Introduction

The Upper Cambrian of North America is punctuated by three sharply defined mass extinction horizons which appear to represent isochronous surfaces (Palmer, 1984). They separate iterative evolutionary sequences in the history of non-agnostid trilobites and are known as biomeres (which are essentially stages; see Palmer, 1984; and Westrop and Ludvigsen, 1987, for discussion). At least two of the trilobite mass extinctions can be recognized in exposures in Yellowstone National Park. Current hypotheses put forth to explain the extinctions include marine cooling, anoxia (Palmer, 1984), marine regression (Lochman-Balk, 1971), and biofacies shifts during transgression (Westrop and Ludvigsen, 1987). Previous hypotheses that lack supporting evidence include extra
terrestrial heating and extraterrestrial impact (Palmer, 1984) and thus some combination of earth-bound causes seems most plausible.

Significant changes in the pattern of sedimentation across biomere boundaries provide important clues that may be used to falsify hypotheses of the extinctions. This is because facies changes observed in vertical stratigraphic sections must reflect local or regional changes in climate, sea level, subsidence and sediment supply. The focus of this paper is the nature of sedimentation patterns across the extinction events that mark the boundaries of the Pterocephaliid biomere in Yellowstone and immediately surrounding areas. Saltzman et al. (1995) revealed significant facies changes across the Pterocephaliid-Ptychaspid biomere boundary in northwestern Wyoming. Deiss (1936), Grant (1965), and Ruppel (1972) have also studied these deposits at various levels of resolution. The primary objectives of this contribution are to: (1) develop a paleogeographic framework for strata deposited in Yellowstone National Park; and (2) integrate newly acquired biostratigraphic and chemostratigraphic data to better constrain the timing of significant stratal surfaces.

Figure 1—Upper Cambrian locality map in the northwestern Wyoming area. Localities mentioned in the text include: CF = Clark Fork; SL = Swamp Lake; FX = Fox Creek; YO = Wyoming Creek; BP = Buffalo Plateau; MC = Mill Creek; TR = Three Rivers Peak. Inset showing generalized Late Cambrian facies map after Saltzman et al.(1995).

Geologic Framework

The Sauk transgression reached northwest Wyoming by Middle Cambrian time in response to a combination of eustatic sea-level rise and flexural bending of the cratonal edge. This initial transgression was marked by deposition of coarse clastic sediments of the Flathead Sandstone over Precambrian basement rocks. Subsequent deposition is characterized by large-scale alternations of fine-grained siliciclastic and carbonate strata. In Yellowstone National Park, the base of the Pterocephaliid biomere is within the uppermost beds of the massive cliff-forming Pilgrim Limestone (Deiss, 1936; Grant, 1965; Ruppel, 1972). This unit is abruptly overlain by the recessive Dry Creek Shale which grades into the carbonates of the Snowy Range (Open Door) Formation. The Pterocephaliid-Ptychaspid biomere boundary, corresponding to the transition between the Elvinia and Taenicephalus Zones, occurs within the Snowy Range (Open Door) Formation. Upper Cambrian strata are unconformably overlain by Middle Ordovician strata in Yellowstone.

Lithofacies Associations

Two sections of mixed carbonate-siliciclastic strata were logged for this study in Yellowstone National Park (Figure 1) at Three Rivers Peak and on the Buffalo Plateau. The sections represent two distinct mappable units: (1) a succession of thrombolite boundstone, shale and flat-pebble conglomerate on the Buffalo Plateau which is similar to the Snowy Range Formation recognized by Grant (1965) in the Cooke City region; and (2) a succession of calcarenite, shale and lime mudstone at Three Rivers Peak which is similar to the Open Door Formation studied by Shaw and Deland (1955) further to the south in the Gros Ventre, Teton and Wind River Ranges. The Snowy Range Formation is generally poorly exposed on the flanks of the Beartooth uplift, where it overlies the cliff-forming Pilgrim Limestone of Crepicephalus and early Aphelaspis Zone age. In marked contrast, deposits of the Open Door Formation are spectacularly exposed in the Gallatin Range. It should be noted however, that the Three Rivers Peak strata, although grouped with the Open Door succession, does differ from the general pattern in that it contains anomalous beds of quartz sandstone at the base, a ~1-meter-thick brecciated olistostrome bed in the middle and abundant chert in the upper wackestone unit (Figure 2).

