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reference section (Kauffman et al., 1985, p. FRS-11). A third mean age of 93.25 Ma was obtained from the early Turonian Pseudaspidoceras flexuosum zone in a bed equivalent to bentonite marker bed 96 (PBC 20; Kauffman et al., 1985, p. FRS-11). The top of the Bridge Creek Limestone Member (lowest part of the Collignoniceras woollgari biozone) has been given an interpolated age of 92.05 Ma based on assigning equal age ranges to ammonite biozones that lie between well-dated levels below and above this biozone (see Figure 3.5).

Based on this set of new 40Ar-39Ar ages, and the calculated time scale that can be constructed between them, if equal age ranges are assumed for intervening ammonite biozones, a duration of 1.95 m.y. is calculated for the Bridge Creek Limestone Member at Pueblo. Here, the member spans the entire set of C-T mass extinction steps (Kauffman, 1988b; Figures 3.4 and 3.6 herein), the complete survival interval, and the recovery interval (sensu Harries and Kauffman, 1990) up to basic recovery of the marine ecosystem in the early to early-middle Turonian. The average durations for ammonite subzones and zones, based on the exclusive ranges of biostratigraphically tested ammonite species, equals 190,000 yr for the C-T interval. This is calculated by dividing the number of ammonite biozones into the time duration between the mean values of bounding radiometric ages.

Because of the dangers in the assumption that ammonite species evolve at relatively constant rates, even through this thin interval, an independent, more highly refined dating system was constructed to test this time scale by using shale (or marl)-limestone (or chalk, calcarenite) bedding rhythms reflecting Milankovitch climate cycles. The scale of this independent chronology is adequate to evaluate the relative importance and interaction of individual physical, chemical, and biological events associated with the C-T mass extinction, survival, and recovery intervals.

At Pueblo, various authors have measured closely spaced sections of the Bridge Creek Limestone Member in great detail, and have reported between 41 and 50 shale (or marl)-limestone (or chalk, calcarenite) bedding couplets. Most sections yield between 44 and 48 bedding couplets that probably reflect Milankovitch cyclicity (Barron et al., 1985; Fischer et al., 1985; Kauffman, 1988a) in the form of climate-regulated dilution cycles (Pratt, 1985) and/or productivity cycles (Eicher and Diner, 1985, 1989).

At the proposed C-T boundary stratotype at Pueblo, Elder and Kirkland (1985) and subsequent work by the author have confirmed 46 Milankovitch-type bedding cycles of all scales in the Bridge Creek Limestone Member, 19 of which are capped by relatively thicker, more resistant limestone or calcarenite beds with extensive basinal dispersion (Figure 3.5; Hattin, 1971, 1985; Elder and Kirkland, 1985; Kauffman, 1988a). These thicker and more pervasive limestone units probably reflect the 100,000-yr Milankovitch orbital eccentricity cycle (Barron et al., 1985; Fischer et al., 1985; Kauffman et al., 1987; Kauffman, 1988a) (Figure 3.5). There are an average of 2.5 smaller bedding cycles between these more prominent limestone beds, suggesting that they may represent the 41,000-yr Milankovitch axial eccentricity cycle (Figure 3.5). These interpretations are subsequently tested and confirmed.

Given the 1.95-m.y. calculated duration for the Bridge Creek Limestone, C-T boundary section at Pueblo, the 19 thicker, more regionally persistent limestone beds have an average duration of 100,000 yr, and logically represent the Milankovitch orbital eccentricity cycle (Figure 3.5). Further, the average duration for the 46 bedding cycles of all scales is 41,300 yr; these are considered representative of the 41,000-yr Milankovitch axial eccentricity cycles (Figure 3.5). This is the optimal level of age resolution for interpreting regional environmental change and the patterns of C-T mass extinction.

These Milankovitch cycle determinations provide an independent means of testing, and recalibrating, the interpolated time scale, based on assigning equal durations to ammonite biozones between radiometrically dated levels. Two methods of calibrating a new C-T boundary time scale, based on integrating radiometric ages with 100,000-yr Milankovitch climate cycles, are presented in Figure 3.5, columns A and B. The first, and most commonly practiced method, starts at a calibrated or radiometric age at the top (or base) of the long Milankovitch cycle sequence, and assigns progressively greater (or lesser) 100,000-yr intervals to the top of each bedding rhythm representing the orbital eccentricity cycle (e.g., Figure 3.5, column B). When this method is applied to the Bridge Creek Limestone Member at Pueblo, the calculated values for the 100,000-yr Milankovitch bedding rhythms deviate from the mean values of 40Ar-39Ar ages within the member by 100,000 to 160,000 yr, and vary from the time scale calculated on the assignment of equal durations to successive ammonite biozones by 150,000 to 360,000 yr.

A second, and preferred, calculated time scale was constructed by utilizing a mean 40Ar-39Ar age from the middle of the Bridge Creek Limestone Member as a starting point (93.25 Ma for the bentonite in the middle of the early Turonian Pseudaspidocerasflexuosum biozone; Figure 3.5, column A), and assigning progressively younger 100,000-yr durations for orbital eccentricity cycles up-section, and progressively older 100,000-yr durations for the same cycles down-section. In this method, the ages calculated from the Milankovitch cycle scale deviate by only 10,000 to 50,000 yr from other 40Ar-39Ar ages within the sequence, and only 10,000 to 50,000 yr from ages calculated by assigning equal range durations to late Cenomanian ammonite biozones, which suggests a relatively constant evo



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