Cretaceous-Tertiary Correlations

The Cretaceous-Tertiary Boundary

Correlation of the Pu0 and Pu1 interval zones with the K-T boundary must be preceded by a means to recognize Cretaceous and Tertiary sediments within both marine and nonmarine sections that span the transition. The K-T boundary is usually placed between the Maastrichtian and Danian stages (but see Voight, 1981). However, the upper limit of the type section of the Maastrichtian Stage (ENCI Quarry, St. Pietersburg, South Holland) is an erosional surface (Felder et al., 1980), while the lower limit of the Danian Stage (Stevns Klint and Faxse, Denmark) disconformably overlies chalks correlated with the Maastrichtian (Berggren, 1964). Subsequently, several sections in Europe were recognized as lacking a major hiatus between the Maastrichtian and Danian stages or as being "relatively complete" (see Dingus, 1983: part 1, for discussion). Recently, the K-T section at El Kef, Tunisia, was proposed as the K-T boundary stratotype by the International Geological Congress in 1989. The marine K-T boundary at El Kef is defined by the first appearances of planktonic forminifera and calcareous nannoplankton (Keller, 1989; with recent modification of FADs and LADs of planktic foraminifera by MacLeod and Keller, 1991a, 1991b).

Biostratigraphic correlation between nonmarine and marine zonations is difficult because in few instances are fossiliferous marine and nonmarine interdigitations preserved and well exposed. This is the case with latest Cretaceous-earliest Tertiary marine (Fox Hills and Cannonball formations) and nonmarine (Hell Creek and Tullock-Ludlow formations) stratigraphic units of the western interior in the northern United States (Figure 2). The Hell Creek Formation is underlain by the Fox Hills Formation, whose mollusk zonation (Cobban, 1958) has been correlated with the European mollusk zonation of the Maastrichtian Stage (Jeletsky, 1960, 1962). The Tullock Formation is laterally equivalent to part of the Ludlow Formation, which overlies the Hell Creek Formation in the Dakotas (Carlson and Anderson, 1966; Moore, 1976; Belt et al., 1984). The Ludlow Formation interfingers with the Cannonball Formation (Jeletsky, 1962), which contains Danian planktonic foraminifera (Fox and Olsson, 1969). Therefore, the marine K-T boundary would fall somewhere within the upper Hell Creek or lower Tullock formations in eastern Montana.

A more precise biostratigraphic determination of the marine K-T boundary in this nonmarine sequence is possible using paleomagnetic and radiometric correlation methods. The marine K-T boundary at El Kef occurs within paleomagnetic anomaly C29R (Keller, 1989). The uppermost Hell Creek and lower Tullock formations in Garfield and McCone counties were deposited in a period of reversed polarity that is correlated with anomaly C29R (Archibald et al., 1982). In eastern Montana, this stratigraphic interval includes the last records of dinosaurs and most genera of Lancian mammals and the first occurrence of Pu0 and Pu1 mammal assemblages (Archibald, 1982; Archibald and Lofgren, 1990). Chron 29R is usually considered to span approximately 500,000 years (Harland et al., 1982), which makes marine-nonmarine correlation still relatively imprecise. Radiometric dating techniques provide little additional precision because when applied to latest Cretaceous samples standard errors can exceed 500,000 years.

Because of difficulties in identifying the stratigraphic level in nonmarine sequences that temporally corresponds to the marine K-T boundary, a number of operational definitions have been employed to identify the K-T boundary in terrestrial sediments in eastern Montana. These are: (1) the base of the first coal zone (the "Z" coal) above the highest occurrence of dinosaur remains (Brown, 1952); (2) the stratigraphically highest occurrence of dinosaur remains (Archibald, 1982); (3) palynofloral extinction or disappearance (Tschudy, 1970); and (4) iridium enrichment (Smit and Van der Kaars, 1984; Smit et al., 1987). These criteria for boundary recognition are reviewed below.

