Cathodoluminescence of carbonate-cemented Holocene beachrock from the Togo coastline (West Africa): an approach to early diagenesis

Abstract

Carbonate-cemented Holocene beachrock outcrops sporadically along more than 100 km of the Togo and Benin coasts. The sands are siliciclastic with subsidiary carbonate skeletal fragments and intraclasts. 14C radiometric dating indicates the sand deposits have formed within the last 5000 years. Beachrock has been indurated in phreatic environments undergoing, afterwards, several stages of burial and subaerial exposure. Cathodoluminescence (CL) proved a useful tool in unravelling the intricate cementation of this beachrock which can be subdivided into three stages. Firstly, magnesian calcite was precipitated in a marine-phreatic active zone. Very dull orange and very dull blue colours emitted in isopachous rims of needle-like crystals and peloidal micrite (CL1-fabric) reveal different Eh and pH precipitating conditions in this environment. Secondly, bright yellow-orange luminescing acicular and equant spar cement (CL2-fabric) is interpreted to have formed in a mixed-water phreatic zone. The third stage of cementation occurred dominantly in the landward part of the beachrock and accompanied neomorphism of the earlier marine cements. The resultant CL3-fabric, dull blue, medium orange luminescing equant spar is typical of continental fresh-water phreatic zones. The progression from marine to fresh-water pore fillings accompanied sand-bar progradation during a stable sea-level stage. Ultimately there was erosion stage of the sand-bar deposits which led to subaerial exposure and partial erosion of the beachrock. Recognition of the effects of repeated shoreline progradation during the early diagenesis of this Holocene beachrock indicates the potential value of diagenetic studies in aiding recognition of marine regressions and transgressions in the geological record.