Ordovician Conodonts from the Watonga Formation, Port Macquarie, Northeast New South Wales

2007·3.3 MB·English

by  David J Och

Ordovician Conodonts from the Watonga Formation, Port
Macquarie, Northeast New South Wales
David J. Och^"^, Ian G. Percival^ and Evan C. Leitch'
' Environmental Sciences, University ofTechnology, Sydney 2007, NSW; and^Geological Survey ofNSW,
Department ofPrimary Industries, 947-953 Londonderry Road, Londonderry 2753, NSW.
Och, D.J., Percival, I.G. andLeitch, E.G. (2007). Ordovicianconodonts fromthe WatongaFormation,
PortMacquarie, northeastNew SouthWales. Proceedings oftheLinneanSocietyofNewSouth Wales
128,209-216.
Conodonts ofMiddle to Late Ordovician age, obtained from cherts ofthe Watonga Formation exposed
in the Port Macquarie Block ofthe Mid North Coast region ofNew South Wales, establish this unit as
the oldest biostratigraphically-dated part of the southern New England Fold Belt subduction-accretion
complex. Correlation ofthe Watonga FormationwiththeWoolomin Formation, faunas from which are no
older than Pridoli, cannot be sustained. This revised age provides evidence ofpossible early Palaeozoic
subduction-accretion in this region at the same time as arc magmatism, volcaniclastic sedimentation and
exhumationofhigh-pressure metamorphic rocks wereproceeding furtherwest.
Manuscriptreceived 16November2006, acceptedforpublication 15 January 2007.
KEYWORDS: Conodonts,NewEnglandFoldBeh, Ordovician, Silurian, Watonga Formation, Woolomin
Formation
INTRODUCTION
overlainbyearlyTriassicrocks ofthe CamdenHaven
Port Macquarie is situated approximately 350 Group but is faulted against all earlier stratifiedrocks
km north of Sydney, geographically located in the (Leitch 1980). A Late Silurian to Late Devonian age
Mid-North Coast region of New South Wales and hasbeenassignedtotheWatongaFormation,basedon
geologically in the southern New England Fold lithological correlationwiththeWoolominFormation
Belt (Fig.la). South of the township, the Watonga of the Tamworth region (Leitch 1980), undescribed
Formation (Leitch 1980), that has been invaded by palaeosceniid radiolarians from chert at Watonga
minorintrusionsandserpentinitebodies,makesupthe Rocks (Ishiga et al. 1988a) and conodonts reported
eastern portion ofthe fault-bounded Port Macquarie from chert at Tacking Point (Ishiga et al. 1988a, b).
Block of Leitch (1974). The Watonga Formation Inthis paperwe presentnew conodont datathatmore
is well exposed along the coastline between Port accurately constrains the age ofWatonga Formation
Macquarie and Tacking Point (Fig.lb) where it chert,andreassesspreviousmicrofossilidentifications
comprises mostly broken formation inferred to result from that formation and its possible correlatives in
from disruption of a once-stratified sequence of orderto reinterpret its tectonic significance.
basalt, chert, siliceous mudstone, siltstone, sandstone
and conglomerate. Several chert-dominatedunits can
be mapped west from the coast, and locally little- CHERT IN THE WATONGAFORMATION
disrupted basalt forms sections at least several tens
of mefres thick. Previous descriptions of the rocks ChertoccurswidelyintheWatongaFormation,in
ofthe Watonga Formation include those ofBarron et mappablechert-richzones(Fig. lb), inshear-bounded
al. (1976), Leitch (1980) and Och et al. (2005), who blocks associated with basalt and/or mudstone,
all concluded that the formation comprises a part of and as discrete tectonically isolated blocks. Mass
the accretionary- subduction complex that is widely flow deposits, although uncommon, also contain
exposed in the New England Fold Belt east of the chert clasts. Most exposures occur as 'ribbon chert'
Peel-Manning Fault System (Fig. Ic). characterised by discontinuous stratification with
mm mm
The Watonga Formation is ;inconformably individual beds 5 to 200 thick intercalated
- ' .