"Z" Coal

This simple definition of using the first coal ("Z" coal) above the highest dinosaur was admittedly crude, but was proposed as a practical solution to end the prolonged debate during the first half of the twentieth century on where to place the K-T boundary in terrestrial sections in the western interior of North America (Brown, 1952). Subsequently, the "Z" coal was employed to approximate the K-T boundary in eastern Montana (Sloan and Van Valen, 1965; Van Valen and Sloan, 1977). However, the use of lithostratigraphic criteria to define a chronostratigraphic datum is both improper usage (Archibald, 1982; Fastovsky, 1987) and unrealistic. In a dynamic fluvial system, such as the one which deposited sediments spanning the K-T transition in eastern Montana, coal-swamp facies are not regionally persistent and coals are demonstrably discontinuous (Sholes and Cole, 1981; Archibald, 1982; Fastovsky, 1987).

This can be demonstrated in the McGuire Creek study area, where the TL and MCZ lignites are mappable for many kilometers but are not regional in extent. For example, the MCZ thins rapidly when traced to the northwest margin of the study area and grades laterally into black organic-rich mudstone (sections S, NN, OO, and HH, Plate 4), and correlations between sections OO and HH become highly questionable. Also, at Section OO (Plate 4), the "Z" coal of the Bug Creek drainage and the MCZ of the Black Spring Coulee drainage are apparently two different units (or part of a series of thin lignites that comprise the upper "Z" coal complex), and the "Z" coal is truncated by channeling. Therefore, both the concept of a regionally extensive "Z" coal bed and the use of the "Z" coal to define the K-T boundary should be discontinued.

Highest Occurrence of Dinosaur Remains

This means of boundary recognition assumes that the passing of the last dinosaur signaled the close of the Cretaceous (Brown, 1952). Whether the timing of dinosaur extinction and the appearance of microfossils used to define the marine K-T boundary are synchronous is unknown. Also, if this means of boundary recognition were to be employed, then all local faunas containing unreworked dinosaur remains would be Cretaceous in age. However, the possibility that dinosaur remains in Pu0 and Pu1 assemblages from channel facies are reworked (and the difficulty in identifying reworked fossils) suggest that the highest occurrence of dinosaur remains may actually significantly postdate dinosaur extinction. Conversely, in any local stratigraphic section composed of fine-grained deposits of floodplain origin, the highest preserved occurrence of dinosaur remains may significantly predate the actual time that dinosaurs succumbed to extinction. Finally, to avoid circular reasoning, it is desirable to determine the timing of dinosaur extinction using independent criteria. Therefore, the use of the highest occurrence of dinosaur remains is neither demonstrably precise nor logically appropriate to employ for K-T correlations.

Palynology

It has long been known that a palynological change could be recognized in local stratigraphic sequences in the western interior of North America that was roughly coincident with R. Brown's formula (1952) for the terrestrial K-T boundary (i.e., first coal above highest dinosaur) (Norton and Hall, 1969; Oltz, 1969; Leffingwell, 1970; Tschudy, 1970). More recently, discovery of an iridium enrichment at the marine K-T boundary in many sections worldwide (Alvarez et al., 1980; Orth et al., 1981, 1982; Alvarez et al., 1982, 1984; many others) has provided a means to globally correlate boundary events if the iridium enrichment represents the product of a single bolide impact (or another brief cataclysmic event) on earth. Recent K-T palynological research indicates that extinction of pollen species and an iridium enrichment are stratigraphically linked in many local sections throughout the western interior, from Saskatchewan and Alberta (Nichols et al., 1986; Jerzykiewicz and Sweet, 1986; Lerbekmo et al., 1987), Montana (Hotton, 1984, 1988; Bohor et al., 1984; Smit and van der Kaars, 1984; Smit et al., 1987), Wyoming (Bohor et al., 1987b), and Colorado-New Mexico (Orth et al., 1981; Tschudy et al., 1984). However, many other sections that are known to span the K-T transition in the western interior have been sampled for iridium but do not yield anomalous concentrations at the stratigraphic level that corresponds to the palynological K-T boundary.

