Quartz Cement: The Miller's Tale

Summary

The Brae Formation sandstones of the Miller Field, UK Continental Shelf, are quartz cemented quartz arenites. The only parts of the sandstone not cemented by quartz lie within calcite concretions. These concretions, precipitated soon after deposition of the sand, preserve the original fabric and chemistry of the sand. Comparison of the calcite cemented rock with the quartz cemented rock reveals that the sandstone gained SiO₂ during subsequent diagenesis.

The Brae Formation sandstones are overlain and interbedded with mudstones of the Kimmeridge Clay Formation. However, the mudstones do not have a uniform chemical composition. Mudstones that are interbedded with the Brae Formation sandstones contain less SiO₂ than mudstones that occur above the Brae Formation. The contrast between mudstones which are relatively rich in silica and those which are relatively poor in silica cross-cuts chronostratigraphic surfaces. This indicates that those mudstones which are interbedded with the sandstones lost (exported) silica during diagenesis.

The volume of silica exported from the mudstones matches that imported by the sandstone. Moreover, the date of quartz cementation in the sandstone was much the same as the inferred timing of clay mineral transformations in the mudstones. Such clay transformations could have liberated silica for cementation.

The average size of the isochemical, diagenetic system with respect to quartz was small. Indeed the system is better described as short since the important parameters are a vector and a measure of net (sandstone) to gross (sandstone plus mudstone). The average distance travelled for a silica quantum was 1.15 ± 2.02 m, in a direction normal to the stratigraphy and in a period of 2–20 Myr. Thus the silica flux during cementation was in the order of 10⁻⁴–10⁻² moles m⁻¹ yr⁻¹.