Maitai Group Research Articles

Origin of dispersed Permian–Triassic fore-arc basin terranes in New Zealand: Insights from zircon petrochronology

Permian–Triassic fore-arc basin terranes are exposed in New Zealand, but their original positions and tectonic configurations along the eastern Gondwanan margin are not fully understood. To better constrain late Paleozoic and Mesozoic reconstructions, we investigated the provenance of Permian–Triassic marine sandstone units from the Dun Mountain-Maitai Terrane (Maitai Group) and the Kaka Point Structural Belt (Willsher Group). The recognition of abundant volcanic lithic fragments in the sandstone samples, combined with the pattern of detrital zircon ages (unimodal to bimodal 280–240 Ma age distribution), demonstrate that the upper Permian to Middle Triassic volcaniclastic successions were derived from a proximal arc source. The detrital zircon age spectra match magmatic pulses in the adjacent Tuhua Intrusives (Median Batholith), a conclusion similar to that recently proposed for the Brook Street Terrane (Grampian Formation) and Murihiku Terrane (Murihiku Supergroup). Trace-element data from the dated zircon grains provide further evidence for a Median Batholith source and cross-terrane provenance links. The data indicate that 275–230 Ma zircon grains from the Maitai Group, Willsher Group, and Murihiku Supergroup were derived from a common magmatic source, and that the late Permian Longwood Suite (261–252 Ma) in the Median Batholith was a source region for these terranes. Based on the cross-terrane provenance links, we suggest that the Brook Street and Murihiku terranes were deposited in the proximal part of a fore-arc basin, whereas the Dun Mountain-Maitai Terrane represents the distal part of the same basin. Sedimentation in the Maitai Group ceased during the Middle Triassic (∼238 Ma), likely in response to a period of orogenesis at 235–230 Ma (Gondwanide Orogeny) that is widely recognized throughout the southwest Pacific.

Chapter 10 Sedimentary geochemistry used to infer the provenance of Permian–Triassic marine sandstones related to the SE Gondwana active continental margin, South Island, New Zealand

Abstract Major, trace and rare earth element data for sandstones and conglomerates from the Mid-Permian–Mid-Triassic Maitai Group are compared with other tectonostratigraphic units, using discrimination diagrams and comparisons with potential source terranes. Maitai Group sandstones reveal a mainly ophiolitic–oceanic-arc source during the Mid-Permian, followed by a mixed continental margin-arc–terrigenous source during the Late Permian. Latest Permian–Early Triassic sandstones mainly came from little-evolved continental margin-arc extrusives, tending to more evolved (but variable) during the Triassic. Source volcanism of the Murihiku Terrane sandstones was magmatically evolved relative to the Maitai Group generally (except during the Late Triassic). The Maitai Group and Murihiku Terrane are restored as proximal and more distal parts, respectively, of the SE Gondwana forearc basin. The localized Willsher Group shows some Maitai Group affinities. Sandstones in two melanges that formed in an outer forearc–subduction trench setting mainly indicate a mixed terrigenous–continental margin-arc source, similar to the Late Permian Maitai Group. The Caples Terrane, a Triassic accretionary prism, received detritus from little-evolved, to evolved continental margin-arc volcanics and terrigenous sources. Much of the arc-related material in all units is compatible with derivation from the latest Permian–Triassic Median Batholith, or a lateral equivalent along the SE Gondwana active margin.

Chapter 11 Geochemistry used to infer source characteristics and provenance of mudrocks of the Permian–Triassic Maitai Group and the associated Patuki Melange, South Island, New Zealand

Abstract Chemical and mineralogical evidence is reported, first for mudrocks from the Mid-Permian–Mid-Triassic Maitai Group and, secondly, for Late Permian(?) mudrocks from the structurally underlying Patuki Melange. Weathering and alteration indices indicate increased source weathering and aluminosilicate input stratigraphically upwards in the Maitai Group. The Maitai Group exhibits an upward change from a relatively enriched continental magmatic arc source (and related country rocks) during the Late Permian, to a relatively depleted continental magmatic arc source (and related country rocks) during the Triassic. The melange mudrocks have a similar provenance to the Late Permian mudrocks of the Maitai Group. The melange mudrocks are, however, generally less altered, probably because of additional, local, ophiolite-related input. Red iron-rich mudrocks accumulated widely in two Triassic Maitai Group formations and also locally in the Patuki Melange. The iron oxide was derived by continental weathering under warm conditions, and then accumulated relatively slowly under oxidizing seafloor conditions. The chemical evidence, as a whole, indicates sources for all of the mudrocks similar to the Median Batholith and associated country rocks, or non-exposed equivalents along the SE Gondwana active continental margin. Accumulation took place during a change from an icehouse to a hothouse world.

