The relationships between sedimentary structures, transport directions and dune types in Mesozoic aeolian sandstones, Atacama Region, Chile



Extensive occurrences of Upper Jurassic to Lower Cretaceous aeolian sandstones form part of a thick sequence of terrigenous red beds in northern Chile. These sediments accumulated in a N-S-elongated intermontane back-arc basin on the landward side of an active volcanic chain. This volcanic chain, which was produced by subduction beneath the continental margin, provided the source of almost all the clastic sediments.

The aeolian sands formed isolated dune fields covering only part of the depositional basin at any one time. Cosets of cross-bedded sandstone up to 40 m thick represent the deposits of linear draas. Cross-bedded sets, which average between 0.5 and 1 m in thickness, were produced by the migration of small dunes down and along the gentle lee slopes of the draas. The majority of foreset laminations in the cross-bedded sets are thin, parallel and persistent. They are the product of the lateral migration of dunes which developed oblique to the prevailing winds. Foreset laminations dip at low angles (averaging 19°), well below the angle of repose of dry sand. Most of the laminations originated by tractional processes, rather than by grainflow (avalanching) or grainfall. This conclusion indicates that the dunes were gently undulating with no slip faces.

The orientation of the direction of maximum dip of foreset laminations does not provide a reliable indicator of the palaeowind direction. Although individual sets or cosets commonly show a unidirectional pattern, significant variations have been recorded between sets of the same age at nearby locations and vertically between one coset and the next. Variations in vertical sections were produced by the superimposition of draas with different orientations.