Ice rafts, debris flows and slumps along a glaciated basin margin: the Carboniferous El Imperial Formation in the San Rafael basin, western Argentina

Abstract

This study developed a novel, detailed sedimentological analysis for the complex interactions between rainout, iceberg rafting, tractional underflows, and settling of fines along a glacially influenced basin margin. The glaciomarine interval of the El Imperial Formation (Pennsylvanian, Serpukhovian–Bashkirian) in the San Rafael basin comprises massive to stratified diamictites, interpreted as rainout tills, thinly bedded diamictites, associated with cohesive debris flows, and mudstones containing ice-rafted debris (IRD), all capped by postglacial, transgressive, fine-grained sediments. The rhythmic intercalation of IRD-bearing (dropstone mudstones) and IRD-free (mudstones) intervals likely indicates variations in debris content within the ice margins, the on-and-off switching of ice streams, or dynamic oscillations of the ice terminus. The glaciomarine deposits exhibit soft sediment deformation on both large (metric to decametric) and small (centimetric) scales. This contribution refines previous interpretations of the soft sediment deformation, discerning between loading and slope triggered deformation. Large-scale deformation is characterized by coherent slump folds with low dispersion in the orientations of fold axial plane vergence and fold b-axes. Downslope-verging folds indicate a northward paleoslope, consistent with paleoflow indicators from flute casts found in sandstone turbidite beds. The diamictites affected by the large-scale soft sediment deformation are interpreted as rainout tills with a variable degree of gravity remobilization. Their association with thinly bedded diamictites and laminated mudstones with dropstones suggests that ice rafting played a significant role in the deposition of this succession.

Graphical abstract