Chapter 13 Permian–Triassic felsic tuffs in South Island, New Zealand: significance for oceanic and active continental margin subduction

Abstract Felsic tuffs play an important role in the Permian–Triassic geology of the Eastern Province in South Island. In the Brook Street Terrane, primary felsic tuff is minor in the south (e.g. Takitimu Mountains) but abundant in the north (Grampian Formation, Nelson area). Felsic fallout tuff dominates one interval of the Maitai Group (Early Triassic Kiwi Burn Formation), south of the Alpine Fault, but is otherwise mainly redeposited by gravity flows. The Murihiku Terrane is characterized by two main intervals of felsic fallout tuff, the Middle Triassic Gavenwood Tuffs and the Late Triassic Bare Hill Tuff Zone, south of the Alpine Fault (e.g. Hokonui Hills and south Otago coast). Counterparts north of the Alpine Fault (Richmond Group) are mainly reworked, with terrigenous admixtures. Tuffaceous sediments are also abundant in the late Middle–early Late Triassic Willsher Group (south Otago coast). Based on combined field, petrographical, semi-quantitative X-ray diffraction (XRD) and chemical evidence, the felsic tuffs of the Brook Street Terrane in the south are interpreted as small-scale eruptions of fractionated oceanic-arc-type magmas. In contrast, the Triassic felsic tuffs of the Murihiku Terrane, Willsher Group and Maitai Group erupted violently and episodically in proximal to distal segments of the SE Gondwana continental margin.

Chapter 5 Correlations between a heterogeneous mantle and multiple stages of crustal growth: a review of the Dun Mountain ophiolite, New Zealand

Abstract The mantle and crustal sections of the Permian Dun Mountain ophiolite (DMO) in New Zealand's South Island have similar and potentially related tectonic histories. The mantle section records a four-part sequence of mid-ocean ridge basalt, and boninite melt extraction, followed by refertilization from supra-subduction melts and, finally, the intrusion of mafic–felsic island-arc tholeiite dykes. The crustal sequence represents a progression from mid-ocean ridge basalts to transitional island-arc tholeiites and, finally, mafic–felsic, possibly calc-alkaline, lavas; overlain by the 6 km-thick sedimentary Maitai Group. Plagiogranite zircon geochronology shows that the crustal ocean-ridge volcanism had commenced by 277.6 ± 3.3 Ma, and the youngest granites and granodiorites crystallized by 269.3 ± 4.5 Ma. All lithologies of the DMO are inferred to have formed in a broad forearc setting, representing a sequence from subduction initiation to magmatic arc formation. Sedimentary blocks within the structurally underlying ophiolitic Patuki Melange are similar to the Maitai Group, supporting a formational relationship with the DMO.

Chapter 12 A 251 Ma diorite clast from a Maitai Group conglomerate and a c. 259 Ma microdiorite sill intruding the Productus Creek Group, Southland, New Zealand: note

Abstract We present the results of U–Pb dating of zircons of two Late Permian igneous rocks from the Dun Mountain–Maitai and Brook Street terranes of New Zealand's South Island. A conglomerate clast of hornblende diorite from the Early–Middle Triassic Stephens Subgroup, Maitai Group, gives an age of 251.1 ± 1.6 Ma. A sample of hypabyssal andesite–microdiorite (Weetwood Formation) that intrudes the Late Permian Productus Creek Group of the Brook Street Terrane gives an age of 258.8 ± 7.9 Ma. The intermediate and subalkaline geochemistry of the two samples is not distinctive of specific tectonic settings, but their ages match plutons of the I-type, subduction-related Longwood Suite of the nearby Median Batholith. Assuming that correlation with the Longwood Suite is correct, the Weetwood sample increases the known areal extent of intrusion of the Longwood Suite; and the Stephens Subgroup Formation clast is consistent with a detrital link between Gondwana and the Dun Mountain–Maitai Terrane by the Middle Triassic. However, because of uncertainties regarding the timing of magmatism along other parts of the Gondwana margin, the data in this paper do not establish any specific provenance links.

