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Recent studies of contemporary basins and ancient strata have provided a basis for expanding and refining early oxygen-related marine biofacies models. New observations permit higher resolution in palaeoecological, palaeoceanographic, and basin analyses. Five oxygen-related biofacies are currently recognized. (1) Anaerobic biofacies: well-laminated strata lacking in situ macro- and microbenthic body fossils and microbioturbation; may contain well-preserved remains of nektonic vertebrates, epiplanktonic or otherwise transported invertebrates, and faecal material of planktonic and/or nektonic origin. (2) Quasi-anaerobic biofacies: laminated strata, subtly disrupted by microbioturbation, containing microbenthic body fossils but lacking in situ macrobenthic body fossils; allochthonous body fossils and recognizable planktonic faecal material may be common. (3) Exaerobic biofacies: laminated strata similar to that of anaerobic or quasianaerobic biofacies but containing in situ epibenthic macroinvertebrate body fossils. (4) Dysaerobic biofacies: bioturbated strata characterized by low-diversity assemblages of relatively small, poorly calcified macrobenthic body fossils or absence of body fossils altogether. (5) Aerobic biofacies: bioturbated strata (where physical processes do not dominate) containing diverse assemblages of relatively large, heavily calcified macrobenthic body fossils. Recognition of these biofacies facilitates the recognition of relative palaeo-oxygenation gradients along the seafloor, vertically across the sediment-water interface, and through time. Application of sensitive trace-fossil models permits even more detailed reconstructions of benthic oxygenation histories for bioturbated strata, particularly those that fall within the previously defined dysaerobic realm.

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Spectral analyses of complementary lithologic and paleoecologic data sets from the Cenomanian-Turonian Bridge Creek Limestone are used to test for Milankovitch periodicities. Because the analyses quantitatively assess variance in the data sets through the study interval, they also offer a new method for evaluating relationships between different components of the depositional system. The analysis was made possible by highresolution sampling for geochemistry, ichnology, and microfossils from a complete section of the Bridge Creek Limestone Member in the USGS #1 Portland core, coupled with a detailed chronostratigraphic framework established for the interval in a recently published compilation ofnew radiometric dates and biozones. The analyzed data included weight percent carbonate (% CaCO3) and organic carbon (% OC), grayscale pixel values, ichnologic measures such as maximum burrow diameter and ichnocoenosis rank, and relative abundance values for selected nannofossil taxa. The lithologic parameters produced significant spectral responses for all three major orbital cycles (eccentricity, obliquity, and precession) in the upper 6 meters of the study interval. Spectra for ichnologic data are similar to those for % OC, possibly because both of these variables are dominantly controlled by benthic redox conditions. Spectra for some nannofossil taxa show results similar to % OC or % CaCO3 but are less definitive due to preservational effects. A new model to explain the Bridge Creek cycles is developed based on the spectral results. The model combines dilution and productivity mechanisms by suggesting that obliquity predominantly forces dilution through its effect on high-latitude precipitation, whereas precession predominantly forces carbonate productivity through its effect on evaporation and nutrient upvvelling in the Tethyan realm to the south. The two influences mix in the shallow Western Interior epicontinental basin, where they result in constructive and destructive interference (because of the different frequencies of obliquity and precession) to produce the complex bedding pattern observed in the Bridge Creek Limestone.Variations in % CaCO3 and % OC of pelagic and hemipelagic strata commonly are interpreted to reflect the interplay between clastic dilution, primary production, carbonate dissolution, and bottom-water oxygen content, factors that may be influenced or controlled by climate cycles (Arthur et al., 1984). These data were successfully employed in spectral estimates of Stratigraphy and Paleoenvironments of the Cretaceous Western Interior Seaway, USA, SEPM Concepts in Sedimentology and Paleontology No. 6 Copyright 1998 SEPM (Society for Sedimentary Geology), ISBN 1-56576-044-1, p. 153-171. 153Milankovitch forcing in Italian limestone/marlstone cycles (Park and Herbert, 1987). Variations in trace-fossil content provide an independent assessment of changes in benthic redox and substrate conditions (Savrda and Bottjer, 1991), and were successfully employed recently in spectral analyses of Cretaceous lhnest...

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