Beacon Supergroup

The Beacon Supergroup and diabase intrusions.

Beacon Supergroup in Taylor Valley. The tan bands are Beacon Sandstone layers, and the dark layers are diabase sills, intruded about 180 million years ago.

The Beacon Supergroup is a geological formation exposed in Antarctica and deposited from the Devonian to the Triassic (400 to 250 million years ago). 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. ^[1]

Setting in time and space

Named after Beacon Heights.^[2] First named 1907, type section described in 1963.^[3] Originally dubbed a formation, with scope left (and later used) to expand to group, then supergroup, as better mapped and understood.^[3]

Age

Upper Devonian to Triassic

Exposure

Antarctica: McMurdo sound, shores of Ross Bay;^[4]

also southern Victoria Land, Ross desert.^[5]

Described by Scott & team on way to South Pole.^[6]

The series is over 1 km thick in places,^[6] and extends for over 1,000 miles.^[6]

The beds are almost flat lying,^[6] dipping at about 3° to the north;^[7] many are interleaved with dolerite sills.^[6]

Location

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.^[8]

Sedimentology

Cross-bedding in sandstone of the Beacon Supergroup suggests a fluvial environment.

The unit is a "Fine grained, arkosic quartz sandstone".^[9] It is composed of shales, coals, conglomerates, and in places the occasional thin limestone bed.^[9]

Lithofacies

Originally divided into 3 subunits,^[6] further refined into five facies, listed below from oldest to youngest:^[10]

Brown hills conglomerate

Basal. Grades into Junction sandstone. Variable thickness; (0-5^[3]/17^[11]/80^[12] m),^[11] overlies pre-Devonian plutonic rocks, of igneous and metamorphic nature, with over 30 m erosional relief.^[3] Contains igneous and metamorphic clasts.^[11]

Poorly sorted at base, influxes of coarser material.^[11] Coarseness is laterally variable - pebbles in places, sands in others, at same horizons.^[12] Planar beds, trough cross-bedding, flaser bedding, mud-drapes on some ripples; U-shaped burrows & escape structures; fining up cycles topped by desiccation cracks in places.^[11]^[12]

Probably alluvial fan. Unidirectional flow & sheet-like deposition point to braided channels.^[12]

Junction sandstone

Part of Taylor group.^[11] Gradational boundaries at top and bottom.^[11] up to 540m thick.^[12] Skolithos abundant.^[12] Intermediate between Brown Hills Conglomerate and Hatherton sandstone.

Hatherton sandstone

Part of Taylor group.^[13] 250-300m thick.^[12] Divided into upper and lower subunits.^[10]

95% Quartz.^[9]
Abundant: Zircon, limonite. Common: garnet, magnetite. Present in places: Shell fragments (Brachiopod / bivalve)^[9]

Lower: white/yellow sandstone.^[10] Layers of grit/conglomerate at base, silt at top, of some beds, which reach 15m thickness.^[10] Trough cross beds.^[13]

Upper: Similar, but rust-weathering, current rippling.^[10]

Dates to late Middle Devonian, by correlation to the well constrained (by fish fossils) Aztec Siltstone nearby.^[13]

Abundant ichnofauna.^[10]

Common bedforms: planar beds, bi-modal cross-beds, hummocky cross-stratification (HCS), laminated seds.^[11] Drainage to north east.^[12]

Presumed marine for a long time on the basis of trace fossils such as Skolithos, and typically marine HCS. But sedimentologists kept pointing out subaerial features such as desiccation cracks (polygonal jointing?), rain drop impressions, surface run-off channels, muddy veneers, and redbeds; also, river-like features such as unidirectional currents and small channels.^[14] The confusion was rectified when it was realised that HCS^[12] and the ichnofacies could in fact be marine.^[14]

Beacon heights orthoquartzite

Only known in north.^[11]

Sometimes just referred to as top 30m of Hatherton sst.^[12]

Well sorted and cemented. Grains medium to coarse. Trough cross-beds.^[11] Haplostigma irregulare - lycopod remnants. Constrain to early Middle Devonian.^[12] Contact on Hatherstone sandstone is sharp, irregular, and in places cobbly - so erosional.^[11]

Aztec siltstone

The Aztec siltstone bears interbedded sandstones and fish-bearing shales (providing late Mid Devonian age). Palæosols abundant and well developed, implying subaerial periods.^[13]

Only known in north.^[11] Top 7.5m contains dewatering structures - result of loading by tillites.^[15] This implies that the sediments were not consolidated in Permian times, and indeed that the area did not undergo glaciation during the Carboniferous ice age.^[15]

Minimum thickness 135m.^[11] Coarse sands and finer muds; cross-bedded channels up to 12m wide.^[11] Small and large roots.^[11] Psilophytes, lycopod stems, logs.^[11]

Darwin tillite

Base of Victoria group.^[13] Also known as Metschel tillite.^[13] Overlays Hatherton and Aztec^[12] unconformably,^[10] resting on "Maya" erosion surface,^[13] which has only "slight" relief.^[16] Underlying sands thumped by granitic clasts, which form load structures.^[15]

This erosion surface was formed by downcutting streams, later scoured by glacial ice.^[13] Permian in age.^[7] Erosion surface covered with pebbly mudstone.^[16] Features rhythmic, varved layers, with some channel and sheet sandstones.^[13] Main unit is diamictite.