The palynological K-T boundary in eastern Montana is recognized by the disappearance of Cretaceous indicator species, not by new appearances in the earliest Tertiary (Hotton, 1988). Absence of Cretaceous species can be employed to differentiate Cretaceous from Tertiary strata in local stratigraphic sections in eastern Montana (Sloan et al., 1986; Rigby et al., 1987; Smit et al., 1987; Hotton, 1988). Therefore, extinction of pollen species is an effective means to differentiate Cretaceous and Tertiary sediments, and was employed at McGuire Creek to determine the age of channel and floodplain facies yielding vertebrates.

Iridium

Iridium enrichment (or concentrations of iridium above background levels) may provide the ideal criterion for correlating marine and nonmarine K-T boundary sections worldwide (Berry, 1984). High concentrations of iridium were first reported by Alvarez et al. (1980) at the marine K-T boundary (as recognized then) in Italy and Denmark, and later elsewhere worldwide (Alvarez et al., 1982, 1984). Alvarez et al. (1980) proposed that the iridium was too concentrated to have been terrestrially derived and therefore must be the record of a bolide impact. The effects of this impact on latest Cretaceous organisms were interpreted to be the causal factor in terminal Cretaceous extinctions, in both marine and terrestrial realms.

According to an impact scenario, extraterrestrially derived iridium would have been dispersed into the air after impact, and eventually incorporated into sedimentary basins within 100 years or less. Iridium-enriched sediment at K-T boundary sections worldwide would be the record of this virtually instantaneous event in earth history, and would thereby provide a basis for worldwide correlation (Berry, 1984). However, others argued that K-T iridium enrichment was deposited during a period of intense volcanism lasting 10,000 to 100,000 years (Officer and Drake, 1985; Officer et al., 1987; Crocket et al., 1988). Reports of high concentrations of iridium during recent volcanic emissions from Kilauea Volcano, Hawaii (Zoller et al., 1983), support the possibility that K-T iridium enrichment may be terrestrially derived. Also, the concentrating effects of micro-organismal activity may have had a role in the formation of iridium anomalies (Dyer et al., 1989).

Shocked quartz (Bohor et al., 1984; 1987a) and microspherules (Smit and Klaver, 1981; Montanari et al., 1983; Montanari, 1986) are often associated with iridium anomalies and may represent impact-derived products. However, shocked quartz may originate from intense volcanism (Carter et al., 1986; but see Alexopoulos et al., 1988), and microspherules may have a volcanic origin as well (Naslund et al., 1986). Also, microspherules from three marine K-T sites have been interpreted to represent diagenetic infillings of organic spheres, not impact or volcanism products (Hansen et al., 1986).

The source(s) of the iridium, shocked quartz, and microspherules near the K-T boundary no doubt will be debated for years. In this study, iridium enrichment is used as a correlation tool, whatever its source. Iridium enrichment is present at the proposed K-T boundary stratotype at El Kef, Tunisia (Kuslys and Krahenbulh, 1983). It also has been reported from many of the terrestrial stratigraphic sections known to span the K-T transition in the western interior of North America (Orth et al., 1981, 1982; Smit and Van der Kaars, 1984; Nichols et al., 1986; Lerbekmo and St. Louis, 1986; Lerbekmo et al., 1987; Bohor et al., 1987b). Other sections spanning the K-T transition, most notably those in eastern Garfield and western McCone counties in eastern Montana (where Bug Creek local faunas are located), have been sampled for iridium, but have not yielded a high level of iridium enrichment. In any case, the critical premise is that the marine and nonmarine iridium enrichments are records of the same discrete event, which allows precise correlation.

Several challenges to worldwide iridium anomaly correlations have been proposed. For example, one hypothesis suggests that K-T extinctions were caused by multiple impacts, perhaps comet showers (Hut et al., 1987). Also, the marine (worldwide distribution) and terrestrial (North American distribution) K-T iridium anomalies may have been deposited by different mechanisms (Schmitz, 1988). If the marine and nonmarine anomalies were formed by different events, or if they were caused by multiple impacts, temporal correlation becomes suspect. However, iridium enrichment could still be an effective correlation tool for terrestrial K-T sections within North America.

