Geology of the Monowai-Waitutu area

Turnbull, I.M.; Uruski, C.I. 1995 Geology of the Monowai-Waitutu area. Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences geological map 19 68 p.

Abstract: Geological maps Monowai and Waitutu cover an area of complex geology along the southeastern margin of Fiordland. Topography ranges from flat-lying coastal terraces to rolling hills in the Waiau and Waitutu depressions, separated by the up-faulted Hump Ridge. Glacial lakes Monowai, Hauroko and Poteriteri lie across the boundary between low-lying land formed on Tertiary sedimentary rocks, and high country of Fiordland which forms the northwestern boundary of the mapped area. The maps straddle the Moonlight Tectonic Zone, a zone of faults with associated depositional basins which strikes northeast along the eastern margin of Fiordland. Within the zone, faults show evidence of strike-slip movement, and northward-directed thrusting is inferred in the northern Waitutu-Sub-basin. Hump Ridge is controlled by southeast striking faults of the Hump Ridge-Stewart Island Fault System. The Moonlight Tectonic Zone has been active since the mid Eocene and has controlled sedimentation and basin development in the region. Phases of development of the Moonlight Tectonic Zone are related to the history of the Alpine Fault which lies 80km to the west across Fiordland. Paleozoic and Mesozoic plutonic and metamorphic rocks lie west of the Hauroko Fault and form smaller areas on Hump Ridge. Metasediments and metavolcanics of the Loch Burn Formation (in part Triassic) are exposed north of Lake Monowai and on Hump Ridge. Older calc-silicate metasediments occur west of Lake Poteriteri, and non-calcareous metasediments form inclusions in younger plutonic rocks. Lower Cretaceous intrusive rocks include granite of the Poteriteri pluton, Green Lake Granodiorite, and smaller granite, aplite, and pegmatite intrusions. These intrude a background of biotite granitoid orthogneisses coeval with or younger than a suite of diorite, gabbro and amphibolitic gneisses, and the distinctive aegirine arfvedsonite alkali Electric Granitoid Gneiss of the Electric pluton. Mafic and granitic intrusives also occur on the southern end of Hump Ridge and form the offshore Mid Bay High and Mid Bay Reef. Cretaceous sediments are inferred to underlie Cenozoic sediments in the Waiau Basin and the offshore Solander Basin. West of the Hauroko Fault, basal Balleny Group sediments of late Eocene age underlie a small area. East of the Hauroko Fault and west of Hump Ridge, the Waitutu Sub-basin contains six formations, dominated by mudstone, which range in age from Eocene to Pliocene, and have distinctive sedimentary features which reflect tectonic instability. Between the Blackmount and Hauroko faults north from Lake Hauroko, Waiau Group sediments infill the Monowai Sub=basin of the Waiau Basin. Although dominated by submarine fan sandstones and basin mudstones, this sub-basin also includes a thin marginal shelf facies west of the Hauroko Fault which is dominated by bioclastic limestone. The main Waiau Basin extends east from Hump Ridge, and contains several thousand metres of Waiau Group sediments. The olde st sediments are upper Eocene subaerial breccia and conglomerate at Sand Hill Point. Overlying these is a thick unit of nonmarine to shallow marine sediments overlain by a submarine fan facies which grades into basin mudstone. The Clifden Subgroup lies within the background mudstone. Upper Waiau Group sediments include limestone resting on a middle Miocene unconformity in western Te Waewae Bay, and shallow marine Pliocene mudstone and sandstone. A sequence of Quaternary marine terraces which follows the southern coast in interleaved with outwash terraces related to late and early Otiran and Waimean glacial advances. Outwash terraces can be traced upstream to moraines at Lake Hauroko and Lake Poteriteri. Both lakes have had more than one outlet, and terrace sequences have formed in several underfit valleys. Recent deposits include peat mounds, alluvial fans, and landslide deposits. Offshore geology has been mapped from seismic and side scan sonar data, and most offshore units and structures can be correlated with the onshore area. The economic potential of the map areas lies chiefly in hydrocarbons, but remains largely untested. Other economic deposits are peat, limestone, and roading aggregate. The area lies within a zone of moderate seismic hazard, due to the proximity of the Alpine Fault. A recent fault is known at Lake Hauroko, and Quaternary offset is inferred on the Hauroko and West Hump faults. Deformation is ongoing in the Waitutu Sub-basin. Other hazards include landsliding and coastal erosion. The likelihood of further landsliding is high on steeper mudstone slopes. (auth)