ORDOVICIAN CONODONTS FROM PORT MACQUARIE
89 •SOSiB -H 92 93 —94— '95*=.
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LITHOSTRATIGRAPHIC UNITS
Quarry
MjtfdfeJoLaleOr€ovidan
Roads Dominantly pillowand massivebasalt
River, aeekand ponds Watonga
Dominantlychert
Cenozoic Formation
Laterite, and alluvial, swamp, anddune cwnplexes. Undifferentiated rocks
I I
HPermiantoTnassic (?)EarlyCambrian
Mafic intrusives Serpentinite
f//l
Figure 1. (a) Location ofPortMacquarie in tiie southern partofthe New England Fold Belt, (b) Geologi-
cal map ofthe northeast corner ofthe Port Macquarie Block showing localities sampled for conodonts.
Map grid is AMG-66, (Mapping by D. Och). (c) The Port Macquarie Block and adjacent tectonic ele-
ments of the southern New England Fold Belt. Pale grey (Tableland Complex) is mostly accretionary
- subduction complex terranes, grey (Manning Block and Nambucca Slate Belt) Early Permian overlap
sequences, and dark grey (Tamworth Belt) Palaeozoic arc and forearc deposits. Widespread latest Car-
boniferous-Triassic granite bodies omitted.
with thinner recessive dark mudstone units (some common thicker beds (up to 1 m) are more laterally
mere films), andtogether forming irregularflat lenses continuous but change thickness along strike and
m
rarely exceeding 0.5 in length (Figs 2a, b). Less mostly lens out or are terminatedby faults and shears
210 Proc. Linn. Soc. N.S.W., 128, 2007
OCH, PERCIVALAND LEITCH
D.J. I.G. E.G.
Figure 2. (A) Ribbon chertshowing discontinuous characterofstratification, thickness variation in indi-
viduallayers, and irregularfolds, all suggestive ofsoft sediment deformation. TackingPoint(Grid refer-
ence 493930 mE 6517660 mN, location ofsample Ptmk 433). (B) Broken formation ofprominent chert
lenses and recessive sheared mudstone matrix offset by a late fault. Town Beach (Grid reference 492600
mE 6522550 mN, location ofsample Ptmk 434).
within individual exposures. and west of Port Macquarie, and recognised the
The chert is highly variable in colour, with chert-rich Watonga Formation as having lithological
white and grey predominating but including red, similaritiestotheWoolominFormationexposedsouth
orange-brown, green and black varieties. Primary of Tamworth (Fig. Ic). On this basis he assigned a
sedimentary structures other than relic bedding are generalised early Palaeozoic age to the Watonga
absent. Widespread disharmonic mesoscopic folds, Formationalthoughbiostratigraphicallyuseful fossils
lacking any signs of axial surface structure, of had not been obtained fi:om these rocks. Prior to the
variable orientation, and ofa style ranging frombox- present investigation, biostratigraphic data for the
shaped to fluidal, are interpreted as pre-consolidation Watonga Formation in the Port Macquarie Block
slump structures. Ghosts ofradiolarians arepreserved was limited to a record of the conodont Belodella
in some specimens but others consist solely of fine spp. (indicative of a generalised Late Silurian-Early
anhedral quartz grains with a dusting of iron oxide Devonian age) recovered from red cherts at Tacking
minerals. Samples collected from Tacking Point Point,andlateFrasnian(LateDevonian)palaeosceniid
(localitiesPtmk432&433)arestronglyrecrystallised radiolariaincludingPalaeorubus hastingensis Ishiga,
due to local thermal metamorphism, whereas those 1987 (in Ishiga et al. 1987) from chert at Watonga
from Flyrms Beach and Town Beach (Ptmk 436 and Rocks (Ishiga et al. 1988a; b). Although this material
Ptmk434,respectively),althoughhighlyveined,show was never illustrated, these ages have been widely
only the effects of the low grade regional alteration accepted as dating the Watonga Formation.
thathas affected all ofthe WatongaFormation.
