In southern South America, mid-Cretaceous compressional deformation transformed an Early Cretaceous back-arc basin-slope-platform setting into a protocordillera-foreland basin. The Lower Cretaceous sedimentary rocks exposed in the Andean foreland fold and thrust belt at lat. 51°S record an abrupt transition from back-arc basin-slope facies to foreland-basin submarine-fan facies.

The Zapata Formation of Portlandian to Albian age represents deposition on the slope of a coeval back-arc basin which formed to the west. The 600-m section consists predominantly of rhythmically interbedded and extensively bioturbated mudstone and siltstone. The siltstone beds are laterally continuous and contain sedimentary structures suggesting deposition by turbidity currents. Sporadic turbidite sandstones occur as shallow channel-fill sequences or as thin, laterally persistent graded beds rich in coarse detrital mica. Slumped intervals are common, and show a northwest-southeast trend of slump fold axes, which parallels the axis of the back-arc basin to the west.

The overlying Punta Barrosa Formation of Albian to Cenomanian age was deposited contemporaneously with deformation in the cordillera to the west. It represents a rapid change in depositional regime, signaled by an abrupt increase in sandstone beds. The thick sandstone beds are lenticular, commonly amalgamated, and show thickening/coarsening-upward and thinning/fining-upward cycles indicative of deposition in a submarine-fan environment. NNW to SSE paleocurrent trends indicate a longitudinal basin-fill pattern; this persists through the Upper Cretaceous flysch sequence. The Punta Barrosa Formation thus represents the establishment of submarine-fan deposition associated with the initiation of a foreland basin on the site of the preexisting back-arc basin slope.

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