Beacon sandstone

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The Beacon Sandstone and diabase intrusions.
The Beacon Sandstone and diabase intrusions.

The Beacon sandstone is a geological formation exposed in Antarctica and deposited from the Devonian to the Triassic (~400 to - million years ago 225 million years ago[verification needed]). 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.

Contents

[edit] Setting in time and space

Named after Beacon Heights.[1] First named 1907, type section described in 1963.[2] Originally dubbed a formation, with scope left (and later used) to expand to group, then supergroup, as better mapped and understood.[2]

[edit] Age

  • Upper Devonian to Triassic

[edit] Exposure

  • Antarctica: McMurdo sound, shores of Ross Bay;[3]

also southern Victoria land, Ross desert.[4]

  • Described by Scott & team on way to South Pole.[5]

The series is over 1km thick in places,[5] and extends for over 1,000 miles.[5]

The beds are almost flat lying,[5] dipping at about 3° to the north;[6] many are interleaved with dolerite sills.[5]

[edit] 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 communities that form the modern inhabitants.[7]

[edit] Sedimentology

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

The unit is a "Fine grained, arkosic quartz sandstone".[8] It is composed of shales, coals, conglomerates, and in places the occasional thin limestone bed.[8]


[edit] Lithofacies

Originally divided into 3 subunits,[5] further refined into five facies, listed below from oldest to youngest:[9]

[edit] Brown hills conglomerate

Basal. Grades into Junction sandstone. Variable thickness; (0-5[2]/17[10]/80[11] m)[10], overlies pre-Devonian plutonic rocks, of igneous and metamorphic nature, with over 30 m erosional relief.[2] Contains igneous and metamorphic clasts.[10]

Poorly sorted at base, influxes of coarser material.[10] Coarseness is laterally variable - pebbles in places, sands in others, at same horizons.[11] 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.[10][11]

Probably alluvial fan. Unidirectional flow & sheet-like deposition point to braided channels.[11]

[edit] Junction sandstone

Part of Taylor group.[10] Gradational boundaries at top and bottom.[10] up to 540m thick.[11] Skolithos abundant.[11] Intermediate between Brown Hills Conglomerate and Hatherton sandstone.

[edit] Hatherton sandstone

Part of Taylor group.[12] 250-300m thick.[11] Divided into upper and lower subunits.[9]

  • 95% Quartz.[8]
  • Abundant: Zircon, limonite. Common: garnet, magnetite. Present in places: Shell fragments (Brachiopod / bivalve)[8]

Lower: white/yellow sandstone.[9] Layers of grit/conglomerate at base, silt at top, of some beds, which reach 15m thickness.[9] Trough cross beds.[12]

Upper: Similar, but rust-weathering, current rippling.[9]

Dates to late Middle Devonian, by correlation to the well constrained (by fish fossils) Aztec Sandstone nearby.[12]

Abundant ichnofauna.[9]

Common bedforms: planar beds, bi-modal cross-beds, hummocky cross-stratification (HCS), laminated seds.[10] Drainage to north east.[11]

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.[13] The confusion was rectified when it was realised that HCS[11] and the ichnofacies could in fact be marine.[13]

[edit] Beacon heights orthoquartzite

Only known in north.[10]

Sometimes just referred to as top 30m of Hatherton sst.[11]

Well sorted and cemented. Grains medium to coarse. Trough cross-beds.[10] Haplostigma irregulare - lycopod remnants. Constrain to early Middle Devonian.[11] Contact on Hatherstone sandstone is sharp, irregular, and in places cobbly - so erosional.[10]

[edit] 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.[12]

Only known in north.[10] Top 7.5m contains dewatering structures - result of loading by tillites.[14] 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.[14]

Minimum thickness 135m.[10] Coarse sands and finer muds; cross-bedded channels up to 12m wide.[10] Small and large roots.[10] Psilophytes, lycopod stems, logs.[10]

[edit] Darwin tillite

Base of Victoria group.[12] Also known as Metschel tillite.[12] Overlays Hatherton and Aztec[11] unconformably,[9] resting on "Maya" erosion surface,[12]which has only "slight" relief.[15] Underlying sands thumped by granitic clasts, which form load structures.[14]

This erosion surface was formed by downcutting streams, later scoured by glacial ice.[12] Permian in age.[6] Erosion surface covered with pebbly mudstone.[15] Features rhythmic, varved layers, with some channel and sheet sandstones.[12] Main unit is diamictite.


[edit] Misthound coal measure

Part of Victoria group.[12] Overlays tillite unconformably,[9] resting on "Pyramid" erosion surface which was formed by reworking of the tillite.[12] Dominated by Gangamopteris. Cross-bedded sandstones, with some mudstones, carbonaceous shales, and of course coal.

