The Aysen Basin of southern Chile is an elongate sedimentary basin situated to the east of the Patagonian batholith in an ensialic back-arc environment. It contains marine sediments of the Coyhaique Group which provide an important stratigraphic marker between similar volcanic successions. The Coyhaique Group comprises, limestones (Toqui Formation), black shales (Katterfeld Formation) and sandstones (Apeleg Formation), which outcrop over an area of approximately 150km N/S and 75km E/W with an estimated thickness of more than 1200m. The Toqui Formation limestones are discontinuous across the basin and were laid down in a marine transgression which occurred during the Tithonian to Berriasian. In the NE, oyster banks indicate a NW /SE aligned shoreline which probably marked the margin of the basin. By the beginning of the Hauterivian, black shales of the Katterfeld Formation had become regionally extensive. They represent a restricted basin with fluctuating levels of oxygen which controlled the fauna! communities. Fossil assemblages indicate a connection to the more southerly basins and a clear link to South African fauna. The influx of the higher energy sandstones of the Apeleg Formation during the late Hauterivian marked the opening of the basin from the south and east to normal marine conditions. Trace fossils indicate a shallow marine setting. This formation probably represents tidal sandstones deposited in a shelf environment. The sediments of the Aysen Basin reflect gentle subsidence of the back-arc basin, probably due to thermal relaxation related to a period of volcanic quiescence, from the lower Berriasian until at least the late Hauterivian, possibly early Barremian. Unlike the basins to the north and south, there is no evidence of subduction related rifting during the formation of the basin.