The first iridium-bearing terrestrial K-T boundary sections were discovered in the Raton Basin of southeastern Colorado and northeastern New Mexico (Figure 7.2). The K-T boundary is preserved in the Raton Formation, an entirely nonmarine unit of Maastrichtian and early Paleocene age (Figure 7.3). The Raton Formation is composed of sandstone, siltstone, mudstone, coal, and minor conglomerate. Thin to thick coal beds are common in the lower and upper parts of the formation, but are scarce to absent in the middle "barren series,'' which is composed largely of sandstone. The K-T boundary is present in the upper part of the lower coaly interval, in some places just below sandstone of
the barren series. It is also present in numerous locations on the eastern side of the basin, and in many places the boundary can be recognized by a distinctive, thin (1-2 cm) claystone unit visible in outcrop, even at a distance (Figure 7.4). In most places where the boundary claystone has been identified, a thin coal bed closely overlies it. An important difference in the stratigraphic setting of the K-T boundary in the Raton Basin from that in the Williston Basin is this: in the Raton Basin, the boundary is entirely enclosed within a coal-bearing interval, the Raton Formation. The fine-grained rocks of the Raton Formation contain abundant plant megafossils and palynomorphs, but with the exception of a few dinosaur tracks, vertebrate fossils are absent, likely an artifact of depositional environments unfavorable for their preservation.
In Maastrichtian time, the fluvial coastal plain in which sediments of the Raton Formation accumulated was vegetated by a diverse, angiosperm-dominated forest in which broad-leaved evergreen species were dominant and conifers were rare. The climate was warm and sub-humid. Following the K-T boundary event, the vegetation changed radically to a low-diversity forest, and
Basalt flows Uerf ano Formation Middle Tertiary intrusives Cuchara Formation Poison Canyon Formation Raton Formation Vermep F ormation
Trinidad Sandstone and Pierre Shale undivided Pierre SNiobr ara undivided Precambrian undivided
Colorado New Mexico
Figure 7.2 Geological map of the Raton Basin showing approximate positions of K-T boundary localities discussed in the text. In all, 13 fully documented boundary localities are known in the basin.
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