Chapter 9 Sedimentary development of the Mid-Permian–Mid-Triassic Maitai continental margin forearc basin, South Island, New Zealand

Abstract The Mid-Late Permian–Mid-Triassic Maitai Group is interpreted as the distal forearc basin of the SE Gondwana active continental margin. The basin initially received very coarse detritus (Upukerora Formation) from the recently emplaced, nearby Dun Mountain ophiolite and related oceanic-arc rocks. Early tectonic subsidence accommodated up to 1000 m of bioclastic gravity-flow deposits from an adjacent carbonate platform, together with terrigenous and volcanic arc-derived material (Wooded Peak Formation). Basin-levelling turbidites then accumulated, composed of mixed terrigenous and arc-derived igneous material, with bottom-current reworking (Tramway Formation). Latest Permian–earliest Triassic gravity-flow deposits (locally absent) are characterized by relatively basic volcanic material (Little Ben Formation). Overlying turbidites accumulated in a relatively oxygen-poor, deeper-water setting (Greville Formation). Fine-grained background sedimentation then switched to well-oxidized, with traction-current reworking (Waiua Formation). The overlying Early–Mid Triassic Stephens Subgroup included the accumulation of lenticular sandstone turbidites, channelized conglomerate, well-oxidized deep-sea mud and felsic tuff. Permian and Early Triassic marginal carbonate platforms collapsed and were emplaced as localized exotic blocks. Extrusive and intrusive clasts within channelized conglomerates (Snowdon Formation) were derived from the adjacent continental margin arc. The forearc basin was subsequently displaced to its present position, possibly with up to 3000 km of southwards translation.

About this title - Paleozoic–Mesozoic Geology of South Island, New Zealand: Subduction-related Processes Adjacent to SE Gondwana

This volume presents a set of research papers that provide new data and interpretations of the Permian–Triassic terranes of SE Gondwana, now exposed in South Island, New Zealand. Following an introduction for general readers, a historical summary and a review of biostratigraphy, the individual papers primarily focus on the Permian magmatic arc of the Brook Street Terrane, the classic Permian Dun Mountain ophiolite and the Permian–Triassic Maitai Group sedimentary succession. The new results emphasize the role of subduction and terrane displacement adjacent to the Permo-Triassic Gondwana margin, and present fundamental insights into three crustal processes: subduction initiation, supra-subduction zone oceanic crust genesis and forearc basin evolution. The volume concludes with a wide-ranging summary and synthesis of the regional Cambrian to Early Cretaceous tectonostratigraphy of New Zealand's South Island in relation to the wider areas of Zealandia, East Australia and West Antarctica. The volume will interest geoscientists, including stratigraphers, sedimentologists, palaeontologists, igneous petrologists, geochemists, geochronologists and economic geologists, and is aimed at professional geologists and advanced students of geology.

Geology of Stephens Island and the age of Stephens formation

On Stephens Island, northernmost Marlborough Sounds, New Zealand, 1100 m of southeast-dipping sedimentary rocks, ranging from mudstone with limestone lenses to volcanogenic sandstone and conglomerate, comprise Stephens Formation, the youngest formation within the Maitai Group. The rocks form the most northerly exposure of the Nelson Regional Syncline; younging directions show that they are overturned and therefore are on the eastern limb of the syncline. They are well indurated with widespread metamorphic prehnite, pumpel1yite and lawsonite. No top or bottom to the formation is exposed. The formation is subdivided into Queens Beach, Incline, Takapourewa, and Titapua Members. The oldest rocks (Queens Beach Member) comprise 150 m of rounded cobble conglomerate which encloses angular blocks of coarse volcanogenic sandstone. One such block, 4 m long and up to 1.5 m thick, contains a diverse but indifferently preserved marine fossil assemblage dominated by brachiopods and pectinid bivalves of Late Permian age....

