Triassic (Rhaetian) El Puquén Formation, Chile

Synsedimentary graben, soft-sediment deformation, volcanism, and U-Pb Zircon ages in a near-arc basin

Abstract

The Triassic to Early Jurassic fore-arc basin successions of the coastal region of Central Chile (approximately 32°14' South latitude) form a continuous belt of sedimentary and volcanic rocks, younging to the south. This belt includes, from north to south, the El Quereo, Pichidangui, El Puquén, and Los Molles formations. This note addresses: i) the facies associations and tectonic setting of the El Puquén Formation and the uppermost strata of the underlying Pichidangui Formation, and ii) the first two Rhaetian zircon U-Pb laser-ablation dates of approximately 201 to 208 Ma on volcanic samples from the former, which are younger than the currently accepted Norian-Carnian ages. Basaltic-andesitic lava domes, hyaloclastites and peperites in the uppermost facies of the Pichidangui Formation were emplaced in a subaqueous environment (lacustrine or marine?). Currently these are considered a subduction-related, bimodal volcanic succession synchronous with rifting. Seven facies associations are identified in the conformably overlying lacustrine deposits of the El Puquén Formation, including storm deposits accumulated in a synsedimentary graben approximately 15 m wide, slump deposits, turbidites, pyroclastic intercalations, sedimentary dikes, peperites, and hyaloclastites.