ORIGIN OF NODULES IN MIXED SILICICLASTIC-CARBONATE SANDSTONES, THE LOWER EOCENE RODA SANDSTONE MEMBER, SOUTHERN PYRENEES, SPAIN

Calcite cement is heterogeneously distributed in the Eocene Roda Sandstone Member. Strongly cemented nodules and layers, intercalated within less well-lithified sandstones, are common. The nodules yield valuable information about the factors that controlled the onset and the degree of cementation. All nodules are associated with abandonment surfaces, formed when sedimentation ceased for a variable length of time. The nodules represent a first stage in the development of continuous hardgrounds. The initiation of early cementation was dependent on a combination of several parameters, such as grain-sorting and the presence and amount of carbonate nucleation centers. Early cementation began at distinct points, now manifested by nodules, aggrading into layers in cases where sedimentation had ceased for a sufficient length of time. The resulting variability in the amount of cement determines the present-day difference in weathering behaviour between nodules and the adjacent, less well-cemented sandstones. © 1990.