Carboniferous conodonts from Rakaia Terrane, East ...

New Zealand Journal of Geology and Geophysics

ISSN: 0028-8306 (Print) 1175-8791 (Online) Journal homepage: http://www.tandfonline.com/loi/tnzg20

Carboniferous conodonts from Rakaia Terrane, East Otago, New Zealand P. Jane Forsyth , Gregory M. Martin , Hamish J. Campbell , John E. Simes & Robert S. Nicoll To cite this article: P. Jane Forsyth , Gregory M. Martin , Hamish J. Campbell , John E. Simes & Robert S. Nicoll (2006) Carboniferous conodonts from Rakaia Terrane, East Otago, New Zealand, New Zealand Journal of Geology and Geophysics, 49:3, 329-336, DOI: 10.1080/00288306.2006.9515171 To link to this article: http://dx.doi.org/10.1080/00288306.2006.9515171

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Date: 29 January 2016, At: 09:08

New Zealand Journal

of Geology

0028-8306/06/4903-0000

& Geophysics,

2006, Vol. 49Forsythetal.—RakaiaTerraneCarboniferousconodonts: 3 2 9 - 3 3 6

329

© The Royal Society of New Zealand 2006

Carboniferous conodonts from Rakaia Terrane, East Otago, New Zealand

P. JANE FORSYTH GNS Science Private Bag 1930 Dunedin, New Zealand

Downloaded by [139.0.27.242] at 09:08 29 January 2016

GREGORY M. MARTIN Formerly Department of Geology University of Otago PO Box 56 Dunedin, New Zealand HAMISH J. CAMPBELL JOHN E. SIMES GNS Science PO Box 30368 Lower Hutt, New Zealand ROBERT S. NICOLL Department of Earth and Marine Sciences Australian National University Canberra, ACT, Australia 0200 Abstract Conodonts of Late Carboniferous (Pennsylvanian) age have been recovered from marble within a 100 m thick sequence of pure and impure marble, hematitic and chloritic slate, and metachert in Torlesse Supergroup semischist of the Rakaia Terrane. This sequence is exposed near Conical Peak in the Kakanui Mountains, Otago. Bands of marble contain microscopic metavolcanic clasts, quartz grains, and conodont elements in a groundmass of recrystallised calcite. The conodonts include at least three genera: Idiognathodus, Streptognathodus, and Gondolella, which together suggest an age of mid-Middle Pennsylvanian to latest Pennsylvanian. The red and green slate represents metamorphosed volcaniclastic beds, and grey slate represents background deposition. Bedding indicators, where present, are subparallel to the plane of cleavage development. No facing directions are recognised. Sandstone and mudstone have been metamorphosed to pumpellyite-bearing semischist of textural zone IIA. The calcareous, siliceous, and metavolcanic sedimentary rocks are interpreted as deep-sea turbidites, sourced from a topographic high, probably a seamount. Associated turbiditic sandstone and mudstone are of continental provenance. Keywords conodonts; Idiognathodus; Streptognathodus; Gondolella; Late Carboniferous; Pennsylvanian; Blue Mountain Formation; Rakaia Terrane; Torlesse Supergroup; semischist; marble; Kakanui Mountains; Blue Mountains; Kakahu Marble; Otago; New Zealand

G05043; Online publication date 26 July 2006 Received 25 August 2005; accepted 30 May 2006

