PHYSICAL STRATIGRAPHY OF K-T BOUNDARY STRATA

Lithology and Mode of Deposition of Ojo Alamo Sandstone

The Paleocene Ojo Alamo Sandstone is a prominent stratigraphic unit throughout the New Mexico part of the San Juan Basin. This formation forms the striking, massive, 100 m high vertical cliffs along the south side of the San Juan River on the south side of the city of Farmington in the west-central part of the basin (Figure 1). The Ojo Alamo is a coarse-grained, conglomeratic sandstone that crops out around the periphery of most of the New Mexico part of the San Juan Basin but is absent in the northern part (mostly in Colorado, Figure 1). The Ojo Alamo was deposited on a basin-wide erosion surface in early, but not quite earliest, Paleocene time by south-to-southeasterly flowing, high energy, braided streams (Fassett 2000, Fassett et al. 2002). A hiatus of nearly 8 m.y., at (or in a few places, slightly below) the base of the Ojo Alamo, separates Cretaceous and Tertiary rocks in the southern part of the basin (Fassett 1982, 1987, 2000, Fassett and Steiner 1997, Fassett and Lucas 2000, Fassett et al. 2002).

The Ojo Alamo is a multi-storied conglomeratic sandstone with highly varied internal architecture and thicknesses throughout the basin (Fassett et al. 2002, figures 4 and 5). Conglomerate clasts range from near-boulder size in the northwest part of the basin to small pebbles and grit in the southeast part. The rock-stratigraphic definition and age of the formation have been characterized differently by various workers over the years as discussed in numerous papers; those discussions are summarized and referenced in Fassett et al. (2002). Figure 2 shows the principal differences in the ways the Ojo Alamo has been characterized in its type area and the way the name is used in this report. The so-called middle, "shaly" part of the Ojo Alamo in the type area of the Ojo Alamo Sandstone is a mischaracterization of this interval because it contains multiple sandstone beds, and these sandstones represent a significant part of the interval. In the type area, the sandstones of the middle part of the Ojo Alamo are white and relatively friable rather than having the rusty-brown color of the harder lower and upper benches, thus these beds do not typically form cliffs or ledges. Photographs of the Ojo Alamo Sandstone at several outcrop localities are included in this report and show the nature and variability of the lithology of this formation.

The uppermost sandstone bench of the Ojo Alamo Sandstone at most exposures, is rusty brown in color, tightly cemented, and forms a vertical cliff face. In some exposures, such as south of Farmington (Figure 1), where the Ojo Alamo consists of as many as four, stepped-back benches, the uppermost sandstone bed capping each bench is also rusty brown, tightly cemented, and forms a vertical cliff. At many localities, sandstone beds of the Ojo Alamo lying below the upper, cliff-forming, rusty-brown bed, are less well cemented, are whiter in color, and weather into gentler slopes.

This phenomenon is probably the result of downward-moving ground water, containing more iron in solution, moving laterally and selectively through the relatively more permeable, uppermost Ojo Alamo sandstone beds, thus cementing them more tightly and giving them their distinctive rusty-brown color. In the southern part of the San Juan Basin, this relationship has misled some workers into thinking that there is a continuous uppermost sandstone bed of the Ojo Alamo that is widespread throughout large parts of the basin, whereas in reality, these upper beds are separate lenses of the Ojo Alamo that happen to be rusty brown and more tightly cemented. This misconception has been exacerbated by the presence of numerous sand-filled arroyos that cut through the Ojo Alamo outcrop preventing the continuous tracing of this rock unit.

Relation of Ojo Alamo Sandstone to Underlying Strata

The stratigraphy of the rocks adjacent to the Cretaceous-Tertiary interface in the San Juan Basin has been discussed in numerous papers since about the beginning of the 20th century. The first publication to describe the geometry of these strata over a large part of the basin was Reeside (1924). Reeside presented a series of 20 measured sections around the north, west, and south edges of the basin; these sections showed thinning of the Fruitland-Kirtland interval from 561 m northwest of Farmington (Figure 1) to 0 m northeast of Cuba, New Mexico. Reeside (1924, p. 52, 53, figure 3) suggested that the thinning of Kirtland-Fruitland strata, from northwest to southeast across the basin, was the result of uplift and erosion of Cretaceous strata that was much greater in the southeast part of the basin. Reeside acknowledged that at some outcrops, the base of the Ojo Alamo Sandstone appeared to be concordant with underlying Cretaceous strata, but in spite of that, he was convinced that there was a significant erosional hiatus at or below the base of the Ojo Alamo.

