Vo1canic history of the Lower Tertiary plateau basalts In the Scoresby Sund region, East Greenland

The extensive plateau basalt lava pile in the Scoresby Sund region has a stratigraphic thickness of 3200 m and an overall average thickness of 1500 m. The pile thins inland from the Atlantic coast and laps onto basement gneisses and Jurassic sediments in the in ner fjord region. The lavas are divisible into five formations which form two separate lava sequences. The lower sequence is best developed in the inner fjord region, while the upper sequence dominates the regions near the Atlantic coast. The sequences are inter preted as produced in two vo\canic episodes in connection with failed rifting episodes during the opening of the North Atlantic Ocean. At the Atlantic coast remains of a third separate lava sequence apparently forrned during active spreading.


Introduction
Extensive areas in central East Greenland are covered~ith plateau basalts. These were erupted in Lower Tertiary time (Upper Paleocene -Lower Eocene) in connection with the initiation of sea-floor spreading and the formation of the North Atlantic Ocean. Large and smal1 remnants of vo1canic activity in the form of lava flows, dykes, silis and central intrusions are found along the Atlantic coast of East Greenland from 66°N (Angmagssalik) to 75°N (Shannon) (Deer, 1976;Noe-Nygaard, 1976). However, the largest area with coherent basalt cover is the region at 68°-70 o N, beteen Kangerdlugssuaq and Scoresby Sund ( fig. 1).
The basalts in the Scoresby Sund region were mapped during GGU expeditions to East Greenland in the years [1968][1969][1970][1971][1972]. The original programme only covered the area north of 70 o N, but since key areas to the basalt stratigraphy were found further south, mapping was subsequently extended south of 70 o N.

General geology
The map and cross-section ( fig. l) show the main structure of the basalt pile in the Scoresby Sund region. The basalts lap on to the elevated area in the north-west and their base drops below sea-level to the east. They are essentiaIly flat-lying, with a regional dip of !-1°S E. A 10-30 km broad strip along the Atlantic coast is extensively block faulted with downthrows to the south-east in excess of 1500 m. The faulted coastal area hosts a 200 km long swarm of coast-parallel basalt dykes while in the inland areas ENE-trending dykes occur over a broad region in Milne Land, Gåseland and south of Gåsefjord ( fig. 2).
The aggregate thickness of the lava pile is 3200 m. Actual thicknesses are greatest at the Atlantic coast and decrease inland to 1500 m around Gåsefjord and to 300-800 m on Gåseland and Milne Land. The overall average thickness is 1500 m. All the basalts were extruded sub-acrially, and red. scori,lCcouS tlmv tops are commol1. Single lava flows are llsually large. 1()....50/TI Ihid. Wilh voJunl(~.s rangillg from n fcw <.:ubic kiJometres to al kasl 300 km-\.

Basalt slraligraph}
Duc to th!,; disseeted and partty glaciJted character uf the area, with man y glacier tongues rcaching from tile high land plalcaux down ro the coast. single lava flows ean usually only he follmvcd hltcrally for shor! distances in the fieid, and marker horizons are conspicuously absent.
The basalts were rnappcd by detailed measuring and sarnpling af numerous profiles. and hy corrclation bet\veen these of characteristic propcrtit:s, such as flow thicknesses and morphnlngy. phcnocryst content and mode, colour and wcathering appearancc. and, later. chernical cornposition. In this way characteristic tlows and scquences of flows ean be fol-low~d latcmlly ovcr large arcas.
Fivc IW5\alt formations of regional ex/en I have becn establishcd: tITe M,1gga, Milnc Lwd. Gcikic. Rømer Fjord and Skraenterne Formations. A sixth and youngcst formation, the Ig- Thc 1\1,'0 major basalt scquen<:cs show systematic compositional zoning. Fig. 3 shows some imponilnt eompositional patterns within a <:omposite profile through the completc lava sequencc. The individuality af [he various formations is seen in lhe alternation bctwccn COnlpositionally helerogeneous <.lnd hOlTlogcncous formations. If tbe lowest formation (Magga) is cxccptcd as an initial phasc. the [wo major basalt sequenees show ve ry similar systematic dcvelopment in composition with heighl: a ph<1se with very variahlc eruption produets (Miine Land/Rømer Fjord Formations) is sun:eeded by a monotOllOUS phase (Geikiel Skraenter nc Formations) \vhich shows systemati<: step-like decrease in TiO, \Vith height. and cnds in a reversal to higher TiO~contents. Thc stcp-likc <.:omposilional p<1tterns are found in all profiles through these formations. and they are thereby subdivided info units as indieated in rig. J. Each of the two major basalt sequences are interpreted as having been produced in a cyelic episode of volcanic activity (Larsen & Watt, 1985). At the beginning of each cyele the situation was unstable and a variety of both strongly and less differentiated magma types were produced. Then the situation apparently stabilised and the basalts became more uniform and systematically less differentiated with height, signalling increasing vo1canic activity and shorter residence times in the magma chambers. The low-Ti units forrned at the peak of activity, and the uppermost, 'reverse' units in each sequence forrned when the activity waned and died. The two vo1canic cycles are viewed as produced in two temporary rifting episodes, while the third sequence is part of the products from a third rifting episode. These episodes can be related to events during the opening of the North Atlantic Ocean.

