Ordovician System in Antarctica
Claudio Alberto PARICA1
Facultad de Ciencias Exactas y Naturales.UBA. Ciudad Universitara. Buenos Aires
SYSTEM IN ANTARCTICA.
Ordovician rocks were recognized in the Trans Antarctic Mountains, Ross orogen,
inside the sector claimed by Argentine, and close to this area, in the Ellsworth
Mountains. In the Trans Antarctic Mountains coarse sediments of a probable
Ordovician age and a dated volcanism were recognized. Pensacola and Patuxent
Mounts comprise the Neptune Group, which consists largely of conglomerate to
quartzose sandstone. In Shackleton Range the Blaiklock Group overlies the
Shackleton Range Metamorphic Complex. Some other areas like Bowers Terrane and
Robertson Bay Terrane display remarkable Ordovician outcrops. The Ross Orogen is
largely represented with several magmatic exposures.
En el Territorio
Antártico, el Ordovícico ha sido reconocido en localidades en el ámbito de
los Montes Transantárticos, el denominado orógeno de Ross o Sistema de Ross
dentro del sector reclamado por la República Argentina y en áreas cercanas
tales como los montes Ellsworth. En el ámbito del los Montes Transantárticos
se ha reconocido sedimentos gruesos de probable edad ordovícica y vulcanitas
con dataciones radimétricas. En los Montes Pensacola y Patuxent se encuentra el
Grupo Neptuno, el cual incluye conglomerados y areniscas cuarzosas. En los
Montes Shackleton el Grupo Glaciar Blailock se apoya sobre el Complejo Metamórfico
Montes Shackleton. Algo más alejadas, se han encontrado afloramientos ordovícos
en los Terrenis Bowers, y Bahía Robertson. La orogenia Ross se encuentra
ampliamente distribuida con presencia de rocas magmáticas.
Ordovician System. Antarctica. Claimed sector and
Ordovíco. Antártida. Sector reclamado y áreas relacionadas.
outcrops in Antarctica are recognized in the Transantarctic Mountains, known as
Ross Orogen too. Shackleton Range and Pensacola and Patuxent Mountains. These
localities are included into the sector claimed by the Argentine Republic, which
is limited by 25 to 74° W and 60° to South Pole (Parica, 1999). Some other
outcrops are found in the Ellsworth and Herbert Mountains, the Bowers Terrane
and the Robertson Bay Terrane.
new paleogeographic sketch was developed by Aceñolaza et al. (2002) for
the Proterozoic and early Paleozoic where these authors stablished interesting
links with the fauna and environmental evolution of the paleocontinents of
Gondwana and Laurentia.
isotopic ages obtained for the Ross Orogeny, let us to observe quite similar
evolution between this tectomagmatic episode and the ages for the Sierras
Pampeanas (Rapela et al., 1999).
of lower – mid paleozoic rocks in Antarctica are restricted, with minor
exceptions, to the present sites of the Transantarctic Mountains, Ellsworth
Mountains and Marie Bird Land. Although these regions are extensive and widely
separated, a Mountains from the Shackleton Range to Byrd Glacier were part of a
single major pattern is emerging to suggest that the Ellsworth Mountains and
Transantarctic sedimentary-tectonic province, encompassing similar sedimentary
environments and tectonic events. Marie Byrd Land and much of northern Victoria
Land formed part of another, contrasting probably allochtonous province.
the Transantarctic Mountains, from Byrd Glacier to the Shackleton Range, lower
Paleozoic rocks from two unconformity sequences. The older one consists
dominantly of Cambrian shallow marine sediments and silicic volcanics resting on
a Proterozoic basement and the younger, of probably Ordovician-Early Devonian
age, comprises coarse-grained, mainly non-marine, clastic postorogenic deposits.
These latter are unconformably overlain by Devonian or younger rocks of the
the Thiel, Horlick and Queen Maud Mountains, the sequences contain prominent
silicic volcanics, giving a Rb-Sr whole rock isochron of 502 ± 6 Ma in the
Thiel Mountains (Pankhurst et al., 1988) and a whole-rock Rb-Sr age of
483 ± 9 (recalculated) in the western Queen Maud Mountains (Faure et al.,
1979), dating the volcanism as close to the Cambro Ordovician boundary.
