Subchronlevel resolution

Magnetostratigraphy allows for marine-nonmarine correlation at higher resolution than at the stage level (Figure 2.1). Magnetic polarity sub-chrons in the Upper Cretaceous range from very short duration (C30r is estimated at 10 000 years) to very long (the Cretaceous Long Normal C34n lasted more than 40 million years), but the 11 subchrons of the Maastrichtian-Danian average about 800 thousand years (Cande and Kent 1995) and C29r, which spans the K-T boundary, is between 570 and 833 thousand years (Cande and Kent 1995, D'Hondt et al. 1996). Thus, the addition of magnetostratigraphy to terrestrial sections can tighten resolution to within a million years and thus bring correlation to the outcrop scale. In many terrestrial sections, sediment accumulation rate is on the order of tens to hundreds of meters per million years, and individual subchrons can be measured in a single stratigraphic sequence. In both nonmarine and marine sections, magnetostratigraphy relies upon biostratigraphy or geochronology to place normal and reversed paleomag-netic intervals within a chronostratigraphic framework. Examples of applications of magnetostratigraphy to K-T boundary studies in nonmarine rocks include Lerbekmo and Coulter (1984), Lerbekmo (1985), Erben et al. (1995), Hicks et al. (2002,2003), and Barclay et al. (2003). Because the duration of polarity subchron C29r is estimated at 570 to 833 thousand years and the K-T boundary in marine rocks occurs in the upper half of the subchron, magnetostratigraphy can be used to locate a potential K-T boundary to within a half-million years.

Geochronologically, the age of the K-T boundary is estimated at 65.5 ± 0.3 Ma (Gradstein et al. 2004). This estimate is based on several 40Ar/39Ar ages derived from Haitian tektites and sanidine crystals from K-T boundary clay layers in Montana. Obradovich in Hicks et al. (2002) recalculated the K-T age at 65.51 ±0.1 Ma. Given that U-Pb ages are typically 0.5% older than 40Ar/39Ar ages from the same rock, U-Pb estimates of the age of the K-T will likely be around 66 Ma (note that the difference between the two systems is currently much greater than the precision within either system). As currently practiced these geochronologic techniques offer temporal resolution on the same level as or slightly more precise than the geomagnetic time scale. Recent developments suggest that geochronology will soon offer resolution on the order of less than 20 thousand years. If realized, this will offer a level of resolution intermediate between subchron and impactite. Resolution at the 20 000-year-scale is now available from cyclostratigraphy but is largely restricted to marine strata and thus is only indirectly relevant to our study.

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