Relationships between sequence stratigraphy, mineralogy and geochemistry in Cenozoic sediments of the northern North Sea

Abstract

Abstract Cuttings of Cenozoic sediments from selected wells in the northern North Sea have been analysed both mineralogically and geochemically, and compared with seismic sequence stratigraphic units. Variations in the composition with respect to clay minerals, feldspars, major elements and selected trace elements have been used to establish changes in the provenance of the Cenozoic sequences. The Eocene sequence consists of a thickness of several hundreds of metres of almost exclusively smectitic clays with high Ni and Zn contents and little or no quartz. This represents a strong input from subaerial volcanism related to the rifting and initial spreading of the Norwegian-Greenland Sea. The Eocene-Oligocene smectitic mudstones have high porosity and low seismic velocity (<2km s −1 ) compared with the underlying Paleocene sediments and overlying Miocene-Pliocene sequences. The Plio cene and Pleistocene mudstones are coarse-grained clays with an immature mineralogy, which compact much more rapidly than the underlying smectitic sediments. The seismic velocity may then reach 2.5–3.0 km s −1 and thus cause a strong velocity inversion. This sequence is partly glacial and mineralogically immature, but may contain some kaolinite and smectite, which indicate reworking of lower Tertiary and Mesozoic sediments. Our obser vations indicate that sequence boundaries are closely related to tectonic movements and to changes in the sediment supply from adjacent areas. Both mineralogical and geochemical data can be used as additional tools for correlation in Cenozoic sediments in the northern North Sea. The mineralogy of these sediments also determines the rate of compaction and other factors relevant for basin modelling.