Synthesis of published and unpublished palynologic data for Cretaceous-Tertiary boundary strata; San Juan Basin, New Mexico
Palynology has been the most precise biochronologic tool for locating the stratigraphic position of the Cretaceous-Tertiary (K-T) boundary in the Western Interior of North America. In the Raton Basin, 230 km east of the San Juan Basin, the K-T boundary was located to within a few centimeters on the basis of the last occurrence of the Cretaceous index palynomorph Tschudypollis (formerly named Proteacidites). The end-Cretaceous asteroid-impact fall-out layer was subsequently discovered within that same centimeters-thick interval by
Orth et al. (1981,
1982). The K-T fall-out layer has now been found at numerous localities throughout the Western Interior of North America just above the last occurrence of Tschudypollis and (or) other Cretaceous index palynomorphs, validating the value of these index fossils for determining the precise location of the K-T interface.
Nichols and Johnson (2002, p. 100) stated that:
In southwestern North Dakota, as elsewhere in the Western Interior region of the United States and Canada, the K-T boundary is defined by the disappearance (local or total extinction) of certain palynomorph taxa. . . . In this study, the K-T boundary was determined on the basis of palynology to be between the highest sample that yielded K taxa [Cretaceous index palynomorphs] and the next sample above that lacks K taxa.
One of the key "K taxa" listed by
Nichols and Johnson (2002) was Tschudypollis. In the following discussion, it is shown that the last occurrence of the Cretaceous index palynomorph Tschudypollis precisely locates the stratigraphic level of the K-T interface at the base of the Ojo Alamo Sandstone at several localities in the San Juan Basin. At a few localities, rare specimens of Tschudypollis have been identified from Ojo Alamo Sandstone rock samples.
Nichols and Fleming (2002, p. 240) discussed the reworking of palynomorphs from older strata into younger strata and concluded that "The age of a contaminated [palynomorph] assemblage is determined by the youngest species present, one that has a restricted stratigraphic range, and does not occur in older rocks." In every instance where rare Tschudypollis specimens have been found in Ojo Alamo Sandstone samples, the younger Paleocene guide fossil Momipites tenuipolus is also present. M. tenuipolus is a known Paleocene guide fossil in the Western Interior of North America (Nichols and Johnson 2002) and has never been found in Cretaceous strata in the southern part of the Western Interior, thus the presence of this palynomorph in the Ojo Alamo Sandstone confirms the Paleocene age of these "contaminated" assemblages.
Obtaining outcrop rock samples productive of palynomorphs has been a difficult challenge in the San Juan Basin. Commonly, samples that looked promising in the field that is, with evidence of abundant organic material in them proved barren, when analyzed. For that reason, palynologic sampling from Mesa Portales, the Ojo Alamo Sandstone type area, and other localities (discussed below), has been spaced out over decades. For every set of samples collected, more than half, typically, turned out to be barren of palynomorphs, so recollecting was necessary to try to find productive material to fill in gaps. This problem has been especially acute in the Ojo Alamo type area (Figure 4), where abundant dinosaur bone is present in the lower part of the Ojo Alamo Sandstone. Numerous attempts to obtain rock samples productive of palynomorphs from dinosaur-bearing strata within the Ojo Alamo Sandstone in the type area, by various workers (including the author), have still not succeeded. Productive samples yielding diverse palynomorph assemblages from dinosaur-bearing strata would contribute greatly to the biochronologic data-base for the Ojo Alamo Sandstone in the southern San Juan Basin.
Good results, however, have been relatively recently obtained at the San Juan River site, where multiple palynomorph-productive samples were obtained a few meters below a large hadrosaur femur in the Ojo Alamo Sandstone (Fassett and Lucas 2000) and at the Barrel Spring locality where the Paleocene index palynomorph Momipites tenuipolus was identified from samples just below the base of the dinosaur-bearing Ojo Alamo Sandstone (Fassett et al. 2002). Excellent results have also been obtained from Cretaceous-Tertiary (K-T) strata at Mesa Portales where multiple, productive, palynologic sample sites closely bracket the Cretaceous-Tertiary interface. Even though studies of the palynology of the Ojo Alamo Sandstone and adjacent strata have been conducted at numerous localities, no synthesis of all available data has been published heretofore.
The aim of this appendix is to present, chronologically, all available published and unpublished palynologic data for rock strata adjacent to the K-T interface in the San Juan Basin.
Anderson (1960) was the first to publish palynologic data for strata adjacent to the Cretaceous-Tertiary boundary in the San Juan Basin. In his introduction, he summarized the conflicting paleontologic data relating to the age of the Ojo Alamo Sandstone in the southern San Juan Basin: paleobotanical data indicated that the Ojo Alamo was Paleocene and abundant dinosaur fossils indicated that this formation was Cretaceous. Anderson wrote (p. 1): "One objective of this study is to determine what bearing the pollen and spore evidence has on the controversy."
Anderson (1960) collected rock samples from the Ojo Alamo Sandstone and adjacent strata at five localities in the southeastern part of the basin near Cuba, New Mexico (Figure 21): the Kirtland Shale, Ojo Alamo 1 and 2, and Nacimiento 1 and 2 localities. (Anderson's palynomorph lists from these localities are in
Figure 47 is a
composite stratigraphic diagram showing the relative stratigraphic positions of Anderson's Kirtland and Ojo Alamo sample sites. The sample locality for the "Kirtland shale florule" is in a badland amphitheater where the upper part of the Kirtland Formation and Ojo Alamo Sandstone are well exposed (Figure 21,
Figure 48). This locality is 220 m west of County Road (CR) 11 (Figure 21) and is 4.0 km (2.4 mi.) south of the intersection of CR 11 and US Highway 550. Anderson's sample was collected from ". . . the upper carbonaceous zone in a medium-gray micaceous mudstone approximately 37 feet [11.3 m] below the base of the Ojo Alamo Sandstone." (Anderson 1960, p. 5). The key Cretaceous index palynomorphs identified by Anderson from this sample were Proteacidites retusus, P. thalmani, and P. sp. (his tables 2, 3). (The genus name "Proteacidites" was recently changed to Tschudypollis by
Nichols, [2002, p. 443-444]). The complete list of Anderson's palynomorphs constituting the "Kirtland shale florule"
is in Table 5.
Figure 48 is a
photograph of the "Kirtland shale florule" locality. The Ojo Alamo Sandstone is about 18 m thick here and consists of two ledges of white sandstone separated by a thin bed of siltstone and mudstone. The Ojo Alamo is underlain by the "Kirtland shale" of Anderson; this rock unit was mapped as Kirtland Shale and Fruitland Formation, undivided, in this area by
Fassett and Hinds (1971),
Woodward et al. (1972), and
Woodward et al. (1973).
Fassett and Hinds (1971, figure 9, plate 2), however, show that the Fruitland-Kirtland interval is beveled at the top from northwest to southeast across the San Juan Basin, resulting in the Kirtland being absent along the east side of the basin, including the area of Anderson's "Kirtland shale florule" (see
Figure 21). Thus, the rock unit underlying the Ojo Alamo Sandstone at Anderson's localities is more properly named the Fruitland Formation. (See
Figure 1.2 for a basin-wide cross section depicting the convergence of the basal contact of the Paleocene Ojo Alamo with underlying Cretaceous strata.) The base of the lower Ojo Alamo Sandstone bench is stratigraphically higher on the right side (north) on
Figure 48 than on the left (south) side. The interface between Cretaceous and Paleocene strata here is probably at the base of the sandy interval just above the top of the carbonaceous zone in the Fruitland Formation and not at the base of the rock-stratigraphic Ojo Alamo.
Anderson's "Ojo Alamo 1 florule" was collected from "a thin carbonaceous layer within a light brownish-buff siltstone lens at the base of the Ojo Alamo sandstone"
(Anderson 1960, p. 5). He stated that "The siltstone lens from which the sample was taken could be considered a part of either the Ojo Alamo Sandstone or the Kirtland shale." This locality is in a road cut in the Ojo
Alamo just east of CR 11, 1.5 km (0.95 mi.) south of the intersection of CR 11
and US Highway 550 (Figure 21,
Figure 49). Although Anderson stated that this sample came from "the base of the Ojo Alamo Sandstone," subsequent mapping by
Baltz (1967) and
Woodward et al. (1973) showed that Anderson's Ojo Alamo 1 florule was actually collected from a mudstone and siltstone layer within the Ojo Alamo. The Ojo Alamo 1 florule from this locality does not contain the key Cretaceous index fossil: Tschudypollis (Proteacidites).
Anderson (1960, p. 5) stated that: "The florule is very different from the underlying one in the Kirtland shale . . ." but concluded (p. 9) that "The Ojo Alamo 1 florule has a 'Tertiary' aspect but is not necessarily Tertiary from the standpoint of common forms." Anderson's complete list of palynomorphs from this locality is in
Anderson's (1960, p. 5)"Ojo Alamo 2 florule" (Table 5) was ". . . found in a carbonaceous zone at the base of a middle shale unit within the Ojo Alamo Sandstone." Anderson's road directions to this locality cannot be literally followed to arrive at this collection site; however, the placement of this site on his map (figure 1) is accurate. (The map on
Figure 21 of the present report clearly shows how to access this site via NM State Highway 126 and CR 13: go 0.86 km (0.54 mi) east of US Hwy 550 on NM Hwy 126 and then 3.7 km (2.3 mi) north and east on CR 13); the site is about 200 m west of CR 13. This sample site is within a lower bench of the Ojo Alamo Sandstone about 4 m above the Ojo Alamo Sandstone-Fruitland Formation contact.
Figure 50 shows two photographs of the Ojo Alamo 2 florule site:
Figure 50.1 shows the position of Anderson's "carbonaceous zone" within the lower bench of the Ojo Alamo;
Figure 50.2 is a close-up view of the sample site. The lower bench of the Ojo Alamo Sandstone here caps an east-trending ridge, but further west, this ridge merges with the northeasterly trending Ojo Alamo Sandstone outcrop (Figure 21). There, the Ojo Alamo consists of this lower sandstone bench, a middle claystone-to-siltstone layer, and an upper sandstone bench.
Anderson did not find the Cretaceous index palynomorph Tschudypollis in this florule, however he did find that it contained the Paleocene index fossil Brevicolporites colpella. (See
Nichols and Johnson 2002, for a comprehensive discussion of the palynology of Cretaceous-Tertiary boundary rocks in the Western Interior of North America.) The disappearance of Tschudypollis (a "K taxon" of
Nichols and Johnson 2002) going upward in a stratigraphic section provides evidence that the Cretaceous-Tertiary boundary has been crossed. Moreover, also finding the Paleocene index fossil B. colpella (Nichols and Johnson 2002) present and K taxa absent from the same strata provides unequivocal evidence of the Paleocene age of the strata in question (Nichols, personal commun., 2005).
