LITHOSTRATIGRAPHY

In the San Juan Basin, the Ojo Alamo Sandstone (=Formation of Lucas and Sullivan 2000) yields the supposed Paleocene dinosaur fossils and, according to all workers except Fassett, includes strata of both Cretaceous and Paleocene age. The Ojo Alamo Sandstone as used by Fassett (2009) and by us is the same unit originally defined by Bauer (1916); we do not use the term Ojo Alamo Sandstone as did Baltz et al. (1966), who restricted the term to the upper part of the original rock formation (Kimbeto Member of Figure 1).

In the southwestern San Juan Basin (between Hunter and Betonnie Tsosie washes/arroyos), where most of the dinosaur-bearing localities of the Ojo Alamo Sandstone are located, the lower part of the Ojo Alamo is a basal conglomerate overlain by a finer-grained, poorly consolidated sandstone-to-shale interval ("shale" to many workers), which is the Naashoibito Member (Figure 1). The Naashoibito Member contains the only known articulated dinosaur bones in the Ojo Alamo Sandstone (e.g., Hunt and Lucas 1991; Sullivan pers obser.). The overlying conglomeratic sandstone interval is the Kimbeto Member, which contains only isolated dinosaur bones and bone fragments – these are the stratigraphically highest dinosaur fossils in the New Mexico portion of the San Juan Basin (Fassett et al. 1987; Powell 1973). The Naashoibito Member is only recognized in the southwestern part of the San Juan Basin (e.g., Baltz et al. 1966; Lucas and Sullivan 2000). Elsewhere, to the northwest, the Ojo Alamo Sandstone is mostly a complex, multistoried stack of conglomeratic sandstone sheets that we refer to as the Kimbeto Member.

Fassett (2009) did not recognize distinct Naashoibito and Kimbeto members of the Ojo Alamo Sandstone in the southwestern San Juan Basin and thus referred to the unit basinwide as the Ojo Alamo Sandstone. However, the distinction between lower and upper intervals of the Ojo Alamo Sandstone is an important one, whether or not formal lithostratigraphic nomenclature is applied to it. This is because, other than Fassett, there has been long and broad agreement among geologists/paleontologists (e.g., Baltz et al. 1966; Lehman 1981, 1985; Lucas 1981; Lucas et al. 1987; Newman 1987; Hunt and Lucas 1992; Sullivan and Lucas 2003, 2006; Sullivan et al. 2005b; Williamson and Weil 2008) that in the southwestern San Juan Basin, the base of the Paleocene is within the Ojo Alamo Sandstone, at the base of its upper (Kimbeto) member (Figure 1). However, elsewhere in the San Juan Basin, it is possible that the lower part of the Kimbeto Member is Cretaceous. Fassett (2009), however, did not distinguish lower and upper parts of the Ojo Alamo Sandstone, so he combined all fossils from the Ojo Alamo into one fossil assemblage, which he assigned a Paleocene age. Fassett (2009) thus created the impression that the Ojo Alamo Sandstone is a single unit yielding palynomorphs and dinosaur fossils that can be assigned a single age. It has long been recognized, however, that the Ojo Alamo in the southwestern San Juan Basin encompasses two distinct lithosomes that yield fossil assemblages of different ages (Figure 1).