Synthetic heavy mineral stratigraphy: applications and limitations

Abstract

A multi-disciplinary study of the dominantly fluvial Lower Triassic Otter Sandstone Formation (east Devon, UK) is presented. The approach integrates a variety of methods including sedimentology, conventional heavy mineral analysis, single grain microprobe analysis and elemental whole rock geochemistry (ICP-AES and ICP-MS). A species-level conventional heavy mineral study is compared to a whole rock geochemical analysis for the corresponding samples to investigate whether or not heavy mineral compositions can be discriminated from elemental whole rock data. The comparison provides sufficient evidence that the absolute and relative abundance of some of the heavy minerals; apatite, iron–titanium oxides, monazite and zircon can be discriminated geochemically. The established element–mineral relationships are used to construct both synthetic heavy mineral assemblages and synthetic heavy mineral index values from the geochemical data, which show considerable similarity to those of the conventional heavy mineral analysis. The presented synthetic heavy mineral logs display intraformational packages that enable subdivision of this barren continental succession. Furthermore, the observed subdivision is correlatable over a considerable distance when extended into the offshore Wessex Basin. The chemical approach unquestionably lacks the sensitivity of conventional heavy mineral analysis, however, by comparison it is rapid. This novel approach demonstrates that synthetic heavy mineral stratigraphy based, as here, on a well constrained stratotype has considerable potential for creating high-resolution correlations in both outcrops and the subsurface and may be of significant value to people working with reservoir characterisation.