Sequence Stratigraphy

The change from quartz sandstone to a cherty wackestone at Three Rivers Peak is interpreted to reflect a relative rise in sea level. This is consistent with the pattern observed during this time period elsewhere (Osleger and Read, 1993; Saltzman et al., 1995). The section is however, unique in the presence of a brecciated olistostrome bed between the quartz sandstone and cherty wackestone. This bed may have formed as a result of a short-term, rapid sea-level fall that exposed the carbonate platform or, alternatively, may have formed in response to a seismic event that fractured the platform. The evidence for the drowning of the carbonate platform at the Buffalo Plateau locality is consistent with a tectonic event associated with a eustatic rise in sea levels rather than exposure of the carbonate platform in northwestern Wyoming. Nonetheless, the paleogeographic and bathymetric significance of the Three Rivers Peak breccia remains unclear at this time and future field and petrographic work is planned.

Paleontology

Figure 2—Measured section of Upper Cambrian rocks from Three Rivers Peak section in Yellowstone National Park (TR in Fig. 1). Stages and trilobite zones indicated. d13C data measured in per mil relative to PDB scale.

At Three Rivers Peak, trilobites assigned as cf. Pterocephalia sp. occur at the top of the brecciated olistostrome bed. This species marks the presence of the Elvinia Zone. Grant (1965) found specimens of Linnarssonella girtyi and Dellea suada which mark the presence of the Elvinia Zone several kilometers away at Crowfoot Ridge. Two meters above the Pterocephalia sp. horizon at Three Rivers Peak, trilobites assigned to Taenicephalus shumardi mark the Taenicephalus Zone. Grant (1965) collected specimens of Taenicephalus shumardi one meter above Elvinia Zone trilobites at Crowfoot Ridge. No trilobtes have yet been found on the Buffalo Plateau, although trilobite taxa found at similar sections in the nearby Cooke City area (Fox Creek, Swamp Lake, Wyoming Creek, Mill Creek and Clark Fork) indicate the presence of the Elvinia and Taenicephalus Zones.

Carbon-Isotope Stratigraphy

Changes in the d¹³C of limestones across the Pterocephaliid-Ptychaspid biomere boundary potentially provide a means of correlation, independent of biostratigraphy. Carbonate samples from Three Rivers Peak were analyzed for stable-isotope ratios. Homogeneous micrite identified in thin section was microsampled from polished slabs by using a microscope-mounted drill assembly. Care was taken to sample micrite with no visible cements or skeletal grains, although ~15% of the samples contain sparry calcite or skeletal material. Sample preparation procedures and analytical error are discussed further in Saltzman et al. (1995).

The d¹³C stratigraphic profile for sections in the Gros Ventre and Wind River Ranges were presented in Saltzman et al. (1995), along with profiles from two sections in the Great Basin. These profiles reveal a positive shift in d¹³C across the Pterocephaliid-Ptychaspid biomere boundary. The d¹³C stratigraphic profiles for the Three Rivers Peak section is presented in Figure 2. The highest d¹³C ratios recorded at Three Rivers Peak are in the Taenicephalus Zone, consistent with earlier studies of this trilobite zone. In addition, the most negative values occur at the biomere boundary (Irvingella major zone). These results are consistent with the biostratigraphic controls at Three Rivers Peak and suggest that the extinction event was associated with changes in carbon cycling (Saltzman et al., 1995). In particular, it seems that the burial ratio of organic carbon to carbonate carbon was increased following the extinction. This may have occurred as a result of increased nutrient fluxes to the surface oceans during sea level rise and subsequent increases in primary production. The significance of the minima at or near the extinction event is unclear. Either there was a sudden decrease in primary productivity related to initial toxicity of upwelling waters, or a sea-level change led to increased erosion of isotopically light organic carbon compared to its preservation before and after the event.

Conclusions

This study provides additional chemostratigraphic and sequence stratigraphic analyses from a section in Yellowstone National Park, consistent with the notion of a significant paleoceanographic event across the Pterocephaliid-Ptychaspid biomere boundary. Future studies in this area focused on better constraining the paleontologic, sedimentologic and chemostratigraphic contexts of this extinction event will be useful in testing hypotheses of extinction. In particular, investigation should focus on the nature of the unique brecciated beds at Three Rivers Peak.

Acknowledgments

Field study was supported in part by research grants from Sigma Xi, the American Association of Petroleum Geologists, the Geological Society of America, the Institute for Cambrian Studies, the J. David Love Field Geology Foundation of the Wyoming Geological Association, and the Ken Watson Memorial Fund of UCLA. Kyger C. Lohmann provided very
generous use of his Stable Isotope Laboratory at the University of Michigan, Ann Arbor. Thanks also to A.R. Palmer for help in critical fossil identifications and Bob Lindstrom for help with obtaining permission to collect rock samples and for help in the field.

References

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