Compaction-sensitive accretionary lapilli: a means for recognizing soft-sedimentary deformation

Abstract

In a region of negligible tectonic overprint, soft-sedimentary folding has affected beds of accretionary lapilli. The folds, which range from upright convolute to intraformational recumbent, are truncated by the overlying beds and their pre-lithification origin is further confirmed by their association with forcefully injected clastic dykes of accretionary lapilli, and fluidized channel-ways. Shape factor analysis using Elliott polar graphs shows that the accretionary lapilli define weak to moderate planar fabrics that are coincident with the regional bedding attitude and which have a compactional strain symmetry (X = Y ≥ Z) . The lapilli fabrics were superimposed on the folds and clastic dykes and hence are axial planar to the recumbent folds and transect the convolute lamination. The geometrical relations and strain pattern establish a pre-tectonic timing for the folds and for their formation in unconsolidated sediment. The fabrics were not generated during folding by processes akin to tectonic dewatering, but were later superimposed on the folds. Compaction-sensitive strain markers give an additional criterion for distinguishing between soft-sedimentary and tectonic structures; post-lithification deformation would be recognized by the dispersal of the planar compaction fabrics defined by the accretionary lapilli.