Stratigraphy of the lower Maitai group at west dome, southland, New Zealand

Five formations (Upukerora Formation, Wooded Peak Limestone, Tramway Formation, Little Ben Sandstone, and Greville Formation) representing the lower to middle Maitai Group are mapped on the southern flanks of West Dome, northern Southland. The rocks are dominantly redeposited volcaniclastic sediments and include breccia, conglomerate, sandstone, mudstone, and tuff, as well as redeposited atomodesmatinid (bivalve) limestone. Felsic tuffs are unusually abundant in the Tramway Formation and Wooded Peak Limestone at West Dome. A new unit, the Waitaramea Member, is introduced for distinctive strata in the upper part of the Wooded Peak Limestone characterised by devitrified felsic tuff, red mudstone, and reddish green sandstone. The biostratigraphic age of the lower Maitai Group on West Dome remains poorly constrained but bryozoan genera described from fossiliferous conglomerate lenses in the Wooded Peak Limestone confirm a late Early Permian to Late Permian age. These incude Dyscritella, Paralio‐clema, Fenestella, Maychella, Shulgapora, and others. Trace fossils are locally abundant in the Wooded Peak Limestone and Tramway Formation, and include Scalarituba, Pla‐nolites, and Lophoctenium. Lower Maitai strata at West Dome are overturned, dipping steeply to the north. They are weakly metamorphosed and contain abundant fine‐grained authigenic minerals including both lawsonite and laumontite.

The Nelsonian Stage: A new Early Triassic local stage for New Zealand

A new stage, herein named the Nelsonian Stage, is formally defined for sparsely fossiliferous New Zealand sequences of Early Triassic age. In terms of the local New Zealand stage scheme, the Nelsonian Stage is part of the Gore Series and occurs above the Late Permian Makarewan Stage and below the Middle Triassic Malakovian Stage. The base of the Nelsonian Stage is defined by the first appearance of the ammonoid Durvilleoceras within sequences of the Maitai Group that are exposed in the Nelson and Southland regions of the South Island of New Zealand. The Nelsonian Stage is attributed a middle Induan to early Anisian age range in terms of global stages, and represents a time‐span of about eight million years.

δ13Corg chemostratigraphy of the Permian‐Triassic boundary in the Maitai Group, New Zealand: Evidence for high‐latitudinal methane release

Carbon isotopic studies from marine organic matter of the Permian‐Triassic Maitai Group, New Zealand, reveal a significant δ13Corg shift toward more negative values within the Little Ben Sandstone Formation. These isotopic data chemostratigraphically define the previously debated position of the Permian‐Triassic boundary in the Maitai Group. The Permian‐Triassic record of the Maitai Group is also important because of its high paleolatitudinal setting and the deposition at intermediate depths in the ocean (c. 400 m) within a volcanic arc‐related basin. Marine Permian‐Triassic strata deposited at water depths deeper than shelf areas are rare. High latitude Permian‐Triassic boundary sections document a significantly larger isotopic offset across the boundary compared with lower latitude settings. Carbon isotopic values decrease rapidly by an average of 7%0 from homogeneous values (x ‐25%0) in the Tramway and lower Little Ben Sandstone Formation to highly fluctuating and very depleted values (x ‐32%0) within the Little Ben Sandstone Formation. The lowermost Big Ben and Tramway Formations are considered to be Permian in age, based on their homogeneous and comparably heavier carbon isotopic values and supported by fossil atomo‐desmatinid bivalves. Based on the distinct δ13Corg excursion toward negative values and the concurrent onset of strong isotopic fluctuations, the Permian‐Triassic boundary is placed in the lower half of the Little Ben Sandstone Formation. Very depleted δ13C values in the Little Ben Sandstone Formation of‐38%0 indicate a contribution from isotopically light methane. A possible methane source is clathrates, released by large submarine slides or warming‐induced melting of permafrost. The Little Ben Sandstone Formation has been interpreted as a massive event deposit from a submarine slide (Landis 1980). This hypothesised methane release could have been in part responsible for the larger Permian‐Triassic isotopic shift in high latitudes compared with low latitudes because large volumes of clathrates are trapped in continental shelves and high‐latitude permafrost.

Uranium-lead ages from the Dun Mountain ophiolite belt and Brook Street terrane, South Island, New Zealand