[edit] Ellis formation

Comprises a conglomerate, sand- and silt-stone.[9]

[edit] Palæontology

[edit] Body fossils

The Aztec sandstone contains units bearing body fossils of Fish:[9] Phyllolepid placoderms,[16][17] and thelodonts;[18]abundant in fish beds; and conchostracans.[13]

Also: Charred wood remnants,[1] and the plants Glossopteris[2] and Haplostigma.[13]

Wood bears clear rings, so environment must have been very seasonal.[2] Large enough to represent temperate climate, at least.[2] Glacial just before Beacon deposition.[2]

Nothing else though.[13]

[edit] Trace fossils

Burrows in the Beacon sandstone.
Burrows in the Beacon sandstone.

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.[10] Size of arthropod tracks (<91cm!) taken to imply that water must have been required for support.[10] In Hatlerton, Skolithos density decreases.

Present include:

  • Fodinichnia: feeding burrows, probably of marine polychaetes, featuring evidence of rhythmic defecation.[9]
    • Narrow, sinuous, near-surface forms on flat bedding surfaces
    • Longer, larger forms, reaching 13cm across and 1 m in length.
  • Walking trackways of arthropods (Repichnia).[9]
    • 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.[9] Occasionally branch.[10]
    • Wider spaced grooves (~3cm); small footprints visible. Implies many walking limbs and an approximately rectangular shape - reminiscent of the Trilobites.[9] B. barretti? Extend laterally up to 1.7m; burrow "deeply" into sediment.[10] Probably produced by a very different arthropod to B. antarcticus.[10]
    • Large (~30cm 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[9]
  • Diplichnites trackways - double rows of fossils - previously attributed to marine trilobites. Clearly not - so perhaps annelids / myriapods?[13] Here appear on metre-scale crossbeds: sub-fluvial dunes?[13]

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,[9] 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.[13]

  • 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.[13]
  • Skolithos - again, traditionally thought to be marine, but there are lots of examples where it isn't.[13]

[edit] Ichnofacies

Main article: ichnofacies
  • Scoyenia ichnofacies implies freshwater aquatic nature.[13]

[edit] Depositional environment

Sedimentological and palæontological data point to a shallow marine depositional environment.[9]

The well-sorted nature of the unit suggests that it was probably deposited close to the shoreline, in a high energy environment.[8] 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.[8] Ripple marks and cross bedding show that shallow water was also commonly present.[8]

[edit] Source rock

  • Too few minerals to come from local granites and schists, unless a long period of subaerial weathering preceded deposition.[8]
  • Could have been transported; would have to be a long distance to produce such a clean sandstone.[8]

[edit] Thermal history

  • Heat from burial modest.[19]
  • Heated to 160+° by intrusion of dolerite sills,[5] dykes and lenses during the early Jurassic, related to break up of Gondwana 180 million years ago.[19] - the Ferrar Dolerite.[9] Reached T of 200-300°C in places.[19]
  • 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.[5]

[edit] Utility

[edit] Biology

  • The rock is low in phosphorous,[20] creating difficulties for organisms living on it.
  • Mostly supports lichens; has its own endogenous community

[edit] Hydrocarbon potential

  • Great source: but nothing to trap oil.[5]

[edit] Further reading

[21] [22] [23] [24] [25]

[26] [27]

