PALEOBOTANY

Leaf Fossils

Knowlton (1917, 1924) discussed leaf fossils he identified from the Fruitland and Kirtland Formations in the San Juan Basin and also discussed the significance of leaf fossils he identified from the Ojo Alamo Sandstone at several localities. Knowlton stated that Ojo Alamo Sandstone fossils were completely different from those found in underlying beds and even though these fossil leaves did not definitively fix the age of the Ojo Alamo as Tertiary, Knowlton found them to be more Tertiary-like than Cretaceous. As far as is known, Knowlton's work is the only published report on leaf fossils from the Ojo Alamo Sandstone. Reeside (1924) addressed the age of the Ojo Alamo Sandstone and cited the paleobotanical studies of Knowlton (1917, 1924) as tentatively supporting the Paleocene age of this formation.

Palynology

The palynology of the rocks adjacent to the Cretaceous-Tertiary boundary in the San Juan Basin is discussed in detail in Appendix of this report. All palynomorphs identified from these rocks are listed in the Appendix tables and the paleochronologic significance of these fossils is discussed therein in detail. The following section of this report summarizes the palynologic data presented in Appendix.

Palynology has been a valuable and precise biochronologic tool for defining the Cretaceous-Tertiary boundary in continental strata of the Western Interior of North America. For nearly 50 years, index palynomorphs such as Proteacidites spp. and many species of Aquillapollenites, for example, have been the primary Cretaceous index fossils in the Western Interior with the last occurrence of these key taxa unequivocally marking the end of the Cretaceous Period (Anderson 1960, Tschudy 1973, Nichols and Johnson 2002, among others). (Proteacidites was renamed Tschudypollis in Nichols 2002.) The K-T boundary was located within centimeters in the Raton Basin of New Mexico and Colorado by R.H. Tschudy (USGS) on the basis of the last occurrence of Tschudypollis spp. That work enabled Orth et al. (1981) to find the iridium-enriched, end-Cretaceous, asteroid-impact, fall-out layer within that same centimeters-thick interval. Because the fall-out layer is only about 25 mm thick and not conspicuous in most exposures, the use of palynology to narrow the stratigraphic interval of interest has been critical in locating this bed at numerous sites in the Western Interior. The identification of the end-Cretaceous fall-out layer in the Raton Basin (Orth et al. 1981, 1982) was the first discovery of this important geochron in continental rocks anywhere in the world. Subsequently, the fall-out layer has been found at numerous other localities in continental strata in the Western Interior using palynology to zero in on the appropriate rock strata. The discovery of the asteroid-impact fall-out layer has not only established for the first time the presence of a physical rock layer marking the Cretaceous-Tertiary boundary, but equally importantly, has also validated the precise age significance of Late Cretaceous and Paleocene index palynomorphs found below or above it in the Western Interior of North America.

The use of fossil pollen to fine tune ages within Upper Cretaceous or lower Paleocene strata, however, has proved to be useful, but less precise. As the discussion in Appendix indicates, palynologic data in the San Juan Basin suggests that the Maastrichtian Stage is missing throughout most of the basin, however, this Stage lasted 5 m.y. (Gradstein et al. 2004); thus, this finding did not have the precision of the end-Cretaceous-boundary determination. (In actuality, as Figure 35 shows, the Cretaceous-Tertiary hiatus in the southern San Juan Basin spans 7.8 m.y.; a time period that includes about 2.5 m.y. of late Campanian time, all of Maastrichtian time, and about 0.3 m.y. of earliest Paleocene time.) The first appearance of Paleocene index palynomorphs, such as Momipites tenuipolus and Brevicolporites colpella, in the southern part of the Western Interior (Anderson 1960, Tschudy 1973, Nichols and Johnson 2002, Nichols 2003) has also been useful in determining the ages of strata adjacent to the K-T interface in the San Juan Basin. Palynologists agree (Nichols, personal commun., 2005) that rock samples that yield diverse palynomorph assemblages, contain no specimens of Cretaceous index fossils (such as Tschudypollis spp.), and contain Paleocene index palynomorphs (such as B. colpella or M. tenuipolus), are "unquestionably" Paleocene in age.

