In the following section we describe stratigraphic correlations based on tracing various marker units, such as calcium carbonate layers and gravels. Faunal evidence is provided for support where appropriate, but is never the primary source of correlation. Twelve stratigraphic sections were measured between Keefe and Fox Canyons using a Jacob's staff and tape; corrections for dip of the rocks were applied where warranted. The stratigraphic sequence in this area is characterized by a lower, thick sand and gravel unit locally at least 9.0 m thick that is usually consolidated and cross-bedded near its top. We call this unit informally the ?Bishop gravel,? named after a large mammal locality associated with this horizon in EOA Canyon. A second sand and gravel unit, the Wolf gravel, is located approximately 13.5-17.0 m above the Bishop gravel. It is mostly unconsolidated, with some consolidated and cross-bedded ledges at various levels. Between these two gravels we recognize three calcium carbonate layers: (1) a lower massive white layer (unit CC1), with charophytes and ostracods visible in its upper half in some hand samples, (2) a middle, grayish, pitted layer, which contains root traces but no charophytes (unit CC2), and (3) an upper layer that where well exposed may be expressed as wavy carbonate stringers of varying thickness, associated with mudstones in the development of vertisol structure, or as isolated carbonate nodules in poor exposures (unit CC3). These units may vary locally in thickness and in appearance. Finally, a 2.0 m thick calcium carbonate layer, informally called the ?Alien Head caliche,? is present 3.0 m above the Wolf gravel. The name derives from the similarity of the caliche caprock at the head of Alien Canyon to the head of the monster by the same name in the movie Alien (fossil quarries and some gullies in EOA Canyon are named after characters in the Alien movie series [Martin et al ., 2000]). Below, we describe stratigraphic correlations in the five major canyons in our study area, moving from west to east, beginning with Keefe and ending with Fox Canyon.
We measured six sections in Keefe Canyon (figure 2). The Bishop gravel crops out in the southern most measured section (Dugout section; section 7), where the canyon is most deeply cut. Higher in section 7 is CC1, which is traceable up canyon as a prominent white ledge through the six measured sections. It is our primary correlational unit in Keefe Canyon. When sediments are well exposed immediately above CC1, large mammal fossils are usually present. Three fossil localities in Keefe Canyon represent this horizon (Raptor 3-5). CC1 is the same unit as the massive caliche 2.0 m above KU Loc. 22 (KC) described by Hibbard and Riggs (1949). In Keefe Canyon there are three fossil localities at this distance below CC1; KC (figure 2, section 2), KC Turtle (figure 2, section 3), and Raptor 1C (figure 2, section 6). A mollusk zone at least 0.3 m thick, Raptor 1A, is present in silty sands 2.4 m below the Raptor 1C level, the same distance as Hibbard and Riggs (1949, p. 833) reported for a mollusk zone below KC. This stratigraphic correlation is further supported by the record of Sigmodon holocuspis , a primitive sigmodontine rodent, from both Raptor 1C and KC. Good exposures of sediments between the Bishop gravel and CC1 show lateral facies changes interpreted as cross-cutting channels and their associated overbank deposits. Near KC (figure 2, section 2), 8.3 to 8.8 m of sandstone, siltstones, and calcium carbonate layers separate CC1 from the overlying Wolf gravel. CC2 and CC3 are found in this interval. Unit CC3 is well-exposed at the top of measured sections 4-6 (figure 2) as a distinctive, pale orange-brown siltstone and silty sandstone containing thin, undulating layers of calcium carbonate nodules and stringers interpreted as a vertisol.
Correlations of units between the localities in Keefe Canyon and those in the canyons to the east are established by following units CC1, CC2, and CC3 across the divide on the south side of Table Mesa, in NE1/4, SE1/4, NW1/4 sec. 3, T. 35 S., R. 30 W. Correlations have also been established by following the Wolf gravel unit north of Table Mesa. As in Keefe Canyon, we found both scattered large mammal bone and quarryable concentrations of bone (the rich Wiens locality) in Alien Canyon in sediments directly above CC1. East of the Wiens locality, CC1 and CC2 are visible as a couplet that can be traced across the hill-slopes, through Alien Canyon, and into the succession of localities in EOA Canyon. Measured section 1 (figure 2) is from Alien Canyon, and also includes the measured levels of the new fossil localities in EOA Canyon. The section in Alien Canyon is the most complete that we have found north of the Cimarron River. Due to a 2° to 3° dip of Pliocene sediments toward the river in Alien and EOA Canyons, erosion does not penetrate into the lower portions of the sediments, even in the lower reaches of the canyons. The lowest fossil level is Bishop, a large mammal site in mudstones immediately on top of the Bishop gravel. Lateral to the Bishop fossil site the underlying sand and gravel is cross-bedded, but immediately beneath the site the sandstone is convoluted. This disturbed bedding may have been the result of bioturbation by animals, or due to water escape at the site of an ancient spring. Support for the latter idea is found at the Camel Pod locality in this canyon about 100 m east of Bishop. Here, lenticular masses of highly calcareous sandstone, some with mammal bones, are connected by three dikes to the sand and gravel unit underlying Bishop. We believe that these dikes represent the conduits to a spring, and that the sand filling these dikes was injected from the underlying sand and gravel unit.