Dane (1936), however, on the basis of surface mapping of Cretaceous and Tertiary rocks in the southeastern part of the basin, concluded that there was no erosional unconformity at the base of the Ojo Alamo. He reached this conclusion because he found places where a sandstone bed at the base of the Ojo Alamo thinned laterally and pinched out. At those localities, the shale above thickened and appeared to merge with the underlying Kirtland Shale. Dane indicated that at such places, the base of the Ojo Alamo should be shifted upward to the base of the next-highest sandstone bed. Dane thus concluded that there was continuous deposition across the Kirtland-Ojo Alamo contact, and thus there was no significant hiatus at this contact.

The geometry of the Ojo Alamo Sandstone at Mesa Portales (Figure 1, Figure 3) illustrates the exact situation described by Dane. There, a lower bench of the Ojo Alamo Sandstone pinches out into mudstones to the east, and its basal contact is then apparently at the base of the higher sandstone bed (Fassett 1966, Fassett and Hinds 1971, figure 12). At this locality, however, the actual erosion surface at the K-T interface is about 22 m below the base of the rock stratigraphic Ojo Alamo at the base of a sandy interval (Fassett and Hinds 1971). Farther east on Mesa Portales, where the lower sandy interval marking the unconformity pinches out, the Cretaceous-Tertiary interface becomes difficult to discern, although it usually can be found with diligent searching.

Following Dane's 1936 paper, dozens of local studies of the rocks adjacent to the Cretaceous-Tertiary interface were published, with authors taking varying positions regarding the presence or absence of an unconformity at (or near) the base of the Ojo Alamo. These different interpretations were reviewed in Fassett and Hinds (1971); Fassett (1973, 1987, 2000); and Fassett et al. (2002). Fassett and Hinds (1971) presented a synthesis of all previously published data and included an analysis of hundreds of geophysical logs of drill holes throughout the basin to precisely map the subsurface relations of uppermost Cretaceous and lowermost Paleocene strata and to assess the coal resources of the Fruitland Formation throughout the basin. This study confirmed Reeside's interpretation of the thinning of the Fruitland-Kirtland interval from northwest to southeast (Figure 1).

Fassett and Hinds (1971) concluded that Fruitland and Kirtland rocks were deposited by streams flowing northeastward toward the retreating shoreline of the Western Interior Seaway, and that this shoreline trended generally northwest throughout the time it was retreating northeastward across the San Juan Basin area. Moreover, they challenged an earlier contention by Silver (1950) that a basin of deposition had existed in the northwestern part of the San Juan Basin in Kirtland Formation time (named by Silver the "Kirtland basin"). Silver had inferred the presence of this basin solely on the basis of an isopach map of the Fruitland-Kirtland interval that showed much greater thicknesses of these rocks in the northwestern part of the basin. Fassett and Hinds (1971, figure 11) produced a more detailed Fruitland-Kirtland isopach map that showed in much greater detail how these rocks thinned southeastward across the basin. They also concluded that Silver's concept of a Kirtland basin was incorrect because the strata beneath the Ojo Alamo had been truncated from northwest to southeast across the basin during a pre-Ojo Alamo erosion cycle.

The only published study of paleo-current directions for the Fruitland-Kirtland interval was by Dilworth (1960) who measured cross-bedding in the Farmington Sandstone Member of the Kirtland Formation at five localities west of Farmington (Figure 1). Dilworth observed that streams depositing the Farmington Sandstone flowed from southwest to northeast. Dilworth's paleo-current study supports the findings of Fassett and Hinds (1971) that Fruitland and Kirtland strata were deposited by northeast-flowing streams.

Two comprehensive studies of paleo-current directions of the Ojo Alamo Sandstone (Powell 1973, Sikkink 1987) showed that the Ojo Alamo Sandstone was deposited by high-energy streams flowing from the north or northwest. Those conclusions are supported by the fact that the conglomerate clasts of the Ojo Alamo Sandstone become smaller from north to south and from west to east across the basin. Sandstone beds in Fruitland-Kirtland strata are fine to very-fine grained, whereas Ojo Alamo Sandstone beds are coarse-grained sandstone and conglomerates containing near-boulder-size clasts in the northwest part of the basin. These contrasting lithologies have made the mapping of the basal contact of the Ojo Alamo Sandstone on the outcrop and in well logs straightforward and uncontroversial.

Butler and Lindsay (1985) resurrected Silver's (1950) "Kirtland basin" model and argued for a northwest sediment source for the Kirtland Formation and the Ojo Alamo Sandstone. They named this the "clastic-wedge" model for Fruitland-Kirtland deposition. This "clastic-wedge" model was based primarily on the assumption that there had been continuous deposition across the Kirtland-Ojo Alamo contact. These authors, however, stated that their model would effectively be disproved if precise dating of Kirtland strata proved that a substantial hiatus was present at the base of the Ojo Alamo Sandstone. Subsequent radiometric dating of altered volcanic ash beds in the Kirtland Formation, to within 5 m of its upper contact with the Ojo Alamo Sandstone by Fassett and Steiner (1997), demonstrated that nearly 8 m.y. are missing from the rock record at the Kirtland-Ojo Alamo contact in the southern San Juan Basin. Thus, the "clastic wedge" model of Butler and Lindsay (1985) has been refuted by their own suggested test of their model.

