2-1 Rethinking the Comondú Group: Early Miocene Extension in a Transitional Forearc Setting

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

Presenting Author:

Mariano Cerca

Authors:

Cerca, Mariano ¹, Aranda-Gómez, José Jorge ², Pérez-Venzor, José Antonio³, Ocampo-Díaz, Yam Zul⁴, Dávila-Harris, Pablo ⁵, Chávez-Cabello, Gabriel⁶, Ramírez-Peña, César Francisco⁷

(1) Universidad Nacional Autonoma de Mexico, Instituto de Geociencias, Juriquilla, Queretaro, Mexico, (2) Universidad Nacional Autónoma de México, Instituto de Geociencias, Juriquilla, Queretaro, Mexico, (3) Universidad Autónoma de Baja California Sur,, Departamento de Ciencias de la Tierra, La Paz, Baja California, Mexico, (4) Universidad Autónoma de San Luis Potosí, Facultad de Ingeniería, San Luis Potosí, San Luis Potosí, Mexico, (5) Instituto Potosino de Investigación Científica y Tecnológica, Geociencias Aplicadas, San Luis Potosí, San Luis Potosí, Mexico, (6) Universidad Autónoma de Nuevo León, Facultad de Ciencias de la Tierra, Linares, Nuevo León, Mexico, (7) Universidad Autónoma de Nuevo León, Facultad de Ciencias de la Tierra, Linares, Nuevo León, Mexico,

Abstract:

The Comondú Group of Baja California Sur has been central to debates regarding the tectono-sedimentary evolution of the southern Baja California margin. Early interpretations described the Comondú as a classical Miocene volcanic arc succession related to subduction of the Farallon Plate, composed of distal to proximal arc facies dominated by pyroclastic deposits, lava flows, domes, and volcaniclastic breccias. In this framework, the Comondú Basin was commonly viewed as a broad, sag-type forearc basin formed by flexural subsidence of the overriding plate before the final cessation of the Farallon plate subduction at ~12.5 Ma. Subsequent work, however, emphasized the basin-like character of the succession and highlighted the dominance of sedimentary processes and lateral facies variability.

Recent stratigraphic and geochronologic data in the region of San Juan de la Costa, on the Lower Comondu member, suggest that these traditional models oversimplify the complexity of the Comondú succession. Available age data indicate that deposition of the Lower Comondú member occurred during a period of early development of extensional structures. Under these conditions, extension became the dominant driver of basin formation, resulting in fault-controlled subsidence and tilting of the basin margins.

Volcanic rocks, mainly distal pumice-bearing tuff deposits of the Sierra Madre Occidental, are a prominent component of the Lower Comondú member. The ages indicate distal input pulses at 30, 27-25, 21, 19, and 18 Ma. Volcanic debris avalanche deposits (VDAD) and lavas with mafic composition, indicating a closer origin, have ages of 23 and 21 Ma. However, a large fraction of the succession consists of thick, laterally variable sedimentary deposits, including coarse-grained alluvial, fluvial, lacustrine, and shallow-marine systems. These deposits record dynamic sediment routing networks and evolving paleotopography within structurally confined basins bounded by active normal faults. The geometry of facies associations, stacking patterns, and spatial distribution of depositional systems, however, remains poorly constrained.

This work reconstructs the volcanic and sedimentary architecture of the Comondú Basin in the southern Baja California Sur through detailed stratigraphic logging, facies analysis, and structural data. Our results support the interpretation of the Comondú Basin as an extensional accretionary-type forearc basin, in which extensional deformation in the upper plate coexisted with contraction in the frontal prism. This model explains observed fault-controlled subsidence, migrating depocenters, and sustained syn-volcanic sedimentation. PAPIIT AG101523.

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

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