The palynology of rock strata in the San Juan Basin adjacent to the K-T interface has been addressed in a number of publications beginning with Anderson (1960). In addition, a large amount of palynologic data exists for these same strata in the form of unpublished USGS "Reports on Referred Fossils" in the files of the author. Appendix of this report contains a detailed discussion and synthesis of all of these palynologic data.

Palynology has fixed the Cretaceous-Tertiary (K-T) interface at (or just below) the base of the Ojo Alamo Sandstone at five principal localities in the New Mexico part of the San Juan Basin: 1) the Cuba, New Mexico area; 2) the Gasbuggy core; 3) the Mesa Portales area; 4) the Ojo Alamo Sandstone type area; and 5) the San Juan River site (see Figure 1 for locations). At one locality in the Colorado part of the basin, near Durango (Figure 1), palynology has bracketed the K-T interface at the contact between the Cretaceous Kirtland Formation and the Paleocene Animas Formation.

Cuba, New Mexico Area. Anderson (1960) was the first geologist to use palynology to determine the location of the Cretaceous-Tertiary boundary in the San Juan Basin. He collected rock samples from the uppermost Kirtland Formation and from within the Ojo Alamo Sandstone in the southeast part of the basin near Cuba, N.M. (Figure 1) and concluded that the palynomorph assemblages from the Ojo Alamo Sandstone were all Tertiary in age and the assemblages from the underlying Kirtland Formation were Cretaceous in age. Based on these data, Anderson (1960) placed the K-T boundary at the base of the Ojo Alamo Sandstone. Anderson was aware that the Ojo Alamo contained abundant dinosaur bone in other parts of the basin and concluded that (p. 13) either those dinosaur fossils had been reworked or misidentified or that: "Alternatively, pre-Lance-type dinosaurs persisted into a Tertiary environment."

Gasbuggy Core. Core chips from 52 levels were collected from the Gasbuggy core (Figure 1) in 1967 by the author from a drill hole in the east-central part of the San Juan Basin (Fassett 1968a, 1968b). These samples were from the Lewis Shale, Pictured Cliffs Sandstone, Fruitland Formation, Ojo Alamo Sandstone, and Nacimiento Formation (depths of 4,263 to 3,437 ft). These samples were submitted to R.H. Tschudy (USGS) for palynologic analysis, and he reported (1973, p. 131) that 30 samples yielded sufficient specimens for a percentage count. Tschudy found that all of the Fruitland Formation samples contained abundant specimens of Proteacidites spp. (Tschudypollis) and, that the overlying Ojo Alamo Sandstone contained the Paleocene index fossil Maceopolipollenites tenuipolus (Momipites tenuipolus) and a few reworked specimens of Proteacidites spp. On the basis of these palynologic data, Tschudy placed the Cretaceous-Tertiary boundary in the GB-1 core at the base of the Ojo Alamo Sandstone.

Tschudy (1973) also compared the palynomorph assemblages from the Gasbuggy core with those of other Western-Interior basins, including, the nearby Raton Basin of northeastern New Mexico and southwestern Colorado. On the basis of those comparisons, Tschudy concluded that uppermost Campanian-age and all Maastrichtian-age rocks were missing in the Gasbuggy core indicating the presence of a significant hiatus separating the Paleocene Ojo Alamo Sandstone from the underlying Campanian Fruitland Formation in the San Juan Basin.

