nichols and Johnson 2002). Strong increase in abundance of pteridophyte spores just above K-T boundary is the fern-spore "spike." Reprinted by permission.

Mudstone Lignite Carbonaceous Variegated shale beds

Figure 6.20 Numbers of K taxa (pollen restricted to Maastrichtian rocks) and relative abundances of K taxa, pteridophyte spores, and gymnosperm pollen in part of the Mud Buttes section (from Nichols and Johnson 2002). Strong increase in abundance of pteridophyte spores just above K-T boundary is the fern-spore "spike." Reprinted by permission.

samples to verify the latest Maastrichtian age of the dinosaur. It also proved to be an interesting K-T boundary locality because of the absence of coal or coaly facies near the boundary. Note that occurrences of spores and gymnosperm pollen as well as angiosperm pollen are plotted in Figure 6.21.

Terry's Fort Union Dinosaur (locality 10; Figure 6.22) is particularly interesting with regard to the occurrence of dinosaur remains near the K-T boundary. The boundary at this locality is 260.5 ┬▒ 2.5 cm above the Hell Creek-Fort Union contact, within strata of Fort Union lithology but latest Maastrichtian age. An associated partial skeleton of a ceratopsian dinosaur (Pioneer Trails Regional

Torosaurus Section m

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