Evolution of syn‐ to early post‐rift facies in rift basins: insights from the Cretaceous–Paleocene of the Great South Basin, New Zealand

Abstract

AbstractEvolution of rift basin fill and geometry depend on the complex interactions between fault growth, sediment supply, base level changes and pre‐existing basement fabric. This study integrates multiple datasets in the Great South Basin (GSB), southeast New Zealand, and provides key insights into the evolution of depositional environments in rift basins, including the interplay between normal faulting, sediment supply and sediment dispersal patterns. It also examines the control of pre‐existing basement fabric on rift geometry and sediment distribution in the syn‐ and post rift successions. The syn‐rift is up to ~5.5 km thick in the GSB, and is underlain by several different basement terranes. Three syn‐rift stages are recognised; c.105–101, 101–90 and 90–83 Ma. During the initial syn‐rift, isolated northeast‐trending graben developed, with resultant alluvial fan/fan delta, fluvial, coastal and lacustrine sediment fill. The balance between sediment supply and accommodation space exerted considerable control on facies, especially the presence of lacustrine facies. During the later stages of syn‐rift, marine transgression occurred and connectivity between the graben developed, with shelfal, shoreface and marginal‐marine facies deposited. With marine transgression across the hinterland, sediment supply was significantly reduced in the northeast of the basin, leading to underfilling of graben, and preservation of rift topography for up to ~20 Myr after the cessation of faulting. In the west, where sediment supply was higher, rift topography was quickly filled. The NW‐trending basement terrane boundaries controlled accommodation space development during the initial stages of graben formation. Later in the syn‐ and post rift stages, these terrane boundaries formed long‐lived sediment input points into the basin, and controlled the position of repeated large deltaic depositional units.