Normal and reverse unroofing sequences in syntectonic conglomerates as evidence of progressive basinward deformation

Abstract

When a new mountain belt is generated by tectonics, the denudation of the mountain front may cause the development of alluvial fans at its base. The retreat of the mountain front by denudation processes and a widening of the drainage network in the source area create adjacent alluvial-fan conglomerates with an inverted vertical clast distribution. This vertical clast distribution represents a normal unroofing sequence. Renewed uplift of a previous inverted vertical clast distribution can produce a reverse unroofing sequence. If continued shortening occurs in the same area, a rejuvenation of the source area and basin margin may produce a new generation of clasts from the source area and a partial or total reworking of previously deposited conglomerates to supply second-generation clasts. If this type of reworking process is repeated as fault uplift steps basinward, several new inverted and stratigraphic vertical clast distributions may arise.