Folded onlap geometries: implications for recognition of syn-sedimentary folds

Growth strata are usually analysed from geological maps, outcrop geometry or seismic sections. Many growth folds have been defined from geological maps, especially in areas where plunging structures allow the syn-tectonic sequence to be displayed at the surface. The geometrical arrangement of these syn-tectonic sequences can also be defined from the relationships between pre-growth and growth strata, as obtained from geological maps. In this paper, from theoretical models and natural examples from the southern Pyrenees, we argue that some cartographic patterns of strata associated with variably plunging folds, traditionally ascribed to syn-tectonic sedimentation with thinning of sedimentary units toward the anticlinal hinge zone, can be explained as completely post-sedimentary folds of sedimentary units lapping onto an inclined bedding surface, and linked to basin margin, or downlap geometries on the basin floor. We conclude that 3-D analysis of syn-tectonic structures, combining data from independent sources (i.e. geological maps and seismic reflection profiles) is essential to determine relationships between sedimentary units and structures.