13-8 Miocene to Present Paleogeography of the Central Baja California Peninsula (Mexico) Confirms No Transpeninsular Seaway

Session: Earth Life Sciences across the Cordillera

Presenting Author:

Scott Bennett

Authors:

Bennett, Scott E.K.¹, Dorsey, Rebecca J.², Darin, Michael³, Gardner, Kevin⁴, Hausback, Brian⁵, Grandy, Sam⁶, Martinez-Gutierrez, Genaro⁷, Heizler, Matthew T.⁸, Schmitt, Axel Karl⁹, Stelten, Mark Evan¹⁰, Wilder, Benjamin¹¹, Munguia-Vega, Adrian¹², Dolby, Greer Andersen¹³

(1) Geology, Minerals, Energy, and Geophysics Science Center, United States Geological Survey, Portland, OR, USA; Portland State University, Portland, OR, USA, (2) University of Oregon, Eugene, OR, USA, (3) University of Oregon, Eugene, OR, USA, (4) University of Oregon, Eugene, OR, USA, (5) California State University, Sacramento, CA, , (6) California State University, Sacramento, Sacramento, CA, USA, (7) UABCS, Dept of Earth Sciences, La Paz, BCS, Mexico, (8) New Mexico Bureau of Geology and Mineral Resources, Socorro, NM, USA, (9) Curtin University, Bentley, WA, Australia, (10) U.S. Geological Survey, California Volcano Observatory, Moffett Field, CA, USA, (11) Next Generation Sonoran Desert Researchers, Tucson, AZ, USA, (12) Conservation Genetics Laboratory, University of Arizona, Tucson, AZ, USA; Applied Genomics Lab, La Paz, Baja California Sur, Mexico, (13) University of Alabama at Birmingham, Birmingham, AL, USA,

Abstract:

A long-standing Earth-Life science hypothesis for the central Baja California peninsula (Mexico) is that a marine seaway transected the peninsula during Plio-Pleistocene time. Such a seaway would have created a physical barrier to floral and faunal gene flow between a shorter northern peninsula and a separate southern island. For decades, biologists have called upon this transpeninsular seaway hypothesis to explain the genetic divergence observed at the latitude of the central peninsula in dozens of low-dispersal plants and animals. To test this hypothesis, our interdisciplinary team (Baja GeoGenomics consortium) conducted detailed geologic mapping of a ~4,000 km² region at the latitude of San Ignacio-Santa Rosalia (~27.4°N), obtaining ~150 geochronologic ages (Ar/Ar, U-Pb), ~350 XRF geochemical analyses, and paleocurrents in sedimentary and volcanic rocks. The study area encompasses the two lowest passes (405 m and 520 m asl) along the central peninsula topographic divide. We combine our new mapping, published mapping, and analysis of satellite imagery to construct paleogeographic maps of the central Baja California peninsula every 2 million years back to 16 Ma. At each time step, we delineate regions of eroding volcanic highlands, active volcanism, low-relief coastal plains, marine environments, active faulting and basin subsidence, and the evolving Pacific and Gulf of California shorelines. Since Miocene time, the western (Pacific) shoreline has remained west of Mesa San Regis, a site ~12 km northeast of San Ignacio that records only non-marine conditions (Gardner et al., this meeting). The eastern (Gulf of California) shoreline was established during 6.3-Ma marine flooding of the Gulf and has remained roughly co-located with fault systems that control the western rift margin. Detailed mapping confirms that marine deposits occur below ~200 m asl, except where locally uplifted up to ~300 m asl by magmatic inflation related to Quaternary volcanoes. Marine deposits do not exist at or near the 405 m and 520 m asl topographic passes. We therefore reject the transpeninsular seaway hypothesis at the latitude of San Ignacio-Santa Rosalia and conclude that climatic and/or topographic factors likely controlled the observed genetic divergence. This study demonstrates the importance of using geologic data to test hypotheses that make predictions about landscape evolution at geologic timescales, underscoring the need for cross-disciplinary collaboration to better understand co-evolution of life and Earth processes.

Geological Society of America Abstracts with Programs. Vol. 58, No. 3, 2026

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