In summation, present data show that the Fruitland and Kirtland Formations were deposited by streams flowing northeastward toward the retreating Pictured Cliffs Sandstone paleo-shoreline. A single-crystal ⁴⁰Ar/³⁹Ar age of 73.04 ± 0.25 Ma for sanidine crystals from an altered volcanic ash bed in uppermost Kirtland Formation strata (Fassett and Steiner 1997) indicates that a nearly 8 m.y. hiatus exists between the top of the Kirtland Formation and the base of the Ojo Alamo Sandstone.

Animas and McDermott Formations

The Animas Formation was defined by Reeside (1924), and that definition was revised by Barnes et al. (1954). Those authors extended the base of the Animas Formation downward to incorporate the upper part of the underlying McDermott Formation of Reeside (1924) renaming these strata the McDermott Member of the Animas Formation. Barnes et al. (1954) reassigned the lower part of Reeside's McDermott Formation to the upper part of the Kirtland Formation (then named "Kirtland Shale"). The Animas Formation of Reeside (1924) thus became the "upper member of the Animas" as part of this redefinition. The southern extent of the McDermott Member was later restricted by Baltz et al. (1966) to the west side of the La Plata River, northwest of Farmington, New Mexico.

It is here recommended that the original definitions of the Animas Formation and McDermott Formation, as defined by Reeside (1924), be reinstated. Reeside's original McDermott Formation is an easily mappable unit of Late Cretaceous age (upper Campanian to lower Maastrichtian, as demonstrated by Newman 1987). The Animas Formation of Reeside (1924) is also an easily mappable unit of Paleocene age (Knowlton 1924, Newman 1987). An unconformity of several million years, representing the upper part of the Maastrichtian Stage separates these rock units. In retrospect, no useful purpose was served by the Barnes et al. (1954) redefinition of the Animas Formation, thus it is recommended that that redefinition be vacated in its entirety. The southern limit of the McDermott Formation suggested by Baltz et al. is still considered valid and should thus still stand.

The Animas Formation is present mostly in the Colorado part of the San Juan Basin (Figure 1) where it unconformably overlies, from west to east, the Cretaceous McDermott, Kirtland, and Fruitland Formations and is overlain by the Eocene San Jose Formation. The most detailed description of the lithology and stratigraphy of the Animas Formation is discussed in Reeside (1924). The Animas is a volcaniclastic rock unit that consists of coarse-grained to conglomeratic, reddish sandstone beds interbedded with olive-green, finer grained, overbank deposits. Knowlton (1924) presented a detailed study of the fossil leaves in the Animas and concluded that this flora indicated that the Animas was Paleocene.

The lower part of the Animas Formation is equivalent in age to the Ojo Alamo Sandstone. The Ojo Alamo has been mapped separate from the Animas in the northeastern part of the basin north and south of the Colorado-New Mexico State line (Figure 1). The upper part of the Animas is time-equivalent to the Nacimiento Formation in the southern (New Mexico) part of the San Juan Basin. The volcaniclastic content of the Animas is most prominent in the northern part of the basin and the formation grades southward into volcaniclastic-free fluvial and lacustrine sandstones and mudstones of the Nacimiento Formation near the Colorado-New Mexico State line (Figure 1); mudstones dominate the Nacimiento throughout most of the San Juan Basin.

Cretaceous-Tertiary Interface

The striking contrast between the fine- to medium-grained rocks of the uppermost Cretaceous Kirtland and Fruitland Formations and the coarse-grained to conglomeratic strata of the Paleocene Ojo Alamo Sandstone and the Animas Formation has made the mapping of the contact between these formations a relatively easy process in the San Juan Basin. This distinct physical contrast alone is clearly suggestive of a significant hiatus at the K-T interface. When the totally different current directions for rocks above and below the interface are added to the equation, northeast-flowing streams for Cretaceous strata and south- to southeast-flowing streams for Paleocene strata, the case for a substantial hiatus at the K-T interface is strengthened even more. Clearly, significant tectonic events (representing millions of years) must have occurred between the time of deposition of Cretaceous strata and Paleocene strata in the San Juan Basin. The geochronologic data obtained over the past few decades have now allowed us to precisely quantify this hiatus, as discussed below.