Diagenesis and sea-level change: lessons from Moruroa, French Polynesia

An inclined borehole through the margin of Moruroa Atoll, French Polynesia, in which the ages of units are constrained by radiometric dates, provides a unique opportunity to examine the relationship between sea-level change and diagenetic history. The oldest rocks encountered are dated at 355 840 yr bp and the overlying succession records five cycles of deposition and emergence, culminating in the deposition of the Holocene sequence. Successive intervals of emergence provided the driving force for diagenetic change, promoting the through-flow of chemically aggressive fresh waters that were at times either pervasive or confined within relatively isolated pathways on a millimetre to metre scale. These locally varying flows and flow rates defined restricted zones characterized by differences in water chemistry and diagenetic signatures. In contrast to some sequences elsewhere, the upper part of the Moruroa Atoll succession is typified by alternations of intervals in which either several generations of cement growth are represented or only one or two cements are present. Three distinct diagenetic signatures can be recognized as follows: (i) fresh water phreatic zones are reflected in neomorphism and the deposition of cements that occluded porosity, thus preventing the deposition of later cements and conferring relative stability; (ii) vadose zones are characterized by more complex diagenetic sequences reflecting multiple phases of dissolution and cement growth, attesting to the relative duration of exposure and the conditions of ground water flow; and (iii) deep in the core, sequences reflect lowstand deposits that suffered sub-sea diagenesis, generating pores in which multi-layered marine cements apparently formed at more frequent intervals than those subject to subaerial processes. All cements appear to be of narrowly constrained ages. These results highlight the contentious issue of whether the distributions of former diagenetic environments can be related to specific sea-level stands and erosion surfaces. It seems that, in the Moruroa core, some cycles of sea-level change are reflected in distinctive packages of diagenetic features that have not been overprinted by cement growths during later sea-level stands.