Stratigraphy of Jurassic pyroclastic rocks in the Transantarctic Mountains

doi:10.1016/S0899-5362(00)00074-9

Journal of African Earth Sciences

Volume 31, Issue 1, July 2000, Pages 77-89

https://doi.org/10.1016/S0899-5362(00)00074-9 Get rights and content

Abstract

Jurassic pyroclastic rocks cap the Antarctic Gondwana sequence and document an important event in the evolution of the continent. The Hanson Formation, which crops out in the central Transantarctic Mountains, consists of ca 240 m of silicic tuffs, tuffaceous sandstones, and subordinate quartzose sandstones of probable Early Jurassic age. It is overlain by basaltic pyroclastic rocks of the Prebble Formation. Similar basaltic rocks also crop out in Victoria Land and constitute the Mawson and Exposure Hill Formations. These basaltic pyroclastic rocks and the overlying Kirkpatrick Basalt lavas are Middle Jurassic. The pyroclastic rocks comprise thick tuff-breccias with minor lapilli-tuffs and tuffs; sequences are as much as 400 m thick. These silicic and basaltic pyroclastic rocks record the transition from foreland basin fluvial sedimentation of the Antarctic Gondwana sequence to an extensional tectonic regime associated with flood basalts and Gondwana break-up.

Résumé

Les roches pyroclastiques jurassiques recouvrent la séquence gondwanienne de l'Antarctique et illustrent un événement important de l'évolution du continent. La Formation de Hanson qui affleure dans les Montagnes Transantarctiques centrales consiste en 240 m environ de tufs acides, de grès tufacés et, en faible quantité, de grès quartzeux d'âge jurassique inférieur probable. Elle est recouverte par les roches pyroclastiques basaltiques de la Formation de Prebble. Des roches basaltiques semblables affleurent également dans la Terre de Victoria et constituent les Formations de Mawson et d'Exposure Hill. Ces roches pyroclastiques basaltiques et les laves sus-jacentes des Basaltes de Kirkpatrick sont du Jurassique Moyen. Les roches pyroclastiques comprennent des brèches à tufs épaisses avec un peu de tufs de lapilli et de tufs; les séquences ont jusqu'à 400 m d'épaisseur. Ces roches pyroclastiques acides et basaltiques enregistrent la transition de la sédimentation de bassin fluviatile d'avant-pays de la séquence gondwanienne de l'Antarctique à un régime tectonique extensif associé aux basaltes des plateaux et à la dislocation du Gondwana.

