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possibly warmer climate phases, more normal marine surface conditions, and a dominance of biogenic, hemipelagic sedimentation with great reduction in clay delivery to the basin. The Milankovitch climate cycles represent a predictable aspect of Cretaceous global change, but are especially enhanced in lithological, geochemical, and biological expression during the middle Cretaceous (Barron et al., 1985; Kauffman, 1988a).

Both planktic and benthic biotas associated with the Cenomanian-Turonian boundary interval at Pueblo represent warm, normal to near-normal marine environments (excepting intervals of benthic oxygen restriction). These biotas paleobiogeographically belong to the warm-temperate to subtropical Southern Interior Subprovince and its northern biogeographic ecotone with the Central Interior Subprovince—the principal endemic center for molluscan evolution in the Western Interior Basin (Kauffman, 1984b).

Against this dynamic environmental backdrop of global sea-level fluctuations, frequent changes in stratification, temperature and water chemistry, active regional tectonism, basin subsidence and volcanism, and more predictable changes in climate related to Milankovitch cyclicity, the global Cenomanian biota became progressively more stressed and underwent accelerated extinction as a series of ecologically graded (tropical to temperate) steps or events during the last million years of the Cenomanian (Kauffman, 1984a, 1988b; Elder, 1985, 1987a,b; Johnson and Kauffman, 1990; Figures 3.4 and 3.6 herein). The final 520,000 yr of this extinction interval are especially well preserved in temperate facies of the Western Interior Cretaceous Basin. Each extinction event was tied to a major short-term shift in ocean temperature and/or chemistry, indicated by the geochemical profiles at Pueblo (Figures 3.3 and 3.6). Each event represents a breached biological threshold for a particular suite of ecosystems, starting with tropical Caribbean Province reef-associated communities (Johnson and Kauffman, 1990). The high-resolution data base on global environmental change, as exemplified by the Pueblo, Colorado section, provides important insights into the rates, patterns, causes, and consequences of the global C-T biotic crisis at scales comparable to those used for long-term sensing of Quaternary global change. This, in turn, allows critical comparisons between Quaternary icehouse and Cretaceous greenhouse worlds.

ESTABLISHING A CHRONOLOGY FOR ENVIRONMENTAL DECLINE AND MASS EXTINCTION ACROSS THE C-T BOUNDARY

To compare the rates, patterns, and magnitude of environmental changes and biological stress leading to global late Cenomanian-early Turonian mass extinction, with the Quaternary record of global change, a highly detailed real-time scale must be established for the C-T interval. Integration of this time scale with physical, chemical, and biological event surfaces/beds, and with short-term cyclostratigraphic units (e.g., Milankovitch climate cycle deposits) of regional extent, has resulted in the construction of a high-resolution chronostratigraphic system (Kauffman, 1988a; Kauffman et al., 1991), which allows precise regional correlation and interpretation, individually or collectively, of C-T boundary mass extinction events. In the C-T boundary chronology developed for the Western Interior Basin of North America, biostratigraphic zonation has been resolved to 0.18-0.33 m.y. per biozone; Milankovitch cyclostratigraphy to 41,000 yr per cycle, and event chronostratigraphic resolution to 17,000 yr per event-bounded interval. There are few systems of chronology and correlation that match this level of resolution, which has similarly been developed in western Europe and northern South America.

Figures 3.3 and 3.4 show the physical, chemical, biological, and cyclostratigraphic details of the late Cenomanian-early Turonian section at the Pueblo, Colorado standard reference section and proposed boundary stratotype (Kennedy and Cobban, 1991). The Bridge Creek Limestone spans the C-T boundary (Figure 3.5) and contains 24 bentonite beds and 13 additional persistent limonite beds, probably representing altered bentonites. These bentonites record the late phases of one of the most active intervals of volcanism in Cretaceous history (Kauffman, 1985, Figure 4). Thirteen of these bentonites (PBC 4, 5, 11, 17, 19, 20, 30, 32, 44-46, 48, and 50, in Kauffman et al., 1985, p. FRS-11) are much thicker and more highly persistent than the others, and form regional chronostratigraphic marker beds (Hattin, 1971, 1985; Elder, 1985; Kauffman, 1988a). These have yielded materials for radiometric dating. Utilizing single sanidine crystal 40Ar-39Ar dating techniques, Obradovich (1993) has established high-resolution geochronologic tie points to ammonite biozones of Cobban (1985, 1993; Figure 3.4 herein) that allow a calculated time scale to be constructed for the entire interval, if equal time durations are assumed for all intervening ammonite biozones between dated Cenomanian-Turonian bentonite levels (Figure 3.5, second column). This commonly used calibration technique assumes constant evolutionary rates within ammonite lineages, however, which is improbable. Obradovich (1993) has dated the lowest ammonite zone in the Bridge Creek Limestone Member (Vascoceras diartanum), bed 63, as having a mean value of 93.9 Ma, from a bentonite that does not occur in the condensed section at Pueblo (Figure 3.5). A second age of 93.56 Ma has been obtained from the latest Cenomanian Neocardioceras juddi zone in a bed equivalent to the ''Neocardioceras bentonite" of Elder and Kirkland (1985) and Elder (1985, 1987a), unit 80, PBC-11 of the Pueblo



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