Figure 47 shows the stratigraphic relations of Anderson's sample-collection sites near Cuba, New Mexico. The stratigraphically lowest sample (about 11 m below the base of the Ojo Alamo Sandstone) yielded his "Kirtland shale florule" which is notable for the presence of the Cretaceous index fossil Proteacidites thalmani (Tschudypollis) "as the dominant dicotyledon" plus other Proteacidites species (Anderson 1960, p. 5 and tables 2, 3). Anderson's "Ojo Alamo 2 florule" is from a sample about 4 m above the base of the Ojo Alamo; this florule contains no Proteacidites (Tschudypollis) species but does contain the Paleocene index fossil Brevicolporites colpella. Going up section to the middle part of the Ojo Alamo and Anderson's "Ojo Alamo 1 florule," again there are no Proteacidites (Tschudypollis) species and according to USGS palynologist D.J. Nichols (personal commun., 2006): "Anderson's OA florule 1, which lacks Tschudypollis spp. as you note, does appear to be Paleocene in age." It thus seems clear that the Ojo Alamo Sandstone in Anderson's study area near Cuba, New Mexico, is Paleocene in age in its entirety.
Anderson (1960, p. 13) concluded that palynologic data suggest that "most of the Ojo Alamo sandstone is Tertiary, but the basal part may be either Cretaceous or Paleocene. " As for the abundant Ojo Alamo dinosaur fossils in the type area, Anderson stated that they may have been reworked, or "Alternatively, pre-Lance-type dinosaurs persisted into a Tertiary environment." Anderson thus became the second geologist (after Reeside 1924) to suggest that dinosaurs may have lived on into the Paleocene in the San Juan Basin.
Anderson's Nacimiento 1 and 2 florule localities are shown on
Figure 21 but are not discussed in detail in this report, because the Paleocene age of the Nacimiento Formation has never been questioned.
Anderson (1960, p. 8) stated that the "Nacimiento 1 florule" was collected from a "medium-gray, micaceous, carbonaceous mudstone, 1 foot above the Ojo Alamo sandstone"; the "Nacimiento 2 florule" was from "an 8-inch coal bed about 115 feet above the Nacimiento 1 florule." Palynomorph assemblages from these localities were determined by Anderson to be Paleocene in age. Anderson's palynomorph lists from his two Nacimiento localities are included in
Baltz et al. (1966) conducted a detailed study of the Ojo Alamo Sandstone in the Ojo Alamo type area (Figure 3,
Figure 51). This study included collecting rock-samples from the uppermost Kirtland Formation, upper part of the Ojo Alamo Sandstone, and the lowermost part of the Nacimiento Formation for palynologic analysis. These authors also redefined the Ojo Alamo Sandstone in their paper (see
Figure 52 shows the stratigraphic levels of the three palynologic collections of
Baltz et al. (1966); because these samples were collected from different localities, their placement on one column on
Figure 52 is diagrammatic. The palynomorphs identified from these three collections are listed in
Collection 3 of
Baltz et al. (1966) was obtained from a "lignite" (carbonaceous mudstone) bed in the uppermost part of the Kirtland Formation (Figure 52) just north of Alamo Wash in the extreme NW
Sec. 7, T. 24 N., R. 11 W. (BAA-3 on
Baltz et al. (1966) (p. D17) stated that:
The florule of collection 3 from the Kirtland is strikingly different from collections 1 and 2 and from any of the Cretaceous and Tertiary florules described by
Anderson (1960) from the eastern part of the basin. The dominant forms of collection 3 are polypodiaceous spores and a monosulcate grain with echinate-clavate sculpture. Pinaceous conifer pollen are [sic] common. Dicotyledon grains are much fewer than in any of the eastern florules, and the dominant form is a tricolpate, reticulate, brevaxial grain with intersemiangular to intersemilobate outline. Smooth and warty trilete spores are present in collection 3, and there are many cystlike structures with hollow processes that resemble some hystrichosphaerids. The florule contains Liliacidites leei Anderson which occurs in the Kirtland, Ojo Alamo, and Nacimiento florules of the eastern part of the basin and Liliacidites hyalaciniatus? Anderson which occurs in the Kirtland and Ojo Alamo 1 florules of the eastern part of the basin. Proteacidites thalmani Anderson is the only really distinctive form in collection 3 that is found also in an eastern florule. It occurs in
Anderson's (1960) Kirtland Shale and Lewis Shale florules and suggests a Cretaceous age for collection 3.
Collection 1 was from a "lignitic shale" (carbonaceous mudstone) interbed in the upper part of the Ojo Alamo Sandstone (Figure 52). This locality was reported to be "on the mesa about one-eighth mile [200 m] north of Barrel Spring" by
Baltz et al.
1966 (p. D17). The location for this sample collection cannot be correct because 200 m north of Barrel Spring is not "on the mesa" and not in the uppermost Ojo Alamo Sandstone, but rather is in the drainage-way of De-na-zin arroyo in the upper Kirtland Shale. The actual location for this locality appears to be in the west-central part of Sec. 16, T. 24 N., R. 11 W. at the edge of the mesa about 70 m north of Barrel Spring (labeled BAA-1 on
Collection 2 was from an extensive bed of "lignite" (carbonaceous mudstone) in the lower part of the Nacimiento Formation (Figure 52) in the southwest part of Sec. 10, T. 24 N., R. 11 W. in Barrel Spring Arroyo (Figure 4). Barrel Spring Arroyo of
Baltz et al. (1966) is now named De-na-zin Wash (Alamo Mesa East, 1/24,000 USGS topographic quadrangle map); the collection 2 palynologic locality is labeled BAA-2 on
Baltz et al.
, p. D17) stated that:
Collection 1 from the Ojo Alamo and collection 2 from the Nacimiento are similar to each other and contain common to abundant grains of Ulmoideipites tricostatus Anderson and Podocarpus
sp. These are the two dominant types of grains in
Anderson's (1960) Ojo Alamo florules from the eastern part of the basin. Several kinds of Momipites grains are present in collections 1 and 2; these also are common in
Anderson's (1960) Ojo Alamo and Nacimiento florules from the eastern part of the basin. The florule of collection 1 contains many large inaperturate semihexagonal grains, some monosulcate grains, monolete and trilete spores, triporate pollen, and pinaceous conifer pollen. The florule of collection 2 contains Quercus? sp., Arecipites cf. A. reticulatus (Van der Hammen), Cupaneidites cf. C. major Cookson and Pike, Paliurus triplicatus?
Anderson, and some spores, all of which are present in
Anderson's (1960) Ojo Alamo or Nacimiento florules from the eastern part of the basin.
Complete palynomorph lists for the three palynologic localities of
Baltz et al. (1966) were not provided.
Baltz et al. (1966, p. D17) concluded that: "The palynology does not directly fix the age of the Restricted Ojo Alamo Sandstone . . ." but then added the somewhat contradictory statement: "In summary, the palynologic and physical-stratigraphic evidence of the [Paleocene] age of the restricted Ojo Alamo Sandstone are in agreement." As for the differences in palynomorph assemblages in the Ojo Alamo type area and in
Anderson's (1960) collections near Cuba, New Mexico, on the east side of the basin,
Baltz et al. (1966)suggested that the different florules:
. . . allow for the possibility that rocks equivalent in age to the upper shale member of the Kirtland (colln. 3) at Ojo Alamo may be absent from the eastern part of the basin. This interpretation would be consistent with the physical evidence for a hiatus between the deposition of the Kirtland and the Ojo Alamo.
Subsequent work, demonstrating the presence of a nearly 8-m.y. hiatus at the Kirtland-Ojo Alamo contact (Fassett and Steiner 1997;
Fassett 2000), and the thinning of Cretaceous strata by more than 650 m from northwest to southeast across the San Juan Basin (Figure 1), shows that the upper Kirtland Formation strata sampled at Ojo Alamo Arroyo indeed are not present east of Cuba, New Mexico. The stratigraphic cross sections of
Figure 34, and
Figure 35 conclusively show that the uppermost Kirtland Formation strata in the Ojo Alamo type area (drill-hole 2) are not present in the Cuba, New Mexico, area (drill-hole 6). Thus, the different palynomorph assemblages at the two places are the result of sampling of strata of different ages deposited in quite different environments: relatively near to the regressing Pictured Cliffs Sandstone shoreline near Cuba vs. far inland from the paleoshoreline at the Ojo Alamo Sandstone type area.
Fassett and Hinds (1971, table 1) published a palynomorph list for samples from eight localities in the San Juan Basin. At a locality in the northeast part of the basin in Colorado, the lowermost Fruitland Formation was sampled; at another locality, south of Mesa de Cuba, the lower part of the Nacimiento Formation was sampled. The other six localities were in the Mesa Portales study area (Figure 21), where multiple samples were collected from the undivided Fruitland-Kirtland Formation and from the Ojo Alamo Sandstone. All these samples were collected by the author between 1964 and 1968 and were analyzed by R.H. Tschudy, U. S. Geological Survey, Denver, Colorado; Tschudy's data were provided in written communications in 1966, 1967, and 1968. Complete palynomorph lists for these localities were presented in table 1 of
Fassett and Hinds (1971); that table is reproduced herein as
Table 7. (The stratigraphic positions of the Mesa Portales sample localities of
Fassett and Hinds
(1971) are shown on Figure 22 and
Figure 23; the other palynologic sample localities shown on these figures were collected later and are discussed in a subsequent section of this appendix.) Palynomorph assemblages in
Fassett and Hinds
(1971) from Mesa Portales made clear that the Cretaceous-Tertiary boundary there was located below the base of the rock-stratigraphic Ojo Alamo Sandstone in the 13 m interval between samples D3738-C and D3738-B (Figure 22 and
All of the
Fassett and Hinds
(1971) samples from the undivided Fruitland-Kirtland interval in the Mesa Portales study area contained abundant specimens of the Cretaceous index fossil Proteacidites (Tschudypollis).
Table 7 lists Proteacidites retusus Anderson from samples D3738-C, D4017-A, D4017-B, and D4017-C (Figure 22 and
Figure 23). Tschudypollis spp. has come to be universally accepted as one of the premier index palynomorphs for uppermost Upper Cretaceous rocks throughout much of the Western Interior of North America. For example,
Tschudy (1973, p. 133) wrote:
The genus Proteacidites throughout the Rocky Mountain region is limited to the Late Cretaceous. In no place, except as isolated redeposited specimens, has it been found in the
Samples D3738-A and D3738-B from the Ojo Alamo Sandstone (Figure 22 and
Figure 23) contained no Proteacidites specimens. In his discussion of these samples in
Fassett and Hinds (1971, p. 33) Tschudy wrote of sample D3738-A: "This assemblage is clearly of Paleocene age and is equivalent to the assemblages found by
Anderson  in the Ojo Alamo Sandstone." And for sample D3738-B he wrote that it was "from the Tertiary." Tschudy further wrote in
Fassett and Hinds (1971, p. 33):
It is possible to postulate a hiatus between the Cretaceous and Paleocene at this locality. The genus Araucariacites (table 1) has not been found in rocks younger than Campanian in the Rocky Mountain Region. Moreover, the Cretaceous assemblages found in your samples are different from those found in the latest Cretaceous of the Raton Formation. However, it must be emphasized that we do not have enough control data from your area to do more than guess at a possible hiatus. For example, the closest area from which we have control on the occurrence of Araucariacites in the Upper Cretaceous is northern Colorado. Furthermore, we know that several floral provinces existed during Late Cretaceous time.
