Fig. 2

SEM backscatter image with Nanomin overlay of two laminae from McManus-1. a SEM backscatter image revealing textural difference and highlighting porosity (35% of image area) in the upper laminae vs. the occluded porosity (4% of image area) in the lower image. Grain size of detrital material is similar, but diagenetic pyrite and apatite are confined to the uncemented laminae. Scale bar = 40 μm. b Nanomin mineral map overlain on the continuation of the image in a to reveal the mineralogy showing 40% illite in the upper laminae vs. 60% illite present as interstitial quartz and illite cement in the lower laminae. c Detail of the contact between the laminae within box c reveals that the primary intergranular porosity in the upper laminae is largely preserved except in some areas where it is filled by minor authigenic clay crystals, apatite (A) and pyrite. Porosity in the lower lamiae is largely occluded by illite and silica (chert) cement (SC). Secondary mouldic porosity resulting from the dissolution of feldspar (F) in the cemented laminae postdates formation of cement with subsequent growth of authigenic illite (AI) within it. (DM detrital mica, Q quartz). Scale bar = 10 μm. d Mineral map of only apatite and euhedral pyrite with no underlying BSE layer identifies these minerals are limited to the open porosity laminae and implies a later timing of formation following the reduction of porosity in the lower laminae. Scale bar = 40 μm