Orogeny in the Transantarctic Mountains During late Cambrian Early
Ordovician times, the entire length of the Transantarctic Mountains from the
Shackleton Range to northern Victoria Land experienced uplift, folding and
metamorphism (Ross Orogeny) and, over most of the region, intrusion by
calc-alkaline granitoid plutons (Granite Harbour Intrusives). The Ross orogeny
has been interpreted as a cratonization even associated with an east-dipping
subduction zone along the paleo Pacific margin of Antarctica (Stump, 1982, Borg,
1983, Schmidt, 1983 and Borg et al., 1987). Following the Ross orogeny, a
new cycle of erosion and deposition occurred, resulting in the formation of
coarse clastics of mainly continental origin over much of the region.
the Shackleton Range, the youngest of the folded sedimentary and metasedimentary
sequences, the Blaiklock Glacier Group, unconformably overlies the Proterozoic
Shackleton Range Metamorphic Complex (Clarkson, 1982). The group consists
dominantly of feldespathic sandstone with subordinate grit and conglomerate beds
inferred to have been deposited under terrestrial-deltaic conditions (Clarkson, et
al., 1979). A Rb-Sr isochron age of 475 ± 40 Ma (early Ordovician) is
presumed to reflect diagenesis of the sediments soon after deposition (Pankhurst
et al., 1983). This is reliable with their unconformable relationship
upon schists recording a 500 ± 5 Ma metamorphism possibly associated with the
Ross Orogeny (Pankhurst et al., 1983).
In the Pensacola Mountains, the Neptune Group overlies the Cambrian succession with marked
This group has been divided into four formations, (Brown Ridge Conglomerate,
Elliot Sandstone, Elbow Formation and Heiser Sandstone) which consist largely of
conglomearate and quartozose sandstone (Schmidt and Ford, 1969). The two oldest
units are considered to represent continental orogenic and alluvial fan
deposits, whereas the Elbow Formation and Heiser Sandstone indicate more
quiescent conditions of sedimentation, perhaps at shallow depths an epineritic
environment on a slowly subsiding continental shelf (Williams, 1969). The
Neptune contains no diagnostic fossils and is of uncertain age. However, since
the group rests unconformably on rocks dated as Late Cambrian and its upper
formation is disconformably overlain by the Dover Sandstone (Beacon Supergroup)
with plant fossils of probably late Devonian age (Schmidt and Ford, 1969) an
Ordovician Devonian age is inferred.
Ordovician Ross Orogeny (Rowell et al., 1988). It was deformed and eroded
prior to deposition of the unconformably overlying Middle or Upper Devonian
Beacon Supergroup (Rowell et al., 1988). Thus, the age falls within the
range Early Ordovician Middle Devonian. No post-Ross Orogeny Lower Paleozoic
strata are known from either southern or northern Victoria Land.
region that has most stratigraphic similarity to the Transantarctic Mountains is
the Ellsworth Mountains, which contain the only Lower Paleozoic sediments with
shelly fossils known from west Antarctica. Here more than 10000 m of Lower
Paleozoic sediments are exposed, overlain for more than 2000 m of Upper
the situation in the Transantarctic Mountains, the Ordovician and younger rocks
of the Ellsworth Mountains rest without apparent regional break on the Cambrian
strata. There is however a marked facies change in latest Cambrian times from
the marble or argillite is considered to have been emplaced under dominantly
shallow-marine conditions, but with periodic emergence suggested by horizons of
Terrane (Cambro Ordovician sequence)
Bowers Terrane comprises mainly the Early Paleozoic Bowers Supergroup (Laird
& Bradshaw, 1983). This consists of three groups, the Slededgers (oldest),
Mariner and Leap Year (youngest) groups, totalling over 10000 m, as well as
fault-involved conglomerates of uncertain affiliation.
middle Cambrian Sledgers Group consists of a volcanic association (Glasgow
Formation) and an interfingering clastic sedimentary sequence (Molar Formation).
