The Ordovician System in Antarctica

Claudio Alberto PARICA1

1 Facultad de Ciencias Exactas y Naturales.UBA. Ciudad Universitara. Buenos Aires

Abstract: THE ORDOVICIAN 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.

Resumen: EL SISTEMA ORDOVÍCICO EN ANTÁRTIDA. 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.

Keywords: Ordovician System. Antarctica. Claimed sector and related areas.

Palabras Clave: Sitema Ordovíco. Antártida. Sector reclamado y áreas relacionadas.


Ordovician 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.

A 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.

The 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).

Transantarctic Mountains.

Outcrops 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.

In 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 Beacon Supergroup.

In 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.

-Ross 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.

-Ordovician-early Devonian sequences

In 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

unconformity. 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.

Early 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.

Ellsworth Mountains

The 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 Paleozoic rocks.

Unlike 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 desiccation cracks.

Bowers Terrane (Cambro Ordovician sequence)

The 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.

The 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.

Although 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).

Robertson Bay Terrane (Cambro Ordovician sequence)

The 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

to 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.

The 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 Tremadocian.

-Ross Orogeny

Late 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.

The 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.

Under 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 setting.

The 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.

A good comparison can be established between the Ross Orogeny and the Sierras Pampeanas.

Most 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 Orogenies.

Herbert Mountains

From 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 Mountains.

Shackleton Range

The 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 Jurassic time.

About 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 (Thomson, 1972).

The 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.

Radiometric 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 suggest that these beds range no higher than Middle Ordovician.


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Recibido: 6 de Noviembre de 2002

Aceptado: 18 de Diciembre de 2002