The Beacon sandstone is a geological formation exposed in Antarctica and deposited from the Devonian to the Triassic . The sandstone was originally described as a formation, and upgraded to group and supergroup as time passed. It contains a sandy member known as the Beacon heights orthoquartzite.

Setting in time and space

Named after Beacon Heights

Beacon Heights

The Beacon Heights are a small cluster of peaks between Beacon Valley and Arena Valley in Quartermain Mountains, Victoria Land, rising to in West Beacon, and also including East Beacon and South Beacon. They were named by Hartley Ferrar, geologist with the British National Antarctic Expedition...

. First named 1907, type section described in 1963. Originally dubbed a formation, with scope left (and later used) to expand to group, then supergroup, as better mapped and understood.

Exposure

• Antarctica: McMurdo sound, shores of Ross Bay
  Ross Bay
  Ross Graham Bay is the current Bishop of Auckland in the Anglican Church of New Zealand since 17 April 2010.Bay was educated in Papatoetoe and at the Bible College of New Zealand. He was ordained in 1989 and began his ministry with a curacy at Whangarei. Later he was priest assistant at St...
  
  ;

also southern Victoria Land

Victoria Land

Victoria Land is a region of Antarctica bounded on the east by the Ross Ice Shelf and the Ross Sea and on the west by Oates Land and Wilkes Land. It was discovered by Captain James Clark Ross in January 1841 and named after the UK's Queen Victoria...

, Ross desert.

• Described by Scott & team on way to South Pole.

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

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

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 focussing on the lichen

Lichen

Lichens are composite organisms consisting of a symbiotic organism composed of a fungus with a photosynthetic partner , usually either a green alga or cyanobacterium...

 communities that form the modern inhabitants.

Sedimentology

The unit is a "Fine grained, arkosic

Arkose

Arkose is a detrital sedimentary rock, specifically a type of sandstone containing at least 25% feldspar. Arkosic sand is sand that is similarly rich in feldspar, and thus the potential precursor of arkose....

 quartz sandstone". It is composed of shale

Shale

Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...

s, coals, conglomerates

Conglomerate (geology)

A conglomerate is a rock consisting of individual clasts within a finer-grained matrix that have become cemented together. Conglomerates are sedimentary rocks consisting of rounded fragments and are thus differentiated from breccias, which consist of angular clasts...

, and in places the occasional thin limestone bed.

Lithofacies

Originally divided into 3 subunits, further refined into five facies, listed below from oldest to youngest:

Brown hills conglomerate

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

Poorly sorted at base, influxes of coarser material. Coarseness is laterally variable - pebbles in places, sands in others, at same horizons. Planar beds, trough cross-bedding, flaser bed

Flaser bed

Flaser beds are a sedimentary bedding pattern created when a sediment is exposed to intermittent flows, leading to alternating sand and mud layers. While flaser beds typically form in tidal environments, they can form in fluvial conditions - on point bars or in ephemeral streams. Individual sand...

ding, mud-drapes on some ripples; U-shaped burrows & escape structures; fining up cycles topped by desiccation cracks in places.

Probably alluvial fan. Unidirectional flow & sheet-like deposition point to braided channels.

Junction sandstone

Part of Taylor group. Gradational boundaries at top and bottom. up to 540m thick.
Skolithos

Skolithos

Skolithos is a common trace fossil ichnogenus whose original form consisted of approximately vertical cylinders. One well-known occurrence of Cambrian trace fossils is the famous 'Pipe Rock' of northwest Scotland...

abundant.
Intermediate between Brown Hills Conglomerate and Hatherton sandstone.

Hatherton sandstone

Part of Taylor group. 250-300m thick. Divided into upper and lower subunits.

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

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

Upper: Similar, but rust-weathering, current rippling.

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

Abundant ichnofauna

Ichnology

Ichnology is the branch of geology that deals with traces of organismal behavior, such as burrows and footprints. It is generally considered as a branch of paleontology; however, only one division of ichnology, paleoichnology, deals with trace fossils, while neoichnology is the study of modern traces...

.

Common bedforms: planar beds, bi-modal cross-beds, hummocky cross-stratification

Hummocky cross-stratification

Hummocky cross-stratification is a type of sedimentary structure found in sandstones. It is a form of cross-bedding usually formed by the action of large storms, such as hurricanes. It takes the form of a series of "smile"-like shapes, crosscutting each other. It is only formed at a depth of...

 (HCS), laminated seds.
Drainage to north east.

Presumed marine for a long time on the basis of trace fossils such as Skolithos

Skolithos

Skolithos is a common trace fossil ichnogenus whose original form consisted of approximately vertical cylinders. One well-known occurrence of Cambrian trace fossils is the famous 'Pipe Rock' of northwest Scotland...

, 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. The confusion was rectified when it was realised that HCS and the ichnofacies could in fact be marine.

Beacon heights orthoquartzite

Only known in north.

Sometimes just referred to as top 30m of Hatherton sst.

Well sorted and cemented. Grains medium to coarse. Trough cross-beds. Haplostigma

Haplostigma

Haplostigma is a genus of arborescent lycopod, found only in the early part of the Middle Devonian....

 irregulare - lycopod remnants. Constrain to early Middle Devonian. Contact on Hatherstone sandstone is sharp, irregular, and in places cobbly - so erosional.

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.

Only known in north. Top 7.5m contains dewatering structures - result of loading by tillites. 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.