Thus, the palynological data of
Fassett and Hinds (1971) from the Mesa Portales study area established the presence of the Cretaceous-Tertiary interface below the base of the Ojo Alamo Sandstone (Figure 22 and
Figure 23), confirmed the age of the Ojo Alamo in its entirety as Paleocene, and suggested the presence of a hiatus at the K-T interface representing all of post-Campanian (Maastrichtian) time.
The Gasbuggy project was initiated in February 1967 with the drilling of the Gasbuggy 1 (GB-1) core hole in the east-central part of the San Juan Basin (Figure 1.1). The objective of the project was to explode a nuclear device in the Pictured Cliffs Sandstone about 1,200 m (4,000 ft) deep as an experiment to determine whether or not natural gas production from the Pictured Cliffs, a relatively impermeable rock unit in that part of the basin, could thereby be significantly increased. A continuous core was cut starting in the lower part of the Nacimiento Formation, through 60 m of massive Ojo Alamo Sandstone, 73 m of the Fruitland Formation, and 192 m of Pictured Cliffs Sandstone and underlying Lewis Shale (Fassett 1968a,
1968b). This continuous core offered an unprecedented opportunity to obtain a large number of closely spaced rock samples across the Cretaceous-Tertiary interface from unweathered core material.
Samples from core hole GB-1 were collected at about 6 m intervals by the author in 1968, from the Nacimiento Formation down through the upper part of the Lewis Shale. Core chips from 52 levels were collected and submitted to R.H. Tschudy for analysis. Tschudy reported that: "Thirty-nine samples yielded some palynomorphs and thirty of these yielded sufficient specimens for a percentage count."
Figure 53 is a copy of figure 1 from
Tschudy (1973) showing the distribution of the GB-1 core samples.
Tschudy (1973, p. 142) also included a table listing all of the palynomorphs identified from the GB-1 core samples;
Table 8 is a modified version of Tschudy's table showing palynomorphs identified from the Fruitland, Ojo Alamo, and Nacimiento Formations. It is interesting to note, that even in the unweathered core material, only slightly more than half of the samples yielded meaningful numbers of palynomorphs, and 12 samples were barren of palynomorphs.
Tschudy's GB-1 palynomorph list (Table 8) shows that the Cretaceous index fossil Tschudypollis spp. (Proteacidites spp. on Tschudy's list) averages nearly fourteen specimens per Fruitland Formation slide. The two lowermost samples from the Ojo Alamo Sandstone; D4665-A and D4666-K (Figure 53,
Table 8) yielded (Tschudy 1973, p. 133) ". . . very sparse specimens of Proteacidites . . . possibly . . . due to redeposition of Cretaceous pollen in Tertiary rocks near the Cretaceous-Tertiary unconformity." Tschudy further stated that the sample at 3515.6 feet [D4665-D,
Figure 53] "contained Maceopolipollenites tenuipolus [now, Momipites tenuipolus], a fossil that elsewhere in the Rocky Mountain region is limited to the Paleocene but is not present in the lowermost Paleocene."
On the basis of palynologic data, Tschudy placed the Cretaceous-Tertiary boundary in the GB-1 core between samples D4778-D and D4666-K (Figure 53,
Table 8); essentially, at the base of the Ojo Alamo Sandstone.
Figure 54 (after
Tschudy 1973, figure 3) shows the stratigraphic distribution of selected palynomorphs identified from samples of the Gasbuggy core. The hiatus at the Cretaceous-Tertiary (Campanian-Paleocene) boundary is marked by the termination of the first 11 palynomorphs (or groups) at the contact between the Fruitland Formation and overlying Ojo Alamo Sandstone. Isolated specimens of Proteacidites spp., Aquilapollenites spp., and Tricolpites sp. are present above this contact, but as articulated by Tschudy, above, the presence of these isolated specimens is probably due to "redeposition of Cretaceous pollen in Tertiary rocks." The Ojo Alamo was deposited on a vast erosion surface that beveled Upper Cretaceous rocks across the entire San Juan Basin (Figure 1). It is thus not surprising that a few, random, Cretaceous palynomorphs redeposited from underlying Cretaceous strata are present in the lowermost part of the Ojo Alamo. Lowermost Ojo Alamo sediments were deposited by high-energy streams flowing from the north or northwest across this widespread erosion surface, and it would indeed be more remarkable if a few, random, Cretaceous palynomorphs had not been transported by wind or water into the lowermost Ojo Alamo Sandstone's channel-sandstone and over-bank deposits.
Figure 54 also shows the emergence of three new species in the Paleocene Ojo Alamo: Periporopollenites sp., Tricolpites anguloluminosus, and Maceopolipollenites tenuipolus (now Momipites tenuipolus). In addition, Ulmipollenites sp., identified in only three isolated samples in Cretaceous strata, is found to be continuously present through most of the Ojo Alamo and into the Nacimiento Formation.
Tschudy compared the Cretaceous palynomorph assemblages in the GB-1 core with palynomorph assemblages from uppermost Cretaceous rocks in northern Montana and Wyoming. On the basis of those comparisons he concluded that uppermost Upper Cretaceous palynomorph assemblages present in that region were missing from the GB-1 core. In addition, Tschudy compared the GB-1 palynomorph assemblages with those he had identified from drill-core samples in the Raton Basin, only 230 km east of the San Juan Basin and concluded that the uppermost-Cretaceous pollen assemblages in the Raton Basin were not present in the GB-1 core hole.
Tschudy (1973, p. 131) summed up this situation by stating:
A section of the Upper Cretaceous, present in the upper part of the Cretaceous in the Raton Basin, is absent from the Upper Cretaceous of the Gasbuggy core. This confirms the presence of a marked hiatus at the top of the Cretaceous, as previously postulated in the San Juan Basin
(in Fassett and Hinds 1971, p. 33).
Figure 55, modified from
Tschudy (1973, figure 5), is a cross section showing the correlation of palynomorph assemblages from the Raton Basin to the San Juan Basin. This diagram shows that several palynomorph zones present in the uppermost Cretaceous strata of the Raton Basin are missing in the San Juan Basin.
Fassett et al. (1987) surveyed all of the known localities in the Ojo Alamo Sandstone where either dinosaur bone or palynomorphs had been documented. They discussed the palynomorphs that had been identified from the Ojo Alamo Sandstone by R.H. Tschudy at three places in the basin: 1)
near Barrel Spring, 2) at Mesa Portales, and 3) in the Gasbuggy 1 core.
Barrel Spring Locality. The Barrel Spring locality was sampled by C.J. Orth, Los Alamos National Laboratory, in 1982; Orth's samples were submitted to R.H. Tschudy for analysis. According to
Orth et al. (1982, p. 427), this locality is 2 km east of Barrel Spring on De-na-zin Arroyo (probably in the SE1/4 SE1/4 Sec. 9, T. 24 N., R. 11 W.,
Figure 51). The sample, labeled USGS paleobotany locality number D6391 (Tschudy, personal commun., 1982) was collected from a claystone layer about 3 m below the top of the Ojo Alamo Sandstone. Tschudy reported that:
This assemblage is clearly of Paleocene age. Several taxa including Momipites tenuipolus were not recorded by Anderson from the Ojo Alamo, but were recorded from the overlying Nacimiento. I have not seen M. tenuipolus in any basal Paleocene samples from the Western Interior. This occurrence suggests that the sample is not from the basal Paleocene but rather from the upper Lower or Lower middle Paleocene.
Mesa Portales locality.
Fassett et al. (1987, p. 30, 31) referred to a new Ojo Alamo Sandstone palynologic sample locality on Mesa Portales (D6583-B,
Figure 23) but did not list all of the palynomorphs identified from that locality. They did state, however, that R.H. Tschudy had reported the presence of the Paleocene index palynomorph Momipites tenuipolus in that assemblage. (The complete list of palynomorphs from this locality is in
Table 9, and Tschudy's comments about this assemblage are given in full in the "This Paper" part of the "Palynology" section of this report.)
Gasbuggy Core. The results of
Tschudy's (1973) study of the palynomorphs identified from the Gasbuggy-core samples were summarized in
Fassett et al. (1987), and the Ojo Alamo Sandstone part of the core was illustrated in a stratigraphic column. Tschudy's comments to the effect that the palynomorphs from the Ojo Alamo Sandstone in the Gasbuggy core indicated that it was Paleocene in its entirety were cited. The complete list of palynomorphs identified from the Gasbuggy core samples is in
K.R. Newman (1987) published a comparison of the palynology of several Western Interior basins with that of the San Juan Basin.
Newman and C. Manfrino (1984) had conducted extensive palynological studies of uppermost Cretaceous and lowermost Tertiary strata in the northern San Juan Basin in the Animas River valley south of Durango, Colorado, and compiled a robust palynologic data set there. The Ojo Alamo Sandstone is absent in the northern part of the San Juan Basin and the lowermost Paleocene formation there is the Animas Formation, thought by
Reeside (1924), and most subsequent workers, to be the same age as the Ojo Alamo Sandstone in the New Mexico part of the basin. (The Animas Formation is discussed in separate sections of this paper.)
Figure 56 shows Newman's interpretation of the strata adjacent to the Cretaceous-Tertiary interface in the northern San Juan Basin based on palynology.
Newman also carried out detailed studies of the palynology of the Fruitland Formation in the southern part of the San Juan Basin. These studies were based on core samples from the Fossil Forest area (Figure 3);
Newman (1987, p. 159) wrote:
Ten samples from 51 m of cored Fruitland Formation have yielded an excellent assemblage of palynomorphs including the guide fossils Trudopollis meekeri, Myrtaceoipollenitius peritus, and Pseudoplicapollis sp. . . . Therefore, the combination of the ammonite and palynomorph zones indicates late Campanian age for the upper Lewis, Pictured Cliffs, and Fruitland Formations in this area, just as at Durango.
Newman did not publish a complete list of identified palynomorphs from this core.
Newman (1987, p. 159) discussed a rock sample from the upper Kirtland Formation at Pot Mesa (Figure 1) and stated that it contained the Maastrichtian palynomorphs Proteacidites (Tschudypollis), Balmeisporites, Interpollis, Gunnera, Kurtizipites, and Ulmoideipites spp. (Newman stated that this sample was from the Ojo Alamo Sandstone, but subsequent studies (Fassett et al. 2002) now place this sample in the uppermost Kirtland Formation.)
Fassett et al. (2002) published a list of palynomorphs identified by D.J. Nichols from a separate sample from this same interval; a comparison of that palynomorph assemblage with Newman's is discussed in the
"Fassett et al. (2002)" section of this appendix.
Newman also reported on a sample he claimed was from the Ojo Alamo Sandstone from a locality "near Farmington" (no more specific location was provided). The author had escorted Newman to the San Juan River Hadrosaur-bone site in 1984 and was informed by Newman at that time that he (Newman) had collected a rock sample "from a lower stratigraphic level" (well below the level of the Hadrosaur bone), but Newman was not specific as to the exact stratigraphic level of that sample. (It is assumed that Newman's "near Farmington" locality is the San Juan River site of this report.)