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Geological and tectonic evolution of the Transantarctic Mountains, from ancient craton to recent enigma
2020, Gondwana Research
The Transantarctic Mountains (TAM) are one of Earth's great mountain belts and are a fundamental physiographic feature of Antarctica. They are continental-scale, traverse a wide range of latitudes, have high relief, contain a significant proportion of exposed rock on the continent, and represent a major arc of environmental and geological transition. Although the modern physiography is largely of Cenozoic origin, this major feature has persisted for hundreds of millions of years since the Neoproterozoic to the modern. Its mere existence as the planet's longest intraplate mountain belt at the transition between a thick stable craton in East Antarctica and a large extensional province in West Antarctica is a continuing enigma. The early and more cryptic tectonic evolution of the TAM includes Mesoarchean and Paleoproterozoic crust formation as part of the Columbia supercontinent, followed by Neoproterozoic rift separation from Laurentia during breakup of Rodinia. Development of an Andean-style Gondwana convergent margin resulted in a long-lived Ross orogenic cycle from the late Neoproterozoic to the early Paleozoic, succeeded by crustal stabilization and widespread denudation during early Gondwana time, and intra-cratonic and foreland-basin sedimentation during late Paleozoic and early Mesozoic development of Pangea. Voluminous mafic volcanism, sill emplacement, and layered igneous intrusion are a primary signature of hotspot-influenced Jurassic extension during Gondwana breakup. The most recent phase of TAM evolution involved tectonic uplift and exhumation related to Cenozoic extension at the inboard edge of the West Antarctic Rift System, accompanied by Neogene to modern glaciation and volcanism related to the McMurdo alkaline volcanic province. Despite the remote location and relative inaccessibility of the TAM, its underlying varied and diachronous geology provides important clues for reconstructing past supercontinents and influences the modern flow patterns of both ice and atmospheric circulation, signifying that the TAM have both continental and global importance through time.
From the transantarctic basin to the ferrar large igneous province-new palynostratigraphic age constraints for triassic-jurassic sedimentation and magmatism in East Antarctica
2014, Review of Palaeobotany and Palynology
Citation Excerpt :
Radiometric age constraints: No radiometric ages have been obtained from the Shafer Peak Formation to date (Elsner, 2010). However, four samples of tuff layers from the Hanson Formation, a local equivalent of the Shafer Peak Formation in the CTM, have yielded a poorly constrained middle Pliensbachian age (186 ± 8 Ma; Faure and Hill, 1973; see Elliot, 2000). Although this date is considered uncertain due to widespread zeolitization (Elliot, 2000), it completely matches the palynological age determined here.
We present new palynological data from the Transantarctic Mountains that clarify the timing of sedimentary and magmatic processes in the transition from continental deposition of the Beacon Supergroup to emplacement of the Ferrar Large Igneous Province. Samples were collected from twenty-three Triassic and Jurassic sections in the southern area of north Victoria Land (NVL), East Antarctica. Recovered palynomorph assemblages are correlated with the widely used, although informal palynostratigraphic framework established for eastern Australia by Price. The associated Late Triassic–earliest Jurassic zone, APT5, is modified here with a proposed new subdivision: Lower APT5 (“APT5L”; middle–late Norian), Middle APT5 (“APT5M”; Rhaetian), and Upper APT5 (“APT5U”; Hettangian–earliest Sinemurian). We further propose a modification unifying the relevant formal eastern Australian and New Zealand palynostratigraphic zones, with a new Polycingulatisporites crenulatus Association Zone (new zonal status) that includes the P. crenulatus Association Subzone (new subzone; equivalent to APT5L) and the following Foveosporites moretonensis Association Subzone (new subzonal status; equivalent to APT5M). Our palynostratigraphic dating of the NVL assemblages demonstrates that the onset of sedimentation was diachronous in this part of the Transantarctic Basin, ranging from at least the Rhaetian to, in places, early Sinemurian. By lack of evidence for rocks containing APT5U assemblages and by analogy with the few coeval sections in Australia, we infer that the Hettangian interval in NVL is probably consumed by unconformity. Deposition of ashes from distal silicic volcanism commenced in the early Sinemurian and reached a peak phase beginning in middle Pliensbachian (ca 187 Ma), coinciding with the first major magmatic interval of the silicic Chon Aike Province in Patagonia and West Antarctica. Two major episodes of phreatomagmatic activity, driven by shallow-level sill intrusion into sandstone aquifers, occurred during the middle Pliensbachian and during the late Pliensbachian–early Toarcian. The latter episode was closely followed by the first pillow extrusion and local lava effusion. Contrary to some previous studies, we further conclude that all available palynological evidence is compatible with a short-lived emplacement of the plateau-forming Kirkpatrick Basalt at around 180 Ma during the early Toarcian.
The Eisenhower Range, Transantarctic Mountains: Evaluation of qualitative interpretation concepts of thermochronological data
2013, Chemical Geology
Citation Excerpt :
