Middle Eocene to middle Miocene evolution of the Te Anau basin, western Southland, New Zealand

Abstract

After spreading ceased in the Tasman Sea in the Late Cretaceous, reorganisation of the Pacific-Australia plate boundary within the New Zealand region saw the development of several sedimentary basins during an extensional phase in the Middle Eocene to Oligocene. The Te Anau Basin was one of these basins. During this period, it opened adjacent to the eastern margin of Fiordland, a crystalline basement complex of Paleozoic to Cretaceous age, and is the main subject of this thesis.

A detailed study of sedimentary facies occurring within the up to 7000 m thick basin-fill identified a vast range of lithofacies. Depositional environments represented include alluvial fans and braided and meandering rivers feeding deltas along the margins of the basin. Estuarine conditions are indicated by a diagnostic mollusc fauna. Shallow marine facies include an extensive limestone shelf represented by the Tunnel Burn Formation. A whole series of submarine fans such as the upper Sandfly Formation or the Turret Peaks Formation formed in deeper marine settings. These fans can be observed grading into a hemipelagic background mudstone mapped as Waicoe Formation.

Faults controlling the Te Anau Basin are rarely exposed, but a comparison of sediment sequences allows reconstruction of kinematics, preferred orientation and timing of several fault systems. A NNE striking system, parallel to the basin axis and including the faults controlling the overall halfgraben geometry of the basin, directly reflects the regional tectonic setting. Pre-existing sets of NE and NW trending faults, cutting through Fiordland in straight lines, influenced sedimentation along the western basin margin where they cut it at high angles and produce distinct depocenters.

Paleogeographic reconstruction of SW New Zealand shows the Te Anau and Waiau basins as separate entities throughout much of their history. The Te Anau Basin is shown here to have opened from the south as a north-south trending halfgraben, defining the eastern boundary of Fiordland. It is inferred to have been separated from the Waiau Basin to the east by an elongated basement high, which was subsequently destroyed during Pliocene to Recent compression. A northward connection of the Te Anau Basin with the West Coast Basins as proposed by several previous authors seems unlikely.

The overall tectonic regime in which the Te Anau Basin developed is well constrained from seafloor data. This allows the sedimentary record of basin evolution to be compared directly to an independent plate tectonic model. The "tectonic signal" is isolated from the sedimentary record of the basin and compared with global models for extensional and strike-slip basins. Aspects of both are recognised, compatible with the transtensional origin indicated from plate tectonics. Changing tectonics towards strike-slip and finally transpression are also recognised within the sedimentary record.

A possible modern analogue is the Gulf of California. However, basins opening at the head of the Gulf at present are strike-slip dominated. Only the southern Gulf underwent an early, rifting phase, followed by subsequent transtension. Basins related to this early rifting, like the Loreto Basin, are seen as the closest analogues to the Te Anau Basin.