4 Sedimentary Systems and Provenance in the Western Cordillera

This session invites contributions on sedimentary processes, depositional environments, stratigraphic frameworks, and provenance evolution across western North America. Topics may include basin analysis, source-to-sink studies, and applications of stratigraphy to energy and environmental questions.

Wednesday, 22 April 2026
8:00 AM - 12:00 PM
LMH, Fiesta Terrace Salon

Sedimentary Environments and Provenance of the Cenozoic Yanhuitlán Formation, Oaxaca: Implications for the Tectonic Evolution of Southern Mexico

Paper number: 4-9
Program: Technical Sessions
Time: 10:40 AM – 11:00 AM

Authors

Abstract

The Yanhuitlán Formation is situated in the northwestern part of Oaxaca and consists of redbeds, including siltstone and sandstone, conglomerate, and tuff. These deposits were previously thought to be lacustrine and were dated as pre-Middle Eocene (40.3-45.4 Ma) based on cross-cutting intrusives. This study aims to reconstruct the depositional environments of the Yanhuitlán Formation, determine its age, contact relationships, provenance, and its relationship to regional tectonic events.

The area experienced complex Cenozoic tectonism involving: a) east-west shortening during the Late Cretaceous-Paleocene Orogeny (Laramide equivalent); b) separation of the Chortis Block (if located adjacent to southern Mexico) and its southeastward displacement during the Paleogene; c) initiation of subduction and development of the modern Mesoamerican subduction zone; and d) north-south left-lateral strike-slip faulting.

The Yanhuitlán Formation shows stratigraphic and tectonic contacts: it unconformably rests on older subaerial conglomerate, Cretaceous limestone, and Proterozoic Oaxaca Complex, following a paleorelief. It is overlain by volcanic rocks to the north, and by the Cántaros and Sosola conglomerates to the east. It is in fault contact with the Cretaceous limestone at the southwest margin.

Detailed sedimentological sections and reconstruction of depositional environments based on facies associations revealed a large, mostly fine-grained alluvial fan system. In the southern sector, proximal fan facies consist of conglomerate and sandstone, interpreted as sediment-gravity-flow deposits, channel fills, and sheet-flow deposits. The central sector records mid-fan facies dominated by sandstone and siltstone, interpreted as downstream accretion bars and channel fills. In the northern sector, distal fan facies include siltstone, sandstone, and paleosols, interpreted as floodplain, levees, and crevasse-splay deposits, interbedded with fallout tuffs. Lacustrine facies with evaporites occur only locally in the northernmost area. Facies distribution and paleocurrent data indicate that the alluvial fan extended from south to north. Sandstone is mostly quartz-feldspathic with plutonic, metamorphic, and volcanic lithics.

The youngest detrital zircon U–Pb age population yields an average age of 57 Ma, which is interpreted as the maximum depositional age at the Paleocene–Eocene boundary. Additional zircon populations range from 60 to 80 Ma, 230 to 280 Ma, and 950 to 1,200 Ma, indicating sources in southern Oaxaquia and the Chortis Block. Magmatic zircon ages constrain the end of sedimentation of the Yanhuitlán Formation at 48.6 Ma and record the onset of magmatism.

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

doi: 10.1130/abs/2026CD-410995

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