The mainly conformably overlying Mariner Group of Late Middle Cambrian-late Late
Cambrian age represents a slowing of subsidence and infilling of the basin. The
overlying Leap Year Group is separated from the older units by a marked erosion
surface. Most of the succession consists of red-brown or buff colored quartzose
sandstone (commonly trough cross-bedded) quartzose conglomerate, and minor
mudstone at least 4000 m thick, with local development of basal polymictic
conglomerate. The group is considered to be dominantly fluvial in origin,
although profuse in trace fossils at some horizons near the base of the
succession suggest restricted marine influence.
there is a marked erosion surface separating the Leap Year Group from older nits
in the Bowers Supergroup, angular discordance can only be detected on a regional
scale, and tectonic activity appears to have been restricted to uplift and
tilting, without folding, prior to deposition of the Leap Year Group. No shelly
fossils are known from the group, but an upper age limit is given by a K-Ar date
of 482 ± 4 Ma; this probably represents the time of cleavage formation during
Early Ordovician folding (Adams and Kreuzer, 1984).
Bay Terrane (Cambro Ordovician sequence)
Bowers Terrane is separated from the Robertson Bay Terrane by a complex tectonic
contact zone in which major overthrusting of the sequence over the other has
occurred, and in which local schistosity is present (Bradshaw, 1987). The
Robertson Bay Group, which occupies most of the terrane, consists almost
entirely of alternating quartzose sandstone and mudstone. Several thousand
metres thick. These beds are interpreted to represent turbidites deposited on a
submarine fan (Field Brodie, 1987). The K-Ar age pattern for the Robertson Bay
Group shows an age range from 455-505 Ma, probably reflecting the stages of post
orogenic cooling after low grade metamorphism (Adams and Kreuzer, 1984). Thus
the minimum age of metamorphism of the group is also close to the
Cambro-Ordovician boundary. The maximum age of the Robertson Bay Group is
uncertain, but it probably ranges from Cambrian to earliest Ordovician.
Burret and Findlay (1984) reported Upper Cambrian to Lower Ordovician conodonts in a limestone block within rocks interpreted as Robertson Bay Group. The discovery should be related
pre Mesozoic tectonic reconstructions Between Antarctica and Australia, the
Lower Ordovician Greenland Group, New Zealand, and the Upper Cambrian-Lower
Ordovician Wierah Formation of Southern Tasmania.
conodont fauna includes: Prooneotodus tenuis, Prosagittoduntus aff.
dunderbergiae, Furnishina spp., Problematoconites sp., Westergaardodina
aff. bicuspidate Müller, Proconodontus aff. muelleri, Proconodontus
posterocostatus and Iapetognathus indicative of the lower
Proterozoic rocks of the Wilson Terrane are intruded by granitoids of the
Granite Harbour Intrusives, as in most of the Transantarctic Mountains. Rb-Sr
whole-rock isochrons calculated on granitoid samples from different areas
yielded dates mainly between 515 and 478 Ma (Vetter et al., 1983),
likewise, isochrons calculated on gneisses give an age of 490 ± 33 Ma (Adams,
1986), indicating a late Cambrian early Ordovician age of high grade regional
metamorphism and associated synmetamorphic plutonism.
K-Ar age pattern for the Robertson Bay Terrane is similar to that of the Wilson
Terrane, and a limited age range from 455 to 505 Ma is apparent. In the Bowers
Terrane, there is much more even spread of ages from 510 to 275 Ma: however,
there is a minor but significant grouping of ages at the older end, in the range
510-470 Ma (Adams and Kreuzer, 1984) This suggests that although the three
terranes may not have been immediately adjacent at the time of the Ross orogeny,
they were sujected to a common tectonic and metamorphic (but not igneous) event
during the late Cambrianearly Ordovician.
tectono-stratigraphic provincialism it could be considered that the Ellsworth
Mountains and the Transantarctic Mountains sequences are characterized by:
shallow-water non marine and marine shelf environments for the Cambrain strata,
and unconformity or abrupt facies change close to the Cambro ordovician
boundary; and non marine to very shallow marine coarse grained clastic
sediments, either sandstones or conglomerates, during Ordovician-(?) early
Devonian times. The Cambrian-early Ordovician sediments and volcanics of the
Bowers Terrane constrast with both (Marie Byrd Land and Robertson Bay Terrane),
having been deposited in a rapidly subsiding basin in an active island arc
stratigraphy and structure of the Ellsworth Mountains are closely similar in
many respects to those of the Transantarctic Mountains, in particular to the
Pensacola Mountains which do not have an angular discordance separating
Ordovician – Early Devonian (?) stratafrom overlying Beacon Supergroup. It has
been suggested by various writers that the Ellsworth Mountains originally formed
part of the Transantarctic Mountain block, having been rotated and translated
into its present position during the break-up of Gondwana in post-mid Jurasic
times (Watts and Bramall, 1981; Gronow et al., 1987). And there is an
agreement with the idea that the Ellsworth Mountains seem adjacent to the
Pensacola Mountains-Shackleton Range region.
