Archaen
BasementThe geological setting of the area around Ikka Fjord is listed below in geochronological order, oldest first:
The basement rocks of South-West Greenland are comprised of migmatites and variable banded gneisess (granodioritic gneisses, gabbro-anorthsite bearing gneisses and quartzo-feldspathic gneisses of probable sedimentary origin), in which amphibolites with ultrabasic lenses, (the amphibolites themselves being derived from basic lavas and tuffs mixed with normal sediment material), occur as impersistant bands concordant with the foliation of the gneisses.
These basement rocks are of pre-Ketilidian age (approx. 2600 m.y.) and show metamorphism in the Almandine-Amphibolite (or Garnet-Metabasite) facies, i.e. High P. / High T. Regional Metamorphism and have been affected by 4 or 5 episodes of pre-Ketilidian folding. The basement rocks have been intruded by swarms of dolerite dykes from the Pre-Ketilidian to the Mesozoic Age. and more importantly by rocks of the Gardar Intrusive Province.
The Gardar magmatism is thought to represent cratonic activity on the foreland of the Grenville orogenic belt of North America. It encompassed a period of intense volcanism and widespread faulting The province comprises 14 alkaline intrusive complexes and various dyke swarms. The province is remarkable for the broad range of alkaline rock types represented including some extreme if not unique compositions, especially in the Ilimaussaq (around Narsaq) intrusion, and for the widespread igneous layering which is found in rocks of many types.
The Gardar alkaline igneous complexes of Kungnat, Ivigtut, Ikka-Gr?nnedal, Ilimaussaq and Igaliko all lie within the same major structural fault block, i.e. the Ivigtut-Igaliko block; this fault block was the result of a period of compressional shearing, which largely gave rise to major WNW or W trending sinistral wrench faults (like those bounding the Ivigtut-Igaliko block to the North and South), and less extensive dextral wrench faults of a subsidiary nature, that trend from NW through N to NE. It is suggested that the major fault observed transecting Ikka fjord is one of these.
The Gardar magmas follow a single evolutionary path from basalt to syenite but then diverge to either oversaturated or undersaturated components (the latter being enriched in alkalis). Petrogenic, geochemical and field data suggests that the magmas evolved by differentiation in situ, or at low crustal levels from a basaltic parent magma (a primitive upper-mantle source). The divergence is not however related to either the age of, or the geographical location of, or the mode of emplacement of the body.
Three periods of emplacement are recognized within the province, (Early / Mid / Late Gardar), emplacement being by substantial stoping of the country rock. The oldest plutonic centre is the Ikka-Gr?nnedal complex, at around 1,327 +/-17 million years (m.y.) It forms an elongated (8 x 3 km) fault deformed complex of nephaline syenites and carbonatite.
The other complexes in the general area represent Mid-Gardar episodes, with ages of 1,245 m.y. +/-17m.y. at Kungnat (near Arsuk) and 1,248 +/-25m.y. at Ivigtut. The Ivigtut complex is the smallest known member of the province and was subjected to a Late Gardar mineralization leading to the formation of a large, ore grade cryolite deposit. This deposit, in which Pauly (the discover of ikaite) was employed as a mining geologist, was of such strategic importance - being essential in the manufacture of aluminium - that during the second world war, about 200 american troops were assigned to guard it.
Intermediate to (and/or contemporaneously with) these early- and mid- Gardar intrusive complexes came the intrusion of first a series of NW trending Lamprophyres and then a series of Olivine-Dolerite dykes (BDn's on Emeleus' map), emplaced along E-W trending dilational fractures. The oldest Lamprophyre cuts the Ikka-Gr?nnedal complex, but is cut by the BDn's and the Ivigtut complex; whilst the BDn swarms are themselves cut by the Kungnat complex.
The Ikka-Gr?nnedal Complex This complex is a nephaline syenite suite encompassing a wide range of petrographic types, these types grade very quickly into each other, often in the space of a hand specimen. The carbonatite is the youngest of the complex and consists of variable proportions of Calcite / Siderite / Magnetite and minor apatite. Numerous dykes cut the complex and extend well into the country rock. The amount of Magnetite is such that there was a period of iron ore mining in the area, and the phosphate content (apatite) was sufficient to be considered for mining.
According to Emeleus' map, the syenites exposed in the fjord are members of the upper series of the igneous complex and are 'Foyaites and Pulaskites'. The area is one of gneiss and metasediments, intruded first by a series of nephaline syenites and then carbonatites (the igneous complex), the area was then cut by a number of minor intrusions.
Though carbonatites are often associated with the metasomatic alteration of the country rocks they intrude, which in gneiss results in the formation of nephaline syenites and foyaites - a process known as fenitization - in Ikka, field evidence such as, the sharp contact between the syenites and the gneiss, show that this is not the case.
Following the emplacement of the lamprophyre dykes in the mid Gardar episode 1,245 m.y. +/-17 million years ago, there is no evidence of any geological activity, until the intrusion of a series of dolerite dykes in the Mesozoic. A period in excess of 1000 million years! and then nothing again until the Quaternary.
The Quaternary period brought with it a series of widespread glaciations & glacial erosion. The latter resulting in the production of corries of various heights or fjord bottoms of various depths - Arsuk Fjord, which is adjacent to Ikka Fjord reaches depths of 600m. And of course lead to the formation of Ikka Fjord itself.
The glaciations further shaped the landscape with the production of a number of typical glacial features such as roche moutonnees. The retreating ice dumped its load, leaving behind erratics and moraines and/or raised beaches (due to a sea level rise).
The level to which the sea rises, following the melting of a retreating ice sheet, is countered by the isostatic rebound (the amount the land rises after the weight of ice is removed) & ocean basin subsidence (due to the increase in the weight of water). The magnitude of changes in water level might conceivably be reduced by as much as a third, by this 'hydro-isostatic' factor - were the Greenlandic and Antarctic ice sheets to melt today, it is estimated that they would return 65m of water to the oceans, however after the 'hydro-isostatic' factor had been accounted for, this would result in only a 40m rise in sea level.
However, it is currently thought that neither ice sheets disappeared during recent interglacial periods, nor deviated from their present dimensions, so it is unlikely that sea level attained, much greater heights than it is today.
At the present, Ikka Fjord is considered a periglacial environment, with the watershead in a zone of discontinuous permafrost.
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