SEM petrography of samples of the Lias Group of England and Wales

This report summarises work undertaken in support of the Ground Movements: Shrink/Swell Project. It provides petrographical descriptions acquired using scanning electron microscopy (SEM) of a suite of samples (predominantly mudrocks) from the main Lias Group depositional basins across England and Wales. The main Formations sampled are the Scunthorpe Mudstone (n = 5), the Blue Lias (n = 8), the Charmouth Mudstone (n = 21 including 15 samples from weathering profiles), the Marlstone Rock (n = 4), and the Whitby Mudstone (n = 5). Small numbers of samples from the other Lias Formations are also described. The analyses largely confirm the observations made by earlier workers, and are broadly consistent with the results of XRD analyses. The Scunthorpe Mudstone Formation samples are typically laminated with mineralogies probably dominated by illite and/or smectite (although XRD analysis would be required to confirm this). Very finely crystalline gypsum/anhydrite occurs throughout. A sample from the Barnstone Limestone Member is typical, dominated by fine grained calcite, with minor dolomite and clay. A sample from the Frodingham Ironstone Member is also typical being a goethitic oolitic ironstone. The Blue Lias Formation samples are typically massive, hard, dense, highly calcareous mudrocks. Some less calcareous samples preserve lamination, and minor framboidal and euhedral pyrite is present. SEM analysis confirms that variations in surface area (Appendix Table 1.1) broadly correspond to variations in the relative proportions of carbonate and clay, with variations matching a regional trend in calcite content reported by Kemp and Mc Kervey (2001). Clay mineral assemblages in the two southernmost samples from this formation contain major illite, with minor kaolinite, chlorite and illite/smectite, whereas in the samples from further north, illite tends to be less dominant of the clay mineral assemblage and smectite and/or illitesmectite become more significant, suggesting either a difference in the primary makeup of the sediment, or that the smectite to illite transformation is more advanced in the most southernmost samples. The Charmouth Mudstone Formation is characterised by well-laminated mudrocks with only minor amounts of silt and sand-grade material. EDXA analysis suggests a predominance of Kbearing (illitic) clays, although XRD indicates that the clay mineral assemblages comprise major kaolinite, with minor illite, chlorite and possible illite-smectite, with minor smectite also present in the samples from the Worcester basin. Samples of this formation from the Dorset Coast contain well-developed calcite veining, with cone-in-cone (beef) fabrics present. Gypsum/anhydrite is locally developed along lamination surfaces and early framboidal pyrite is present. The samples from weathering profiles at Dimmer and Blockley reveal development of locally abundant gypsum/anhydrite, nodularisation, rootleting and oxidation towards the surface as seen in hand specimen, but these features are poorly defined on the scale of SEM stubs. No appreciable, systematic differences clay mineralogy is noted through the weathering profiles. The Marlstone Rock Formation is peloidal/ooidal with thick coatings of probable chloritic or glauconitic clay on well-rounded grains in a fine clay matrix. Cone-in-cone calcite cement is locally present. The Whitby Mudstone Formation is confirmed as a typically well-laminated mudrock with minor silty material. EDXA suggests a predominance of illite and/or illite-smectite (K and KCa-bearing) compositions, consistent with earlier, quantitative, XRD data on samples from the Cleveland Basin by Kemp and Mc Kervey (2001) which indicates that the smectite to illite transformation is well advanced. Qualitative XRD on these samples indicates major kaolinite with minor illite, illite-smectite and chlorite.