ABSTRACT

Terrestrial siliciclastic deposits of the lower part of the Nevremont Formation (Lower Givetian) in the Belgian Ardennes include sandstones and conglomerates, all of which have a predominantly quartzose composition. Diagenesis of these clastic sediments was strongly influenced by the terrestrial depositional environment. Postdepositional alterations range from the pre-burial to a deep-burial realm. Pedogenic processes include mechanical infiltration of clay minerals often associated with iron compounds. Infiltration occurred along permeable water channels, created by both pedoturbation and bioturbation. This clay-sized material formed cutans which enveloped detrital framework grains or surrounded the permeable vugs and channels. Thick cutans prevented the nucleation of quartz cement, hich in other places frequently filled nearly all available pore spaces and caused the lithification of many of the sediments during early burial diagenesis. The relation between cutanic features and early burial quartz cement suggests that the necessary silica was most probably derived from chemical weathering in the source area and at the depositional surface. Only minor amounts of quartz cement were formed during intermediate to deep-burial stages. The silica for this second-cementation phase was produced by intergranular pressure solution and clay-mineral diagenesis. Furthermore, various stages of mechanical compaction and pressure solution were observed, depending on the amount and timing of the introduction of quartz cement. Compaction played an important role in the diagenesis whe ever cementation was absent or not completed during early burial. The cutanic features and the occurrence of detrital matrix partly controlled the distribution of quartz cement and compaction.