THE LAGUNA DEL HUNCO FORMATION; LACUSTRINE AND SUB-AERIAL CALDERA FILL, CHUBUT PROVINCE, ARGENTINA

The Laguna del Hunco Formation is the caldera fill formation within the Middle Chubut River Volcanic and Pyroclastic Complex. This study supplements previous studies on the caldera's magmatic evolution, mammal fauna, and the world's most diverse Eocene flora by examining the stratigraphy and geochronology of the sub-aerial and lacustrine caldera fill. The trapdoor style caldera is 30-35 kilometers in diameter with crescent shaped moat deposits. An ignimbrite that conformably overlays the caldera forming ignimbrite and is overlain by caldera fill deposits yields an ⁴⁰Ar/³⁹Ar age of 52.05±0.23 Ma. The lowermost member of the caldera-fill includes non-welded tuffs, reworked Paso del Sapo sandstone, glass flows, and conglomerates. Facies and bedding geometries suggest changing drainage pathways and sheet flow over an inclined depositional slope. These beds progressively onlap the caldera-floor ignimbrites with increasing angularity suggesting syn-depositional formation of an arch in the southeast corner of the caldera. In the latter stages of this basin division into an east and south moat, a massive yellow tuffaceous member with basalt clasts and pillow basalts was deposited in the east basin by both air-fall events and sub-lacustrine canyons moving out from the resurgent dome. White tuffaceous deposits follow and are distributed caldera wide. These deposits overlay the southeast arch and eventually the caldera rim. The northeastern white units contain shallowing upwards basin floor lacustrine debris-flows, turbidites, ashes, and mudstones. Tens of meters tall coherent soft sediment folds verge to the east. The southeast and southwest white units are sub-aerial or shallow sub-aqueous with silicified trees, glass domes, and ash flow tuff aprons. Three mechanisms of flora fossilization are observed. The best preserved leaves are found in extremely fine tuffs that are widely distributed laterally but constrained stratigraphically to the middle of the white tuffaceous member. The last caldera fill member consists of post-white sub-aerial massive tuffs, welded ignimbrites, volcanic flows, and tuffaceous fans. Mammals are found within this member in an extra-caldera valley-fill deposit between an ignimbrite dated at 49.51±0.32 Ma and basalt dated at 47.89±1.21 Ma.