Beacon Supergroup

The Beacon Supergroup is a geological formation exposed in Antarctica and deposited from the Devonian to the Triassic (). The unit was originally described as either a formation or sandstone, and upgraded to group and supergroup as time passed. It contains a sandy member known as the Beacon heights orthoquartzite.
Overview.
The base of the Beacon Supergroup is marked by an unconformity, and is composed of the Devonian Taylor Group, a quartzose sandstone sequence, and a Late Carboniferous to Early Jurassic Victoria Group, consisting of glacial beds, sandstone, shale, conglomerate, and coal. The Beacon Sandstone was named by Hartley T. Ferrar during Scott's Discovery Expedition (1901-1904), using the Beacon Heights survey points as reference. Glossopteris fossils dated the sandstone as Permian and linked the lithology to similar sequences in neighboring continents. Generally flat lying, the supergroup is up to 3.2 km thick, and is fairly continuous from south Victoria Land to the Beardmore Glacier along the Transantarctic Mountains. The Urfjell Group in Dronning Maud Land, and the Neptune Group in the Pensacola Mountains have been correlated with the Taylor Group. Macrofossils and palynomorph assemblages age date Devonian, Late Carboniferous-Early Permian, Late Permian and Triassic strata. The Ferrar Dolerite intrudes at various levels, while the Mawson Formation and Kirkpatrick Basalts within the Ferrar Supergroup cap the Beacon Supergroup.
The location of the formation in a cold, desert environment, and the lack of nutrients or soil (due to the purity of the sandstone) has led to the beacon sandstone being considered the closest analogue on Earth to Martian conditions, therefore many studies have been performed on life's survival there, mainly focusing on the lichen communities that form the modern inhabitants.
The supergroup originated in a shallow marine sedimentary depositional environment. The well-sorted nature of the unit suggests that it was probably deposited close to the shoreline, in a high energy environment. Features, such as the presence of coal beds and desiccation cracks, suggest that parts of the unit were deposited subaerially, though ripple marks and cross bedding show that shallow water was also commonly present. Heat from burial is modest, though they could have been heated to over 160 ° by intrusion of dolerite sills, dykes and lenses during the early Jurassic, related to break up of Gondwana . The rock is low in phosphorus.
Taylor Group.
The Taylor Group is separated from the overlying Victoria Group by a disconformity called the Maya Erosion Surface. Taylor Group formations in the Darwin Mountains region include the Brown Hills Conglomerate (34 m), which overlies pre-Devonian plutonic rocks, of igneous and metamorphic nature, with over 30 m of erosional relief, and igneous and metamorphic clasts. The conglomerate is poorly sorted at the base, with influxes of coarser material. Coarseness is laterally variable with pebbles in places, and sands in others, at the same horizon. The conglomerate includes planar beds, trough cross-bedding, flaser bedding, mud-drapes on some ripples, U-shaped burrows and escape structures, with fining up cycles topped by desiccation cracks in places. Depositional environment is probably that of an alluvial fan, though unidirectional flow and sheet-like deposition point to braided channels. Equivalent strata in South Victoria Land include the Wind Gully Sandstone (80 m), the Terra Cotta Siltstone (82 m), and the New Mountain Sandstone (250 m), which are separated from the overlying Altar Mountain Formation (235 m) and Arena Sandstone (385 m) by a disconformity.
Within the Darwin Mountains region, the Junction Sandstone (290 m) overlies the Brown Hills Conglomerate, with abundant "Skolithos". This is followed by the Hatherton sandstone (330 m), with brachiopod and bivalve shell fragments in places. Trough cross beds and current rippling are present, with abundant ichnofauna. Drainage was to the north east, with the depositional environment presumed to be marine, though also present are subaerial features such as desiccation cracks, rain drop impressions, surface run-off channels, muddy veneers, and redbeds, besides river-like features such as small channels.
The Beacon Heights Orthoquartzite (330 m) is found in the S. Victoria Land region between the Arena Sandstone and the overlying Devonian Aztec Siltstone. It is well sorted and cemented with medium to coarse grain sizes, trough cross-beds, with "Haplostigma" and "Beaconites" remnants.
The Aztec Siltstone (125–220 m) is found both in South Victoria Land and the Darwin Mountains. The siltstone includes interbedded sandstones, fish-bearing shales, conchostracans, and paleosols implying subaerial periods within an alluvial plain sequence.
Within the Beardmore Glacier region, the Devonian Alexandra Formation (0–320 m), which constitutes the entire Taylor Group, is a quartz sandstone to siltstone.
Victoria Group.
The Victoria Group begins with a diamictite-bearing unit, known as the Metschel Tillite (0–70 m) in South Victoria Land, the Darwin Tillite (82 m) in the Darwin Mountains, the Pagoda Tillite (395 m) at the Beardmore Glacier, the Scott Glacier Formation (93 m) on the Nilsen Plateau, and the Buckeye Tillite (140–308 m) in the Wisconsin Range and Ohio Range. The glacial beds are valley fill or occur as sheets. This is followed by the Misthound Coal Measures (150 m) in the Darwin Mountains and the Permian Weller Coal Measures (250 m) in South Victoria Land. A disconformity separates the Misthound Coal Measures from the overlying Ellis Formation (177 m), consisting of conglomerate, sandstone and siltstone. The Pyramid Erosion Surface separates the Mitschell Tillite and the Weller Coal Measures, which are overlain by the Feather Conglomerate (215 m), the Triassic Lashly Formation (520 m). The Pagoda Tillite is overlain by the MacKellar Formation (140 m) of interbedded black shales and fine sandstones, the Lower Permian Fairchild Formation (220 m) arkosic sandstone, the Upper Permian Buckley Formation (750 m), the Middle-Lower Triassic Fremouw Formation (650 m), the Upper-Middle Triassic Falla Formation (530 m), and the Jurassic Prebble Formation (0–460 m) volcanic conglomerate, tuff and tuffaceous sandstone.
Palæontology.
Body fossils.
The Aztec sandstone contains units bearing body fossils of fish: Phyllolepid placoderms, and thelodonts; abundant in fish beds; and conchostracans. The presence of *"Scoyenia" ichnofacies implies freshwater. Also present are charred wood remnants, and the plants "Glossopteris" and "Haplostigma". The wood bears clear rings, indicating a seasonal environment, and large enough to represent a temperate climate, though glacial just before Beacon deposition.
Trace fossils.
Sparse below, but become common in Hatherton Sandstone. Changes from Skolithos-dominated facies to wide diversity and abundance, including vertical and horizontal burrows, and huge arthropod trackways. Size of arthropod tracks (less than 91 cm) is taken to imply that water must have been required for support:
The presence of crawling traces in such well sorted sands is problematic. The arthropod trackways are thought to have been formed in shallow water, and supersaturated sand has a shallow angle of repose. Thus either a layer of organic matter, perhaps in the form of an algal slime, must have supported the sediment, or the sediment must have been partially dry. In the context of subaerial features such as raindrop marks and desiccation cracks on associated horizons, the best explanation is that the trackways were formed on bedforms produced on a river bed, but while they were exposed by a low-flow period.