New Mexico Geological Society Annual Spring Meeting — Abstracts

A measured stratigraphic section of the Satan Member of the Mancos Shale at La Ventana in Sandoval County in northern New Mexico contains well preserved ammonite zonal taxa and encompasses the Santonian-Campanian boundary. Zones present include the uppermost Santonian Desmoscaphites bassleri Zone and the lowermost Campanian Scaphites leei III Zone. In addition, the ammonite Placenticeras placenta (DeKay) and the inoceramid Cordiceramus ex gr. muelleri (Petrascheck) may be good proxies for the Santonian-Campanian boundary.

Sealey and Lucas (2019) published a monograph on ammonites from the southeastern San Juan Basin that included a measured stratigraphic section (South Fork B section) of the Satan Member near La Ventana in Sandoval County. However, Sealey and Lucas (2019) only mentioned that the Satan Member crosses the Santonian-Campanian boundary in the southeastern San Juan Basin. Prior to and after Sealey and Lucas (2019), no measured section in the Western Interior Basin (WIB) has been published that included the Santonian-Campanian boundary with the taxon range zones of Desmoscaphites bassleri and Scaphites leei III. Gale et al. (2023) stated that in the WIB, the crinoid Marsupites testudinarius occurs in the D. bassleri Zone associated with Scaphites leei II, and the base of magnetochron C33r must fall around or above the contact with the overlying S. leei III Zone, but that there is some doubt as to its exact position. Gale et al. (2020) dated the Desmoscaphites bassleri Zone at 84.43 ± 0.15 and the Scaphites leei III Zone at 83.27 ± 0.11, placing the boundary at 83.65 Ma, which is slightly above the base of the S. leei III Zone. However, Gale et al. (2023) stated that the only certainty is that the boundary must lie between 82.7 and 84.5 Ma, leaving doubt as to its exact position. In the WIB, Cobban (1969) correlated the base of the Campanian with the base of the Scaphites leei III Zone, stating that S. leei II occurs with the late form of Haresiceras mancosense (Reeside), and S. leei III occurs with the early form of Haresiceras montanaense (Reeside). Sealey and Lucas (2023) documented the latter occurrence in the Satan Member in Rio Arriba County. In the La Ventana section, the highest occurrence of S. leei II, Placenticeras placenta and Cordiceramus ex gr. muelleri is approximately 55 m below S. leei III, and S. leei III is approximately 42 m below the Point Lookout Sandstone. Gale et al. (2023) stated that the base of the Campanian at the Tepeyac section in Mexico is identified from the carbon isotope record and coincides approximately with the base of the Menabites (D.) delawarensis Zone. The lowermost part of the M. (D.) delawarensis Zone would therefore be equivalent to the S. leei III Zone.

Placenticeras placenta occurs in the La Ventana section associated with Scaphites leei II and also above Desmoscaphites bassleri. It also occurs in the Scaphites hippocrepis I Zone in the Satan Member at Llaves in Rio Arriba County (Sealey and Lucas, 2023). P. placenta ranges from the uppermost Santonian to the middle lower Campanian in the USA (Sealey and Lucas, 2019). Placenticeras syrtale (Morton), which is associated with S. leei III in our section, has been reported from the lower Santonian to the lower Campanian, and in the southeastern San Juan Basin it spans the middle Santonian Clioscaphites vermiformis to the Scaphites leei III Zone; it also occurs in the S. hippocrepis I Zone in the eastern San Juan Basin and the S. hippocrepis II Zone in the Chama Basin (Sealey and Lucas, 2023). Ifrim and Stinnesbeck (2021) used ammonite interval zones to define the Santonian-Campanian boundary at the Tepeyac section in Mexico. They stated that Placenticeras syrtale and P. placenta are absent in their Tepeyac section but that P. syrtale occurs below and above the boundary in New Mexico and other states in the USA and Chihuahua, Mexico, and the taxon appears to be a good proxy for positioning the boundary in the WIB. Because P. placenta has a shorter chronologic range spanning five ammonite zones, as compared to the eight zones of P. syrtale, it may be a better proxy (secondary boundary marker) for the Santonian-Campanian boundary. Cordiceramus ex gr. muelleri was collected from the same localities as S. leei II, P. placenta and Baculites haresi Reeside. Because it crosses the Santonian-Campanian boundary in Europe and has been collected just below the boundary in Texas and at La Ventana, its highest occurrence may also be a good proxy (secondary boundary marker) for the Santonian-Campanian boundary. Thus, we propose the La Ventana section as a reference section for the Santonian-Campanian boundary in the WIB.

References:

1. Cobban, W. A., 1969, The Late Cretaceous ammonites Scaphites leei Reeside and Scaphites hippocrepis (DeKay) in the Western Interior of the United States: U. S. Geological Survey, Professional Paper 619, 47 p.
2. Gale, A. S., Mutterlose, J., Batenburg, S., Gradstein, F. M., Agterberg, F. P., Ogg, J. G. and Petrizzo, M. R., 2020, Chapter 27 - The Cretaceous Period. In Geologic Time Scale 2020: Elsevier, p. 1023-1086.
3. Gale, A., Batenburg, S., Coccioni et al., 2023, The Global Boundary Stratotype Section and Point (GSSP) of the Campanian Stage at Bottaccione (Gubbio, Italy) and its Auxiliary Sections: Seaford Head (UK), Bocieniec (Poland), Postalm (Austria), Smoky Hill, Kansas (USA), Tepayac (Mexico): Episodes, v. 46, p. 451-490, https://doi.org/10.18814/epiiugs/2022/022048.
4. Ifrim, C. and Stinnesbeck, W., 2021, Ammonoids and their biozonation across the Santonian-Campanian boundary in north-eastern Coahuila, Mexico: Palaeontologia Electronica, v. 24, p. 1-62.
5. Sealey, P. L. and Lucas, S. G., 2019, Late Cretaceous (Cenomanian-Campanian) ammonite systematic paleontology and biostratigraphy, southeastern San Juan Basin, Sandoval County, New Mexico: New Mexico Museum of Natural History and Science Bulletin 80, 245 p.
6. Sealey, P. L. and Lucas, S. G., 2023, Late Cretaceous (Campanian) ammonites from the Satan Member of the Mancos Shale, Rio Arriba County, New Mexico: New Mexico Museum of Natural History and Science Bulletin 94, p. 639-656.