LITHOSTRATIGRAPHY

As discussed in Fassett (2009) and in Fassett et al. (2002), the Ojo Alamo Sandstone is present throughout much of the ~13,000 km² area of the New Mexico part of the San Juan Basin. Those authors of the Lucas et al. paper who have studied the Ojo Alamo Sandstone in the basin have done so at only a relatively few limited outcrop areas in the southern part of the basin. Being vertebrate paleontologists, they have naturally focused on the few places on the Ojo Alamo Sandstone outcrop – a few square km - where dinosaur fossils have been found (the locations of these areas are shown on Figure 1). The Ojo Alamo, is a multi-storied, high energy, fluvial, conglomeratic-sandstone deposit thus it is extremely variable in its lithologic make up and thickness throughout the basin (Figure 1). This figure shows the lithologic diversity of the Ojo Alamo in a northeast-trending, geophysical-log cross section. Geophysical log 1 at the southwest end of this cross section is only about 3 km down dip from the outcropping Ojo Alamo Sandstone in the Ojo Alamo Sandstone type area. In this outcrop area the Ojo Alamo is about 25 m thick and consists of upper and lower ledge-forming sandstone beds with interbeds of less well cemented sandstone beds and overbank mudstone beds (see photographs of Ojo Alamo Sandstone exposures at several localities in Fassett (2009). The Ojo Alamo on log 1 is about the same thickness as at the outcrop in the type area, however on the log, the uppermost part of the formation consists of three massive sandstone beds separated by two thin (~1-m thick) mudstone layers. The lowermost sandstone bed of the Ojo Alamo on log 1 is about 2 m thick, slightly thicker than the average thickness of the lowermost conglomeratic sandstone bed in the Ojo Alamo type area.

Figure 1 shows that the Ojo Alamo is thinnest – 25 m – in log 1 and ranges up to 110 m thick in log 9; the numbers of sandstone beds and mudstone interbeds included in the formation can be seen to vary greatly across this line of cross section. The same variations in the lithologic components of the Ojo Alamo are also seen along its outcrop where the formation ranges in thickness from one bed a few meters thick in the southeast part of the basin to four massive beds aggregating 120 m thick at the San Juan River locality (Figure 1).

Lucas et al. (2009) divide the Ojo Alamo into a lower Naashoibito Member and an upper Kimbeto Member in the type area of the Ojo Alamo Sandstone, whereas in Fassett (2009) it is shown that there are no lithostratigraphic criteria for such a subdivision. The cross section of Figure 1 speaks for itself in showing that there is no such simple, two-part, lithostratigraphic subdivision of the Ojo Alamo throughout the basin. Lucas et al. state that their Kimbeto Member is Paleocene and their Naashoibito Member in the type area is Cretaceous and on their figure 1 they show that an unconformity separates these two members. These authors present no physical or biochronologic evidence for the presence of such an unconformity within the Ojo Alamo either in their current paper or in any previous publications, thus this unconformity appears to be no more than a mental construct.

Lucas et al. (2009) state that Fassett (2009) "combined all fossils from the Ojo Alamo into one fossil assemblage" of Paleocene age. This statement is somewhat misleading. Fassett (2009) listed 103 palynomorph species from Ojo Alamo Sandstone samples collected at numerous localities throughout the basin; these palynomorphs came from samples throughout the formation; from its base to its top. Without exception, all such samples were found to represent Paleocene palynomorph assemblages. All of the unequivocally in-place vertebrate remains identified from the Ojo Alamo are from the lowermost 15 m of this formation at the three principal Ojo Alamo bone areas shown on Figure 1. Because the palynomorph assemblages from the Ojo Alamo are unequivocally all Paleocene, the inevitable conclusion is that the vertebrate remains found in the Ojo Alamo Sandstone are Paleocene as well.

One might argue that dinosaur bones, traditionally thought to be Cretaceous index fossils, should have equal weight wtih pollen and spores as biocrhonologic age indicators in assigning an age to the Ojo Alamo Sandstone. However, the end-Cretaceous, asteroid-impact, fall-out layer has been discovered in the nearby Raton Basin (and at many other localities in the northern part of the Western Interior of North America) precisely coincident with the palynologically defined K-T boundary, and as discussed in Fassett (2009) this validates palynology as the sharper tool to define the K-T boundary in the Western Interior of North America.

Lucas et al. (2009) end their discussion of the lithostratigraphy of the Ojo Alamo Sandstone by stating that this formation "has long been recognized" to encompass "two distinct lithosomes that yield fossils of different ages." These authors offer no citations to substantiate this claim because there are none, other than perhaps Baltz et al. (1966) and some of their own recent publications. It is true that many authors, including Reeside (1924) and Anderson (1960), recognized that plant fossils and vertebrate fossils offered conflicting evidence for the age of the Ojo Alamo Sandstone, but these authors never suggested that these fossils represented "two distinct lithosomes." In addressing these conflicting age-data Reeside (1924) concluded that the plant fossils were the best biochronologic indicator and thus classified the Ojo Alamo as "Tertiary (?)" and Anderson (1960) concluded that because his palynologic studies indicated that the Ojo Alamo was Paleocene, this formation's contained dinosaur fossils must also be Paleocene, if not reworked. Both of these authors thus suggested decades ago that the dinosaurs of the Ojo Alamo Sandstone were Paleocene in age.