9-8 Sedimentary and Tectonic History in the Northeastern Margin of the Oligocene – Early Miocene Juchipila Graben (Western Mexico)

Session: Sedimentary Systems and Provenance in the Western Cordillera (Posters)

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Abstract:

The Juchipila graben, located in the southeastern end of the Sierra Madre Occidental in western Mexico, contains a relatively complete continental Upper Miocene sedimentary record. Adjacent to the northeastern border graben fault, a continuous stratigraphic section ~96 m thick was measured near the Tabasco village. The sedimentary succession comprises volcaniclastic sandstone and conglomerate beds, lacustrine limestone strata, and ash-fall tuffs, which represent a lacustrine-alluvial system. A sample from a 1.75 m tuff bed in the middle part of the section yielded a weighted mean U-Pb zircon age of 9.4 ± 03 Ma (Tortonian; LA-ICP-MS, n = 4 zircon grains; MSWD = 2). This new age indicates that the sedimentary fill of the Juchipila graben began earlier than 6.95 Ma, a previously estimated age based on geochronological and biostratigraphic data. Two main stratigraphic units were distinguished, revealing sedimentary changes likely controlled by tectonic processes. Unit I comprises three lithofacies associations: distal alluvial fan, paleosols, and marginal lake. This unit is characterized by a variety of pedological textures and structures induced by inorganic precipitation and/or biological activity, including mottled texture, glaebules, rhizoconcretions, rootlets, root casts, and rhizoliths. All these elements are primarily produced by roots of higher plants, indicating widespread terrestrial conditions and tectonic stability. Distal explosive volcanic activity supplied a large volume of sediment to the basin, as indicated by tuff beds, and temporarily broke the equilibrium between subsidence and sedimentation. Unit II comprises proximal alluvial-fan deposits. Lithofacies association includes debrites and clast-supported conglomerate deposits that show no internal stratification or bedding. Clast composition indicates that most sediment was derived from the erosion of volcanic units that were exposed on the graben-bounding faults. This unit is interpreted as a result of the activity of normal faults along the northeastern margin of the basin, causing the arrival of a large volume of sediments and the progradation of alluvial deposits. These data indicate that tectonic activity determined the morphology of the graben and controlled the sedimentary evolution of these alluvial and lacustrine systems. Juárez-Arriaga thanks SECIHTI for a postdoctoral grant. UNAM-PAPIIT AG101523 funds this research.

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

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