57-9 Timing and drivers of late Cenozoic marine phosphogenesis in the Caldera forearc basin of northern Chile

Session: Advances and Applications in Geochronology for Interpreting Stratigraphic and Basin Records, Part II

Presenting Author:

Priscilla Martinez

Authors:

Martinez, Priscilla R.¹, Kirk, Jason², Carrapa, Barbara³, Clementz, Mark T.⁴, Worrell, Whitney E.⁵, Hasiotis, Stephen T.⁶, Gutstein, Carolina S.⁷, Muñoz, Fabían A.⁸, Martínez-López, Joao G.⁹

(1) Department of Geosciences, University of Arizona, Tucson, Arizona, USA, (2) Department of Geosciences, University of Arizona, Tucson, Arizona, USA, (3) Department of Geosciences, The University of Arizona, Tucson, AZ, USA, (4) Department of Geology and Geophysics, University Of Wyoming, Laramie, WY, USA, (5) Department of Geology and Geophysics, University of Wyoming, Laramie, WY, USA, (6) Department of Geology, University of Kanasa, Lawrence, KS, USA, (7) Escuela de Geología, Facultad de Ingeniería, Universidad Santo Tomás, Santiago, Chile, (8) Escuela de Minas y Recursos Naturales, Universidad Central de Chile, Santiago, Chile, (9) Ecología y Biología Evolutiva, Facultad de Ciencias, Universidad de Chile, Santiago, Chile,

Abstract:

Neogene marine phosphorites in northern Chile have been attributed to coastal upwelling of deep, nutrient-rich waters during periods of increased primary productivity in the global ocean. These deposits are important for understanding the evolution of late Cenozoic marine ecosystems due to their association with a dense accumulation of exceptionally preserved marine vertebrate fossils. However, the precise timing and depositional conditions that promoted phosphogenesis remain poorly understood. Here, we present new U-Pb ages of phosphorites collected from the Miocene­—Pliocene Bahía Inglesa Formation acquired using a novel, image-based approach to laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). U-Pb phosphorite ages are integrated with in-situ phosphorite geochemistry, U-Pb zircon geochronology, biostratigraphy, and litho-ichnofacies analyses from the Rocas Negras, Mina Fosforita, Chorrillos, and Bahía Salado outcrop localities to generate a more robust chronostratigraphic model for the Miocene—Pliocene Bahía Inglesa Formation and elucidate the drivers of phosphogenesis in the Caldera forearc basin. Our results demonstrate that phosphogenesis initiated at ca. 6.8 Ma during a local marine transgression in continental shelf and upper slope environments. Facies analyses indicate strong episodic tsunami backflow events generated rip-up clasts that served as nuclei for phosphatic nodule formation through subsequent precipitation of carbonate fluorapatite from phosphate-rich porewaters. U-Pb LA-ICP-MS phosphorite age results are mixed, with some samples yielding late Miocene ages whereas others show anomalously young Pliocene to Pleistocene (ca. 4.17—0.48 Ma) ages. This age variability suggests that burial depth may control U-Pb isotopic system closure in phosphorites, and that the younger Pleistocene ages record the cessation of sediment-seawater interaction following coastal uplift of Neogene marine sequences. We hypothesize that phosphogenesis was microbially mediated and enhanced by volcanic ash ocean fertilization, providing insights into the paleoceanographic and paleoecological evolution of the southeastern Pacific margin.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8378

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.