Stac Fada member

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The Stac-Fada-Member is a geological group of strata in the north-west of Scotland , which is counted as part of the Mesoproterozoic Bay-of-Stoer Formation . The member has recently been interpreted as the ejection mass of a meteorite impact .

Type locality and occurrence

The type locality of the Stac Fada member is the Stac Fada of the same name , a rocky outcrop on the northwest coast of Scotland 800 meters west of Stoer . The Stac-Fada-Member stands immediately east of the Coigach Fault and extends in the north from Stoer over the Caillich Head to Gairloch in the south, where it is cut off from the Loch Maree Fault . It thus has a length of around 70 kilometers.

stratigraphy

Accretionary lapilli in the Stac Fada member

The Bay-of-Stoer formation, which belongs to the stenium, is the lowest formation of the Stoer Group , which in turn forms the lower section of the Torridonian Supergroup . The Stac-Fada-Member is between 10 and 15 meters thick and is mainly composed of yellowish-greenish, clayey sandstone . In the sandstone there are accretionary lapilli in the hanging wall of the member and generally up to 30% dark green, vesicular glass fragments of mafic composition, which are now devitrified, distributed over the entire member. At the type locality, allodapic layers of sandstone, torn from their stratigraphic compound, appear in the reclining member. The Stac Fada member lies concordantly on the deck of the Bay of Stoer formation. For its part, it is concordantly overlaid by red tones of the Poll-a'-Mhuilt member (silting sea facies).

Interpretations

The Stac Fada member forms a key horizon within the Stoer Group, which is clearly different from the rest of the sediments in the group. The member has so far mostly been interpreted as a volcanic mass flow ( lahar ), ash stream, reclaimed volcanic tuffs or fluidized peperite . Other siltstone layers within the Stoer Group are also interpreted in this way.

However, this contradicted in March 2008 the results of a united research team from Oxford and Aberdeen, which addressed the Stac Fada member as the ejecta of an asteroid impact, which is said to have had the area around Ullapool in north-west Scotland as the point of impact. If this assumption is true, it would be the first confirmed meteorite impact in Great Britain.

The impact was dated to 1180 million years BP . It melted the rocks underground, formed shock lamellae (PDFs) in quartz and biotite and left behind an iridium anomaly.

The research group suspects the impact crater in the Minch , hidden under sandstone deposits. Its diameter is estimated at 10 kilometers, the energy of the impact at 145,000 megatons of TNT equivalent. The gusts of wind generated by this are said to have had a speed of 420 kilometers per hour even in Aberdeen .

Individual evidence

  1. ^ Young, GM: Stratigraphy and geochemistry of volcanic mass flows in the Stac Fada Member of the Stoer Group, Torridonian, NW Scotland . In: Transactions of the Royal Society of Edinburgh, Earth Sciences . tape 93 , 2002, p. 1-16 .
  2. ^ Sanders, IS and Johnston, JD: The Torridonian Stac Fada Member: an extrusion of fluidized peperite? In: Transactions of the Royal Society of Edinburgh, Earth Sciences . tape 80 , 1989, pp. 1-4 .
  3. ↑ Amor, K., Hesselbo, SP, Porcelli, D., Thackrey, S. and Parnell, J .: A Precambrian proximal ejecta blanket from Scotland . In: Geology . tape 36 , 2008, p. 303-306 .
  4. ↑ Parnell, J., Mark, D., Fallick, AE, Boyce, A. and Thackrey, S .: The age of the Mesoproterozoic Stoer Group sedimentary and impact deposits, NW Scotland . In: Journal of the Geological Society . tape 168 (2) , 2011, pp. 349-358 , doi : 10.1144 / 0016-76492010-099 .