Ripley A, located 2.2 m above the Bishop gravel, is a light greenish-gray, fine-grained silty sandstone containing mollusks. Ripley B, a dark grayish-brown, organic-rich sandstone and siltstone, containing abundant large mammal bones and rodent teeth, is developed directly above Ripley A. Both units likely represent a pond deposit. The top of the Ripley A level is 2.8 m below CC1 (figure 3; measured section 1), compared to a distance of 1.8 m below CC1 for Raptor 1C in Keefe Canyon. The inferred stratigraphic position of Ripley B below Raptor 1C is indirectly substantiated by comparison of cotton rat specimens from Raptor 1C, Ripley B, and the House Mountain locality of the Verde l.f, the type locality of Sigmodon holocuspis . Both S. holocuspis and S . Minor were recorded by Czaplewski (1987) from House Mountain. The cotton rat from Ripley B represents a more primitive morphological grade of S. minor than that from House Mountain (Peláez-Campomanes and Martin, in preparation), whereas Raptor 1C has only S . holocuspis , and is therefore tentatively correlated with House Mountain.
Two rich micromammal localities, Vasquez and Newt, lie between CC1 and CC2 near the head of EOA Canyon. The eastern edge of deposits that produced the Vasquez l.f. is separated by a small gully from the western edge of the locality producing the Newt l.f. It is highly probable that the two were once connected, and therefore represent the same sampling interval. These localities are at the same general stratigraphic level as Wiens (in Alien Canyon) and Raptor 3-5 (in Keefe Canyon).
In Alien Canyon 5.4 m of grayish-orange siltstones and fine-grained silty sandstones containing local concentrations of carbonate nodules and stringers are located between CC2 and the Wolf gravel; this interval is partly equivalent to unit CC3 in Keefe Canyon. The section is capped by a 1.8 m cliff-forming caliche containing abundant root traces and probable burrows that forms the upland surface just north of the river.
Tracing of beds between Fox Canyon and EOA Canyon is hampered by extensive cover and the presence of a collapse basin in section 2, T. 35 S., R. 30 W. This collapse explains the southward dip of Pliocene sediments toward the Cimarron River in Fox Canyon and in the EOA Canyon area. Fortunately, an outcrop in a small canyon (Fractal Canyon; figure 3, section 12) situated between Fox and EOA Canyons, in the SW1/4 sec. 35, T. 34 S., R. 30 W, appears to provide the necessary connection. A prominent 0.55-1.2 m thick ledge of interbedded wavy carbonate and silty sandstone is found there, the top of which is 5.3 m below the Wolf gravel. The former unit is lithologically similar to unit CC3 in Keefe Canyon, and the base of both units (figures 2, 3; measured sections 12 and 4) are nearly the same distance below the Wolf gravel, differing only by about 0.5 m. Therefore, we tentatively conclude that the units are equivalent.
The Fox Canyon locality of Hibbard (1950) is 1.56 km northeast of the EOA localities. Four sections were measured in Fox Canyon (figure 3, sections 8-11). Section 11 was measured about 50 m upstream from Hibbard's Fox Canyon locality (UM-K1-47). About 7.0 m below the base of the Wolf gravel at measured section 11 and 10.5 m below the Wolf gravel at section 8 is a prominent caliche that forms a white band across the hillsides in this area. Locally it forms a massive bench showing root traces. Hibbard (1950: p. 120) reported this caliche to be 6.7 m below the ?Meade Gravels? at the Fox Canyon quarry, and believed that this ?massive caliche zone? was the same as the caliche 2.0 m above UK loc. 22 (unit CC1 in figure 2). He compared the stratigraphic distances of his Fox Canyon and KC sites below these (assumed equivalent) caliches, and concluded that the Fox Canyon site was slightly lower stratigraphically than the KC quarry. Our correlations generally support this interpretation.
The Red Fox mammal locality (figure 3, section 8) is 3.0 m below CC1, and correlates stratigraphically to D. W. Taylor's mollusk locality 4b (Taylor, 1960; = Rexroad loc. 15 of Taylor, 1966) (figure 3, section 10). Using CC1 as a marker unit, these localities are stratigraphically higher than Hibbard's Fox Canyon locality, and may represent the same stratigraphic level that produced the Ripley l.f. in EOA Canyon (figure 3, section 1). Although the collection of rodent molars from Red Fox is small, the absence of Pliophenacomys finneyi , a common rodent from Hibbard's Fox Canyon locality, further suggests a chronological difference from Hibbard's site.
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