PROCESSES CONTROLLING THE DISTRIBUTION OF TI AND AL IN WEATHERING PROFILES, SILICICLASTIC SEDIMENTS AND SEDIMENTARY-ROCKS

The ratio of Ti to Al varies greatly in primary (igneous) source rocks . These elements are considered to be relatively immobile in most weat hering regimes, so that TiO2:Al2O3 ratios have been used to investigat e the provenance of sediments and sedimentary rocks. In modern weather ing profiles that have undergone moderate weathering, Ti:Al ratios rem ain fairly constant so that siliciclastic sediments derived from them should have Ti:Al ratios like that of the source materials. Under extr eme weathering conditions, however, the upper (most weathered) parts o f profiles commonly show a marked increase in Ti:Al ratio that is thou ght to be due to preferential translocation of Al-rich phases. Under s teady-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 set ting (minimum weathering, moderate sorting) there is considerable vari ation in Ti content but Al is nearly constant. This is due to concentr ation of Ti-bearing mafic minerals, such as biotite, in the fine fract ion. In modern sediments derived from moderately weathered and sorted material, the Ti:Al distribution reflects mixing of Al- and Ti-enriche d fine grained materials with sands that are depleted in these element s. When a plot of TiO2 vs. Al2O3 is constructed for a suite of such sa nds and muds, the resultant trend differs significantly from that show n by a similar suite of unweathered glaciofluvial sediments. Compariso n of these results with data from sedimentary rock suites suggests tha t trends shown by plots of TiO2 vs. Al2O3 may provide insight into wea thering and depositional history, in addition to their use as provenan ce indicators.