Newman (1987, p. 159) stated that his sample collected "near Farmington" yielded the same Maastrichtian palynomorphs that he found at Pot Mesa in the uppermost Kirtland Formation. Newman's determination that the "Ojo Alamo" palynomorph assemblage "near Farmington" is Maastrichtian is not in agreement with the three other palynomorph lists from the Ojo Alamo at the San Juan River site (Table 10). Those palynomorph lists all contained the Paleocene index palynomorph Momipites tenuipolus, and one of them also contained the Paleocene index palynomorph Brevicolporites colpella (Table 10). Three of the palynologists found the Cretaceous index fossil Proteacidites (Tschudypollis) in their samples from the San Juan River site, however, D.J. Nichols did not find this index fossil in any of his three samples from that locality. The presence of two Paleocene index palynomorphs in samples from this locality suggests that the Proteacidites specimens found in some of them were reworked.
Fassett and Lucas (2000) and
Fassett et al. (2002) concluded that the Ojo Alamo Sandstone was Paleocene in age at the San Juan River site on the basis of palynologic data.
Newman (1987) concluded, (as did
Tschudy 1973), that the Ojo Alamo Sandstone rests on a significant unconformity in the San Juan Basin and that a hiatus representing most, if not all, of the Maastrichtian and possibly the lowermost part of the Paleocene separated Cretaceous from Tertiary rocks throughout the San Juan Basin.
Newman (1987, figure 10) also showed that more uppermost Cretaceous strata were missing in the southern part of the basin than in the northwestern part.
Fassett and Lucas (2000) published a paper focused on the large hadrosaur femur that had been discovered in the Ojo Alamo Sandstone at the San Juan River site (Figure 1). They reported the results of palynologic studies of three samples collected from a carbonaceous to coaly shale bed located 3.5 m stratigraphically below the level of the hadrosaur femur (Figure 57). (Those three samples are shown as a composite list of samples 6877-A, -B, and -C on
Table 10.) These samples were processed and analyzed by D.J. Nichols who found that the samples contained the Paleocene index fossils Momipites tenuipolus and Brevicolporites colpella and no specimens of the Cretaceous index fossil, Tschudypollis spp. (Nichols, personal commun., 1994). Thus, the palynologic evidence for the age of the Ojo Alamo Sandstone at the San Juan River site showed it to be unequivocally Paleocene. On the basis of this evidence,
Fassett and Lucas (2000, p. 229) stated that the hadrosaur femur found above this palynomorph assemblage must have come from a dinosaur that lived during earliest Paleocene time, and they concluded that: "some dinosaurs in the San Juan Basin survived the 'terminal' end-Cretaceous asteroid impact event only to become extinct a few hundred thousand years (at most) later, in earliest Paleocene time."
As discussed above, Newman (1987) had reported a palynomorph assemblage at his "near Farmington" locality totally different from the assemblages identified by D.J. Nichols at the San Juan River site.
Table 10 shows that there are no palynomorphs in common between the Nichols list and the Newman list. Because the palynomorph assemblages of
Fassett and Lucas (2000, table 1) came from three separate samples and because those results have been independently replicated two other times (Frederiksen personal commun., 1986, and Braman, personal commun., 2000 as discussed below in the
"This Paper" section of this report) it seems evident that
Newman's (1987) palynomorph assemblage could not have come from the Ojo Alamo Sandstone at the San Juan River site. The base of the Ojo Alamo Sandstone is covered by slope wash immediately below the hadrosaur-bone site, and thus its contact with the underlying Kirtland Formation is masked, it may be that Newman's sample containing Cretaceous palynomorphs "near Farmington" may actually have been collected from the uppermost Kirtland Shale and not from the Ojo Alamo Sandstone. Another possibility is that Newman's sample came from a rip-up clast of Kirtland mudstone imbedded in the lowermost part of the Ojo Alamo Sandstone. Such rip-up clasts are not uncommon in the lower meter or two of the Ojo Alamo Sandstone in this area. A third possibility is that if Newman's sample did indeed come from the Ojo Alamo, the Maastrichtian palynomorphs found therein were reworked from underlying Cretaceous strata. And finally, Newman's sample may have come from the Kirtland Formation at an entirely different site from the San Juan River site.
Ojo Alamo Type Area.
Fassett et al. (2002) summarized the study of
Fassett and Lucas (2000) and in addition discussed the palynology of Cretaceous and Tertiary strata in the Ojo Alamo type area (Figure 3,
Figure 4). These authors listed palynomorphs identified by D.J. Nichols (personal commun., 1994) from rock samples in the Ojo Alamo type area. (Figure 4 and
Figure 51 show the locations of the palynologic collection sites in the Ojo Alamo type area;
Figure 51 is a larger-scale map of the Barrel Spring area showing palynologic sample localities in more detail.) One sample (no. 24-5, D6901 of
Figure 51) from a carbonaceous shale bed less than 1 m below the base of the Ojo Alamo Sandstone yielded "a well-preserved assemblage of palynomorphs" including the Paleocene index fossil Momipites tenuipolus (Nichols, personal commun., 1994). (Table 6 contains the published palynomorph lists from the Ojo Alamo type area.) Nichols reported that the palynomorph assemblage from this sample closely resembled the Paleocene assemblage found in the Ojo Alamo at the San Juan River site (Table 10). Sample D6901 also contained the Cretaceous index fossil Tschudypollis, however,
Fassett et al. (2002, p. 318, 319) stated that:
The Proteacidites specimens in this assemblage must be reworked from underlying Cretaceous strata: the reworking of some Cretaceous palynomorphs into this Paleocene assemblage is not unexpected because the early Paleocene swamp in which indigenous Paleocene pollen was accumulating was located on an erosion surface (peneplain) on Kirtland Formation strata of Campanian age and Cretaceous pollen could easily have been transported laterally a few, to a few tens of meters across this surface in wind-blown dust and deposited in the Paleocene swamp.
On the basis of the presence of the Paleocene index fossil M. tenuipolus in this palynomorph assemblage and its similarity to the palynomorph assemblage from the Ojo Alamo Sandstone at the San Juan River locality,
Fassett et al. (2002) concluded that the Ojo Alamo Sandstone, including its contained dinosaur fauna in the Ojo Alamo type area, is Paleocene in age.
Figure 58 is a composite stratigraphic column for the Ojo Alamo type area showing the positions of palynologic samples collected there.
Nichols (1994, written commun.) identified and discussed palynomorphs from the upper Ojo Alamo Sandstone at USGS locality D6880 (labeled sample 24-3C on table 2 of
Fassett et al. 2002). This locality is about 0.6 km east of Barrel Spring (Figure 51) and about 30 m above the base and about 5 m below the top of the Ojo Alamo Sandstone. (Fassett et al. 2002, incorrectly stated that this sample was 15 m above the base of the Ojo Alamo Sandstone.) This sample was from a dark gray mudstone pod completely enclosed within the upper conglomeratic sandstone bench of the Ojo Alamo. Nichols listed the palynomorphs from this sample (Table 6) and stated that it "yielded abundant cutinite as well as palynomorphs" and concluded that "Based on this assemblage, the sample is Paleocene in age."
Fassett et al. (2002) also reported palynomorph identifications from samples collected from the uppermost Kirtland Formation in the vicinity of Barrel Spring from a coaly, carbonaceous shale bed. This sample, numbered 043002 in
Fassett et al. (2002), came from 3 m below the base of the Ojo Alamo Sandstone less than 100 m west of Barrel Spring (Figure 51). D.J. Nichols (personal commun., 2000) reported that this sample (USGS number P4300,
Figure 51) contained a "well-preserved assemblage of palynomorphs indicating Late Cretaceous age." Nichols stated that:
The assemblage identified consists of 12 species, mostly fossil pollen, including three that are restricted to the Upper Cretaceous and none that are known to occur only in the lower Tertiary. The Cretaceous species are Tricolpites interangulus, Proteacidites retusus, and P. thalmannii. Tricolpites interangulus is the most common single species in the assemblage, and an estimated 300 specimens are present on the slide examined. This species is known from the upper Campanian-lower Maastrichtian interval in Colorado and New Mexico. The species of Proteacidites are well known Upper Cretaceous guide fossils throughout the Western Interior region.
The complete list of palynomorphs from this sample is in
Fassett et al. (2002, table 2) and is shown on
Table 6 of this report.
Fassett et al. (2002, p. 319) also stated that:
Additional samples from the same level in this bed (a few meters below the base of the Ojo Alamo) a few hundred meters northwest of the 043002 sample site also yielded Campanian to lower Maastrichtian palynomorphs (Nichols,personal commun., 2000).
The locations and palynomorph lists obtained from those samples are provided in the "This Paper" section of this report.
Pot Mesa. At Pot Mesa (Figure 3,
Figure 59) the Ojo Alamo Sandstone has been considered by some investigators to consist of just one sandstone bench and by others to contain two sandstone benches, as discussed in
Fassett et al. (2002, p. 324). In that report, Fassett et al. concluded that the Ojo Alamo consisted only of the uppermost of the two benches in question at Pot Mesa. A rock sample collected for palynologic analysis from about 9 m below the base of the upper bench was found to be unquestionably Cretaceous in age (Nichols personal commun., 1994).
Fassett et al. (2002) did not list the palynomorphs identified at this site, but that listing is provided in the
"This Paper" section of this report.
Lucas, and Braman (2005) reviewed "the stratigraphic position of the Cretaceous-Tertiary (K/T) boundary in the San Juan Basin" and included a palynomorph list from the uppermost Kirtland Formation in the vicinity of Barrel Spring from the same coaly carbonaceous shale bed as the P4300 sample of
Figure 51. Their sample was given locality number SGL 00-046 (SGL 046 on
Figure 51) and contained a list of palynomorphs identified by D.R. Braman (personal commun., 2006) as shown on
Table 6. Braman commented as follows regarding these palynomorphs:
The above assemblage is made up of mostly species that span the Cretaceous-Tertiary boundary. The exception is Proteacidites retusus and Proteacidites thalmani which are thought to have been two species that went extinct at the boundary (Nichols 1994;
Nichols et al. 1992;
Nichols et al. 1990). Using this observation then would indicate that the sample is Cretaceous in age. The species occurs in Campanian and Maastrichtian deposits, but the presence of Pandaniidites typicus and Ulmoideipites krempii suggests a Maastrichtian age for the sample. The sample is dominated by bisaccate conifer pollen and the species Tricolpites reticulatus.
This paper presents unpublished palynologic data from Mesa Portales, Pot Mesa, the Ojo Alamo type area, the San Juan River site, and other localities.
