Do marine faunas track lithofacies? Faunal dynamics in the Upper Cretaceous Pierre Shale, Western Interior, USA
Introduction
The distributions of marine faunas in both time and space are known to be correlated with changes in environmental conditions, including water depth, substrate, oxygen levels, and temperature (Thorson, 1966; Patzkowsky and Holland, 2012). However, until recently, most paleontological studies have linked marine faunal distributions to lithofacies control based on analyses of faunal and biofacies change in conjunction with varying lithologies (e.g., Kauffman, 1967; Fürsich, 1976; Patzkowsky, 1995). This has resulted in the assumption that an absence of lithofacies change in a stratigraphic section should be reflected in a lack of environmental influence on faunal distributions. This inference has led many studies of mass extinctions and evolutionary patterns (e.g., Hansen et al., 1993; Elder, 1987; Sheldon, 1996; Jin et al., 2000) to prefer to utilize stratigraphic sections with limited lithofacies change under the assumption that these settings will reflect change over time within a single environment rather than those influenced by shifting environments, which could influence faunal patterns and an organisms' phenotype. However, this has been questioned by evidence that marine species are even more highly sensitive to environmental dynamics than that captured in lithofacies change (Brett, 1998; Holland et al., 2001).
Numerous studies linking variation in faunal distribution to lithofacies come from the Upper Cretaceous of the Western Interior. For example, Kauffman (1967) linked faunal distributions in the Upper Cretaceous of Colorado to changing lithofacies patterns in his cyclothem model of sea-level change. Similarly, Waage (1967) documented the relationship between lithofacies and biofacies in a prograding nearshore system represented by the Fox Hills Formation of South Dakota (see also Fürsich and Kauffman, 1984; Kauffman, 2008). However, despite these studies, changes in faunal distributions and biofacies in monotonous lithofacies in the Western Interior have been almost completely ignored (for exceptions see: Sageman, 1989; Elder, 1990, Elder, 1991). This is significant as much of the Upper Cretaceous strata in the Western Interior are represented by extensive monotonous successions of shale, siltstone, and marls that were deposited in offshore settings hundreds of kilometers from the shoreline (e.g., Fig. 1). These offshore successions can extend up to hundreds of meters in thickness and cover thousands of square kilometers. Studies that have examined faunal change in these monotonous lithofacies in the Western Interior have usually attributed them to change over time within a single environment rather than sampling different environments (e.g., Elder, 1987; Ozanne and Harries, 2002).
The primary aim of this study is to examine the sensitivity of marine species to broadly defined environmental change within a monotonous, clay-rich succession. To assess the interplay between lithofacies and faunal sensitivity to environmental change, faunal distributions, as determined by an investigation of fossiliferous concretionary horizons, are examined through four ammonite zones in an offshore setting. These assemblages provide the context from which to investigate the paleoecological, paleobiological, and paleoenvironmental dynamics associated with ocean/climate changes within an epeiric seaway.
Section snippets
Stratigraphic and geographic setting
This study examined the Campanian to Maastrichtian (Upper Cretaceous) Pierre Shale, which is under- and overlain by the Coniacian-Santonian Niobrara and Maastrichtian Fox Hills formations, respectively (Robinson et al., 1964; Waage, 1967; Landman and Waage, 1993). These lithostratigraphic units and their lateral equivalents are well exposed across the Western Interior and vary from non-marine to offshore lithofacies (Gill and Cobban, 1966; Lynds and Slattery, 2017).
This study investigated the
Data
To examine the faunal trends through this monotonous succession, a total of 19 whole or portions of fossiliferous carbonate-cemented concretion (i.e., samples) were collected from separate horizons. Concretions were bulk sampled from two concretionary horizons from the upper 40 m of the lower shale member, from two concretionary horizons within the 10 m Kara Bentonitic Member, and from 15 concretionary horizons spanning the 210 m upper shale member (Fig. 5). To standardize sample volumes for
Occurrence of concretions and fossils
Fossils from the study interval are primarily concentrated in carbonate-cemented concretions, which occur as laterally persistent horizons, surrounded by poorly lithified shale or silty shale (Fig. 5, Fig. 6; also see Gill and Cobban, 1966). These surrounding sediments are typically unfossiliferous (or barren) to poorly fossiliferous. Poorly fossiliferous and barren concretion beds are also found throughout the Pierre Shale, although most of these horizons occasionally preserve mature Baculites
Taphonomy and fidelity of fossil assemblages
All the fossil assemblages display taphonomic features that relate to their composition, mode(s) of death, exposure time on the sea floor prior to final burial, depositional setting, and post-burial diagenetic processes that ultimately resulted in concretion formation. The taphonomic characteristics of the various samples reveal that those analyzed represent time-averaged, within-habitat assemblages.
Paleobiological implications
Our results suggest that substantial temporal variation in biofacies, diversity, and life habits can arise in response to variations in water-depth-correlated environmental factors with limited to no apparent change in lithofacies. This provides empirical support for the hypothesis that fossil taxa are much more sensitive indicators of environmental change than lithofacies. Several studies have documented the sensitivity of marine faunas to environmental change (e.g., Springer and Bambach, 1985
Conclusions
- 1.
The concretionary faunas at Red Bird represent “within-habitat, time-averaged assemblages” (sensu Kidwell, 1998). The parallel alignment of the shells of Baculites and the inclusion of abundant, disarticulated epifaunal shells and articulated infaunal shells indicate that the assemblages were partially reworked and concentrated by storms or currents on the sea floor. The random orientation of the specimens in the shell concentrations is likely the product of both the current winnowing and the
Acknowledgments
We would like to thank the various Niobrara County ranchers and ranches that made this project possible. We especially want to acknowledge and thank the Williams, the Four Three Ranch, the Mengs, as well as one anonymous land owner for granting permission to carry out field work on their land for this project. We would also like to thank I. Montanez, S. Holland, C. Myers, T. Algeo, N. Landman, N. Larson, G. Herbert, and two anonymous reviewers for their helpful comments and suggestions on
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