Cenozoic sediments of the East Hump area, Te Waewae Bay, Southland

McMurtrie, Grant Bevan.

This thesis describes the Eocene to Recent sedimentary sequence of the East Hump area, in the southern Waiau Basin, Western Southland. Cenozoic sedimentation in the East Hump area began in the Late Eocene with the deposition of terrestrial conglomerates and marginal marine sandstone of the Hump Ridge Formation. The Hump Ridge Formation is conformably overlain by deeper marine sandstone/mudstone interbeds of the Hauroko Formation and mudstone of the Waicoe Formation. This Eocene to Miocene sequence is unconformably overlain by the Late Miocene inner shelf conglomerate, sandstone and limestone of the Port Craig Formation, which in turn grades into the outer to mid shelf Late Miocene to Pliocene Te Waewae Formation. The following stratigraphic advances have been made: the boundary between the Hump Ridge Formation and Hauroko Formation is defined; the Hauroko Formation is divided into two members, the sandstone-dominated Breakneck Member, and the mudstone-dominated Hauroko Member; the boundary between the Port Craig Formation and Te Waewae Formation is redefined; the Port Craig Formation is correlated throughout using the lithofacies and biofacies concepts; and a new member (the Waikoau Member) of the Te Waewae Formation is proposed. These lithostratigraphic advances combined with chronostratigraphic data allow new interpretations of two areas of Te Waewae Bay coastal sediments. A chronostratigraphic map produced from foraminiferal and molluscan dating shows an Eocene to Early Miocene (Runangan to Altonian) sequence of Hump Ridge Formation, Hauroko Formation, and Waicoe Formation which youngs and thins to the south. The Port Craig Formation, which unconformably overlies these · sediments, was previously thought to be Tongaporutuan at the base, but foraminiferal dating now interprets it as Kapitean throughout. Dates from above and below the Miocene unconformity suggest it has a maximum duration of 17 million years (part of the Altonian Stage, Southland Series, and Tongaporutuan Stage). The time gap between these units increases to the south to 24 million years at J amiesons Creek, coinciding with decreasing thickness of the Waicoe Formation. The Miocene/Pliocene boundary (Kapitean/Opoitian) probably coincides with Ethological change from the Waikoau Member to the Deadwood Member in the Te Waewae Formation. A diverse macrofauna from the sandstone of the Kapitean Port Craig Formation allows biofacies correlation of outcrops in streams several kilometres apart. In contrast, the gastropod-dominated assemblages of the mudstone of the Te Waewae Formation are more uniform and include taxa which are of more chronostratigraphic ). t IV value. Fossils from both the Port Craig Formation and the Te Waewae Formatio,n are used to interpret paleoenvironment. Petrographic study of conglomerate, sandstone, and heavy minerals from the Hump Ridge, Hauroko and Port Craig formations indicate that Fiordland and cannibalised underlying sediments were the major sources of detritus. Provenance areas are narrowed using petrography, heavy minerals, previous published paleogeographic maps, and paleocurrents. Rhyolite and dacite from the Oligocene Hauroko Formation has been traced to a potential source of the Eastern Fiordland Volcanics around present Lake Te Anau. Similar clasts in the Port Craig Formation are probably reworked from the underlying Hauroko Formation as the East Hump area was surrounded by sediment sinks in the Late Miocene. Heavy minerals indicate the Brook Street terrane was a source of sediment in the Late Eocene and Early Miocene, and Southwest Fiordland was a possible source in the Late Eocene to Early Oligocene. Paleoenvironments of the sediments of the East Hump area are consistent with regional structural trends. The terrestrial to marginal marine Hump Ridge Formation (Runangan), the submarine fan complex of the Hauroko Formation (Whaingaroan), and the bathyal mudstone of the Waicoe Formation (Whaingaroan to Altonian) were deposited during the Eocene to Oligocene; thought to be a phase of oblique extension. The Miocene unconformity probably marks Middle to Late Miocene compression which caused uplift of Hump Ridge resulting in folding, faulting, and erosion of the sedimentary sequence. Back-arc subsidence allowed deposition of the shoreface to inner shelf Port Craig Formation (Kapitean) and the outer shelf Waikoau Member of the overlying Te Waewae Formation (Kapitean). Uplift in the Pliocene is reflected in shallowing to the mid shelf Deadwood Member of the Te Waewae Formation. This uplift continues to the present day, reflected by a flight of preserved Quaternary marine terraces.

xiv, 262 p. : ill. (some col.), maps (some col.) ; 30 cm.

1996McMurtrie

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