ABSTRACT

The ratio of Ti to Al varies greatly in primary (igneous) source rocks. These elements are considered to be relatively immobile in most weathering regimes, so that TiO₂:Al₂O₃ ratios have been used to investigate the provenance of sediments and sedimentary rocks. In modern weathering profiles that have undergone moderate weathering, Ti:Al ratios remain fairly constant so that siliciclastic sediments derived from them should have Ti:Al ratios like that of the source materials. Under extreme weathering conditions, however, the upper (most weathered) parts of profiles commonly show a marked increase in Ti:Al ratio that is thought to be due to preferential translocation of Al-rich phases. Under steady-state weathering conditions, one particular level of the profile may act as the main source of detrital material and may thus control the Ti:Al ratio of sediments produced from it. In a glacio-fluvial setting (minimum weathering, moderate sorting) there is considerable variation in Ti content but Al is nearly constant. This is due to concentration of Ti-bearing mafic minerals, such as biotite, in the fine fraction. In modern sediments derived from moderately weathered and sorted material, the Ti:Al distribution reflects mixing of Al- and Ti- enriched fine-grained materials with sands that are depleted in these elements. When a plot of TiO₂ vs. Al₂O₃ is constructed for a suite of such sands and muds, the resultant trend differs significantly from that shown by a similar suite of unweathered glacio-fluvial sediments. Comparison of these results with data from sedimentary rock suites suggests that trends shown by plots of TiO₂ vs. Al₂O₃ may provide insight into weathering and depositional history, in addition to their use as provenance indicators.