INTRODUCTION The Blue Mountains and Kakanui Mountains are located in eastern Otago, in the South Island of New Zealand (Fig. 1). A sequence of marble, c. 200 m thick and mappable over a distance of 5-10 km, is known from the Blue Mountains, where it is named Blue Mountain Formation (Brown 1959). It lies within typical Rakaia Terrane semischist (Fig. 1) but stratigraphic and structural relationships remain uncertain, despite good outcrop in a large quarry. The age of this large marble body has been assumed to be Permian or younger (Brown 1938; Paterson 1939; Mutch 1963) but remains unsubstantiated, as no identifiable fossils have previously been recovered despite numerous investigations. Conodonts were discovered within an isolated marble sequence exposed near Conical Peak in the Kakanui Mountains, some 15 km west of the Blue Mountains. Conical Peak is approximately along-strike from the Blue Mountain Formation marble, and like that formation is adjacent to the Waihemo Fault System. The conodonts are from a marble bed within a 100 m thick marble and metavolcanic sequence. They indicate a Late Carboniferous (Pennsylvanian) age for the Conical Peak marble, and although stratigraphic relationships remain uncertain, a comparable age for Blue Mountain marble is probable. The Conical Peak area was mapped by G.M. as a BSc (Hons) project (Martin 1999a,b). Previous studies Marble beds in East Otago were collectively named the Blue Mountain Limestone (Cox 1883; Hector 1884; Brown 1938; Paterson 1939; Mutch 1963). Brown (1959) defined the unit as the Blue Mountain Formation from a type locality in the Blue Mountains (Fig. 1). Mutch (1963) depicted an area of marble near the Blue Mountains, and Pringle (1980) described marble associated with metachert, red argillite, and metamorphosed basalts at localities in and near the Kakanui Mountains, notably at Devils Elbow in the Waianakarua River, 5 km northeast of Conical Peak (Fig. 1). Forsyth (2001) showed these and other small areas of metachert, marble, and metavolcanics on a regional-scale map of North Otago and South Canterbury. Cox (1883) suggested the Blue Mountain Limestone was of Early Carboniferous age, while Mutch (1963) considered a Balfour (Late Triassic; Norian-Rhaetian) age likely. Campbell & Warren (1965) mapped the distribution of fossil localities throughout the Torlesse Supergroup, suggesting that sequences towards the eastern margin are Permian. This was supported by the discovery of Permian Atomodesma near Danseys Pass (some 40 km northwest of Conical Peak) by Pringle (1979). More recently, Ford et al. (1999) reported Early Permian conodonts from Rakaia Terrane chert near Meyers Pass in South Canterbury (c. 65 km north of Conical Peak). Nicoll (in press) has suggested these may be of Carboniferous (Pennsylvanian) age.

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Fig. 1 Location of Conical Peak within the Rakaia Terrane of New Zealand's South Island. Inset: Conical Peak relative to the Waihemo Fault System and other marble/metavolcanic localities in the Kakanui Mountains (e.g., Blue Mountains, Devils Elbow). Textural zones in Haast Schist shown (IIA, IIB, III). Partly after Forsyth (2001).

N

Haast Schist 50km

n\


^ * ^ j Meyers Pass ^ ^ ^ Danseys Pass

.DevilsElbow .Conical Peak

~- Conical Peak (~ Waihemo Fault - n d System

Palmerston S h a g Point

Waihemo

47°S

0

100km

Caples/Torlesse '•. boundary Blue Mountain Formation

Waihemo Number One Fault

N

Conical Peak 945m 28 conodontbearing samples

Waihemo Number One Fault Blue Mountain Formation constrained fault moderately constrained fault synform axis antiform axis

1km bedding foliation

Fig. 2 Distribution map of the marble/metavolcanic sequence near Conical Peak. The Waihemo Number One Fault is offset along faults that disrupt the Conical Peak sequence. Axes of fault-related folding are indicated. After Martin (1999b).

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Forsyth et al.—Rakaia Terrane Carboniferous conodonts


Fig. 3 The main outcrop of the marble/metavolcanic sequence occurs in a west-facing cliff (base at grid ref. I42/188447) below a smallpeak(gridref. I42/188449), c. 1.4 km southwest of Conical Peak. Conodonts were recovered from the main outcrop and from another outcrop to the north (left).

Conical Peak

Adams (2003) and Adams & Maas (2004) investigated the age of metasedimentary rocks in this area using K-Ar and Rb-Sr dating methods. Rb-Sr and K-Ar ages of low-grade metasedimentary rocks can be interpreted as dating source area, diagenetic, metamorphic, and/or cooling events. Greywacke and argillite from the Blue Mountain quarry yielded an Rb-Sr age of 205 ± 28 Ma (Adams & Maas 2004), dating the episode of burial metamorphism in the latest Triassic that accompanied the development of the textural zone II foliation. Argillite at the same location was K-Ar dated at 188.5 ± 3.8 Ma (Adams 2003), constraining the subsequent cooling in the earliest Jurassic. Slate (argillite) samples from Conical Peak yielded K-Ar ages of 213.5 ± 4.3 to 198.0 ± 4.0 Ma (likewise Late Triassic to earliest Jurassic; Adams 2003). Carboniferous sedimentary rocks are rare in New Zealand and the conodonts documented herein represent only the second certain occurrence of Carboniferous fossils so far recorded. Jenkins & Jenkins (1971) recovered New Zealand's first Carboniferous fossils from the Kakahu Marble, some 150 km north of Conical Peak, but also within Rakaia Terrane rocks. Genera found by Jenkins & Jenkins (1971) and Hitching (1979) are Idiognathodus, Cavusgnathus, Gnathodus, Gondolella, Hindeodella? and Ligonodina, indicating a Late Carboniferous (Pennsylvanian) age. Nicoll (in press) interprets Cavusgnathus as ?Adetognathus and Gnathodus as Neognathodus. There are three other records of possible or probable Carboniferous fossils from New Zealand: a single conodont element recorded by Pound (1982) from melange within Caples Terrane rocks of Otago; foraminiferans recorded by Meshesha & Black (1989) from Waipapa Terrane rocks in Northland; and the Meyers Pass locality in South Canterbury (Ford et al. 1999; Nicoll in press).