Mesa Portales. The palynology of strata adjacent to the K-T interface at Mesa Portales was discussed in Fassett and Hinds (1971). These authors collected rock samples for palynologic analysis from the uppermost Kirtland-Fruitland Formation (undivided) and from the Ojo Alamo Sandstone on Mesa Portales (Figure 1, Figure 21). R.H. Tschudy analyzed the samples and found (in Fassett and Hinds (1971), p. 31,33, and table 1) that the K-T interface at Mesa Portales was in the uppermost part of the Kirtland-Fruitland interval, 13 m below the base of the Ojo Alamo Sandstone (Figure 23). Tschudy concluded that the uppermost 13 m of the Kirtland-Fruitland Formation and all of the Ojo Alamo Sandstone at Mesa Portales were Paleocene in age (Figure 22, Figure 23, Figure 24, and Figure 25). Additional samples were subsequently collected at Mesa Portales from other parts of the Fruitland-Kirtland Formation and from the Ojo Alamo Sandstone and palynomorph lists identified by Tschudy from those samples are published for the first time in Appendix of this paper. These new data confirmed that the K-T interface at Mesa Portales is 14 m below the base of the Ojo Alamo Sandstone in uppermost Fruitland-Kirtland strata (Figure 23). All samples from below this interface yielded large numbers of Proteacidites spp., and no specimens of this Cretaceous index fossil were found above this level. One sample 5 m below the base of the Ojo Alamo (D6583-B of Figure 23) yielded the Paleocene index fossil M. tenuipolus. Thus the Ojo Alamo Sandstone is unequivocally Paleocene in age in its entirety at Mesa Portales.

Ojo Alamo Sandstone Type Area. The Ojo Alamo Sandstone type area is between Hunter Wash and De-na-zin Arroyo in the southwest part of the San Juan Basin (Figure 1, Figure 4). The Ojo Alamo Sandstone contains abundant dinosaur fossils in this area. As discussed in Appendix, palynologic data from strata adjacent to the K-T interface in the Ojo Alamo Sandstone type area have been presented in several publications. In addition, Appendix also presents unpublished palynomorph lists for this area from the files of the author. These data show that all of the many rock samples from the Cretaceous Fruitland and Kirtland Formations in this area have yielded abundant specimens of Tschudypollis spp. One sample from the uppermost Kirtland Formation (less than 1-m below the base of the Ojo Alamo) has yielded the Paleocene index palynomorph M. tenuipolus and a few reworked specimens of Tschudypollis spp. Several samples from mudstone interbeds in the upper part of the Ojo Alamo in this area have also yielded Paleocene index palynomorphs and no specimens of Tschudypollis spp.

On the basis of these data, the K-T interface is placed just below the base of the Ojo Alamo Sandstone in the Ojo Alamo Sandstone type area.

San Juan River Site. Rock samples collected for palynologic analysis from the Ojo Alamo Sandstone at the San Juan River site from a coaly, carbonaceous shale interbed, 13 m above the base of the Ojo Alamo Sandstone, have yielded the Paleocene index fossil M. tenuipolus; in addition one sample also yielded the Paleocene index fossil Brevicolporites colpella. The carbonaceous shale yielding these Paleocene palynomorphs is 3.5 m below the level of a large hadrosaur femur collected from this locality. Some, but not all of these samples, also yielded rare, reworked specimens of Tschudypollis spp. The presence of two Paleocene index palynomorphs from the Ojo Alamo Sandstone at the San Juan River site provides conclusive evidence that the Ojo Alamo Sandstone is Paleocene in age at this location.

Durango Area. Studies of the palynology of rock strata adjacent to the K-T interface were conducted by Manfrino (1984) and Newman (1987) in an area in the northern San Juan Basin near Durango, Colorado (Figure 1). The results of those studies are summarized in Newman (1987). The rock strata adjacent to the K-T interface in the Durango area are different from those in the New Mexico part of the San Juan Basin because: 1) The Animas Formation rather than the Ojo Alamo Sandstone overlies the K-T interface in the northern San Juan Basin (Figure 1.1), and 2) The McDermott Formation is the stratigraphically highest Cretaceous rock unit in that area. The lower part of the Animas Formation is the same age as the Ojo Alamo Sandstone, even though the two formations are distinctly different, lithologically, and the Animas is much thicker: about 335 m in the Durango area. The upper part of the Animas Formation in the northern San Juan Basin is time equivalent to the Nacimiento Formation in the southern part of the basin even though the two formations are lithologically distinct (Fassett 1985).

