Grain-coating chlorite cements commonly occur within sandstones of late Middle and Upper Miocene age deposited in the North Sumatra back-arc basin. Chlorites from the Lower Keutapang Member contain Ca (maximum 0.75 wt% oxide) and show textural evidence for direct precipitation on grains. However, crystals are subhedral, showing curved faces and often ragged edges, and show a tendency to merge together. In overlying beds of the Upper Keutapang Member, grain-coating chlorite-smectite (20% smectite) cements display an identical morphology but are more siliceous, have a lower octahedral occupancy and contain higher total (Na + Ca + K). It is proposed that chlorite cements in the Keutapang Formation originated as smectite-rich cement rims whose initial precipitation was related to the breakdown of volcanic detritus in the sediments. Transformation to chlorite occurred subsequently during burial, facilitated by a high geothermal gradient in the back-arc basin.

The suite of London Clay mudrock samples may be sub-divided into three lithofacies: laminated silty claystone, partially bioturbated silty claystone, and partially bioturbated claystone. Mineralogically the mudrocks show little bulk variation, but considerable differences in the suites of trace cements which characterize each sample. The concretions also show a range of cement types, even within a single concretion. Preservation of uncompacted burrows and minus cement porosities (MCP) indicate that the concretions formed during early diagenesis. These data have been used to constrain the timing of authigenic mineral formation in both the mudrocks and concretions. Petrographic study of the concretions has revealed that there are significant errors involved in the use of MCP for estimation of depth of formation, which make it of qualitative rather than quantitative use. Stable isotope and chemical data suggest concretion formation occurred towards the base of the sulphate reduction zone and in the methane oxidation zone.