Mesa Portales. Rock samples were collected for palynologic analyses from within and below the Ojo Alamo Sandstone at Mesa Portales (Figure 21) by C.L. Pillmore (USGS) in 1983. These samples were submitted to R.H. Tschudy, and the productive samples were given USGS paleobotany locality numbers D6582, D6583-A, and D6583-B. Localities 6583-A and B, from the Ojo Alamo Sandstone are shown on
Figure 22 and
Figure 23; locality D6582, from the uppermost Fruitland-Kirtland Formation, is shown on
Figure 21. The stratigraphically lowest of the Ojo Alamo samples; D6583-A, was collected to try and narrow the gap in which the Cretaceous-Tertiary interface was located on Mesa Portales. Tschudy (personal commun., 1983) reported that this sample:
. . . yielded abundant finely divided organic fragments plus fusinite. The sample appears to have been oxidized. Few palynomorphs, difficult to identify, were present. The following were tentatively identified: Tricolpites, Podocarpidites cf. P. sellowiformis, Arecipites, Periporopollenites, Ulmipollenites, Alnipollenites, Lycopodiacidites, Trilete fern spores. This sample did not yield any characteristic Late Cretaceous taxa.
Sample D6583-B was collected from the same bed as sample D3738-B (Figure 22 and
Figure 23) of
Fassett and Hinds (1971). Tschudy reported that this sample:
. . . was very poor. Very few palynomorphs were present. . . The presence of Momipites tenuipolus strongly suggests a Paleocene age. We have not observed this taxon in other than Paleocene rocks. Sample D3738-B (Fassett and Hinds  P.P. 676, p. 22) was rechecked and bore some resemblance to this sample but D3738-B although also poor, did not exhibit as much evidence of oxidation.
Sample D6582 was collected from a light-gray claystone about 100 mm below the base of the Ojo Alamo Sandstone on the east side of Mesa Portales (Figure 21). Tschudy stated that:
This sample was oxidized and very poor. Fusinite and oxidized organic material were present, but very few palynomorphs. . . The assemblage, though poor, indicates a Late Cretaceous age. Owing to the poor recovery and the condition of the sample, even though no evidence of Paleocene fossils was evident, one should consider the possibility that the Cretaceous fossils might have been redeposited in Paleocene rocks.
The palynomorphs identified from localities D6582, D6583-A, and D6583-B are listed on
Table 9 for easier comparison with palynomorph lists from other localities on Mesa Portales.
E.M. Shoemaker collected additional rock samples from the lower part of the Kirtland and Fruitland Formations, undivided, for palynologic evaluation in 1983. The samples were processed by R.H. Tschudy in 1984, and the three productive samples were given USGS paleobotany locality numbers D6626-A, -B, and -C (Figure 22 and
Table 9 lists the palynomorphs identified at each of these localities. In his commentary regarding the significance of these palynomorph assemblages, Tschudy (personal commun., 1984) stated:
The palynomorph assemblages from the above three samples were virtually identical. They indicate a Cretaceous age for the samples but not a latest Cretaceous age. The taxa Pristinuspollenites, Rugubivesiculites, Trudopollis, Accuratipollis, and Pseudoplicapollis in particular have not been observed in post-Campanian rocks from the Western Interior but are commonly found in rocks of that age. I am confident that these samples are no younger than Late Campanian. This evaluation is supported by the presence of Aquilapollenites spp., Proteacidites - large, abundant, Araucariacites, and Aequitriradites, taxa with a greater stratigraphic range, but uncommon in terminal Cretaceous rocks.
The presence of Botrycoccus, Lecaniella, Pediastrum (algae) and Balmeisporites (a water fern) indicates lacustrine deposition. The few dinoflagellate and hystrichosphere cysts probably were redeposited from older marine rocks.
In 1985, the author collected a sample from USGS paleobotany locality D6878 from a thin coal bed in the lower part of the Nacimiento Formation about 50 m above the top of the Ojo Alamo Sandstone (Figure 21,
Table 9). D.J. Nichols (personal commun., 1994) reported that this sample "yielded abundant sapropel and inertinite along with palynomorphs." Nichols concluded that: "Based on this assemblage, the sample is Paleocene in age." Palynomorphs identified from this sample are listed on
Pot Mesa. As discussed in a previous section of this report, palynomorphs from the uppermost Kirtland Formation were identified at the Pot Mesa locality (Figure 3,
Figure 59). One set of samples was described by Newman (1987), a second set by Nichols (in Fassett et al. 2002); and a third set was analyzed by R.H. Tschudy (personal commun., 1977). Tschudy's 1977 list of palynomorphs is published here for the first time (Table 11). Although Newman (1987) did not provide the stratigraphic level or exact location of his sample site, he did indicate to the author in the field (Newman, personal commun., 1984) its approximate stratigraphic position and locality which is nearly identical to the sample locality of Fassett et al. (2002), 9 m below the base of the Ojo Alamo, and labeled PPS on
Figure 59. (As stated above, Newman thought his Pot Mesa sample was from the Ojo Alamo Sandstone at the time he collected it.)
R.H. Tschudy (personal commun., 1977) provided palynomorph identifications from drill-core and cuttings samples from a drill hole on top of Pot Mesa (Figure 59). The hole USGS SL 10-1 was drilled by the USGS in 1975 to evaluate Fruitland Formation coal resources in this area. (A geophysical log and description of the lithology of the drill hole are in
Jentgen and Fassett 1977.) An attempt was made to core this drill hole from the depths of 4.5 m to 30 m (15 to 97 ft), however, swelling shales between the depths of 10 m and 20 m (32 and 65 ft) prevented that part of the hole from being cored. The drill hole started in the Ojo Alamo Sandstone at the surface and ended in the Pictured Cliffs Sandstone (Figure 60). The basal contact of the Ojo Alamo was at a depth of about 10 m (32 ft). Unfortunately, samples from the Ojo Alamo Sandstone part of the core were barren, and all palynomorph-productive samples came from the Kirtland and Fruitland Formations. The author was present at the time this hole was drilled and collected the samples from this drill core, which were analyzed by Tschudy.
Table 11 lists the palynomorphs identified by Tschudy (personal commun., 1977) from samples from drill hole USGS SL 10-1; this table is in the format presented by Tschudy in his report. The five productive samples were given USGS paleobotanical locality numbers; D5783-A, -B, -C, -D, and -E (Table 11,
Figure 60); sample depths are provided on
Table 11. Samples D5783-A, -B, and -C were from drill core; the other two samples were from drill cuttings. The two uppermost samples and the lowermost sample were the most productive of palynomorphs; the two middle samples produced sparse numbers of palynomorphs. All five of the samples were productive of the Cretaceous index fossil Tschudypollis (Proteacidites on
Table 11). In his discussion of these samples, Tschudy stated:
The genus Proteacidites is present in the Cretaceous but has not been found in the Paleocene in any samples from the Rocky Mountain region. On this basis I would place the Cretaceous-Tertiary boundary between 93.8 feet and 94.5 feet. However, the samples from these two levels are so similar in their palynomorph recovery, that I suspect that the sample from 93.8 feet is also of Cretaceous age. Both of these samples yielded many taxa common to the Cretaceous, but generally foreign to the Paleocene. Furthermore, no clearly Paleocene taxa were found in the sample from 93.8 feet.
At the author's request, Tschudy later re-examined his slides for the samples from the Pot Mesa drill hole and revised his earlier findings for these samples stating the following (Tschudy, personal commun., 1981):
At your request I re-examined the slides reported on in 1977 from Star Lake Drill hole 10-1. I made a serious error. On re-examination I found Proteacidites pollen on the original slides. The grains were few and light colored, but I shouldn't have missed them, however, they are definitely present for all to see. Thus the uppermost sample from 93.8 feet (D5783-A) becomes palynologically Cretaceous.
Furthermore, all but 3 taxa found in the uppermost samples were present in the next lower sample (D5783-B) which I originally designated as Cretaceous. The three taxa are Liliacidites, Azolla and Pterospermopsis, forms that we now know to exist in both the Cretaceous and Tertiary. Pollen grains that are present in the uppermost Cretaceous such as Gunnera and "Tilia wodehouseii" were not found in these samples. On the basis of the current information I believe that these samples are not near the Cretaceous-Tertiary boundary, but from lower in the Cretaceous, possibly as low as the upper Campanian.
Thus, the total palynomorph assemblage from drill-hole samples at Pot Mesa indicates that all of the Maastrichtian stage and possibly the upper part of the Campanian stage are missing from the Kirtland Formation below the upper Kirtland sandstone bed shown on
Table 12 lists the palynomorphs identified by Tschudy, Nichols, and Newman from the Pot Mesa locality.
Newman (1987, p. 159) stated that his Pot Mesa palynomorph assemblage indicated a ". . . Maastrichtian age. So far, it has not been possible to determine whether the age is early or late Maastrichtian from these assemblages." Nichols (personal commun., 1994) concluded that: "Based on this diverse and well-preserved assemblage, the sample is clearly of latest Cretaceous age." Nichols and Newman identified Gunnera in their samples from Pot Mesa and Nichols also reported "Tilia" wodehouseii in his sample from that locality. According to Tschudy (above) both of these forms are "uppermost Cretaceous" index fossils. In his comparison of Gasbuggy-core palynomorphs with palynomorphs identified below the K-T boundary in the Raton Basin,
Tschudy (1973) showed "cf Tilia woodhousei" was present in the uppermost Cretaceous of the Raton Basin but absent in the highest Cretaceous strata of the Gasbuggy core (Figure 54). Thus, palynologic data at Pot Mesa indicate the possible presence of an unconformity at the base of the upper Kirtland sandstone bed (Figure 60) separating Campanian strata from Maastrichtian strata. The existence of this unconformity is further evidenced by the fact that even though the stratigraphic levels of palynomorph samples above and below the upper Kirtland sandstone bed are only 12 m apart (Figure 60,
Figure 33), only about 9 % of the palynomorph taxa are common to the palynomorph lists for these two samples. These data support the presence of the hiatus shown at the Pot Mesa locality at drill-hole 5 on
Figure 34 and
Figure 35 at the base of the upper Kirtland sandstone bed.
Ojo Alamo Sandstone Type Area. Unpublished lists of palynomorphs identified by R.H. Tschudy and D.J. Nichols in and near the Ojo Alamo Sandstone type area are presented on
Tables 13.1-13.2. These USGS paleobotany localities are shown on
Figure 4, and
Figure 51. Samples range in stratigraphic position from the lowermost Fruitland Formation (sample D6902, about 12 m above the base) to the uppermost Kirtland Formation for the Moncisco
Mesa sample (just below the Kirtland Formation—Ojo Alamo Sandstone contact); a stratigraphic spread of about 370 m (Figure 33). Samples from localities D6900 and D6902 (Table 13.1,
Figure 3) were collected by J.D. Obradovich (USGS) in 1984 and analyzed by R.H. Tschudy (personal commun., 1985). The Moncisco Mesa sample (Table 13.1,
Figure 3) was collected by C.J. Orth in 1982 and analyzed by Tschudy for its palynologic content. The complete report for this sample is not available, but a summary of Tschudy's report is contained in an undated
written communication from Orth (1983?); the palynomorphs listed in that summary are shown on
Table 13.1. Tschudy concluded that the palynomorph assemblage from the Moncisco Mesa sample was an: "Assemblage equivalent to those in Vermejo Fm [in the Raton Basin]. Age represented is Campanian or early Maastrichtian."
