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Paleogeography of Miocene Western Amazonia: Isotopic composition of molluscan shells constrains the influence of marine incursions

H.B. Vonhof^,1, F.P. Wesselingh^,2, R.J.G. Kaandorp^,3, G.R. Davies^,3, J.E. van Hinte^,3, J. Guerrero^,4, M. Räsänen^,5, L. Romero-Pittman^,6 and A. Ranzi^,7

¹ Faculty of Earth and Life Sciences, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, Netherlands
² Naturalis—Nationaal Natuurhistorisch Museum, P.O. Box 9517, 2300 RA Leiden, Netherlands and Department of Biology, University of Turku, 20014 Turku, Finland
³ Faculty of Earth and Life Sciences, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, Netherlands
⁴ Departamento de Geociencias, Universidad Nacional, AA 14490, Bogota, Colombia
⁵ Department of Geology, University of Turku, 20014 Turku, Finland
⁶ Instituto Geológico Minero y Metalúrgico, Avenida Canada 1470, Lima 41, Peru
⁷ Departamento de Biologia, Laboratorio de Paleontologia, Universidade Federal do Acre 69.915-900, Rio Branco, Acre, Brazil

Strontium, oxygen, and carbon isotope compositions of well-preserved mollusks (bivalves) indicate a dominantly freshwater depositional setting for the lower Miocene–upper Miocene Pebas Formation in Western Amazonia. Molluscan ⁸⁷Sr/⁸⁶Sr ratios identify different freshwater sources. Andean runoff was the dominant water source in Miocene Western Amazonia, though there was occasional influx of waters from cratonic catchments.

At only one stratigraphic level, isotope signals indicate increased (mesohaline) aquatic salinities, in concert with a clearly more saline molluscan faunal assemblage.

Strontium isotope–based salinity estimates are surprisingly low when compared to other paleosalinity estimates based on the interpretation of (ichno)faunal assemblages and sedimentological structures. We propose that these seemingly contrasting observations can be unified if Miocene Western Amazonia was occupied by a long-lived (lacustrine) wetland system with a restricted connection, via the Los Llanos Basin, to the Caribbean Sea. Abundant runoff supplied fresh water to this system, which effectively blocked the influx of saline waters through the restricted marine connection to the north. Much like modern Lake Maracaibo, such a system could have been the site of microtidal currents and thus could have hosted brackish-water fauna in a dominantly freshwater depositional system.

Key Words: Miocene, • Amazonas • paleosalinity • paleohydrology • isotope geochemistry

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