Evidence for the continental origin of Pliocene deposits in the central Baja California peninsula, Mexico, and implications for controls on biodiversity

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 hypothesising that the divergence is due to an ancient seaway that crossed the central peninsula and created a physical barrier to gene flow. We test this transpeninsular seaway hypothesis through the integrated sedimentology, stratigraphy, paleopedology and ichnology of Pliocene deposits exposed at Mesa San Regis in the San Ignacio trough, a low‐lying area that coincides geographically with the locus of genetic divergence and the hypothesised seaway. This analysis shows that the San Regis beds accumulated in a sandy ephemeral stream system dominated by sheetfloods and channelised flow. ⁴⁰ Ar/ ³⁹ Ar 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. Measured sections reveal 21 palaeosols containing pedogenic carbonate horizons with rhizoliths formed by periods of non‐deposition, development of carbonate‐bearing aridisols and well‐established vegetation in an arid to semi‐arid environment. A trace‐fossil assemblage including Celliforma, Coprinisphaera, Daimonelix, Fictovichnus, Parawanichnus, Megaichnus and Naktodemasis is consistent with inferred climatic conditions and depositional environments. Integration of ⁴⁰ Ar/ ³⁹ Ar ages and palaeosol chronology shows that the ~32‐m‐thick San Regis beds accumulated in 900.4 ± 160.5 ky, between ca 4.6 and 3.7 Ma. After ca 3.7 Ma, the region experienced long‐term base‐level fall and slow regional‐scale incision punctuated by episodic aggradation of terrestrial alluvial deposits separated by nested unconformities. These data record Middle Pliocene deposition in a continental setting by terrestrial and freshwater processes followed by dominantly uplifting and eroding conditions throughout Late Pliocene and Quaternary. These findings preclude marine conditions for the past 10 My and refute the transpeninsular seaway hypothesis, suggesting climate‐modulated glacial refugia and/or adaptation to regional differences in rainfall and ecology as other potential drivers of genetic divergence.