Stages, the shortest formal chronostratigraphic units, are defined by paleontological content delimited at a stratotype. Stages are typically a few to
Figure 2.1 Subdivisions of the geologic time scale near the K-T boundary at the magnetic polarity subchron level. Normal polarity subchron intervals are shown in black and reversed polarity subchron intervals are shown in white. The K-T boundary is within C29r, the reversed part of Chron (C) 29. Dates in Ma. Data from Gradstein et al. (2004).
several million years in duration. The Maastrichtian Stage (the final stage of the Cretaceous, Figures 1.1 and 2.1), begins at 70.6 ± 0.6 Ma, based on correlation of its stratotype in southwestern France to the marine strontium isotope curve, and ends at the K-T boundary (Gradstein et al. 2004). All or part of six geomagnetic polarity subchrons occur in the Maastrichtian (part of C32n, C31r, C31n, C30r, C30n, and part of C29r). Because C30r is brief (~10 thousand years), the 5.1 Ma of the Maastrichtian is essentially represented by a normal polarity interval bounded by two reversed polarity intervals (Figure 2.1). The Danian Stage (the first stage of the Paleocene, Figures 1.1 and 2.1) begins at 65.5 ± 0.3 Ma and ends at 61.7 ± 0.2 Ma (Gradstein et al. 2004). The Danian, when first defined by Desor (1847), was considered to be the last stage of the Cretaceous. This change has confused workers and the resulting terminology over the last century. Parts of six geomagnetic polarity subchrons occur in the Danian (part of C29r, C29n, C28r, C28n, C27r, and C27n).
Both the Maastrichtian and Danian are defined by their content of marine fossils and thus are terms that are best applied to marine rocks. In addition, the base of the Maastrichtian has been subject to many revisions and has recently ranged from 73 Ma (Harland et al. 1982) to 74 Ma (Harland et al. 1989) to its present definition of 70.6 Ma (Gradstein et al. 2004).
It has been, and continues to be, challenging to refer terrestrial strata to marine stages, and local terrestrial stages have been proposed to address this issue. In North America, a system of North American Land Mammal Ages was defined by the Wood Commission in 1941 to correlate the extensive, mammal-bearing, terrestrial sequences of the Cenozoic. Correlation of terrestrial stages with the time scale has improved with the use of intercalated marine and nonmarine sections and the rise of independent dating techniques such as magnetostratigraphy and geochronology. In the Western Interior of North America, stage-level resolution of terrestrial K-T strata was achieved by constraining the terrestrial K-T boundary interval between Maastrichtian marine mollusks in the Fox Hills Formation and Danian foraminifera in the Cannonball Formation (Fox and Ross 1942). If only stage-level resolution is in place, the precision of statements concerning K-T boundary events is limited. Ultimately, stage-level resolution rests upon the recognition of age-diagnostic fossils and in terrestrial sections these are usually represented by vertebrates, plants, and freshwater mollusks. There are still many terrestrial sections of Late Cretaceous and Paleocene age where stage-level correlation to the marine record has not been achieved and the resolution of K-T boundary discussions carries error bars of several million years.
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