The Dun Mountain ophiolite and overlying Maitai Group is discontinuously exposed for 480 km in South Island, New Zealand. Zircon U/Pb dates from plagiogranite are presented for relatively intact ophiolite, ophiolitic melanges, and for more silicic volcanic-plutonic assemblages in the southern part of the belt where a typical ophiolite association is lacking. Step-wise dissolution experiments on slightly discordant plagiogranite zircon produce more concordant residues that indicate the zircons have lost from ∼1% to more than 5% of their radiogenic Pb relatively recently. High-precision 207 Pb/ 206 Pb dates establish the age of ophiolite formation for at least 350 km along strike to a narrow interval between about 275 and 285 Ma. The zircon U/Pb data confirm correlation of petrologically distinct segments of the Dun Mountain ophiolite and show that mafic-ultramafic ophiolite assemblages and moderately potassic high-level granitoids developed within a short time interval, probably during the extension of a volcanic-arc marginal basin. Thick lenses of polymictic breccia and bio-clastic limestone of the Maitai Group locally rest in depositional contact on relatively intact ophiolite within the Dun Mountain ophiolite. Comparison of inferred biostratigraphic ages from the limestone with the ∼280 Ma ages from the plagiogranites indicate a gap of ∼20 m.y. following ophiolite formation. A granite clast from conglomerate higher in the Maitai Group yielded a near concordant U/Pb date of 265 Ma and provides a maximum age for this part of the sequence. Attenuation of the Dun Mountain ophiolite by extensional faulting and erosion may have occurred during the interval between ophiolite formation and Maitai Group sedimentation. The Dun Mountain ophiolite and overlying Maitai Group are bounded to the west by Triassic and Jurassic volcanogenic sedimentary rocks of the Murihiku terrane, and in turn by the Brook Street terrane, which is interpreted as remnants of an early Permian oceanic arc. A hornblende gabbronorite associated with a layered mafic-ultramafic intrusion in the Brook Street terrane yielded a date of 265 Ma, significantly younger than Dun Mountain ophiolite. Such intrusions may represent the plutonic roots for ankaramitic volcanic rocks that comprise a conspicuous component of the Brook Street terrane, but which are not represented by detritus in the Maitai Group. Biotite granite occurs locally in the Brook Street terrane and is dated at 260 Ma. The absence of any clear stratigraphic correlation or provenance linkage between the Brook Street terrane and Dun Mountain-Maitai terrane suggests strike-slip displacements on intervening terrane boundary faults.

Permian ammonoids from the Greville Formation, Upukerora valley, Southland

Abstract Ammonoids collected from calcareous concretions within laminated siltstones of the Greville Formation near the head of the Upukerora valley, northwestern Southland, are identified as Durvilleoceras woodmani Waterhouse. These fossils are the first occurrence of this taxon south of the Alpine Fault and the first identifiable molluscan fossils to be collected from Greville Formation in Southland. They are important because of the absence of any other identifiable fossil material from Greville rocks and also because they strengthen the existing lithostratigraphic correlation of this unit across the Alpine Fault. To date, Durvilleoceras has not been recorded from rocks other than Greville Formation. Although the age range of Durvilleoceras is unknown, and age of the Greville remains poorly constrained, this study confirms the overall stratigraphic integrity of the Maitai Group. The available data suggest that Durvilleoceras and the Greville Formation are of Late Permian age.

Stratigraphy of Stephens Subgroup (Maitai Group) in the Countess Range — Mararoa River area, northwestern Southland, New Zealand

Abstract Stephens Subgroup (upper Maitai Group) is well exposed (except for its top) for over 30 km in the Countess Range, Snowdon Peak and Bare Peak-Mararoa River areas. The sequence consists of 2750 m of sedimentary rocks which include volcaniclastic sandstone, conglomerate, and siltstones, conspicuous tuffs and occasional limestone lenses. The subgroup is subdivided into five formations; four of them are new. Kiwi Burn Tuffs (new) is characterised by tuff. Acheron Lakes Sandstone (new) is a thick sequence of volcaniclastic sandstones. Cerberus Formation (new) is dominated by red coloured siltstone and sandstone. Eldon Formation (new) is another sequence of volcaniclastic sandstones, and Snowdon Formation is of variable lithology including volcaniclastic sandstone, conglomerate, siltstone and tuff. Fossils are rare in the Stephens Subgroup. However, Permophorus obovatus is found in the Retford Conglomerate Member, which has now been traced to the Mararoa River. This, along with Mellarium and radiometric...

