Recycling and chemical weathering in tectonically controlled Mesozoic–Cenozoic basins of New Zealand

Sedimentation in molasse basins is controlled by tectonics, however, recycling and chemical weathering play a critical role in the compositional evolution of a sedimentary succession. The Cretaceous to Pliocene molasse deposits of Central Otago, New Zealand are excellent examples of tectonically related deposits that were governed by the effects of chemical weathering and recycling. Preserved in fault-controlled basins floored by flysch deposits of the Otago Schist, the clastic successions contain ubiquitous unconformities and lithofacies consistent with alluvial, fluvial and lacustrine depositional settings. Textural analysis of Central Otago sandstones establishes a general quartz enrichment and increased mixing of angular and well-rounded quartz varieties up-section, consistent with a history of sediment recycling. Rare earth element (REE) patterns, which reflect upper crustal compositions, are similar for the flysch-type Otago Schist (Permian–Early Cretaceous), a palaeo-weathering profile, and the overlying molasse deposits. The development of quartz arenites is also consistent with high degrees of chemical weathering, and erosion of the schist basement, which contains numerous quartz veins. Although recycling has occurred, SiO₂ and TiO₂ do not consistently show a negative correlation over time. This reflects erosion of previously deposited quartz-rich sediment and the Otago weathering profile, which produced an inverse stratigraphy. CIA values range from 52 for lithic-rich, coarse-grained sandstones and polymictic conglomerate matrices, to 93 for coarse-grained to pebble-rich quartz arenites. Individual samples were split into finer- and coarser-grained pairs (<2·5φ and 2·5 to −1φ) and were analysed separately. The results show that finer-grained samples contain higher REE abundances and less SiO₂, but the coarser-grained Miocene–Pliocene samples have higher CIA values than their finer-grained counterparts. These coarse-grained deposits are quartz-rich and plot erratically on tectonic discrimination diagrams, implying that using SiO₂-poor samples is more reliable for geochemical analysis. Overall, the petrographic and geochemical results indicate that the main factors controlling the composition of the Central Otago molasse deposits were source composition, chemical weathering and recycling. Studies of this nature can be conducted in Archaean tectonically controlled molasse basins that are affected by similar allocyclic factors.