Farellones Formation

Farellones Formation
Name	Farellones Formation
Type	Geological formation
Period	Miocene–Pliocene
Lithology	Andesitic and dacitic lava flows, pyroclastic rocks, volcanic breccia
Region	Santiago Metropolitan Region, Valparaíso Region
Country	Chile

Farellones Formation The Farellones Formation is a Neogene volcanic and volcaniclastic succession exposed in the Andes, primarily in the Santiago Metropolitan Region and Valparaíso Region of Chile. It records a major phase of arc magmatism related to the subduction of the Nazca Plate beneath the South American Plate during the Miocene and Pliocene. The formation has been studied in relation to regional tectonics involving the Central Volcanic Zone, crustal shortening associated with the Andean orogeny, and hydrothermal systems linked to mineralization near Santiago, Chile.

Introduction

The Farellones succession sits within the framework of Neogene volcanic belts of the central Andes and is spatially associated with volcanic centers such as remnants near Farellones, Chile, exposures adjacent to the Maipo River basin and uplifted crystalline rocks of the Coastal Cordillera. Work on the succession has been carried out by researchers from institutions like the Universidad de Chile, Pontifical Catholic University of Chile and international teams affiliated with the Smithsonian Institution and the Geological Society of America.

Geology and Lithology

The lithology is dominated by intermediate to felsic volcanic rocks including andesite, dacite and rhyolite lava flows, along with widespread pyroclastic deposits, tuffs and volcanic breccias. Primary facies include thick porphyritic andesitic lavas, obsidian-bearing rhyolitic domes, and welded tuff units linked to explosive eruptions comparable to deposits described in studies of the Altiplano-Puna Volcanic Complex and the Atacama Region. Hydrothermal alteration is common in silicified horizons and argillic zones, factors relevant to comparisons with epithermal systems such as those at El Teniente and Chuquicamata.

Stratigraphy and Age

Stratigraphically the succession overlies older Mesozoic basement complexes and Oligocene volcanic units, with a stratigraphic transition to younger Pleistocene volcanic deposits in uplifted blocks. Radiometric ages obtained by potassium-argon and argon-argon methods place emplacement predominantly in the mid to late Miocene and extending into the Pliocene, with reported dates that correlate with regional volcanic pulses contemporaneous with magmatism documented at Cerro Azul, Cerro Arenales and other central Andean edifices.

Depositional Environment and Paleogeography

Depositional settings range from proximal subaerial volcanic centers to more distal pyroclastic aprons and volcaniclastic sedimentary fans grading into lacustrine-fluvial basins. Paleogeographic reconstructions link the Farellones deposits to a coastal-to-interior arc setting influenced by forearc tectonics and basin evolution similar to the depositional frameworks studied in the Central Depression and Maipo River catchment. Paleoclimatic indicators and paleoflora remains from equivalent Neogene sequences have been used to infer climate evolution in concert with uplift phases tied to the Andean uplift.

Tectonic Setting and Structural Features

The Farellones rocks were emplaced in an active continental margin above the subducting Nazca Plate with structural control exerted by thrust faults, reverse faulting and strike-slip structures associated with regional shear zones analogous to the San Ramón Fault and Precordillera fault systems. Folding, brittle deformation and uplift have exposed thick volcanic successions along transverse valleys and fault-bounded blocks, providing key constraints for models of crustal shortening, slab geometry and magmatic migration explored in tectonic syntheses by groups from the International Union of Geological Sciences and regional geological surveys.

Paleontology and Fossil Record

Although primarily volcanic, volcaniclastic horizons and interbedded sedimentary lenses preserve terrestrial plant macrofossils, pollen assemblages and occasional vertebrate remains that contribute to Neogene biostratigraphy in central Chile. These fossiliferous layers have been compared to floras and faunas documented in contemporaneous sequences near Coquimbo, Valparaíso and the Maule Region, aiding correlation with South American Neogene biotic provinces and studies by paleontologists at the Museo Nacional de Historia Natural (Chile).

Economic Importance and Land Use

The Farellones Formation hosts hydrothermally altered zones and silicified veins that are of interest for low-sulfidation epithermal mineralization models similar to those explored at El Teniente, Codelco operations and smaller exploration targets in the Central Chile mineral belts. Volcanic rock strength, slope stability and groundwater interactions influence infrastructure and recreational land use in areas like Farellones ski resort and transport corridors near Santiago Metropolitan Region; studies by regional mining companies, municipal planning agencies and the Servicio Nacional de Geología y Minería inform hazard assessment, water resource management and land-use planning.

Category:Geologic formations of Chile