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A CASE HISTORY: THE PUEBLO, COLORADO, C-T BOUNDARY SECTION

In order to demonstrate the application of high-resolution stratigraphic methodology to the study of Cretaceous global change, and its relationship to a major biodiversity crisis, data are presented (Figures 3.3 to 3.5) and interpreted (Figure 3.6) for the late Cenomanian-early Turonian mass extinction interval as preserved along the north cut face of the Arkansas River where it breaches the Rock Canyon Anticline just west of Pueblo, Colorado. This locality is the standard reference section for the Greenhorn cyclothem (Scott, 1964, 1969; Cobban and Scott, 1972; papers in Pratt, 1985) and is the proposed Cenomanian-Turonian boundary stratotype section (Kennedy and Cobban, 1991). It has further been a focus for paleobiological studies of the C-T mass extinction in North America (Koch, 1977, 1980; Kauffman 1984a, 1988b; Eicher and Diner, 1985, 1989; Elder, 1985, 1987a,b; Leckie, 1985; Watkins, 1985; Watkins et al., 1993), and its evolutionary aftermath (Harries and Kauffman, 1990; Harries, 1993). The most detailed geochemical framework for the C-T boundary interval has also been developed at the Pueblo section, including stable isotope and Corg analyses (Arthur et al., 1985; Pratt, 1985), and trace element analyses (Zelt, 1985; Orth et al., 1988, 1990, 1993), summarized in Figures 3.3 and 3.4. Data from the Pueblo section have played a major role in the development of the highly refined Cretaceous biostratigraphy for the basin (Cobban and Scott, 1972; Kauffman et al., 1976, 1985, 1993; Cobban, 1985, 1993; Eicher and Diner, 1985; Leckie, 1985; Watkins, 1985; Caldwell et al., 1993; Watkins et al., 1993; and references therein). Pueblo is also a key section for the development of a regional system of high-resolution event chronostratigraphy (Elder and Kirkland, 1985; Kauffman, 1988a; Kauffman et al., 1991) and cyclostratigraphy (Hattin, 1971, 1985; Barron et al., 1985; Elder, 1985; Fischer et al., 1985; Kauffman, 1988a) for the C-T boundary interval in the Western Interior Basin of North America. Data resolution from this Cretaceous section compares favorably with Quaternary data for the documentation and interpretation of ancient global change as it relates to a major biodiversity crisis.

Geologically, the Pueblo C-T boundary section lies in the eastern portion of the Axial Basin (Cretaceous tectonostratigraphic zones of Kauffman, 1984b, 1988a) within the Western Interior foreland basin (Figure 3.2). The C-T boundary interval was coeval with active volcanism, thrusting and subsidence in the western part of the foreland basin (Kauffman, 1984b, 1985; Kauffman and Caldwell, 1993), broadly reflecting active middle Cretaceous plate rearrangement, late phases of development of the Pacific superplume (Larson, 1991a,b), active migration of the Caribbean Plate into the proto-Caribbean Basin (Pindell and Barrett, 1990), and numerous large-scale eruptive volcanic centers in both northwestern (Idaho, Montana) and southwestern (Arizona, New Mexico, Texas) localities. These explosive volcanic episodes produced thick, regionally distributed, marine ash or bentonite beds (Kauffman, 1985, Fig. 4). None of these volcanic ashes are associated with iridium enrichment or extinction events (Orth et al., 1987, 1990, 1993).

At Pueblo, the C-T boundary section spans the uppermost few meters of the Hartland Shale Member, and the entire Bridge Creek Limestone Member of the Greenhorn Formation (Figure 3.3) (Scott, 1964, 1969; Cobban and Scott, 1972; Elder, 1985, 1987; Elder and Kirkland, 1985; Sageman, 1985, 1992; Harries and Kauffman, 1990; Harries, 1993). This marks the latest transgressive systems tract, the maximum flooding interval (eustatic sea-level 300 m above present stand), and the earliest highstand systems tract of the Greenhorn second-order sequence, or cycle (Kauffman, 1984b, 1985; Kauffman and Caldwell, 1993). The interval further shows evidence for three third-order relative sea-level fluctuations (Figure 3.3) with regional expression. Paleobathymetry at the site during sea-level highstand has been estimated between 150-300 m and >500 m (Kauffman, 1984b, 1985; and Eicher, 1969, respectively), depending on the evidence used; foraminifers give a deeper signal than do molluscs and sedimentary features.

The Western Interior epicontinental sea was episodically stratified during C-T time, with the upper portion of an expanded oceanic oxygen minimum zone (OMZ) associated with OAE II forming low-oxygen bottom waters stratified beneath a shallower, warm-temperate to subtropical oxygenated water mass; both water masses immigrated into the Western Interior Seaway from the proto-Gulf of Mexico-Caribbean region (Kauffman, 1984b, 1985; Kauffman and Caldwell, 1993), displacing cooler northern water masses poleward. The low-oxygen benthic water mass (expanded OMZ) was neither stable nor stagnant, as indicated by dynamic changes in continuously represented benthic paleocommunities (Kauffman and Sageman, 1990; Sageman et al., 1991, 1994), as well as the frequency of short-term, large-scale geochemical fluctuations (Arthur et al., 1985; Pratt, 1985; Zelt, 1985; Orth et al., 1988, 1990, 1993). Well-defined 41,000- and 100,000-yr Milankovitch climate cycles are represented by shale-limestone, shale-calcarenite, and marl-chalk bedding rhythms throughout the study interval (Arthur et al., 1985; Barron et al., 1985; Fischer et al., 1985; Pratt, 1985; Kauffman, 1988a; Figure 3.5 herein). The shales and marls represent wetter and possibly somewhat cooler climatic phases with high freshwater runoff and clay delivery to the interior basin; the limestones and calcarenites represent drier and



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