BIOSTRATIGRAPHY OF THE WOOLOMIN
PREVIOUSAGE-DATING OF THE WATONGA FORMATION
FORMATION
Inthe lightoftheinferredlithologicalcorrelation
Accurate dating of the rocks of the Port of the Watonga Formation with the Woolomin
Macquarie Block is important in understanding the Formation of the Tablelands Complex to the west
geological evolution of the region, better defining near Tamworth, it is important to reassess available
the timing of accretion, determining relationships data on ages fi-om the latter unit. Major advances
ofthe block to other structural units ofthe southern in dating the Woolomin Formation resulted fi-om a
New England Fold Belt and establishing tectonic collaborative project between geologists from the
relationships with other early Palaeozoic elements University of Sydney and a consortium of Japanese
in eastern Australia. Leitch (1980) first defined universities inthe mid-1980s. Microfossils, including
Palaeozoic lithostratigraphic units in the area south radiolaria and conodonts, were extracted from cherts
Proc. Linn. Soc. N.S.W., 128, 2007 211
ORDOVICIAN CONODONTS FROM PORT MACQUARIE
by dissolution in hydrofluoric acid; the results were
presented in a detailed report (Ishiga et al. 1988b) Figure3 (RIGHT). Conodonts fromWatongaFor-
with significant findings summarised in Ishiga et al. mation chert, Port Macquarie Block
(1988a). Conodonts identified by these authors as A - G from locality Ptmk 432, Tacking Point; A.
MMMC
Ozarkodina eosteinhornensis, Belodella cf. resima Panderodus recurvatusl 4348; B. eobe-
and Walliserodussp. wererecoveredfi-omonelocality lodiniform element ofBe,lodina sp., MMMC 4349;
(WA-50) inredbeddedchertnorthofWoolomin. This C. Paroistodus sp., MMMC 4350; D. drepanois-
assemblage, although limited, is clearly indicative todid element in oblique view, showing strongly
of a Late Silurian (Pridoli) age due to the presence flared basal cavity (specimen intersected by V-
MMMC
of the nominate species of the eosteinhornensis shaped cryptocrystalline vein), 4351; E.
zone, now referred to as Ozarkodina remscheidensis indeterminate coniform element, MMMC 4352;
eosteinhornensis. Additional specimens illustratedby F. drepanoistodid element with expanded base,
MMMC M
Ishigaetal.(1988b)confirmtheidentityofthisspecies. 4353a; G, strongly reclined element,
However, Belodella resima is a characteristic Early possibly related to Ansella sp., MMMC 4353b. H
Devonian form, not known to occur in the Silurian - K from locality Ptmk 434, Town Beach; H. uni-
and typically displaying strong marginal costae dentified platform? element with two prominent
not observed in the specimens fi-om the Woolomin peg-like denticles (remainder of specimen, in up-
Formation. The four specimens illustrated from perpartoffigure,is outofplaneoffocus),MMMC
localityWA-50byIshigaetal. (1988b, pi. 3 figs 9-12) 4355; I. unidentified S element, possibly referrable
are insufficiently well-preserved to show diagnostic to Periodon, showing fine denticles but apparently
MMMC
features ofthe Late Silurian species B. anomalis, so incomplete posteriorly, 4356; J. exten-
they are probably best assigned to Belodella sp. The sively fragmented element oiBelodina, anteriorto
specimen figured by Ishiga et al. (1988b, pi. 3 fig. right,MMMC 4357; K.lateralviewofunidentified
8) as Walliserodus sp. is more likely to be the non- platform? element with discrete peg-like denticles
M MMMC
denticulate element ofthis Belodella as it appears comparable with those shown in H, 4358;
W
to lack the pronounced costae on lateral faces that L - from locality 436, Flynns Beach. L. Pan-
MMMC
characterises Walliserodus. Ishiga et al. (1988a, derodus sp., 4359a; M. Panderodus recur-
fig. 2p) illustrated a sharply-keeled element with vatus? showingcurvatureofcusp comparablewith
triangular cross section, planar to slightly concave specimen depicted in A (although expanded base
MMMC
lateral and posterior faces and a deep basal cavity is more reminiscent of drepanoistodid),
that they also assigned to Walliserodus sp. from the 4360a; N.Strachanognathusparvus, interpreted as
MMMC
same locality; it is here identified as the long-ranging spUt along cusp and dextrally separated,
genus Coelcerodontus sp. Four additional fi-agments 4361; O. unidentified element with two prominent
of S elements ofozarkodinids, depicted by Ishiga et denticles on lateral process, MMMC 4359b; P.