[edit] References

  1. ^ a b Stewart, Duncan, Jr.. "The Petrography of the Beacon Sandstone of the South Victoria Land". Mineralogical Society of America. 
  2. ^ a b c d e f g h Hamilton W & Hayes PT (1963). "Type section of the Beacon Sandstone of Antarctica". US Geol Survey Prof paper 456-A: 1–18. 
  3. ^ Scott's Terra Nova Antarctic Expedition. Retrieved on 2008-04-23.
  4. ^ Friedmann, E.I.; Weed, R. (1987). "Microbial trace-fossil formation, biogenous, and abiotic weathering in the Antarctic cold desert". Science 236 (4802): 703–705. doi:10.1126/science.11536571. 
  5. ^ a b c d e f g h i Elliott, R.B.; Evans, W.D. (1963). "A Beacon Sandstone: its Petrology and Hydrocarbon Content". Nature 199 (4894): 686–687. doi:10.1038/199686b0. 
  6. ^ a b Kamp, P.J.J.; Lowe, D.J. (1982). "Geology and terrestrial age of the Derrick Peak meteorite occurrence, Antarctica". Meteoritics 17: 119–127. 
  7. ^ e.g. Derek Pullan, Frances Westall, Beda A. Hofmann, John Parnell, Charles S. Cockell, Howell G.M. Edwards, Susana E. Jorge Villar, Christian Schroder, Gordon Cressey, Lucia Marinangeli, Lutz Richter, Gostar Klingelhofer. (2008). "Identification of Morphological Biosignatures in Martian Analogue Field Specimens Using In Situv Planetary Instrumentation". ASTROBIOLOGY 8 (1): 119. doi:10.1089/ast.2006.0037. 
  8. ^ a b c d e f g h i Angino, E.E.; Owen, D.E. (1962). "Sedimentologic Study of Two Members of the Beacon Formation, Windy Gully, Victoria Land, Antarctica". Transactions of the Kansas Academy of Science (1903-) 65 (1): 61–69. doi:10.2307/3626470. 
  9. ^ a b c d e f g h i j k l m n o p q r Gevers, T.W.; Frakes, L.A.; Edwards, L.N.; Marzolf, J.E. (1971). "Trace Fossils in the Lower Beacon Sediments (Devonian), Darwin Mountains, Southern Victoria Land, Antarctica". Journal of Paleontology 45 (1): 81–94. 
  10. ^ a b c d e f g h i j k l m n o p q r s t u Bradshaw, M.A.; Harmsen, F.J. (2007). "The paleoenvironmental significance of trace fossils in Devonian sediments (Taylor Group), Darwin Mountains to the Dry Valleys, southern Victoria Land". Antarctica: A Keystone in a Changing World--Online Proceedings of the 10 thISAES X, edited by AK Cooper and CR Raymond et al., USGS Open-File Report 1047. 
  11. ^ a b c d e f g h i j k l Woolfe, K.J. (1993). "Devonian depositional environments in the Darwin Mountains: Marine or non-marine?". Antarctic Science 5 (02): 211–220. doi:10.1017/S0954102093000276. 
  12. ^ a b c d e f g h i j k Woolfe, K.J. (1994). "Cycles of erosion and deposition during the Permo-Carboniferous glaciation in the Transantarctic Mountains". Antarctic Science 6 (01): 93–104. doi:10.1017/S095410209400012X. 
  13. ^ a b c d e f g h i j k Woolfe, K.J. (1990). "Trace fossils as paleoenvironmental indicators in the Taylor Group (Devonian) of Antarctica". Palaeogeography, Palaeoclimatology, Palaeoecology 80: 301–310. doi:10.1016/0031-0182(90)90139-X. 
    • Great images of the different trace fossils
  14. ^ a b c Isbell, J.L.; Lenaker, P.A.; Askin, R.A.; Miller, M.F.; Babcock, L.E. (2003). "Reevaluation of the timing and extent of late Paleozoic glaciation in Gondwana: Role of the Transantarctic Mountains". Geology 31 (11): 977–980. doi:10.1130/G19810.1. 
    • contains images of dewatering structures
  15. ^ a b PR Pinet, DB Matz, MO Hayes (1971). "An Upper Paleozoic Tillite in the Dry Valleys, South Victoria Land, Antarctica: NOTES". Journal of Sedimentary Research 41 (3): 835–838. doi:10.1306/74D7236A-2B21-11D7-8648000102C1865D. 
  16. ^ Allowing dating to late Mid Devonian
  17. ^ Woolfe, K.J. (2004). "Cycles of erosion and deposition during the Permo-Carboniferous glaciation in the Transantarctic Mountains". Antarctic Science 6 (01): 93–104. doi:10.1017/S095410209400012X. 
  18. ^ Turner, S.; Young, G.C. (2004). "Thelodont scales from the Middle-Late Devonian Aztec Siltstone, southern Victoria Land, Antarctica". Antarctic Science 4 (01): 89–105. doi:10.1017/S0954102092000142. 
  19. ^ a b c Bernet, M.; Gaupp, R. (2005). "Diagenetic history of Triassic sandstone from the Beacon Supergroup in central Victoria Land, Antarctica". New Zealand Journal of Geology and Geophysics 48: 447–458. 
  20. ^ Banerjee, M. (2000). "Phosphatase Activities of Endolithic Communities in Rocks of the Antarctic Dry Valleys". Microbial Ecology 39 (1): 80–91. doi:10.1007/s002489900188. 
  21. ^ Plume, R.W. (1982). in Craddock, C: Sedimentology and palaeocurrent analysis of the basal part of the Beacon Supergroup (Devonian (and older?) to Triassic) in south Victoria Land, Antarctica. Madison: University of Wisconsin Press,. 
  22. ^ Geevers TW & Twomey (1982). in Craddock, C: Sedimentology and palaeocurrent analysis of the basal part of the Beacon Supergroup (Devonian (and older?) to Triassic) in south Victoria Land, Antarctica. Madison: University of Wisconsin Press,, 639-648. 
  23. ^ Sherwood, A.M.; Woolfe, K.J.; Kirk, P.A. (1988). "Geological mapping and preliminary paleoenvironmental interpretations of the Taylor Group in the knobhead area, Southern Victoria Land". New Zealand Antarctic record 8 (2): 60–61. 
  24. ^ Plume, RW (1978). "{{{title}}}". New Zealand Journal of Geology & Geophysics 21: 167–173. 
  25. ^ Barrett PJ; Kohn BP (1975). in Campbell KSW: {{{title}}}. Canberra: ANU Press, 15-35. 
  26. ^ Barrett, PJ (1979). "{{{title}}}". Proceedings of the 4th International Gondwana Symposium Calcutta 1977: 478–480. 
  27. ^ Bradshaw MA (1981). "{{{title}}}". New Zealand Journal Geology & Geophysics 24: 615–652. 

[edit] External links