Geology of the Fiordland Area - GNS Science

eorge Sound, and remnants of well-rounded boulderdeposits occur on some offshore stacks and on narrowledges along the outer coast as far north as Poison ay.The terrace surfaces may be wa e-cut rocky erosionplatforms, or may be o erlain by shallow marine orterrestrial deposits. ost surfaces are also draped by peatysoils (e.g. Wright iller 1952; Wardle et al. 1973). pto 13 surfaces ha e been mapped (Ward 1988), althoughthe highest are difficult to identify. The terraces range inwidth from a few metres to kilometres. This difference ismost marked in the icinity of ig Ri er, where surfaces onhard basement rocks expand eastward across the HaurokoFault onto softer Cenozoic mudstones. Howe er, somenarrow surfaces become much wider toward the west(Fig. 63), perhaps reflecting more intense marine erosionon the outer coast. t has not always been possible to mapterrace deposits consistently because of thick egetationco er and poor outcrop. Only where there is reasonablee idence are gra els shown on the map. The oldest, highestsurfaces generally lack associated deposits except in theWaitutu Forest, where large rounded boulders, and gra elsmixed with peaty soils (eQb), o erlie wa e-cut platformsin places. On lower terraces, deposits range from boulderysands to well-rounded and better sorted gra els that maybe beach deposits (Q b, Q b, Qb, Qb, Qb). Twoterrace le els are locally differentiated within Q5 deposits,and are tentati ely assigned to O stages Q5c and Q5e. nthe Waitutu Forest and Te Waewae ay, the lower and moreextensi e Q5 and Q7 terraces are clearly wa e-cut platformseroded into Cenozoic rocks and o erlain by shallow marinegra els, rarely with macrofossils (Ward 1988; ishop 1991;Turnbull ruski 1995; Fig. 53). The gra el clasts aremainly fresh, well-rounded, Fiordland-deri ed, plutonicand high-grade metamorphic rocks, except east of HumpRidge where Cenozoic sedimentary clasts and olcanicpebbles deri ed from rook Street terrane also occur. nthe Waitutu Forest and Te Waewae ay, marine surfacesare interspersed with glacial outwash gra els (Ward 1988;Turnbull et al. 2007; Fig. 63). Rare large granite boulders(3–5 m across) on some surfaces may represent completelydegraded till deposits.The ages of most marine terraces are poorly constrained.Absolute age control comes from 14 C dates on theyoungest surface (6000 yrs P; ishop 1985, 1991) andfrom cosmogenic dating of terraces at West Cape. There,a prominent 65 8 m terrace is dated, using 10 e and26Al isotopes, at ca. 120–130 ka (O stages 5–6; imSutherland 2004). This age gi es an inferred uplift rateof 0.52 0.08 mm yr for the terraces, similar to ratescalculated by Ward (1988), and by Turnbull et al. (2007)for terraces west of the Hauroko Fault. plift rates may bedifferent east of the Hauroko Fault, where there is ongoingLate Cenozoic deformation (Ward 1988; Turnbull ruski1995). The highest marine surfaces may be older than565 ka (Turnbull et al. 2007).OFFSHORE GEOLOGYSince the 1970s, offshore exploration for oil and gashas pro ided much information on the area south ofFiordland. There ha e been se eral seismic sur eys, andtwo exploration wells ha e been drilled (Parara-1 andSolander-1; H PCO 1976; Renton 1986). Limited sidescansonar and shallow seismic data co er Te Waewaeay ( ishop et al. 1992; Turnbull ruski 1995). Thebasement geology of western Fo eaux Strait can be inferredfrom regional geophysical data (Woodward 1976; rant1985) and the two wells, both of which reached basement.eological interpretations of the offshore Cretaceous andCenozoic sequence are gi en by Norris Carter (1980),Turnbull et al. (1993), Sutherland elhuish (2000), andSutherland et al. (2006b). West of Fiordland, plate margintectonic studies ha e also produced detailed data (e.g.Cutress et al. 1999; Wood et al. 2000; arnes et al. 2002,2005). Numerous samples ha e been dredged from the seafloor around Fiordland (Sutherland et al. 2004), and thereare se eral studies of the sediments beneath the waters ofthe fiords (Fleming 1951; Bruun et al. 1955; lasby 1978;Pickrill et al. 1992; Pickrill 1993).The onshore Waiau asin sequence extends offshore into TeWaewae Bay (Fig.46). Te Waewae Bay is floored by a thineneer of gra el and sand (Cullen ibb 1965), o erlyingPliocene silt that is correlated with Te Waewae Formation(Turnbull ruski 1995). iddle iocene uplift of thesouthwestern Waiau asin resulted in erosion of most ofthe Cenozoic sequence and brought basement gabbro closeto sea le el at id ay Reef in Te Waewae ay. asementand possibly Eocene rocks are also exposed on the sea bedjust south of Hump Ridge, and mark the southern limit ofthe onshore Waiau asin Cenozoic sequence.etween the northwest-trending Hump Ridge-Stewartsland Fault System and the northeast-trending HaurokoFault, a thick Eocene to Quaternary sequence is preser edin the Solander asin and its constituent Waitutu andHautere sub-basins (see abo e; Fig. 46). Further west inthe Solander asin, the northeast-trending Solander andLong faults (see Fig. 67) control the now-buried SolanderRidge or horst, where sediments as young as Quaternarylap onto basement rocks. Solander-1, drilled on the ridge,intersected Early Oligocene to Late iocene sedimentaryrocks o erlying altered microdiorite (Renton 1986; Watters1986). Younger sediments ha e been remo ed by marineerosion, and north-trending strike ridges in tilted iocenesediments are obvious on side-scan sonar profiles north ofthe well site. The sediments draped o er Solander Ridgeare folded and the equi alent anticline onshore may still bedeforming, as an O stage 5 marine erosion surface o erlyingit is also gently folded (Turnbull ruski 1995).62