Beacon sedimentation was followed by intrusion and extrusion of magmatic rocks in/on basement and sediments during the Ferrar event at ~ 180 Ma (e.g., Elliot and Fleming, 2008). The subaerial/subaquatic nature of these rocks is recognized by the presence of syn-Ferrar pyroclastic and siliciclastic, partially fossil-bearing sedimentary sequences; pillow lavas; phreatomagmatic structures and diatremes of local hydromagmatic explosive events; vesicles and sediment suspensions in sills; and plastic deformation of Jurassic sediments by Ferrar apophyses (e.g., Elliot, 2000; Schöner et al., 2007; Viereck-Götte et al., 2007; Elliot and Fleming, 2008). West of the study area, basement is locally overlain by Neogene volcanic rocks and/or Pliocene sediments and glacial deposits.
The Transantarctic Mountains (TAM) were one of the first regions where apatite fission track (AFT) thermochronology was applied routinely to study exhumation processes and long term landscape evolution. Pioneering publications from the region introduced or refined interpretation concepts of thermochronological data such as the break in slope in vertical age profiles as qualitative marker for the onset of accelerated rock cooling.
New AFT data were compiled from vertical profiles in the Eisenhower Range, northern TAM, and compared with published data. Samples originally examined by the population technique were re-analyzed via the external detector technique. AFT ages increase from 32 ± 2 Ma at an elevation of 220 m to 175 ± 14 Ma at 2380 m. Geological evidence and thermal history modeling of the AFT data require Jurassic to Late Eocene reheating of the samples and an onset of cooling at ~ 35–30 Ma. This requires the deposition of an ~ 3 to 3.5 km thick sedimentary sequence on the granitic basement subsequent to Jurassic Ferrar magmatism at ~ 180 Ma. The regression of paleotemperatures against sample altitudes infers a high Jurassic geothermal gradient of ~ 60 °C/km related to rifting processes and Ferrar magmatism, and a moderate Cretaceous/Eocene geothermal gradient of ~ 30 °C/km.
Comparison of ages generated with population and external detector technique shows the importance of determining single-grain ages for each sample, even from granitic rocks of the same intrusion, and thus strongly supports previous cases made for the determination of annealing kinetics and grain-age evaluation. Age comparison additionally illustrates that samples above a break in slope record larger deviations between population and external detector ages than samples below a break in slope.
We demonstrate that the position and shape of a break in slope result from various factors, such as the thermal history prior to final cooling, maximum paleotemperatures, cooling rate, and geothermal gradient. A break in slope does not straightly date the onset of final cooling and cannot substitute thermal history modeling. Therefore, earlier studies from the TAM and similar settings elsewhere need to be validated by combining thermal history modeling of thermochronological data and supplementary geological information.
Quantification of vesicle characteristics in some diatreme-filling deposits, and the explosivity levels of magma-water interactions within diatremes
2012, Journal of Volcanology and Geothermal Research
Vesicles within juvenile fragments in mafic pyroclastic deposits contain important information about the state of the magma at the time of fragmentation. There have been few vesicle studies of juvenile pyroclasts from mafic phreatomagmatic deposits, however, and none we can find from maar–diatreme volcanoes. In this paper we document the vesicularity and vesicle-population characteristics of juvenile fragments sampled from non-bedded lithified deposits of the Coombs Hills diatreme complex, part of the Ferrar large igneous province, Antarctica. The diatreme-filling pyroclastic deposits, dominated by lapilli tuffs and tuff breccias, contain typically abundant lithic clasts derived mostly from the enclosing sedimentary sequence, and several types of juvenile clasts ranging from blocky to fluidal or “raggy”.
In the samples measured, 77–80% of the juvenile pyroclasts ranging in size from 0.5 mm to fine lapilli is in the ‘non-vesicular’ to ‘incipiently vesicular’ range (< 20% vesicles). Such low vesicularities are expected for pyroclasts from maar–diatreme volcanoes where fragmentation takes place at depth in the diatreme or root zone due to magma–water interaction. A few juvenile clasts, however, are more vesicular, and seven of these were chosen and sectioned for 2D analysis of vesicle shapes and orientation, vesicle number densities (N_v), and vesicle volume distributions. The shapes of the vesicles in the studied sections are mostly elliptical (sometimes polylobate), with mean aspect ratios ranging between 0.67 and 0.72. Circular statistics are used to test for trends in the vesicle long-axis orientation data; non-uniformity of orientations is found in most cases, but the trends are weak. Vesicle volume distributions are often bimodal due to variable coalescence. Total N_v values range from 1.0 × 10² to 5.7 × 10³ mm^− 3; taking the effects of bubble coalescence into account, these values are similar to those found in pyroclasts from other phreatomagmatic volcanoes, although they also overlap partly with those seen in fire fountain deposits and some basaltic Plinian eruptions.
Fluidal- or rag-shaped juvenile clasts, some circular vesicles, and the lack of microlites all suggest that the Coombs Hills magma had a relatively low viscosity prior to fragmentation, despite the basaltic andesite composition. This low viscosity allowed parts of the magma to be fragmented in a non-brittle fashion during phreatomagmatic explosions and to form fluidal clasts. Phreatomagmatic explosions in diatremes can therefore produce diverse types of juvenile clasts simultaneously, and the proportions of each will depend on the explosivity of the magma–water (slurry) interaction and other factors. Recycling of fragments is also thought to be an important factor in generating mixtures of different types of juvenile fragments in diatremes.
