Stratigraphic nomenclature of rock strata adjacent to the Cretaceous-Tertiary interface in the San Juan Basin
James E. Fassett

Abstract:

During the last two decades, several changes in the stratigraphic nomenclature of rocks adjacent to the Cretaceous- Paleogene (K-T) boundary in the San Juan Basin have been recommended. All of these changes have been suggested by vertebrate paleontologists working in limited areas (those containing vertebrate fossils) in the southern part of the San Juan Basin. Because of their provincial view of the formations under discussion and a clear lack of understanding of the North American Stratigraphic Code, these geologists’ suggested stratigraphic changes do not stand close scrutiny and are thus rejected. Suggested deletion of the lithic component of two formations: the Ojo Alamo Sandstone of Paleocene age, and the Upper Cretaceous Farmington Sandstone Member of the Kirtland Formation is rejected. Recommendations were to eliminate the word “Sandstone” from the names of these two rock units and substitute the word “Formation” in their stead. Because these rock units are characterized by their sandstone components, the use of the word Sandstone in both instances is in accordance with the Stratigraphic Code and thus no change in nomenclature is warranted.

It has been suggested that the Ojo Alamo Sandstone be divided into two members in the southwest part of the San Juan Basin; the Naashoibito and Kimbeto Members, and only consist of one member in the southeast part of the basin. No lithologic criteria have been established for a two-member Ojo Alamo in the southwest part of the basin, thus this subdivision is rejected. Moreover, the suggestion that the Ojo Alamo consists of a single member in the southeast part of the basin is contrary to the Stratigraphic Code. A formation cannot consist of a single member.

It has been recommended that the long established names for subdivisions of the Kirtland Formation: lower shale member, Farmington Sandstone Member, and upper shale member, be changed to the Hunter Wash Member, Farmington Sandstone Member, and the De-na-zin Member, respectively. Because these new names apply to exactly the same lithologic intervals as the old names, there is no valid reason to change the old names that had been in print in various media for 76 years, prior to these suggested name changes. The Stratigraphic Code rejects frivolous changes to long-established nomenclature, therefore, these name changes are rejected. Furthermore, the Hunter Wash name has been previously applied to other rocks in this stratigraphic section in this same area, thus the use of the name “Hunter Wash” for the lower member of the Kirtland Formation has been preempted and is thus improper.

It has been suggested that a sandstone bed named the Bisti Bed marks the base of the Kirtland Formation throughout the San Juan Basin. This implies that the Bisti Bed can be traced continuously throughout the San Juan Basin. It is abundantly clear that no such bed exists in the lower Kirtland Formation throughout the basin. Random fluvial channel-sandstone beds occur sporadically in the uppermost Fruitland Formation and lowermost Kirtland Formation, but field mapping has shown conclusively that none of these sandstone beds are continuous even locally, let alone basin-wide. Fruitland and Kirtland Formation rocks are more than two million years older in the southwest part of the San Juan Basin than they are in the northeast part of the basin, therefore, it is physically impossible that a sandstone bed named the Bisti Bed in the southwest part of the basin could be the same bed in the northeast part of the basin.

It has been suggested that the Fruitland Formation be subdivided into two members: the Ne-nah-ne-zad (lower member) and the Fossil Forest (upper member). There are no lithologic criteria to validate such a subdivision of the Fruitland, thus these new member names are rejected.


Citation:

  1. Fassett, James E., 2010, Stratigraphic nomenclature of rock strata adjacent to the Cretaceous-Tertiary interface in the San Juan Basin, in: Geology of the Four Corners Country, Fassett, James E.; Zeigler, Kate E.; Lueth, Virgil W., New Mexico Geological Society, Guidebook 61st Field Conference, pp. 113-124.

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