al. (1988b, pi. 3 figs. 13-16), are too incomplete for Protopanderodusl sp. with posteriorly-extended
MMMC
identification but are consistent with Late Silurian base, 4359c; Q. indeterminate coniform
species. The Pridoli age inferred for cherts at locality element resembling Dapsilodus, MMMC 4362;
WA-50 contrasts with an early Carboniferous age R. Periodon aculeatus, S element (note that cusp
determined for similar cherts south of Woolomin is displaced into section below plane of focus),
(locality 29 of Ishiga et al. 1988a; b) based on the MMMC 4360b; S. Phragmodusl sp., P element,
occurrence oftheradiolarianAlbaillella sp. Giventhe MMMC 4363a; T. unidentifiedPb element(anteri-
highly imbricate structure ofthis area the possibility orprocess unclear, but note three ormore discrete
MMMC
of infaulting of younger units as occurs north of denticles on posterior process), 4364; U.
Woolomin cannot be discounted (cf Cawood 1982). Pseudobelodina sp., MMMC 4360c; V. Pseudobe-
While the Pridoli age determined for the Woolomin lodina sp., MMMC 4363b; W. unidentified Sa?
MMMC
Formation is generally consistent with the previously (symmetrical) element, 4365. All ele-
accepted age of the Watonga Formation, new data ments illustrated in lateral view unless otherwise
reported here suggest that the Watonga Formation is indicated. Scale bar for each specimen represents
considerably older. 100 |iim. Chertsections are housed in the microfos-
(MMMC)
sil reference collection ofthe Geological
Survey ofNSW, Londonderry Geoscience Centre,
Londonderry 2753.
212 Proc. Linn. Soc. N.S.W., 128, 2007
D.J. OCH, I.G. PERCIVALAND E.G. LEITCH
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Proc. Linn. Soc. N.S.W., 128, 2007 213
ORDOVICIAN CONODONTS FROM PORT MACQUARIE
NEW DATAON THEAGE OF THE WATONGA as Phragmodus sp. by Iwata et al. (1995: figure 2g)
FORMATION from the Ballast Formation east of Cobar in central
NSW, though the Town Beach example is unlikely to
More than 25 conodont elements were observed be that genus (R.S. NicoU, pers. comm.. 2006). The
in cherts collected from outcrops of the Watonga Ballast Formation fauna was tentatively assigned a
Formation on the coast immediately south of Port Late Ordovician age by Iwata et al. (1995).
Macquarie (see Fig. lb for sample locations). Unlike Orange-brown chert from Flynns Beach (sample
previous investigations, these microfossils were not Ptmk 436) was the most prolific of the samples
dissolved out of the cherts using HF. Instead, the examined, yielding 14 conodont elements. Periodon
technique used involved preparing four or five large aculeatus (Fig. 3R) has a range of late Darriwilian
(7.5 X 3.8 mm) thin sections fi-om each sample, cut (late Middle Ordovician) to early Gisbomian (early
parallel to bedding planes of the cherts, and ground Late Ordovician). Pseudobelodina (Fig. 3U,V) and
to a thickness of about 50-60 microns. Both sides Strachanognathusparvus (Fig. 3M) also occur over
ofthe finished thin section were carefully examined this interval, which is consistent with ages of other
with a binocular microscope using transmitted light. genera tentatively identified from this locality. Thus
This method (first developed by Ian Stewart of locality 436 maybe slightly olderthanboth localities
Monash University) has previously been employed 432 and 434 which couldbe identical in age.