Unfortunately, sections spanning the K-T transition in McCone County have been sampled for iridium enrichment without success (Smit et al., 1987). The nearest K-T iridium enrichment is 50 km to the west, at the Lerbekmo site in Garfield County. With the absence of records of iridium enrichment in McCone County, direct correlation of K-T sections to the marine K-T boundary cannot be accomplished. Pollen extinction then becomes the most precise correlation tool available for determining the local K-T boundary.

It has been implied that an iridium enrichment is present in the lower "Z" coal at Russell Basin, McCone County (Sloan et al., 1986; Rigby, 1989), a few kilometers north of the McGuire Creek study area. However, this iridium concentration is 40-80ppt, or only 2-3 times background (Fastovsky and Dott, 1986), and is similar to normal background levels such as those reported from Saskatchewan (5-60ppt, Nichols et al., 1986), North Dakota (25ppt, Johnson et al., 1989), and the Raton Basin in New Mexico (10-30ppt, Orth et al., 1981). A concentration of 40-80ppt is not anomalous, and reference to an iridium anomaly in McCone County should be discontinued.

Age of the Pu0 and Pu1 Interval Zones

Pollen extinction is employed to differentiate Cretaceous and Tertiary sediment at McGuire Creek in the absence of iridium enrichment. Criteria for palynological differentiation of Cretaceous and Tertiary strata were developed from independently dated sequences of overbank deposits from both Garfield and McCone counties (Hotton, 1988). The data presented in an earlier chapter on palynological correlations show that all Pu0 and Pu1 sites at McGuire Creek yield Tertiary palynofloras (Table 3). Therefore, at McGuire Creek, the Puercan-Lancian and the palynological K-T boundaries coincide at the crude level of precision that is available to us: first appearance of Protungulatum = Pu0 interval zone; first record of loss of Cretaceous indicator palynomorphs = Tertiary.

Because all Pu0 and Pu1 vertebrate assemblages were collected from channel facies, and the palynological K-T boundary criteria were developed in overbank sequences, it was important to test whether a facies bias might exist. The composition of the flora contributing to the pollen rain along channel margins might differ significantly from that in overbank facies. The absence of Cretaceous indicator species in a Cretaceous channel might be due to these species' preference for overbank environmental settings. Therefore, a sample from the channel fill containing Matt's Dino Quarry (Section AA, Plate 3), which yielded associated hadrosaur skeletal remains,

was analyzed for pollen. It yielded a Cretaceous palynoflora. This one sample suggests that palynological samples from channel facies are not biased by facies effects.

Erosion of Cretaceous strata and subsequent reworking of Cretaceous fossils into Paleocene channels occurred at McGuire Creek (Lofgren et al., 1990). A related issue is whether Cretaceous indicator palynomorphs were also reworked into these same channels. Paleocene channels yield a few occurrences of Cretaceous indicator species. These are usually less than 2% of total number counted, while the sample from Matt's Dino Quarry yielded 25% Cretaceous palynomorphs. For comparison, Cretaceous floodplain deposits yield 10-40% Cretaceous palynomorphs (pers. comm., C. Hotton, 1988). Therefore, it appears that if significant amounts of Cretaceous palynomorphs were eroded and redeposited, their numbers were diluted by large quantities of Tertiary pollen rain.

Reworking would explain the original conflict (now resolved) in age interpretation between the USGS palynologists and Carol Hotton concerning the Brown-Grey Channel. Thin siltstones in the Brown-Grey Channel were sampled and portions of a single sample were sent for analysis to two palynologists with experience in the K-T boundary problem. Rock sample 88DLL7-14-30 (original field no. 85H7-26-5, collected by J.H. Hutchison, 1985) was analyzed by palynologists at the USGS and subsequently assigned a Cretaceous age (pers. comm., Nichols and Wingate, March, 1988, USGS PL No. D6906). Sample 88DLL7-14-30 was also analyzed by Dr. Hotton, who concluded that it contains a Paleocene palynoflora (pers. comm., C. Hotton, December 1988).