CONICAL PEAK SECTION The Conical Peak area is separated into four blocks by northeast-trending strike-slip faults (Fig. 2). The largest of these blocks is 1.5 km wide, and its southwestern margin is exposed

fossils recovered

main outcrop and measured section

as an 80 m high scarp along a segment of the Waihemo Number One Fault (Fig. 3). At the base of the Conical Peak section (grid ref. I42/188447), marble and argillite are juxtaposed along what is inferred to be an original mudstone/limestone depositional contact. Lithologies The Conical Peak section (Fig. 3, 4) consists of interbedded pure and impure marble, red and green slate, red and green metachert, and semischistose mudstone, siltstone, and sandstone. Layers dip moderately to the ENE except where disrupted by reverse and strike-slip faults (Fig. 2). Pure marble Four beds of very fine grained, massive grey marble contain randomly distributed black fragments (possibly crinoid plates) 1-3 mm in diameter, which make up 5% of the rock. These marble beds occur at the base and in the central part of the measured section (Fig. 4). The groundmass consists of fine-grained, granoblastic polygonal calcite, with a few large (0.4 mm) quartz grains scattered throughout the rock and occasional fibrous, prismatic grains of actinolite. Foliation is incipient in this marble. Conodont elements have been extracted from this rock type. Impure marble Two thin beds of impure marble contain lenses of calcite up to 5 cm long, 3 cm wide, and 6 mm thick (40%) surrounded by smears of red (hematitic) slate (15%), green (chloritic) slate (15%), and light brown slate (10%). This rock has the well-defined foliation and lineation of an L-S tectonite. In thin section, large calcite crystals (up to 0.2 mm) with conjugate deformation twins and syntaxial calcite overgrowth around their edges occur in a fine-grained groundmass. Twenty percent of the rock is composed of evenly scattered quartz grains (0.2 mm) with serrated edges. Local clusters of intergrown polygonal quartz grains up to 0.3 mm are also present. Muscovite grains, oriented parallel to schistosity, occur in the bands between calcite lenses. Oblong metavolcanic clasts up to 4 mm long are found in hematitic slate bands throughout

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the rock. Some of these exhibit volcanic flow-banding in the form of oriented feldspar laths (0.2 mm long) in a dark devitrified glass groundmass. Some feldspars have swallow tails on tabular crystals that indicate a quenched texture. Subequant dark vesicles can be seen in a light coloured groundmass (possibly sideromelane) in some metavolcanic clasts.

Thickness 90m

i

Red and green slate

80m

II = = = = =

hematitic

chloritic

70m


Marble pure (grey) impure (hematitic & chloritic)

60m

50m

40m

1

. ' . ' . ' . ' .'

1

. ' . ' . ' . ' .'

1

. ' . ' . ' . ' .'

fossils recovered

Metachert green/grey (chloritic)

i

I'

I'

i

red (hematitic)

30m

outcrop obscured

Quartzofeldspathic sedimentary rocks mudstone

20m

outcrop obscured

10m

.I". interbedded mudstone/ sandstone

sil

Slate A 16 m thick hematitic slate bed is continuous for 100 m. The slate is very fine grained, highly calcareous, and contains small dark brown mudstone clasts and white calcite lenses up to 5 cm long and 1 cm thick (5%). Foliation and lineation are moderately developed. This slate weathers dark brown with iron staining along joint surfaces. In thin section, very small (0.1 mm) quartz and calcite grains (in 40:60 ratio) appear in a fine black hematitic groundmass. Some specimens contain slightly larger quartz grains up to 0.2 mm and scattered grains of magnetite. Folded quartz veins are shortened perpendicular to foliation. The section includes two 1 m thick beds of very fine grained, green, chloritic slate containing deformed lenses of darker green chloritic material (up to 12 mm long and 3 mm wide) in a lighter green groundmass. The darker green material, interpreted as altered basaltic clasts, contains small (0.1-0.2 mm), round, slightly deformed vesicles filled with calcite. Small, round, red/brown grains (generally