to investigate conodont biostratigraphy in cherts In summary, the maximum age range indicated
NSW
exposed intheNarooma areaonthe south coast for cherts from the Watonga Formation appears to
(Glen et al. 2004), and from the Tamworth Belt south extend from late Darriwilian to somewhere within
of Tamworth (Fig. Ic) fi-om where Stewart (1995) the Late Ordovician, possibly as young as the end
determined a Middle to early Late Cambrian age for ofthe Eastonian (middle Late Ordovician). There is
conodonts in spiculitic chert of the Pipeclay Creek no evidence for Silurian or Devonian ages in these
Formation.Althoughconodonts foundinthiswaycan samples, thus conflictingwiththepreviouslyreported
only be observed in the plane ofsection in whatever occurrence ofBelodella spp. (Ishiga et al. 1988a; b).
random orientation is presented to the viewer, the Several Ordovician conodont elements, such as those
thin section technique has the distinct advantage of ofAnsella and Belodina (the latter newly identified
preserving extensively fractured elements that would from Tacking Point) resemble Belodella, though
be destroyed by dissolution in HF. without illustrations ofspecimens that were obtained
Samples Ptmk 432 and Ptmk 433 were obtained fromtheWatongaFormationbyIshigaitisimpossible
from grey-black chert outcrop at Tacking Point (Fig. to clarify whether or not this was misidentified.
lb). Although the two cherts closely resemble each Palaeosceniid radiolaria obtained from the Watonga
other, sample Ptmk 433 was devoid of microfossils Formation at Watonga Rocks include Palaeorubus
whereas Ptmk 432 yielded eight elements. One of hastingensis, otherwise known only from late
these (Fig. 3B) is identified as the eobelodiniform Frasnian rocks at Yarras in the western part of the
element of the Late Ordovician genus Belodina. Hastings Block, to the west of the Port Macquarie
Associated Paroistodus (Fig. 3C) and unidentified Block (Ishiga 1988). Until more information is
drepanoistodids are consistent with this age. available concerning the age range ofthis form, the
A
fiirther four conodont elements were found in Late Devonian age accorded the unit here cannot be
sectionscutfromsamplePtmk434, agrey-blackchert regarded as well established.
at Town Beach. Although extensively fi"actured, the
belodiniform element ofBelodina (Fig. 3J) is readily
identifiable, and provides support for recognition TECTONIC IMPLICATIONS
of another element from the Belodina apparatus in
sample Ptmk 432. Elsewhere in New South Wales Although no coherent stratigraphic sequence
several species ofBelodinahave been identified from has been demonstrated in the Watonga Formation,
limestones of Gisbomian and Eastonian age (Late the association of chert and mudstone with basalt
Ordovician) in the central part ofthe state. Belodina mainly of mid-ocean ridge magmatic affinity (Och
is also known from limestone olistoliths ofEastonian et al. 2004), and interbedded graded volcaniclastic
age in the Wisemans Arm and Drik Drik formations sandstone and siltstone, is readily interpreted as the
of the Tablelands Complex and Tamworth Belt, product of sea-floor spreading, the accumulation of
respectively, in the New England Fold Belt (Furey- pelagicoceanicrocks, andfrenchdepositiononaplate
Greig 1999; 2000).Anunidentifiedelementillustrated undergoing subduction. The early deformational style
inFig. 31mostcloselyresemblesaspecimenidentified of the Watonga Formation, involving progressive
214 Proc. Linn. Soc. N.S.W., 128, 2007
OCH, PERCIVALAND LEITCH
D.J. I.G. E.G.
