Age constraints on marine-fluvial deposits at Rancho La Palma, Baja California Sur, Mexico
Determinación de la edad de los depósitos marino-fluviales de Rancho La Palma, Baja California Sur, México
Heriberto Rochín-Bañaga1,*, Donald W. Davis1, Tobias Schwennicke2, Atzcalli E. Hernández-Cisneros3
1 Department of Earth Sciences, University of Toronto, 22 Russell Street, Toronto, ON M5S 3B1, 5 Canada.
2 Departamento de Ciencias de la Tierra, Universidad Autónoma de Baja California Sur, 7 Km. 5.5, La Paz, Baja California Sur 23080, México.
3 Instituto Politécnico Nacional, Centro Interdisciplinario de Ciencias Marinas, Av. Instituto Politécnico Nacional Playa Palo de Santa Rita, La Paz, Baja California Sur 23096, México.
Corresponding author: (H. Rochín-Bañaga) This email address is being protected from spambots. You need JavaScript enabled to view it.
How to cite this article:
Rochín-Bañaga, H., Davis, D.W., Schwennicke, T., Hernández-Cisneros, A.E., 2024, Age constraints on marine-fluvial deposits at Rancho La Palma, Baja California Sur, Mexico: Boletín de la Sociedad Geológica Mexicana, 76 (3), A040424. http://dx.doi.org/10.18268/BSGM2024v76n3a040424
Manuscript received: January 30, 2024; Corrected manuscript received: March 30, 2024; Manuscript accepted: April 4, 2024.
ABSTRACT
Coastal marine and fluvial deposits, exposed at Cerro El Divisadero near Rancho La Palma, Baja California Sur, Mexico, contain a fossil record of cetaceans (baleen whales). U–Pb zircon dating of a tuff horizon collected near the base of Cerro El Divisadero yields a 206Pb/238U age of 27.95 ± 0.16 Ma. This suggests that the marine and fluvial beds at the Rancho La Palma locality are late Rupelian in age and not lower Miocene as previously reported. Therefore, the marine deposits from the Cerro El Divisadero section can be correlated with the Oligocene El Cien Formation at San Juan de La Costa and are contemporaneous with the middle part of its San Juan Member, representing a marginal facies of this unit. Our results constrain the age of the baleen whales found at the Cerro El Divisadero section, providing clues for Oligocene cetacean evolution in the North Pacific.
Keywords: marine-fluvial stratigraphy, U–Pb geochronology, zircon dating.