Physical volcanology and geological relationships of the Jurassic Ferrar Large Igneous Province, Antarctica
2008, Journal of Volcanology and Geothermal Research
Citation Excerpt :
In the waning stages of pyroclastic activity, lacustrine sediments were deposited and paleosols formed, indicating confining environments and surface exposure for significant time periods. The switch to lava effusion following an hiatus has been attributed to increased magma supply rates and/or decrease in availability of water for phreatomagmatism, both of which affect magma–water ratios, or magma/water interaction ceased as a result of sealing of magma conduits (Hanson and Elliot, 1996; Elliot, 2000; Ross et al., 2005). Phreatomagmatic activity, however, continued sporadically during lava eruption, and is recorded by tuff–breccia and lapilli–tuff beds low in the lava sequence in CTM (Mt. Bumstead), SVL (Carapace Nunatak, Mt Brooke), and PAM (Tent Peak), and high in the section in the Mesa Range.
The Ferrar Large Igneous Province forms a linear belt for 3500 km along the Transantarctic Mountains, and as a geochemical province extends into southeastern Australasia. The principal components of the Ferrar are: intrusive — Ferrar Dolerite sills and dikes, and Dufek intrusion; pyroclastic — the Prebble, Mawson and Exposure Hill Formations; effusive — the Kirkpatrick Basalt. In terms of the three dimensional architecture of the Ferrar, a range of “facies” can be recognized in each of the principal components. The Ferrar province was initiated with a major episode of phreatomagmatism leading to formation of tephra cones and associated deposits, and near-surface vent structures. Activity switched to predominantly quiet effusion of alternating thick flood basalt flows and thin pahoehoe lobes and flows. Intrusive bodies were emplaced early, given the occurrence of dolerite clasts in tuff–breccias, but most sills were probably intruded after accumulation of extrusive rocks. Pre-existing rift structures played a major role in controlling the transport and distribution of the Ferrar magmas and the apparent centers of extrusive activity. The associated paleohydrology controlled the eruption styles.
Geological evolution of the Coombs-Allan Hills area, Ferrar large igneous province, Antarctica: Debris avalanches, mafic pyroclastic density currents, phreatocauldrons
2008, Journal of Volcanology and Geothermal Research
The Jurassic Ferrar large igneous province of Antarctica comprises igneous intrusions, flood lavas, and mafic volcaniclastic deposits (now lithified). The latter rocks are particularly diverse and well-exposed in the Coombs–Allan Hills area of South Victoria Land, where they are assigned to the Mawson Formation. In this paper we use these rocks in conjunction with the pre-Ferrar sedimentary rocks (Beacon Supergroup) and the lavas themselves (Kirkpatrick Basalt) to reconstruct the geomorphological and geological evolution of the landscape. In the Early Jurassic, the surface of the region was an alluvial plain, with perhaps 1 km of mostly continental siliciclastic sediments underlying it. After the fall of silicic ash from an unknown but probably distal source, mafic magmatism of the Ferrar province began. The oldest record of this event at Allan Hills is a ≤ 180 m-thick debris-avalanche deposit (member m₁ of the Mawson Formation) which contains globular domains of mafic igneous rock. These domains are inferred to represent dismembered Ferrar intrusions emplaced in the source area of the debris avalanche; shallow emplacement of Ferrar magmas caused a slope failure that mobilized the uppermost Beacon Supergroup, and the silicic ash deposits, into a pre-existing valley or basin.
The period which followed (‘Mawson time’) was the main stage for explosive eruptions in the Ferrar province, and several cubic kilometres of both new magma and sedimentary rock were fragmented over many years. Phreatomagmatic explosions were the dominant fragmentation mechanism, with magma–water interaction taking place in both sedimentary aquifers and existing vents filled by volcaniclastic debris. At Coombs Hills, a vent complex or ‘phreatocauldron’ was formed by coalescence of diatreme-like structures; at Allan Hills, member m₂ of the Mawson Formation consists mostly of thick, coarse-grained, poorly sorted layers inferred to represent the lithified deposits of pyroclastic density currents. Meanwhile at Carapace Nunatak, mafic clasts were mixed with detrital material to form the Carapace Sandstone in alluvial and eventually lacustrine environments.
Eruptions then became largely effusive, producing hundreds of metres of flood lavas that covered the landscape (‘Kirkpatrick time’). In places, lava flowed into ephemeral lakes to form pillow-palagonite breccias (base of sequence, Carapace Nunatak) or pillow lavas (top of sequence, Coombs Hills). Several generations of Ferrar intrusions were emplaced during the course of these events; at least three can be distinguished based on field relations. New geochemical data indicates that for the Ferrar province, magma involved in the explosive eruptions had the same major element composition as that which produced shallow intrusions and lavas. We also note the possibility that flood lavas were fed by plugs cross-cutting the Mawson Formation at Coombs Hills, rather than by major dikes extending to the surface. Finally, we infer that eruption plumes were limited to the troposphere and that direct environmental impacts were thus likely restricted to the southern hemisphere.

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Stratigraphy of Jurassic pyroclastic rocks in the Transantarctic Mountains

Abstract

Résumé

Earth Planetary Science Letters

Reviews Pal˦obotany Palynology

Earth Planetary Science Letters

Journal Volcanology Geothermal Research

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