The sample from locality D9157 (Table 13.1,
Figure 33) is 55 m below the Kirtland
Formation—Ojo Alamo Sandstone contact. D.J. Nichols analyzed this sample and reported (personal commun., 2000) that its palynomorph assemblage was early Maastrichtian. Samples D8179 and D8180 (Table 13.1,
Figure 33) were collected from the uppermost Kirtland Formation. Sample D8179-A was from a carbonaceous shale bed about 0.6 m below the base of the Ojo Alamo; sample D8179-B was collected from the same bed, but about 10 m east of the D8179-A locality. Sample D8180 was collected from an organic-rich mudstone bed about 3 m below the D8179-sample level. Nichols reported (personal commun., 1995) that these samples yielded sparse assemblages of palynomorphs.
Because so few palynomorphs were identified from each of these samples. They are listed in a composite list on
Table 13.1. Nichols concluded that this assemblage was Late, but not latest Cretaceous in age.
The palynomorph lists on
Table 13.2 were all provided by D.J. Nichols (personal commun., 2000, 2003). Samples D9156-A and D9156-B (Figure 51)
were collected from the same carbonaceous mudstone bed about 2 m below the
Kirtland—Ojo Alamo contact. Nichols concluded that these two assemblages indicate an early Maastrichtian age. Samples from localities 82403-A, 82303-D, and 82303-E (Figure 51) were collected from a trench excavated through the base of the Ojo Alamo Sandstone and into the uppermost Kirtland Formation; the trench locality is shown on
Figure 51 and
This trench was excavated to try to relocate the bed from which the D6901 palynomorph assemblage (Table 6) had been collected in 1985 in the same area. (An earlier attempt to locate that bed by trenching in the vicinity of sample sites P4300 and SGL 046 (Figure 51) was not successful, as discussed in Fassett et al. 2002, p. 318-321.) The D6901 sample locality (referred to as the 24-5 locality in
Fassett et al. 2002) is important because it produced a Paleocene palynomorph assemblage that confirmed the Paleocene age of the Ojo Alamo Sandstone in the Ojo Alamo Sandstone type area.
Figure 61 is an annotated photograph of the Barrel Spring area showing the location of the trench on the left side of the photograph at the base of the Ojo Alamo Sandstone. The approximate location of the original D6901 sample site is shown to the left of the trench on this Figure.
Figure 62 is an
annotated photograph of the entire trench, showing the stratigraphic level (4 m below the base of the Ojo Alamo Sandstone) of sample 82403-A. (Palynomorphs identified from this sample are listed on
Table 13.2.) This sample is at about the same level as the coaly, carbonaceous shale bed shown on
Figure 61. from which samples P4300 and SGL 046 (Table 6) were collected further to the west (Figure 51,
Figure 61.). The thin, coaly layer is not present in the trench because it pinches out to the east, as shown on
Figure 61. Nichols (personal commun., 2003) concluded that the palynomorph assemblage from sample 82403-A was early Maastrichtian in age.
Figure 63 is a close-up view of the upper trench of
Figure 62 showing the locations of all samples that were collected for palynologic study here. Sample 82303-E, according to Nichols contained a "diverse and well-preserved assemblage of fossil pollen and spores." (Table A13.2) He concluded that this assemblage indicated an early Maastrichtian age. Nichols reported that sample 82303-D "appears to have been weathered in place such that only a few robust palynomorph species survived; species identified have no biostratigraphic value; assemblage also includes cysts of freshwater algae (also lacking biostratigraphic value)." The three palynomorphs identified by Nichols from this sample are Ghoshispora sp., Pityosporites sp., and Taxodiaceaepollenites hiatus.
All of the other samples collected from the upper trench shown on
Figure 63 were barren of palynomorphs, thus this attempt to relocate the bed from which the D6901 sample was collected was not successful. The level of that bed must be between sample 82303-E and the base of the Ojo Alamo Sandstone and is probably within the thin interval between the yellow-dashed line and the base of the Ojo Alamo. It is suggested that the Cretaceous-Paleocene interface may be located at the yellow-dashed line on
Figure 63. The pre-Paleocene unconformity, which, as discussed above, represents a 7.8 m.y. hiatus, is probably on an erosion surface on top of the harder and more massive blue-black strata shown beneath the yellow-dashed line on
Figure 63. The squeeze-ups seen at the base of the Ojo Alamo Sandstone are a clear indication that the sediments between the Paleocene interface and the base of the Ojo Alamo must have been unconsolidated and at least somewhat fluid at the time that the first high-energy streams carrying the gravels of the lower Ojo Alamo Sandstone flowed across the pre-Ojo Alamo erosion surface and rapidly built up to a thickness of sand and gravel of sufficient weight to cause the bed distortion seen on
Figure 62, and
Figure 63 at the base of the Ojo Alamo.
The thin gray unit below the base of the Ojo Alamo Sandstone (Figure 63) may be the bed from which the D6901 sample was collected in 1985 east of the trench in a place where palynomorphs were better preserved. This interval probably represents a soil layer or swamp deposit that filled a low-lying area on the pre-Paleocene erosion surface in early Paleocene time. The few Cretaceous palynomorphs found in the D6901 palynomorph assemblage could easily have been washed or blown into this layer from the adjacent Cretaceous land surface, as discussed in
Fassett et al. (2002, p. 318-319). It is possible, although less likely, that the Cretaceous-Tertiary interface is at a lower level in the upper trench, somewhere above sample 82303-E (Figure 63). There is another (subtle) lithologic break at the base of the blue-black massive shale bed marked by a thin greenish-gray silty layer just below the level of sample 82303-C (yellow-dotted line on
Figure 63) and perhaps the base of this unit represents the K-T interface. There are no other apparent lithologic breaks in the interval between samples 82303-C and 82303-E in the strata exposed in the trench.
Lucas, and Braman (2005) disputed the presence of Paleocene dinosaurs in the Ojo Alamo Sandstone of the San Juan Basin. These authors argued that the Paleocene age of the Ojo Alamo Sandstone on the basis of palynological data obtained in the Barrel Spring area (palynologic sample D6901,
Table 6), published in
Fassett et al. (2002), was invalid because they could not replicate those data; they stated (p. 402) that their "processing of several samples yielded no identifiable palynomorphs." Sullivan et al., however, did not specify where exactly their sampling had been done, did not supply photographs of their sample sites, or state how many samples had been collected and analyzed. It is here suggested that this vague reference to sampling from an unknown locality at an unknown distance below the base of the Ojo Alamo Sandstone with the results being samples barren of palynomorphs does not obviate the data reported by
Fassett et al. (2002). Those data contained the results of palynologic analysis of a rock sample from a specifically identified locality, less than 1 m below the base of the Ojo Alamo, that contained a diverse palynomorph assemblage of Paleocene age. As demonstrated in the discussion above, samples barren of palynomorphs have been frustratingly common throughout the San Juan Basin (see
Figure 63), however, rock samples barren of palynomorphs, from whatever locality, have no geochronologic value to prove or disprove anything.
Lucas, and Braman (2005, p. 402) stated that they found "no physical evidence of an unconformity . . . of at least 7.5 million years" in uppermost Kirtland Formation strata in the Barrel Spring area. Unconformities can be subtle, and it is here suggested that because these authors did not carefully trench the interval in the uppermost Kirtland up to the base of the Ojo Alamo Sandstone in the Barrel Spring area, they did not observe the subtle lithologic changes shown on
Figure 63 and discussed in detail above. The rocks of the uppermost Kirtland Formation, immediately beneath the Ojo Alamo Sandstone in the Ojo Alamo Sandstone type area, are 73.04 Ma based on a 40Ar/39Ar single-crystal sanidine age. In addition, palynomorphs identified from multiple rock samples from within and immediately below the Ojo Alamo Sandstone in the Barrel Spring area have an early, but not earliest Paleocene age. (See
Figure 58 for a depiction of these relations.) Therefore, an unconformity of about 7.8 m.y. must be present at or near the base of the Ojo Alamo in this area, even though this hiatus is not, at first glance, physically apparent.
Sample 110303-D was collected from a coaly, carbonaceous shale bed about 0.3 m below the base of the Ojo Alamo Sandstone at a locality 225 m east of the trench locality (Figure 51). According to Nichols, this sample also yielded a diverse and well-preserved assemblage (Table 13.2) of early Maastrichtian age. At this locality, the K-T interface is apparently at the base of the Ojo Alamo Sandstone. This finding supports the suggestion above that the Paleocene rocks in the uppermost Kirtland Formation in the Barrel Spring area (Figure 63) represent sedimentation in an isolated bog or pond present on the Paleocene erosion surface just before deposition of the overlying Ojo Alamo Sandstone conglomerates began. The squeeze-ups of this unconsolidated and somewhat fluid material (Figure 63) into the basal part of the Ojo Alamo are diagnostic of these Paleocene bog deposits that filled isolated low spots on the pre Ojo Alamo Sandstone erosion surface.
The palynomorphs listed in the column headed: "Locality D6901" ("archival split 24-5") on
Table 14, were identified by D.H. Nichols (personal commun., 2003) from a new analysis of a sample reported by Nichols to be a split of USGS archival sample 24-5 (from USGS Paleobotany Locality 6901,
Figure 56). The label "24-5" is from Fassett et al. (2002). The palynomorph assemblage from the "archival split" appears to differ markedly from the assemblage identified from the original analysis of this sample (column headed "Locality D6901 (sample 24-5)" on
Table 14). There are only 12 common palynomorphs (33%) out of the 36 palynomorphs identified from these two sample preparations, and 24 palynomorphs (67%) are not common. A composite list of palynomorphs from USGS localities D6877-A, -B, and -C from the San Juan River locality (Figure 1) has also been added to
Table 14 for comparative purposes. This Paleocene assemblage has a greater degree of commonality of palynomorphs with the original D6901 list than does the "archival split": 15 common palynomorphs out of 36 (42%) vs. 19 not-common (58%). This higher percentage of common palynomorphs is even more remarkable in consideration of the fact that these two sample localities are nearly 50 km apart. The low percentage of commonality of palynomorphs (33%) from sample D6901 and "archival split 24-5" (reported to be from the original D6901 sample) casts serious doubt as to whether these two palynomorph lists actually came from splits of the same sample. It is suggested that "archival split 24-5" may have come from a different USGS archival sample from Cretaceous strata and not from the original "24-5" sample.
Northeast San Juan Basin. The locations of the four palynologic samples from the northeast part of the San Juan Basin are shown on
Figure 1. Palynomorphs identified by R.H. Tschudy from the D4119 locality were published in
Fassett and Hinds (1971) and are listed on
Table 7 (they are also shown on
Table 15 with the three other palynomorph lists from the northeastern San Juan Basin). The other three samples (D5393, D5394, and D5408) were collected by R.T. Ryder in 1975 and were reported on by Tschudy (personal commun., 1976). Two of these samples (D5393 and D5394) were collected from the lowermost part of the Fruitland Formation: D5393 was from a mine dump from a coal bed 6 m above the base of the Fruitland, and D5394 was collected from a bed 3 m above the base of the Fruitland near an outlier (Klutter Mountain) of Pictured Cliffs Sandstone, Fruitland Formation, and Ojo Alamo Sandstone (Figure 1) about 19 km east of locality D5393. The D4119 locality, also from a coal bed a few meters above the base of the Fruitland is located about 9 km southeast of the D5393 locality (Figure 1). Tschudy wrote of the palynomorph assemblage from locality D5393:
This assemblage is clearly of Late Cretaceous age. The estimation of lower Fruitland is consistent with the palynomorphs found. Kuylisporites has not been found in rocks younger than middle Campanian and is present in the lower part of the Fruitland Formation in the Gasbuggy core.
