The Magallanes Basin is located at the southern edge of the South American plate and is underlain by crust of Paleozoic age. The initial history of the basin is one of extension associated with the breakup of the South American sector of Gondwanaland. Triassic to Late Jurassic extension produced a normal-faulted terrane with numerous grabens and half grabens. This extensional event also resulted in extensive, dominantly silicic volcanism. The basin floor subsided from the Late Jurassic to the Late Cretaceous with decay of the thermal anomaly associated with extension. During the Late Cretaceous and Tertiary, uplift and shortening occurred along the western and southern edges of the basin, forming the Patagonian Andes and the fold and thrust belt of southernmost South America. Subsidence in the basin during this interval of time was the result of lithospheric flexure caused by loading.

The sedimentary fill of the basin is related to three major phases of basin development. The rift-related Triassic to Middle Upper Jurassic succession consists of mostly nonmarine volcanic and volcaniclastic rocks largely restricted to isolated grabens. Upper Jurassic to Upper Cretaceous, largely retrogradational sedimentary units, were deposited while the basin passively subsided on the remnant-arc ride of a small marginal sea. Uppermost Cretaceous and Tertiary units were derived from the south, west, and northwest, and show a progressive onlap geometry from west to east. These deposits mark the onset of sedimentation from the Andes, although subsidence caused by tectonic loading started somewhat earlier in the Upper Cretaceous. Depositional patterns for this interval consist of fanglomerates separated by deep-water shales from an eastern complex of low-sedimentation-rate glauconitic sandstones which onlap a long-lived basement high. The most impressive feature formed during the foreland basin stage is a regional composite unconformity that separates rocks as old as Paleocene from the Mesozoic section.

Production or shows of oil and gas occur in many of the stratigraphic sequences defined in the basin. The producing interval, the Springhill Sandstone, and the major source-rock units were deposited while the basin was a westward-facing remnant-arc margin. Burial of there rocks during the foreland basin stage led to the maturation and migration of hydrocarbons. Thus, the Magallanes Basin is a polyphase foreland basin and each phase of evolution has had a role in making the basin a productive one.

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