disruption of the rocks in a continuum from soft CONCLUSIONS
sediment deformation (Fig. 2a) to shearing and
stretching producing broken formation (Fig. 2b), is Conodont faunas identified in bedding-parallel
consistent with accretionary - subduction tectonics. thin sections from Watonga Formation cherts of
Furthermore the Watonga rocks are of similar the Port Macquarie district are of Middle-Late
structure to that of many units considered to be of Ordovician age, thus making these cherts the oldest
subduction - accretion origin, including pre-Permian biosfratigraphically dated rocks from the extensive
rocks from elsewhere east ofthe Peel-Manning Fault accretionary - subduction complex of the New
System inNew England. England Fold Belt. The dates provide a minimum
TheMiddle-LateOrdovicianchertsamplesPtmk age for the oceanic plate that was subducted, and
432, 434 and 436 are the oldest biostratigraphically raise the possibility that the Watonga Formation was
dated rocks recorded from the New England accreted in the early Palaeozoic at the same time as
accretionary - subduction complex, and rule out arcmagmatismandvolcaniclasticsedimentationwere
correlation of the Watonga Formation with the preceding further west. A reassessment ofthe age of
Woolomin Formation chert from which only Late a conodont fauna from the Woolomin Formation in
Silurian and younger ages are known (see preceding the southwestern part of the accretionary complex
discussion). The Ordovician chert ages provide a indicates itis ofLate Silurian(Pridoli) age, andhence
minimumagefortheoceanicplatesedimentsthatwere substantially younger than the Watonga Formation.
subducted, and raise the possibility that the Watonga Thus the previously conjectured correlation between
Formation was accreted in the early Palaeozoic, the Woolomin and Watonga formations can no
at which time arc magmatism and volcaniclastic longer be sustained. The Watonga Formation was
sedimentation were proceeding fiirther west in the probably moved north in the late Palaeozoic during
New England Fold Belt (Cawood 1983; Furey-Greig displacement ofthe Hastings Blockalong strike from
et al. 2000; Oflfler and Shaw 2006) and in the eastern the southern Tamworth Belt.
LachlanFoldBelt(Glenetal. 1998),andhighpressure
metamorphicrocksnowembeddedwithinserpentinite
bodies at Port Macquarie were being exhumed under ACKNOWLEDGMENTS
blueschist facies conditions (Fukui et al. 1995; Och
et al. (2003). Scheibner (1998) and Glen (2005) have We thank Anthony Och for his valuable assistance
suggested that the blueschists are accretionary rocks in the field. Gary Dargan (Geological Survey of NSW)
that formed outboard of the Macquarie Arc in the prepared the chert thin sections. Conodont microfossils
eastern Lachlan Fold Belt. Ordovician accretion of were photographed by David Barnes (NSW Department
the Watonga Formation rocks would be consistent of Primary Industries). Bob Nicoll (Canberra) and John
with them constituting a fragment of the Narooma Pickett (Londonderry) provided usefiil second opinions
accretionary complex postulated to extend into New on conodont identifications. Constructive reviews by Dick
Glen, Peter Cawood and Yong-yi Zhen were instrumental
Englandbytheseworkers. TheOrdovicianage forthe
in reorganising the manuscript for publication. David
Watonga Formation also provides a potential source Och and Ian Percival publish with permission of the
forchertblocks widespreadinthe (?) EarlyDevonian Deputy Director-General, NSW Department of Primary
WisemansArmFormation(Leitch andCawood 1980; Industries - Mineral Resources. The work of David Och
Furey-Greig 1999). andEvanLeitchwas supportedinpartbyARC LargeGrant
The Watonga Formation is isolated from the A39601646. David Och acknowledges support fi-om a
remainder ofthe New England accretionary complex University ofTechnology, Sydney, FacultyofSciencePhD
by the allochthonous Hastings Block (Fig. lb) that Research Scholarship. Thispaper is a contributionto IGCP
503: OrdovicianPalaeogeography andPalaeoclimate.
was moved north from along strike of the southern
Tamworth Belt in the Late Palaeozoic (Schmidt et al.
1994). It is likely that the Watonga Formation was REFERENCES
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