For the D5394 assemblage, Tschudy reported that:
This assemblage is of Late Cretaceous Campanian age. All taxa have been found previously in the Fruitland Formation. The presence of Phaseolidites stanleyi suggests lower Fruitland, but I am unable to restrict the assemblage to the lower part of the Fruitland.
Tschudy's comments in
Fassett and Hinds (1971, p. 21) regarding sample D4119 are:
Sample D4119 I believe to be Cretaceous. The coal yielded a poor corroded assemblage. I was able to find only two specimens of the marker genus Proteacidites. This assemblage appears to have a closer resemblance to the Trinidad and Vermejo of the Raton Basin than to the assemblage reported by Anderson from the southern San Juan Basin.
As Table 15 indicates, the three palynomorph lists for samples from the lowermost Fruitland Formation in the northeastern San Juan Basin, all relatively close together, have remarkably few common taxa. This would seem to indicate that local environmental conditions had a profound affect on the plant assemblages even at closely spaced localities; at least in this part of the San Juan Basin.
The fourth palynomorph list on Table 15 is for sample D5408; this sample was collected from the Animas Formation about 150 m above its base (Figure 1). Tschudy (personal commun., 1975) wrote that this sample:
. . . yielded a very sparse palynomorph flora . . . The presence of these two species of Momipites [Table 15] in an otherwise very poor assemblage, definitely indicates a Paleocene age for the sample.
Composite Palynomorph Lists by Locality
Mesa Portales and Anderson's Localities Near Cuba, New Mexico.
Table 16 shows composite palynomorph lists for the southeastern part of the San Juan Basin, including all of the palynomorphs identified at and near Mesa Portales (Table 7 and
Table 9) and in Anderson (1960, table 1),
Table 5. These lists consist of palynomorphs identified from the Fruitland and Kirtland Formations, the Ojo Alamo Sandstone, and the Nacimiento Formation. There are 144 taxa listed on
Table 16; of these only 12 (9%) are common to the Cretaceous Fruitland-Kirtland Formations and the Paleocene Ojo Alamo Sandstone. At first glance this
evidence would seem to indicate an enormous die-off of plants at the K-T interface in the southeastern part of the basin. However, because of the 7.8 m.y. hiatus at the K-T boundary, representing all of Maastrichtian, part of Campanian, and also a small part of earliest Paleocene time, this apparent sudden die-off actually represents species that died off over a nearly 8 m.y. period and not suddenly at the end of the Cretaceous. There are 72 taxa in Paleocene strata listed on
Table 16; 22 taxa (31%) are common to the Ojo Alamo Sandstone and Nacimiento Formation. There was apparently continuous deposition across the contact between these formations (Fassett 1966), consequently this relatively small percentage of common forms probably indicates a distinct change in depositional environments in this area from Ojo Alamo to Nacimiento time. There are 37 taxa listed on
Table 16 from the Nacimiento vs. 57 for the Ojo Alamo, thus, this disparity in numbers of taxa preserved and identified from these two formations may be skewing this percentage.
Pot Mesa Locality. The composite list of palynomorphs identified from the Pot Mesa locality (Table 17,
Figure 60) are all from the Cretaceous Kirtland Formation. No samples collected for palynologic analysis from the Paleocene Ojo Alamo Sandstone at Pot Mesa were productive. There are 58 taxa shown on the palynomorph list on
Ojo Alamo Sandstone Type Area.
Table 18 shows composite palynomorph lists for the Ojo Alamo type area and nearby areas to the west (Figure 1,
Figure 3, and
Table 18 lists 112 taxa; of these, 11 of 81 taxa (14%) are common to the Fruitland and Kirtland Formations, 17 of 73 taxa (23%) are common to the Kirtland and the Paleocene uppermost Kirtland, 13 of 52 taxa (25%) are common to the uppermost Kirtland and upper Ojo Alamo, and 5 of 37 taxa (14%) are common to the upper Ojo Alamo Sandstone and lower Nacimiento Formation. Cretaceous taxa total 81, and Paleocene taxa total 52.
San Juan River Locality. The palynomorphs listed on
Table 19 from the San Juan River locality were all originally reported to be from the Ojo Alamo Sandstone, however, as discussed above, the palynomorphs identified by
Newman (1987) from that locality were probably not collected from the Ojo Alamo but were probably from the underlying Kirtland Formation. For this reason, Newman's palynomorph list is shown to be from the Cretaceous Kirtland Formation on
Table 19. There are 48 taxa listed on this table; six are from the Kirtland Formation, and 43 are from the Ojo Alamo Sandstone. The only common palynomorph to the two lists is Tschudypollis and because palynomorph assemblages identified from the Ojo Alamo Sandstone are Paleocene in age, the specimens of Tschudypollis in the Ojo Alamo Sandstone are considered to be reworked. (See discussion of reworking of Cretaceous palynomorphs into Paleocene strata in
Nichols and Fleming 2002.)
Northeast San Juan Basin.
Table 20 lists the palynomorphs identified from the northeastern part of the San Juan Basin from the lowermost Fruitland Formation and from the lower part of the Animas Formation. Of the 36 taxa listed, only two (6%) are common to the Fruitland and Animas Formations. This low percentage of commonality is clearly skewed by the small number (6) of palynomorphs identified from the Animas in this area.
Palynomorph Lists by Formation
Fruitland Formation. Table 21 contains composite lists of palynomorphs from four areas in the San Juan Basin. Two areas—the Ojo Alamo type area and the Mesa Portales area—are in the southern part of the basin, and two areas—the Gasbuggy core and the northeastern San Juan Basin area are in the northern part of the basin. Of the 155 taxa listed on
Table 21, there is a low percentage of common taxa between these areas; the Ojo Alamo type area and the Mesa Portales areas only have six palynomorphs in common (5%) out of 110 taxa from those two areas. This low percentage is probably skewed by the different numbers of taxa identified from these two areas: 29 from the Ojo Alamo type area and 87 from the Mesa Portales area. The Mesa Portales and Gasbuggy core lists have 18 of 121 taxa in common (15%). The Gasbuggy core and the northeast San Juan Basin lists have 12 of 106 taxa in common (11%). These relatively low percentages of commonality of palynomorphs for Fruitland Formation assemblages suggest that depositional environments were variable across the basin during Fruitland Formation time. Moreover, the time-transgressive nature of Fruitland Formation strata—becoming about 2 m.y. younger from southwest to northeast across the basin—probably contributed to the lack of commonality for assemblages in the southwest and northeast.
Kirtland Formation. Palynomorphs collected from the Kirtland Formation were identified from only the two localities shown on
Table 22. The Pot Mesa area is 62 km southeast of the Ojo Alamo type area (Figure 3). The Fruitland-Kirtland interval is about 145 m thinner at Pot Mesa than at the Ojo Alamo type area, thus palynomorphs collected from the upper part of the Kirtland Formation at Pot Mesa are about 145 m stratigraphically lower than the upper-Kirtland samples from the Ojo Alamo type area. The Pot Mesa samples from upper Kirtland strata are thus considerably older than those from the Ojo Alamo type area. That is probably the reason why there are only 11 common taxa (10%) out of a total of 108 palynomorphs identified from the two localities.
Comparison of Fruitland and Kirtland Formations Palynomorphs.
Table 23 lists 206 palynomorph taxa identified from the Fruitland and Kirtland Formations from all localities in the San Juan Basin. Of these, 46 taxa (22%) are common to the Fruitland and Kirtland Formations. Of the 206 total taxa, 106 are present only in the Fruitland, and 53 are present only in the Kirtland. The Cretaceous index palynomorph Tschudypollis spp. is ubiquitous and the most abundant form in all Fruitland and Kirtland samples. A preliminary zonation of the Fruitland-Kirtland is presented in the
section of this appendix, below.
Ojo Alamo Sandstone. The Paleocene Ojo Alamo Sandstone and the Paleocene (uppermost) parts of the Kirtland and Fruitland Formation have yielded 101 palynomorphs in the San Juan Basin (Table 24).
Table 25 shows that the Ojo Alamo Sandstone and underlying Cretaceous rocks have yielded 243 taxa between them; of these taxa, 192 are restricted to Cretaceous strata, 51 are common to Cretaceous strata, and 51 were found only in Paleocene strata. (The two occurrences of reworked Tschudypollis shown on
Table 24 are not included in these numbers.) A comparison of Cretaceous and Paleocene palynomorphs within the San Juan Basin and with other Western Interior basins is presented in the section of this report labeled
"Comparison of Palynomorphs," below.
The guide palynomorphs, Brevicolporites colpella and (or) Momipites tenuipolus have been identified from Ojo Alamo Sandstone samples at numerous localities in the San Juan Basin.
These palynomorphs are known to be restricted to Paleocene-age strata throughout the Western Interior of North America (Nichols and Johnson 2002).
Nichols (2002) discussed the significance of M. tenuipolus as a Paleocene index fossil throughout the Western Interior. Nichols (p. 124), and in his discussion of biozone P1 in the Raton Basin, stated that:
Momipites tenuipolus and M. leffingwellii occur only in the middle to upper part of the biozone [P1], thereby delimiting a basal Paleocene subzone in which M. inaequalis is present but the other species of Momipites are absent. This basal Paleocene subzone of Zone P1 is recognizable in the Denver Basin, as well.
This finding is in accord with statements by R.H. Tschudy in numerous reports to the author (written communs. cited above) that M. tenuipolus "is limited to the Paleocene but is not present in the lowermost Paleocene" in the San Juan Basin.
In his conclusions, Nichols (2003, p. 130-131) emphasized that:
With attention to the influence of paleolatitude, local zonations can be integrated into a comprehensive nonmarine biostratigraphy for the lower Cenozoic of the Rocky Mountains and Great Plains region.
The Raton Basin of northeastern New Mexico and southeastern Colorado is only 230 km east of the San Juan Basin and is at the same latitude. It is thus reasonable to conclude that in the San Juan Basin, M. tenuipolus is also restricted to the upper part of biozone P1 and is absent from lowermost Paleocene rocks. M. tenuipolus has been identified from a sample less than a meter below the base of the Ojo Alamo Sandstone in the Ojo Alamo type area and 8 m above the K-T interface at Mesa Portales. Because M. tenuipolus is only present in the upper part of zone P1, it can thus be concluded that the lowermost part of zone P1 (the lowermost Paleocene) is missing in the San Juan Basin. This finding agrees with paleomagnetic data indicating that the base of the Ojo Alamo Sandstone has an age of about 65.2 Ma (see
Figure 34 and
Figure 35) suggesting a 0.3 m.y. gap in the lowermost Paleocene in the southern part of the San Juan Basin.