Stratigraphic and structural relations of strata enclosing the Dun Mountain Ophiolite Belt in the Arthurton—Clinton region, Southland, New Zealand

Abstract Strata lying between the Caples Group and the Murihiku Supergroup in the region between Arthurton and Clinton, Southland, and previously assigned to the Waipahi and Arthurton Groups, consist largely of Maitai Group rock units. Associated igneous rocks belong to the Dun Mountain Ophiolite Belt. Rock units south of the ophiolite belt consist, from south to north, of the Waiua Formation, Greville Formation, and undifferentiated lower Maitai Group. The last unit is restricted to a series offault blocks adjacent to the ophiolite belt and probably includes parts of both the Tramway and Little Ben Formations. The Livingstone-Macpherson Fault System separates the ophiolite belt and Maitai Group from the Caples Group and is up to 3 km wide in this region. It contains faultbounded blocks of volcanogenic sandstone inferred to belong to the Little Ben Formation and red and green sandstone and siltstone, here termed the Watties Sandstone. The latter shows a mixed silicic volcanic and low-grade metamorphic provenance similar to the Caples Group. Sandstones from the Waiua, Greville, and Little Ben Formations were derived from a little dissected magmatic arc of largely intermediate composition. Tramway Formation sandstones show a mixed silicic volcanic and low-grade metamorphic provenance. Boundaries between rock units are faulted. Strata south of the ophiolite belt young essentially to the south except for some fault-bounded blocks of north-younging strata in the Greville Formation.

Stratigraphic and structural relations of the southern Dun Mountain Ophiolite Belt and enclosing strata, northwestern Southland, New Zealand

Abstract Three lithostratigraphic groupings are recognised to the northwest and southeast of the Five Rivers Plain in northwestern Southland: the lower Maitai Group consisting of the Greville, Little Ben, Tramway, and Upukerora Formations; the conformably underlying Dun Mountain Ophiolite Belt; and the Caples Group which lies to the east of the ophiolite and is separated from it by the Livingstone-Macpherson Fault System. On the northern side of the plain, the Maitai Group and ophiolite belt are deformed into a series of sinistral-verging steeply plunging folds which become extensively disrupted by left-lateral strike-slip faults in the east. The deformational pattern has resulted in acrossstrike repetition of these units and accounts for their apparent offset between West Dome and Lintley and the apparent thickening of the ophiolite belt between West Dome and Black Ridge. Sandstones of the Greville and Little Ben Formations are quartz poor and were derived from an undissected basic-intermediate volcanic ...

Fossil evidence of the age of Brook Street Volcanics Group, Nelson

Abstract Augite-rich rocks of the Brook Street Volcanics Group in east Nelson were, up until 1900, regarded as intrusive into sedimentary rocks mapped as Maitai Group. Subsequently all these rocks, which are largely tuffaceous, have been included in the Brook Street Volcanics. Poorly preserved fragments of atomodesmatinid bivalves and Peruvispira, previously recorded from the base of the group (Grampian Formation) in Nelson City, suggest a Permian age. However, a shellbed within the augite-rich rocks (Kaka Formation) in Speargrass Creek, near Lake Rotoiti 70 km southwest of Nelson City, contains abundant Maitaia obliquata (Waterhouse), indicating a middle Permian (Barrettian, late Kungurian) age for the group.

The Early Permian age of the Rai Sandstone, east Nelson, New Zealand

Abstract The Rai Sandstone contains a bivalve species allied to Aphanaia otamaensis Waterhouse, 1979 from the Waipahi Group of South Otago, and upper Takitimu Group of Southland, of Early Permian age. Previously the Rai specimens had been compared to Maitaia trechmanni Marwick of late Middle Permian age. ?Aphanaia cf. otamaensis, and gastropod ?Mourlonia impressa Waterhouse from the Croisilles Volcanics indicate that the Lee River Group as originally defined is of Early Permian age, older than the Maitai Group, of Middle and Late Permian age.

Little Ben Sandstone, Maitai Group (Permian): nature and extent in the Hollyford-Eglinton region, South Island, New Zealand

Abstract The Upper Permian Matai Group and its constituent formations of the Nelson Regional Syncline may be extended across the Alpine Fault into the Key Summit Regional Syncline, southern South Island. The Bryneira Group and its constituent formations are in most respects indistinguishable from the Maitai, and their usage should be discontinued in favour of the senior synonym. Little Ben Sandstone (previously mapped in Southland and Otago as Key Summit Sandstone) comprises distinctive green, quartz-deficient, vol-canogenic strata which are exposed extensively in the Eglinton-Hollyford region. Stratigraphy, field sedimentology, petrography (sedimentary and metamorphic), and chemistry of Little Ben rocks are described. It is suggested that Little Ben sediments were derived from a basaltic-andesitic volcanic arc and were deposited by mass flow processes in coalescing submarine fans flanking the arc.