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

Darwin tillite

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

This erosion surface was formed by downcutting streams, later scoured by glacial ice. Permian

Permian

The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...

 in age. Erosion surface covered with pebbly mudstone.
Features rhythmic, varved layers, with some channel and sheet sandstones. Main unit is diamictite

Diamictite

Diamictite : through and µεικτός : mixed) is a poorly or non-sorted conglomerate or breccia with a wide range of clasts, up to 25% of them gravel sized...

.

Misthound coal measure

Part of Victoria group.
Overlays tillite unconformably, resting on "Pyramid" erosion surface which was formed by reworking of the tillite.
Dominated by Gangamopteris

Gangamopteris

Gangamopteris is a genus of Carboniferous-Permian seed fern, very similar to Glossopteris. The genus is usually only applied to leaves, making it an form taxon. Gangamopteris dominates some coal deposits, such as those of the Beacon sandstone....

.
Cross-bedded sandstones, with some mudstones, carbonaceous shales, and of course coal.

Body fossils

The Aztec sandstone contains units bearing body fossils of Fish: Phyllolepid placoderms, and thelodonts; abundant in fish beds; and conchostracans.

Also: Charred wood remnants, and the plants Glossopteris

Glossopteris

Glossopteris is the largest and best-known genus of the extinct order of seed ferns known as Glossopteridales ....

and Haplostigma

Haplostigma

Haplostigma is a genus of arborescent lycopod, found only in the early part of the Middle Devonian....

.

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

Nothing else though.

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 trackway

Fossil trackway

A fossil trackway is a type of trace fossil, a trackway made by an organism. Many fossil trackways were made by dinosaurs, early tetrapods, and other quadrupeds and bipeds on land...

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

Present include:

• Fodinichnia
  Fodinichnia
  Fodinichnia are trace fossils formed by deposit feeders as they excavate the sediment in search of food. They tend to have repeated patterns or spreites , the shape reflecting the systematic feeding strategy used by the organism as it scours the sediment...
  
  : feeding burrows, probably of marine polychaete
  Polychaete
  The Polychaeta or polychaetes are a class of annelid worms, generally marine. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. Indeed, polychaetes are sometimes referred to as bristle worms. More than 10,000...
  
  s, featuring evidence of rhythmic defecation.
  • Narrow, sinuous, near-surface forms on flat bedding surfaces
  • Longer, larger forms, reaching 13 cm across and 1 m in length.
• Walking trackway
  Fossil trackway
  A fossil trackway is a type of trace fossil, a trackway made by an organism. Many fossil trackways were made by dinosaurs, early tetrapods, and other quadrupeds and bipeds on land...
  
  s of arthropods (Repichnia).
  • Beaconites
    Beaconites
    Beaconites is an ichnogenus known from the Beacon sandstone, Antarctica, comprising a large, segmented burrow, bearing superficial resemblance to the skeleton of a snake, and probably created by a worm-like organism "shovelling" the substrate out of its way. Some terminate in elliptical pits,...
    
     antarcticus: Narrow, parallel grooves, about an inch apart, disappearing into elliptical pits; created by shovelling the surface sediment aside before burrowing into the sediment. Occasionally branch.
  • Wider spaced grooves (~3 cm); small footprints visible. Implies many walking limbs and an approximately rectangular shape - reminiscent of the trilobite
    Trilobite
    Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...
    
    s. B. barretti? Extend laterally up to 1.7m; burrow "deeply" into sediment. Probably produced by a very different arthropod to B. antarcticus.
  • 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 Xiphosura
    Xiphosura
    Xiphosura is an order of marine chelicerates which includes a large number of extinct lineages and only four recent species in the family Limulidae, which include the horseshoe crabs...
    
    ns

• Diplichnites
  Diplichnites
  Diplichnites is an ichnogenus thought to be made by members of the Phylum Arthropoda. It is a fossil trackway showing two parallel lines of feet impressions. The famous terrestrial arthropod genus Arthropleura produces the trace fossil D. cuithensis...
  
  trackways - double rows of fossils - previously attributed to marine trilobites. Clearly not - so perhaps annelids / myriapods? Here appear on metre-scale crossbeds: sub-fluvial dunes?

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.

• Cruziana
  Cruziana
  Cruziana is a trace fossil consisting of elongate, bilobed, approximately bilaterally symmetrical burrows, usually preserved along bedding planes, with a sculpture of repeated striations that are mostly oblique to the long dimension...
  
  & Rusophycus
  Rusophycus
  Rusophycus is a trace fossil allied to Cruziana. Rusophycus is the resting trace, recording the outline of the tracemaker; Cruziana is made when the organism moved...
  
  : thought to be formed by trilobite
  Trilobite
  Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...
  
  s, 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.

• Skolithos
  Skolithos
  Skolithos is a common trace fossil ichnogenus whose original form consisted of approximately vertical cylinders. One well-known occurrence of Cambrian trace fossils is the famous 'Pipe Rock' of northwest Scotland...
  
  - again, traditionally thought to be marine, but there are lots of examples where it isn't.

Ichnofacies

• Scoyenia ichnofacies implies freshwater aquatic nature.

Depositional environment

Sedimentological and palæontological data point to a shallow marine depositional environment

Sedimentary depositional environment

In geology, sedimentary depositional environment describes the combination of physical, chemical and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will be formed after lithification, if the sediment is preserved in the rock...

.

The well-sorted nature of the unit suggests that it was probably deposited close to the shoreline, in a high energy environment. 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. Ripple marks and cross bedding show that shallow water was also commonly present.

Source rock

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

Thermal history

• Heat from burial modest.
• Heated to 160+° by intrusion of dolerite sills, dykes and lenses during the early Jurassic, related to break up of Gondwana . - the Ferrar Dolerite. Reached T of 200-300°C in places.
• 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.

Biology

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