Nacimiento Formation. Palynomorphs identified from the Paleocene Nacimiento and Animas Formations are listed in
Table 26. Samples from these formations at nine localities in the San Juan Basin yielded 69 identified palynomorphs;
sample collections ranged from near the base to 150 m above the base of these
formations. The two localities labeled "WNW 2008—Kimbeto Arroyo" were discussed in
et al. 2008. These authors determined that (p. 9): "Both palynomorph assemblages, which are reported here, contain palynomorphs that are characteristic of early Paleocene assemblages that are widespread in the Rocky Mountain region." The taxa identified from the samples collected from strata 19 m above the base of the Nacimiento Formation (top of the Ojo Alamo Sandstone) were found to contain "palynomorphs characteristic of P1–P3 pollen zones" and the assemblage from samples 105 m above the base of the Nacimiento contained "palynomorphs characteristic of P3." The P1 through P3 pollen zones mentioned here refer to the Paleocene pollen zonation established in the northern part of the Western Interior of North America by Nichols (2003). The paper by Williamson et al. (2008) is the first to correlate the Paleocene palynomorph zonation of the northern part of the Western Interior with palynomorph assemblages in the southern part in New Mexico.
Williamson et al. (2008, p. 3) identified the palynomorph Momipites triorbicularis in their sample 105 m above the base of the Nacimiento Formation and stated that this taxa "is indicative of palynostratigraphic Zone P3."
Nichols (2003, figure 2), however, shows this guide fossil's first appearance as more specifically being at or near the boundary between the P3 and P4 zones with its range extending up into the middle of zone P5. This diagnostic fossil was also identified from a sample collected 150 m above the base of the Animas Formation at locality D5408 in the northern San Juan Basin in Colorado (Table 26). The presence of this palynomorph 45 m above its presence lower in the Nacimiento Formation would thus indicate that the strata at this higher level are in zone P4, and that the P3-P4 boundary is just above the sample 105 m above the base of the Nacimiento.
Nichols (2003, figure 2) shows the boundary between biozones P2 and P3 to be between the last occurrence of M. inaequalis and the first occurrence of M. triorbicularis. M. inaequalis is present 20 m above the base of the Nacimiento at the D3803 locality (Table 26), and M. triorbicularis is present at the Kimbeto Arroyo locality 105 m above the base of the Nacimiento (Table 26). The boundary between zones P2 and P3 must, therefore, be between these two stratigraphic levels in the San Juan Basin—probably not far above the stratigraphic level of M. inaequalis.
Williamson et al. (2008) also pointed out that the Paleocene index fossil M. tenuipolus was found in their lower sample, 19 m above the base of the Nacimiento Formation, and
Table 26 shows that this palynomorph was also found 150 m above the base of the Nacimiento at the D5408 locality. As this table shows, M. tenuipolus is by far the most common palynomorph identified from Nacimiento Formation samples and is present in seven of the nine palynomorph lists shown.
Palynomorph lists from the Nacimiento Formation and the Ojo Alamo Sandstone are compared in
Table 27. This table shows that 101 palynomorphs have been identified from the Ojo Alamo Sandstone, 69 from the Nacimiento Formation, and 49 palynomorphs are common to both formations. The total number of palynomorphs from both formations is 125. Fifty six palynomorphs are present only in the Ojo Alamo Sandstone, and 24 are found only in the Nacimiento Formation. The high percentage of commonality of palynomorphs for the Ojo Alamo and Nacimiento formations attests to uninterrupted deposition across the boundary between these two formations. These data thus further reinforce the findings presented elsewhere in this report that the Ojo Alamo Sandstone is Paleocene in age.
Table 28 shows the zonation of palynomorphs from the lower and upper parts of the Fruitland Formation and the upper Kirtland Formation in the southern part of the San Juan Basin. As the time-stratigraphic cross sections of
Figure 34 and
Figure 35 show, Fruitland and Kirtland strata are late Campanian in age in the southwest part of the basin; the uppermost part of the late Campanian and all of the Maastrichtian are missing.
Figure 64 shows the stratigraphic positions of these three palynomorph assemblages projected into drill-hole 2 (see
Figure 34, and
Figure 35) near the Ojo Alamo Sandstone type area.
Table 28 shows that 34 palynomorphs from lower Fruitland samples are not present in stratigraphically higher Cretaceous samples, 12 palynomorphs are common to the lower and upper Kirtland samples, and eight palynomorphs are present in the lower Fruitland and upper Kirtland samples. Eight palynomorphs are unique to the upper Fruitland samples, and 21 palynomorphs identified from upper Fruitland samples are not present in upper Kirtland samples. Eight taxa are common to the upper Fruitland and upper Kirtland samples, and three are common to all three sample lists. As for upper Kirtland samples, 40 taxa are found only in these samples.
Table 28 clearly shows the progressive disappearance of palynomorphs and the appearance of new taxa going stratigraphically upward through the Campanian Fruitland and Kirtland Formations in the San Juan Basin. Of the taxa present in the lower Fruitland palynomorph list of this table, only two genera reappear in Paleocene strata: Araucariacites as A. australis and Triporopollenites as T. plektosus and T. rugatus (Table 29). Of the taxa present in the lower and (or) upper Fruitland but absent in the upper Kirtland of
Table 28, five genera also appear in the Paleocene: Gleicheniidites as G. senonicus, Momipites sanjuanensis, Nyssapollenites spp as N. explanatus, Tricolpites spp., and Triporopollenites as T. tectus and T. plektosus (Table 29).
Table 29 compares Cretaceous (Campanian) and Paleocene palynomorph assemblages in the San Juan Basin; 244 palynomorphs are listed on this table. Of these, 50 taxa (20%) are present only in Paleocene strata, 143 taxa (59%) are present only in Cretaceous strata, and 51 taxa (21%) are common to Cretaceous and Paleocene strata. The 23 taxa shown in magenta are Cretaceous index palynomorphs in the Raton Basin (Fleming 1990) and (or) the Northern Great Plains (Nichols and Johnson 2002). The palynomorphs shown in blue are Paleocene index fossils in the Raton Basin or Northern Great Plains. All but one of the Cretaceous index palynomorphs of the Raton Basin and Northern Great Plains Tricolpites spp. are absent in Paleocene strata of the San Juan Basin. Taxa, reported to be Cretaceous index fossils in the Raton Basin are present in Paleocene strata at three different localities in the San Juan Basin.
As Table 29 shows, Brevicolporites colpella and Momipites spp. are Paleocene index palynomorphs in the Northern Great Plains (Nichols and Johnson 2002).
Fleming (1990, p. 247) stated that: "in the Raton Formation [in the Raton Basin], Momipites tenuipolus first appears 16 m above the K-T boundary in the Momipites inaequalis zone and ranges to near the top of the formation." Fleming, however, indicated that B. colpella may have been identified in Cretaceous strata in the Raton Basin. It would appear that both B. colpella and M. tenuipolus are Paleocene index palynomorphs in the San Juan Basin. A more detailed comparison of palynomorph occurrences in the San Juan Basin and other Western Interior basins is made difficult by the fact that in those basins there was apparently continuous deposition across the Cretaceous-Tertiary boundary, whereas in the southern San Juan Basin, there is a nearly 8 m.y. hiatus at the K-T interface with all of the Maastrichtian, the uppermost part of the Campanian, and a small interval of lowermost Paleocene absent. A comparison of palynomorphs found in Campanian strata in the San Juan Basin (Table 28) with palynomorph assemblages of the same age from other Western Interior basins would be instructive, but is beyond the scope of this report.
Summary of Palynology
This appendix synthesizes all published palynologic data for rocks adjacent to the Cretaceous-Tertiary (K-T) interface in the San Juan Basin, and in addition, presents additional new palynomorph lists to add to the basin's published palynologic database. Palynomorphs identified from the Upper Cretaceous Fruitland and Kirtland Formations are twice as abundant as those from the Paleocene Ojo Alamo Sandstone and the Nacimiento and Animas Formations. This is because the collection sites for Cretaceous samples are much more numerous, the samples from the Ojo Alamo have generally yielded far fewer identifiable palynomorphs, and because palynomorph diversity is apparently less for the Ojo Alamo, Nacimiento, and Animas formations.
Composite-palynomorph lists are presented in
Table 24, and
Table 25 synthesizing all available palynologic
data for the San Juan Basin. Table 16,
Table 19, and
Table 20 present composite palynomorph
lists for each locality where such data have been obtained;
Table 23, and
Table 24 compare palynomorph lists for Cretaceous and Paleocene strata, and
Table 25 lists all palynomorphs identified from Cretaceous and Paleocene strata for the entire basin.
Table 26 lists palynomorphs from the Nacimiento and Animas Formations, and
Table 27 compares Ojo Alamo Sandstone with Nacimiento-Animas palynomorphs.
Table 28 shows the palynologic zonation of uppermost Cretaceous strata, and
Table 29 compares the total Cretaceous vs. Paleocene palynomorph lists.
The discussions above address the statistical variations in numbers of palynomorphs identified from the formations adjacent to the Cretaceous-Tertiary interface in the San Juan Basin, and represent the empirical observations of a non-palynologist. A much more nuanced interpretation of these data could, and should, be made by an experienced palynologist. These numbers are skewed by the variable numbers of sample-collection localities in each formation and by variations in the diversity of palynomorphs present in these samples. In addition, going upward in the stratigraphic section from the lower Fruitland Formation through the Kirtland Formation (upper Campanian) the depositional environments change progressively from near shore, swampy conditions, to coastal plane, and ultimately to well-drained, continental, fluvial environments well inland of the Western Interior Seaway's regressive shoreline far to the northeast. These different environments undoubtedly contributed to the stratigraphic variations in palynomorph assemblages in the Fruitland and Kirtland and, to a degree, may cloud the evolution, extinction, and first occurrences of palynomorphs going upward in the section. The depositional environments for palynomorph-sample localities within the Ojo Alamo Sandstone and overlying Nacimiento Formation were probably less variable across the basin.
The last occurrences of 22 taxa in Cretaceous strata of the San Juan Basin are in agreement with last occurrences of these taxa in the Raton Basin (Fleming 1990) and the Northern Great Plains (Nichols and Johnson 2002). These last occurrences unequivocally mark the Cretaceous-Tertiary (K-T) interface in the San Juan Basin. The last occurrence of the principle Cretaceous index fossil: Tschudypollis (formerly Proteacidites); sharply defines the K-T interface in the southeastern San Juan Basin at Mesa Portales, in Anderson's (1960) collecting localities and in the Gasbuggy core. In addition, the Paleocene index palynomorphs Brevicolporites colpella and Momipites tenuipolus have been identified in the Paleocene Ojo Alamo Sandstone (and the Paleocene part of the uppermost Kirtland Formation) at two localities in the San Juan Basin. Because M. tenuipolus is restricted to the upper part of biozone P1 and is not present in lowermost Paleocene strata in the Western Interior of North America (Nichols 2003), the presence of this guide fossil in the lowermost part of the Ojo Alamo Sandstone supports paleomagnetic evidence suggesting that as much as 0.3 m.y. are not represented by Paleocene rocks in the San Juan Basin.