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Why Core?
Outcrop Issues
Figure 4: Opportunities and problems present themselves in the superb outcrops of muddy facies of the Colorado Plateau early Mesozoic, showing near 100% outcrop (above). Mudstones are often bentonitic with a partial volcanic source, which means there are datable ashes, however it also means that the outcrop surfaces present horrendous sampling problems where freshness and competency are at premium as for geochemistry or paleomagnetics. Two examples include: A (above), Brushy Basin Member of the Morrison Formation west of Green River, Utah (from; B, typical “popcorn” surface of bentonitic mudstone of the Brushy Basin Member of the Morrison Formation west of Green River, Utah (same source as A); C, “popcorn” surface of the Chinle Group near Bluewater Creek, NM (Stop 1 of Lucas et al., 2007), with fragmentary weathered metoposaur amphibian dermal bones; D, outcrops of bentonitic Painted Desert Member of the Petrified Forest Formation of the Chinle Group in Petrified Forest National Park with the U-Pb dated Black Forest Bed (Riggs et al., 2003) is the white bed in foreground.

Although the American Southwest, including the Colorado Plateau, is famous for spectacular exposures and striking badlands, the thickest sections of key time intervals are often projected to be in the subsurface. The most continuous sections in outcrop are often exposed as inaccessible vertical cliffs or are heavily weathered and geochemically altered (Figure 4, above), precluding research at the appropriate level of detail. The assembly of sections spanning large stratigraphic thicknesses requires, because of low bedding dips, long-distance traverses that often compromise the essential assumption of superposition because of facies changes. The problems are amply revealed by repeated attempts in the literature at compiling long high-resolution sections that describe the succession in any specific area. Long, continuous cored intervals, tied to outcrop in key areas are required for further substantial progress.