The Appendix contains a summary of Newman's (1987) findings regarding the biochronologic significance of the palynomorphs identified from rock samples collected from uppermost Cretaceous and lowermost Paleocene strata in the Durango, Colorado area. Newman showed that the palynomorph assemblages from the uppermost Lewis Shale, Pictured Cliffs Sandstone, and most of the Fruitland Formation are late Campanian in age, and that most of the Kirtland Formation and the lower half of the McDermott Formation yielded palynomorphs of early Maastrichtian age. He also showed that the Animas Formation contains palynomorphs of early Paleocene age. Newman (1987) placed the K-T interface at the base of the Animas Formation in the Durango, Colorado area. Newman also concluded (p. 158) that based on his palynologic studies in this area: "Approximately the upper half of the Maastrichtian Stage is not represented, and perhaps some earliest Paleocene is missing as well between McDermott and Animas strata."

Newman's finding that the Animas Formation is Paleocene in the northern San Juan Basin supports earlier conclusions by Knowlton (1924) who conducted an extensive study of leaf fossils in the Animas Formation. Knowlton stated (p. 71) that the Animas is:

. . . undoubtedly Tertiary. Not a single species is known to be common to the Animas formation and the Cretaceous exclusively—in fact, there are only five species that extend into the acknowledged Cretaceous anywhere.

Summary

Figure 36 summarizes the palynologic biochronology of strata adjacent to the K-T interface in the San Juan Basin. The six columns show the localities where palynology definitively establishes the age of the Ojo Alamo Sandstone (or lowermost Animas Formation) in the San Juan Basin. The column headed "Durango Area" shows that the Cretaceous-Tertiary (K-T) interface is at the base of the Animas Formation—top of the McDermott Formation based on palynology. The McDermott rests conformably on top of the Kirtland Formation and is only found in a small area in the northwest part of the San Juan Basin (Fassett 1985).

At the San Juan River locality (Figure 36), two Paleocene index palynomorphs M. tenuipolus and B. colpella were identified from samples about 13 m above the base of the Ojo Alamo Sandstone where it rests unconformably on the Kirtland Formation. No samples have been collected from the Kirtland for palynologic analysis at this locality, thus the K-T interface is not bracketed by palynomorph assemblages here. At the Ojo Alamo type area, the basal contact of the Ojo Alamo Sandstone with the top of the Kirtland Formation is closely bracketed by palynologic assemblages indicating that all Ojo Alamo strata are Paleocene in age there.

At Mesa Portales, palynologic data show that not only is the Ojo Alamo Sandstone Paleocene in its entirety, but about 14 m of the underlying Fruitland-Kirtland Formation is Paleocene as well. In the Cuba area and in the Gasbuggy core, Paleocene and Cretaceous palynomorph assemblages closely bracket the basal contact between the base of the Ojo Alamo Sandstone and the top of the Fruitland Formation providing conclusive evidence that the Ojo Alamo at those places is Paleocene in its entirety.

In summary, at all localities where palynomorphs have been identified bracketing the base of the Ojo Alamo Sandstone or Animas Formation, Cretaceous palynomorphs such as Tschudypollis spp. are found in large numbers in underlying Cretaceous strata and Paleocene palynomorphs such as M. tenuipolus and (or) B. colpella are found in the Ojo Alamo and lowermost Animas or Nacimiento Formations. At three localities San Juan River, the Ojo Alamo type area, and the Gasbuggy core hole (Figure 36) rare, reworked specimens of the Cretaceous index palynomorph Tschudypollis have been identified in some samples in the lowermost part of the Ojo Alamo Sandstone but at the other localities shown on Figure 36, Tschudypollis has not been identified in the Ojo Alamo or Animas Formations. The weight of the palynologic evidence thus supports the conclusion that the Ojo Alamo Sandstone and the Animas Formation are Paleocene in age in their entirety throughout the San Juan Basin.

In addition, because M. tenuipolus has been shown to be present only in the upper part of biozone P1 in many Western Interior basins (including the nearby Raton Basin) and absent in lowermost Paleocene strata (Nichols 2003), the presence of this guide fossil in lowermost Paleocene strata in the San Juan Basin suggests that strata representing the lower part of biozone P1 is not present in the San Juan Basin (see discussion in Appendix). This finding supports paleomagnetic evidence (discussed above) suggesting that the lowermost Paleocene strata in the San Juan Basin are 63.2 Ma (Figure 34, Figure 35).