Misthound coal measure

Part of Victoria group.^[13] Overlays tillite unconformably,^[10] resting on "Pyramid" erosion surface which was formed by reworking of the tillite.^[13] Dominated by Gangamopteris. Cross-bedded sandstones, with some mudstones, carbonaceous shales, and of course coal.

Ellis formation

Comprises a conglomerate, sand- and silt-stone.^[10]

Palæontology

Body fossils

The Aztec sandstone contains units bearing body fossils of Fish:^[10] Phyllolepid placoderms,^[17]^[18] and thelodonts;^[19] abundant in fish beds; and conchostracans.^[14]

Also: Charred wood remnants,^[2] and the plants Glossopteris^[3] and Haplostigma.^[14]

Wood bears clear rings, so environment must have been very seasonal.^[3] Large enough to represent temperate climate, at least.^[3] Glacial just before Beacon deposition.^[3]

Nothing else though.^[14]

Trace fossils

Burrows in the Beacon Supergroup.

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.^[11] Size of arthropod tracks (<91 cm!) taken to imply that water must have been required for support.^[11] In Hatherton, Skolithos density decreases.

Present include:

Fodinichnia: feeding burrows, probably of marine polychaetes, featuring evidence of rhythmic defecation.^[10]
- Narrow, sinuous, near-surface forms on flat bedding surfaces
- Longer, larger forms, reaching 13 cm across and 1 m in length.
Walking trackways of arthropods (Repichnia).^[10]
- Beaconites antarcticus: Narrow, parallel grooves, about an inch apart, disappearing into elliptical pits; created by shovelling the surface sediment aside before burrowing into the sediment.^[10] Occasionally branch.^[11]
- Wider spaced grooves (~3 cm); small footprints visible. Implies many walking limbs and an approximately rectangular shape - reminiscent of the trilobites.^[10] B. barretti? Extend laterally up to 1.7m; burrow "deeply" into sediment.^[11] Probably produced by a very different arthropod to B. antarcticus.^[11]
- Large (~30 cm wide) trails with a scrape mark from a central tail. Three to four footprint pits diverge from these tracks at a high angle. The feet making the footprints had spines on their rears. These may have been formed by eurypterids but aren't a perfect match to known eurypterid trails; they may also have been formed by Xiphosurans^[10]
Diplichnites trackways - double rows of fossils - previously attributed to marine trilobites. Clearly not - so perhaps annelids / myriapods?^[14] Here appear on metre-scale crossbeds: sub-fluvial dunes?^[14]

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,^[10] 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.^[14]

Cruziana & Rusophycus: thought to be formed by trilobites, whose body fossils are only found in marine assemblages. Could they also be made by other arthropods, or could the lower parts of the Beacon sandstone have been marine? They have been found in many other non-marine instances.^[14]
Skolithos - again, traditionally thought to be marine, but there are lots of examples where it isn't.^[14]

Ichnofacies

Main article: ichnofacies

Scoyenia ichnofacies implies freshwater aquatic nature.^[14]

Depositional environment

Sedimentological and palæontological data point to a shallow marine depositional environment.^[10]

The well-sorted nature of the unit suggests that it was probably deposited close to the shoreline, in a high energy environment.^[9] This is backed up by the absence of clay-sized particles, and the rounded, spherical shape of quartz grains.

Features, such as the presence of coal beds and desiccation cracks, suggest that parts of the unit were deposited subaerially.^[9] Ripple marks and cross bedding show that shallow water was also commonly present.^[9]

Source rock

Too few minerals to come from local granites and schists, unless a long period of subaerial weathering preceded deposition.^[9]
Could have been transported; would have to be a long distance to produce such a clean sandstone.^[9]

Thermal history

Heat from burial modest.^[20]
Heated to 160+° by intrusion of dolerite sills,^[6] dykes and lenses during the early Jurassic, related to break up of Gondwana 180 million years ago.^[20] - the Ferrar Dolerite.^[10] Reached T of 200-300 °C in places.^[20]
Volatiles would have migrated outwards from the hot aureole, condensing when they reached rock cooler than their boiling point. This results in the "steam distillation" of the volatiles.^[6]

Utility

Biology

The rock is low in phosphorus,^[21] creating difficulties for organisms living on it.
Mostly supports lichens; has its own endogenous community

Hydrocarbon potential

Great source: but nothing to trap oil.^[6]