ABSTRACT

Petrographic study of stained Pleistocene-Recent biolithites from Erromango reveals an advanced state of diagenesis. Micrite is produced by grain diminuton of algal colonies, and probably by fragmental abrasion. Other fabrics developed include granular mosaics and simple syntaxial rims. In samples collected from carbonates sandwiched between volcanic ash bands, two phases of cementation are apparent. The first phase cement is composed of ferroan calcite, which is followed by a second phase of non-ferroan calcite cement. Ferroan calcite is previously unrecorded as a cement in Recent carbonates, and in addition it is very rare as a first stage cement in ancient carbonates.

Diagenesis of these carbonate sediments has produced in a very short time a hard, compact, partially recrystallized rock. Reasons for this advanced state of diagenesis are unknown, but in all probability they are connected with the high heat values in the rocks resulting from contemporaneous volcanicity.