This apparent conflict in age interpretation of 87DLL7-14-30 (=85H7-26-5) is a reflection of how the palynological boundary is defined. Earliest Paleocene palynofloras are depauperate and are recognized primarily on the absence or rarity of Cretaceous indicator species, not by new appearances in the earliest Paleocene (Hotton, 1988). Both analyses of the same rock sample indicate the presence of Cretaceous indicator species, but they are low in relative abundance (2.5-3.5% of the total sample), indicating a Tertiary age (pers. comm., C. Hotton, 1989). The USGS specialists made their Cretaceous interpretation based on the presence of Cretaceous indicator species. Therefore, the palynological boundary was recognized on different criteria by different palynologists: (1) the presence of Cretaceous indicator species no matter what their relative abundance; and (2) the relative abundance of Cretaceous indicator species.

In an attempt to resolve this disparity in approach, Dr. Nichols analyzed more samples of 88DLL7-14-30 and also 88DLL7-14-13, both from the Brown-Grey Channel. He concludes that because of the rare occurrence of characteristic Cretaceous palynomorphs in the samples, they still appear to be latest Cretaceous in age. However, the samples could be earliest Paleocene containing a few reworked Cretaceous palynomorphs. Identification of a species of pollen in the assemblage that may be restricted to the lowermost Paleocene in Wyoming and the Dakotas suggests this may be correct and earliest Paleocene is probably the most reasonable interpretation (pers. comm., D. Nichols, 1990). Therefore, Hotton and Nichols now agree that it is probable that the Brown-Grey Channel yields a Tertiary palynoflora with a low abundance of

reworked Cretaceous palynomorphs (pers. comm., C. Hotton, 1989, and D. Nichols, 1990). This is similar to what may be occurring with the vertebrate assemblage from the same channel (i.e., reworking of dinosaurs and Lancian mammals).

Elsewhere in McCone County, Montana, where vertebrate sites yielding Pu0 and Pu1 have been sampled for pollen, most Pu0 and Pu1 sites yield Paleocene palynofloras (Sloan et al., 1986; Rigby et al., 1987; Newmann, 1988). However, Cretaceous palynofloras are claimed to be associated with Pu0 assemblages from channel fills at Bug Creek Anthills (Newmann, 1988) and Grenda's Horn/Doc's Folly (Rigby, 1989). If this is true, then the Pu0 interval zone would span the K-T boundary. However, detailed stratigraphic or palynological data were not given in these reports. The position within the channel fill of the Cretaceous pollen or from what lithology the pollen was collected (mud clast, or a siltstone lens, etc.), are not stated, nor are abundances/ numbers of Cretaceous indicator species. Therefore, evaluation of claims of Cretaceous Pu0 assemblages (Newmann, 1988; Rigby, 1989) are not possible from the data given.

Similarly, two reportedly Pu0 assemblages (Frenchman 1 and Long Fall) from channel facies in Saskatchewan which yield dinosaurs and Lancian mammals are claimed to be Cretaceous in age, on the basis of relative stratigraphic position (Frenchman 1) or the presence or association with "unreworked" dinosaur and Lancian mammal remains (Frenchman 1 and Long Fall) (Johnston and Fox, 1984; Fox, 1987, 1989, 1990). However, Paleocene channels may contain reworked Cretaceous fossils (Lofgren et al., 1990). Also, palynological analysis of sediment within either channel fill has not been reported. Therefore, Pu0 assemblages from Frenchman 1 and Long Fall, as well as those from Bug Creek Anthills and Grenda's Horn/Doc's Folly, may be Cretaceous, but this remains to be demonstrated. If these Pu0 assemblages are Cretaceous, the Pu0 interval zone would span the K-T transition, and the Lancian-Puercan and Cretaceous-Tertiary boundaries would be diachronous.