Caracterización de los rizolitos en el Cenomaniense Superior de Torrelaguna (Madrid)significado sedimentológico y estatigráfico

Abstract

It has been studied the rhizoliths located in the base of the Utrillas Fm. (Cretaceous), near Torrelaguna (Madrid), the presence of rhizoliths has been recognised at the top of the Atienza Mb. (Upper Cenomanian). The Atienza Mb. is composed of terrigenous sediments deposited in low-energy coastal environments with ephemeral channels. The rhizoliths are cylindrical tubes with a ferruginous envelope corresponding to external moulds of roots, infilled with coarse sands with ferruginous cement. Rhizoliths formation include: 1) the presence of vegetation and the precipitation of an earlier ferruginous cement around roots, originating thick- and hard-walled tubes, and 2) the disappearance of the vegetation leaving the internal hole of the tube, which is subsequently infilled by coarse terrigenous sediments that are cemented by a later ferruginous cement. The presence of a rhizoliths horizon at the top of the unit reflects the existence of pedogenetic processes and the existence of a major sedimentary break, which linked to the coeval development of a thick ferruginous crust stands out the importance of this sedimentary discontinuity. Based on the stratigraphic position the discontinuity is interpreted as a type-II sequence boundary between two depositional sequences assimilated to 3rd-order sedimentary cycles.