New Discovery of Pliocene Tidal Deposits Near the Crest of the Central Baja California Peninsula: A Key Constraint on Vertical Crustal Motions and Genetic Divergence

Abstract:

An important class of vertical crustal motions are those that shape regional-scale landscape and biological evolution, but are not detectable with modern analytical methods because exhumation is insufficient to reset low-T thermochronometers, or the magnitude of change is less than uncertainties in paleoelevation proxies. One approach to reconstructing paleotopography that helps fill this gap is recognition and dating of sedimentary deposits formed at or near sea level. The Baja GeoGenomics consortium is a collaboration of Earth and life scientists working to understand the controls on a poorly understood pattern of genomic divergence between northern and southern populations of land plants and animals along the Baja California peninsula. Previous workers proposed that an ancient seaway across the central peninsula isolated northern and southern species, creating divergent genetic lineages, but the existence and age of such a seaway is debated. We have identified a previously unrecognized unit of tidal deposits, informally named the Dátil beds, that overlies the ca. 10-Ma basalt of Esperanza and is capped by ~3-Ma basalts. The age of the Dátil beds is constrained by new detrital zircon data that indicate deposition after ~4 Ma. These deposits are well exposed east of San Ignacio and observed up to ~ 90 km inland from the Pacific coast at elevations up to 320 m. Compelling evidence for tidal conditions includes planar rhythmic bedding in fine sands and muds, mangrove root networks embedded in silt, Ophiomorpha burrows, shallow sand-filled channels, and well sorted grain-size segregated sand to fine gravel and pumice with horizontal stratification and small-scale dune cross bedding. The close association of these features indicates deposition in a tidal flat system that formed a large embayment in the mid-peninsular region, likely creating an effective barrier to N-S gene exchange during Pliocene time. Preliminary mapping suggests a narrow isthmus may have separated the tidal embayment from open marine waters of the Gulf of California in the NE, rather than a full seaway connecting the Gulf of California and Pacific Ocean. These results provide evidence for at least ~320 m of post-Pliocene uplift along the crest of the central Baja peninsula, and will be integrated with other tests of genomic divergence in the region.