Sedimentary architecture of a Late Cretaceous under‐filled rift basin, Canterbury Basin, New Zealand

Abstract

The Canterbury Basin in southeastern Zealandia was initiated during the late Albian (ca. 105 Ma) as a rift system, prior to the onset of seafloor spreading between Zealandia and eastern Gondwana at ca. 85 Ma. Basin‐fill architecture and sediment types have been determined from interpretation of 2D and 3D seismic‐reflection lines tied to five wells and compared to outcrop data from the literature. These data show that Cretaceous syn‐rift basin‐fill architecture was controlled by normal faulting, which produced basin and range topography that persisted for more that ca. 30 Myr after the cessation of faulting. Initial sedimentation was dominated by short drainage systems sourced from within the basin to produce alluvial fans along fault scarps, which inter‐fingered with axial‐flowing braided river conglomerates, coal measures and mudstone‐rich lake deposits in more central portions of the basins. Marine incursion of the basin from the east commenced during rifting and onset of Gondwana breakup, with maximum water depths achieved in the Oligocene. Post‐rifting, detrital sediments were mainly sourced locally from structural highs and augmented by pelagic sedimentation, which collectively draped and eventually buried most of the earlier‐formed horsts by the Early Eocene. The temporal persistence of basin and range topography reflected the low rates of erosion of horst blocks compared to the rates of fault displacement. The lack of substantial sediment input from outside of the rift basin was a key factor in the under‐filling of the Canterbury Basin. This research emphasises the key role played by sediment supply in rift basin filling. Despite an abundance of active faulting at initiation, some rift basins may fill slowly over tens of million years after rift cessation.

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

“…For example, in one northeastern GSB half‐graben the cumulative syn‐rift fault displacement was ~5400 m (~2800 ms), but the syn‐rift sediment thickness was only ~3200 m (~1700 ms; Sahoo et al, 2020). Similar underfilled syn‐rift basins were also recognised in the contiguous CB to the north (Barrier et al, 2021).…”

“…Similar underfilled syn-rift basins were also recognised in the contiguous CB to the north (Barrier et al, 2021).…”

“…In the filled graben, the development of fault‐controlled accommodation space roughly equals the thickness of the growth strata. However, in the underfilled graben, fault‐controlled accommodation space outstripped sediment supply, and therefore growth strata failed to fill up the graben (Barrier et al, 2021).…”

“…By the end of rifting at ~83 Ma (Barrier et al, 2021; Laird, 1993, 1994; Sahoo et al, 2020; Strogen et al, 2017; Sutherland et al, 2001) most syn‐rift graben in the GSB were filled with sediments (e.g. Figure 11).…”

“…Most Zealandia basins developed during mid‐Cretaceous intra‐continental rifting, and were subsequently filled by the end of rifting (~83 Ma; Gaina et al, 1998; Sahoo et al, 2020; Strogen et al, 2017). However, some of the graben in the north‐eastern GSB and the adjacent CB to the north remained underfilled at 83 Ma and some structural highs were emergent until the mid Paleocene (Barrier et al, 2021). By the end of the Paleocene, all graben that had formed during the rift phase in the GSB were filled with sediments.…”