Abstract

Titania (TiO₂) minerals are minor diagenetic phases in many sandstone–shale successions, but the conditions of Ti mobility and titania authigenesis are poorly known. Here, new modes of occurrence of authigenic titania minerals, filling pores and replacing phytodetritus, are documented from Lower Cretaceous prodeltaic sandstones in the offshore Scotian Basin of southeastern Canada. Textural relationships suggest that titania minerals formed at the same time as kaolinite, most commonly below lowstand sequence boundaries, related to flux of meteoric water. Such authigenic titania is most abundant in thin-bedded sandstones interbedded with thick prodeltaic mudstone in which fine-grained detrital Ti-rich minerals are concentrated. Textures in titania that has replaced uncompacted phytodetritus suggest microbial mediation in near-surface eodiagenesis. Eogenetic titania is enriched in phosphorus (~ 0.6% P₂O₅) and may co-occur with authigenic fluorapatite and crandallite crystals. Such titania resulted from prolonged flow of meteoric water that dissolved P from apatite and feldspar, thereby enhancing Ti solubility. In basins with supply of immature sediment from granitic terranes and important diagenesis by meteoric water, the assumption that Ti is an immobile element and that rutile or other titania minerals are exclusively of detrital origin may not be valid. Probable mesodiagenetic titania has low P content, suggesting that Ti mobility under these conditions was facilitated by some other agent, probably organic acids. The results of this study illustrate that even in cases where titania minerals are volumetrically insignificant, their microtextures and trace-element chemistry can provide important insights into the character of fluid flow during diagenesis of siliciclastic rocks.