2-3 Oligocene-Miocene volcano-sedimentary record in southern Baja California: implications for a rift-related origin for the Comondú Group

Session: Tectonic setting of Comondú group: a discussion on alternative interpretations

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Abstract:

We present an updated synthesis of the Eocene to mid-Miocene volcano-sedimentary record of southern Baja California based on new field observations and U–Pb and ^40Ar/^39Ar geochronology. Detrital zircon populations from the Eocene and Oligocene San Gregorio, El Salto, and El Cien Formation show remarkably similar age distributions from the Concepción Peninsula to La Paz. These populations are dominated by two Late Cretaceous peaks at ~100–90 Ma and ~80–70 Ma, correlated with the Peninsular Ranges Batholith (PRB) and the Sinaloa batholith/Tarahumara Formation, respectively, with subordinate Early Cretaceous, Middle Jurassic, and Proterozoic components. Late Cretaceous zircon populations continue to persist throughout the sequence and locally rival Cenozoic components.

U–Pb dating reveals an increasing abundance of syn-volcanic Oligocene zircons (~34–25 Ma) upward through the succession, with progressively younger maximum depositional ages. Thin ignimbrites dated at ~27 and ~25 Ma are interbedded within eolian sandstones of the upper El Salto Member north of Loreto. The lower member of the Comondú Group exhibits a similar pattern, with intercalated ignimbrites dated at ~24–19 Ma from the Sierra Madre Occidental (SMO) that are the source for increasing contributions of Early Miocene detrital zircons.

The middle member of the Comondú Group marks the onset of local volcanism characterized by hornblende-bearing dacitic to rhyodacitic domes and lavas that fed thick resedimented breccias and sandstones. This volcanic episode is tightly constrained by multiple ages between ~19 and 18.6 Ma and constitutes the dominant detrital zircon population in the middle and upper members of the Comondú Group. Nevertheless, the ubiquitous presence of Late Cretaceous zircons in the Comondú Group implies continuous exposure of the PRB to the east and challenges the traditional interpretation of a high-standing “Comondú arc” located at the site of the future Gulf of California. Instead, the Cenozoic volcano-sedimentary record of southern Baja California is best explained by a two-stage evolution: (1) development of a flanking basin to the SMO from ~34-19 Ma recording direct and resedimented pyroclastic material; and (2) focusing of effusive, intermediate volcanism at ~19–18 Ma, with emplacement of silicic to intermediate domes—now exposed in both the Loreto region and Sinaloa-Nayarit —within a new extensional basin bounded to the east by PRB horsts, remnants of which are preserved in the Gulf islands.

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

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