The Toqui Formation in the Northern Austral Basin, or Ayse ' n - Rio Mayo Basin in Central Patagonia (43 degrees-49 degrees S), is the basal member of the transgressive-regressive Coyhaique Group of Tithonian-Aptian age. A revision of the basin fill, focusing on the early transgressive units exposed in Chile and Argentina at 45 degrees-46 degrees S, is presented. In order to explain the lithological variability of the mixed calcareous-volcaniclastic Toqui Formation and the evolution of its depositional environments, a detailed sedimentological-petrographic analysis of two outcrops and four drill cores was executed at the type locality, El Toqui Mine (45 degrees S). Twenty-three lithofacies were recognized, arranged in five facies associations. The Toqui Formation comprises three facies associations (Calcareous-; Muddy-, and Sandy Volcaniclastic), and is here redefined as conformed by two members: Mantoand Mina San Antonio Members. Its Manto Member conformably overlies volcanic rocks of the Ib ' an similar to ez Fm., settled as subtidal Gilbert-type delta foresets, conforming a proximal-medial volcanic apron (KimmeridgianValanginian). The Manto Member includes mixed calcareous-volcaniclastic sandstone, bioclastic wackefloatstone, and oyster boundstone, depicted as patch-reefs developed in a carbonate mid- and outer-ramp. The Manto Member is covered by tuffaceous mudstone and volcanic sandstone of the Mina San Antonio Member, representing a prograding, eruption-fed shelf delta, conforming a medial-distal volcanic apron. Upsection, the volcaniclastic succession retrogrades into outer-shelf hemipelagic mudstone of the Katterfeld Formation. Overall, the Ayse ' n Basin depicts a not fully developed marginal basin, i.e. formed by extension but without onset of seafloor spreading. The early marine transgression is linked to a relative sea-level rise caused by tectonic subsidence (back-arc extension), and the subsequent flooding of continental, intra-arc volcanic terrains comprising a transgressive system tract. Carbonate platforms aggraded as highstand system tracts over the subtidal volcanic topography, suggesting periods of tectono-volcanic quiescence. Ramps were covered by prograding deltaic deposits, reflecting a normal regression caused by high volcaniclastic input and local, shallow intrusion-related volcanic uplift (inflation). Upwards, the volcaniclastic succession grades to prodelta and hemipelagic deposits of a major transgressive system tract. The onset of pelagic conditions depicts an important relative sea-level rise, regarded as caused by "post-rift" thermal subsidence, possibly enhanced by the Valanginian high global eustasy. Lithological differences between the Toqui Formation in Chile and the partly coeval transgressive units exposed in Argentina (44 degrees 50 '-47 degrees S), i.e. Cotidiano, Tres Lagunas, and Springhill Formations, is considered as caused by diachronic development of ramps, lateral facies changes, and differential volcaniclastic input. Deposition of the Toqui Formation thus reflects a complex interaction between tectonic subsidence, coastal volcanism, and patchreef growth.
