13-9 Evidence for the continental origin of the Pliocene San Regis beds in the central Baja California Peninsula, Mexico, and implications for controls on biodiversity

Session: Earth Life Sciences across the Cordillera

Presenting Author:

Kevin Gardner

Authors:

Gardner, Kevin K¹, Dorsey, Rebecca J.², Hasiotis, Stephen T.³, Antinao, Jose Luis⁴, Darin, Michael⁵, Bennett, Scott E.K.⁶, Heizler, Matthew T.⁷, Dolby, Greer Andersen⁸, Hausback, Brian⁹

(1) Deparment of Earth Sciences, University of Oregon, Eugene, Oregon, USA, (2) Department of Earth Sciences, University of Oregon, Eugene, OR, USA, (3) University of Kansas, Lawrence, KS, , (4) Indiana University, Bloomington, IN, USA, (5) Department of Earth Sciences, University of Oregon, Eugene, OR, USA, (6) United States Geological Survey, Portland, OR, USA, (7) New Mexico Bureau of Geology and Mineral Resources, Socorro, NM, , (8) University of Alabama at Birmingham, Birmingham, AL, , (9) California State University, Sacramento, CA, USA,

Abstract:

Previous studies have documented genetic divergence within the last few million years between northern and southern populations of terrestrial plants and animals along the Baja California peninsula, with many studies hypothesizing that the observed divergence is due to an ancient seaway that crossed the central peninsula and created a physical barrier to gene flow. In this study we test the transpeninsular seaway hypothesis using an integrated study of sedimentology, stratigraphy, paleopedology, and ichnology.

The San Regis beds are Pliocene-aged deposits exposed at Mesa San Regis in the San Ignacio trough, a low-lying area that coincides geographically with the hypothesized transpeninsular seaway. Our analysis via process sedimentology shows that the San Regis beds accumulated in a sandy braided river similar to the modern depositional system. ⁴⁰Ar/³⁹Ar dating yields an age of 4.097 ± 0.076 Ma for an interbedded mafic tephra bed and 3.635 ± 0.013 Ma for a capping basaltic andesite lava. Detailed measured sections reveal 20 paleosols that contain extensive pedogenic carbonate-dominated paleosols reflecting stage I-II and stage III BK horizon development with plant (rhizoliths, rhizocretions, and rhizohaloes) and animal trace fossils formed by periods of nondeposition and development of well-drained soils and well-established vegetation in an arid to semiarid environment. The animal trace fossils include (in order abundance): Fictovichnus (cocoons of wasp and beetle larvae), Celliforma (reproductive cells of soil bees), Coprinisphaera (dung balls and nests of dung beetles), Termitichnus (termite nests), Parawanichnus (ant nests), Naktodemasis (backfilled meniscate burrows of beetle larvae and soil bug nymphs),  Megaichnus (giant armadillo burrow?), Daimonelix (helical ground-dwelling mammal burrow), Reniformichnus (inclined burrow of a tortoise or lizard), and coprolites (carnivore or omnivore feces). This assemblage is also consistent with the climatic conditions inferred from soil development and depositional processes.

Integration of ⁴⁰Ar/³⁹Ar ages, measured sections, and time estimates of soil formation shows that the ~ 30-m-thick San Regis beds accumulated over 397 ± 111 ky, between ca. 4.43 and 4.04 Ma. These data record deposition by terrestrial and freshwater processes in a continental setting, refuting the marine conditions hypothesized for this period of time. We therefore reject the transpeninsular seaway hypothesis. Other potential drivers of genetic divergence include climate-modulated glacial refugia and adaptation to regional differences in rainfall and ecology.

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

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