good comparison can be established between the Ross Orogeny and the Sierras
of the geochronological data obtained have a good relationship with the
Famatinian ages described in several papers, one of them by Rapela et al. (1999).
Also, the sketch for Ordovician times presented by Aceñolaza et al. (2002),
can be support the link in the tectomagmatic episode for the Ross Orogeny, or it
could be seriously considered the correlation between the Famatinian and Ross
the Herbert Mountains a series of five garnet bearing mica schists (Herbert
Series), taken from a nunatak 4 km SSE of Sumgin Buttres gave a two point
isochron. Indicating an age of 1414 ± 184 Ma and a three point isochron with
470 ± 36 Ma (Hofmann et al., 1981). The first isochron is interpreted as
the age of pre-Middle Riphean regional metamorphism of the Herbert Series. And
the second age, 470 Ma, is interpreted as a representative of the Ross Orogeny
which reset the radioactive clocks of the metamorphic rocks on the Herbert
Shackelton Range lies east of the Ice Shelf at the head of the Weddell Sea. The
greater part of the range is formed of basement rocks of the Shackleton Range
Metamorphic Complex, overlain by the Turnpike Bluff Group in the Northwest
(Clarkson, 1972, 1982). This group is considered to be Cambro-Ordovician from
the evidence of fossiliferous erratics (Thomson, 1972) assumed to be derived
form intermediate strata hidden beneath the Blaiklock and Straton Glaciers.
Dolerite dikes were intruded during Ordovician time (457±18 Ma, K-Ar) according
to Rex (1972) into schists of the basement Glacier Group. A number of dikes have
chemical affinities with the dolerite intrusions of Western Dronning Maud Land
and the Transantarctic Mountains, suggesting a third period of intrusion during
40 specimens of Lingulella sp. were found on slabs of a dark grey silty
shale from a moraine at latitude 80° 25’ 26” S 29° 53’ 00” W situated
about 3 km south of Mount Provender in the western part of the Shackleton Range
age of the Blaiklock Glacier Group has been a problem ever since its discovery,
Stephenson (1966) doubtfully included it in the Permian later in the same paper
he noted that it was on petrographic features distinct from the Upper Devonian
to Early Mesozoic Beacon Supergroup and could be of appreciably different age to
latter. Finallly (Thomson, 1972) describe the Stephenson (1966) comparisons of
this group to some conglomerates and sandstones in the Neptune Range which lie
above a sedimentary sequence containing Cambrian fossil. A similar comparison
was made by Williams (1969), who more specifically suggested a correlation with
the Middle Paleozoic Dover Sandstone, although he also noted that the Blaiklock
Glacier Group was more feldespathic than the latter. On a composite geological
map of Antarctica, Craddock (1972) tentatively included the Blaiklock Glacier
Group with Upper Precambrian sedimentary and volcanic rocks of the Ellsworth
Mountains, Transantarctic Mountains and East Antarctica.
dating carried out by Rex (1972) places the age of the Blaiklock Glacier Group
somewhere between the late Precambrian and the Upper Carboniferous. It rests
unconformably on the Shackleton Range Metamorphic Complex (cut by a diorite dyke
dated at 1446 ± 60 Ma) and is itself cut by a dolerite dyke at The Dragons Back
(Clarkson, 1972) dated at 297 ± 12 Ma. The stratigraphical value of the
Brachiopoda described here is limited by the fact that they are known by only
form loose slabs on a moraine. However, assuming that the evidence for referring
the brachiopod-bearing shale fragments to unexposed intermediate beds of the
Blaiklock Glacier Group is justified, then the identification of the brachiopods
as obolids would restrict the age of these beds to the Lower Cambrian-Upper
Ordovician, while their identification as a species Lingulella would
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6 de Noviembre de 2002
Aceptado: 18 de Diciembre de 2002