The substratum of the basalts
During Mesozoic time a N-S elongated sedimentary basin developed in East Greenland, partly bounded by faults towards the Caledonian and older highlying crystalline basernent to the west (Surlyk et al., 1981). Sediments of this basin occupy extensive areas in Jameson Land, immediately to the north of the basalts, and are also found on Milne Land where they are partly covered by basalts ( fig. 1). A small exposure of Cretaceous sediments is found below the basalts on the south side of Scoresby Sund at Kap Brewster (R. A. Fensome in Watt & Watt, 1983), but the extension of the sedimentary basin to the south, below the basalts, is unknown. Cretaceous sediments underlie the basalts at Kangerdlugssuaq (Deer, 1976;Nielsen et al., 1981) 400 km to the south, and it is believed that the sedimentary basin is continuous and underlies the whole eastem half ofthe lava pile. This concept is supported by the occurrence of methane, presumably sediment-derived, in the warm springs issuing from the lavas at Rømer Fjord on the Blosseville Kyst, and the occurrence of migrated hydrocarbons in the lavas at Savoia Halvø (Watt & Wrang, 1984).
At the onset of vo1canism, the landscape thus consisted of two very different elements: a flat, low-Iying sedimentary plain in the east and south-east, and in the west and north-west a mountainous gneiss country with deeply incised valleys leading down to the plain. The mountains and valleys have been preserved beneath the basalt cover, and where this has been removed by later erosion, the old landscape elements have reappeared. Thus, Gåseland as a mountain ridge and Gåsefjord as a valley are elements of Paleocene age.
One of the main boundary faults between the highland and the plain runs approximately NNE-SSW through the eastem part of Milne Land. It cannot be followed farther south.

Magga Formation
The vo1canism started in the inland region around Gåsefjord. Here, the Magga Formation occurs as an up to 300 m thick sequence of thin, aphyric, grey-brown flows and flow units ( fig. 4). Most of these lavas were confined to the lowland areas, but some lavas were apparently erupted from local vents in the high-Iying areas of Gåseland. Pillow breccias are frequent in this formation; they forrned where the lavas inundated moist valley bottoms and river beds and dammed up the rivers.
The formation is compositionally uniform and consists of moderately differentiated ba-

Milne Land Formation
This formation is up {O 350 In thick and consists af (hicker, trap-forming flows af mostly feldspar-phyric basalt ( fig. 4). Many Jlows have very well-dcvclopcd colonnadcs (Noc-Nygaard. 1976, fig. 342). The lower bOllndar)' af the formation is irregular due to the irregular substratum. ancl pillow breccias still formed in valley bottoms and iakes. During thc buiJd-up af this formation, the lavas compleleJy engulfcd and ovcrOowcd the old Gåscland mountain ridge and spread to the west and norIh. One af tile firsl lava flows to do so was the volurninolls I1jØrnedal marker Ilow wilich is both petrographically and compositionaJly distinclive and has been found in all profiles which cover the appropriate stratigraphic level. This flow covers a minimum area af 11 500 km' and has a minimum volume af 285 km3; like the other flows in this formalion. its extension towarcis the east and south-east cannot be c1elimiled becallsc it drops bclow sea-Ievel. Extrusion siles for this formation are not known but are beJieved to bc centred in the Gåscfjord area.
Compositionally, the formation is heterogeneous. It includes a flow af Mg-rich basalt cornpositionally simiiar to the basalts af the Mikis Formation in the luwer basaltl'i in Kangercllugssuaq (Nielsen el al.,19R I). and also includes the first Ti-rich, strongly differentiated basalts. Olle af these Ti-rich flow's has a calculated minimum volume af 300 km 3 . alld the develupmcnt af such a large volume af strongly diffcrentiated basalt requires that grcat magma chambers must by then have come into existence. The occasional presence of high-Si flows shows that wall rock contamination still took place intermittently. Most lavas are still quartz normative ( fig. 3), probably a long-term effect of this contamination.

Geikie Formation
This is the most voluminous and widespread basalt formation. It is up to 1100 m thick and consists of thick, trap-forming flows of aphyric or sparsely porphyritic basalt with a clear division in many flows into a lower colonnade and an upper entablature. The lavas of this formation spread out over the level basalt plain from east to west and overstepped further the gneiss highland on Milne Land. The inland dykes on Milne Land, Gåseland and south of Gåsefjord-Scoresby Sund ( fig. 2) are compositionally identical to the lavas of the Geikie Formation, and are believed to be feeders for this formation. The whole first vo1canic episode thus seems to be centred in the inland region around Gåsefjord.
Compositionally, the Geikie Formation ean be subdivided into five units ( fig. 3) which are not mappable in the field but can be recognised in all analysed profiles. As interpreted by Larsen & Watt (1985), during the build-up of this formation magma input into the crustal magma chambers increased, leading to shorter residence times in the magma chambers and eruption of successively less differentiated lavas. The activity culminated with the formation of low-Ti basalts, similar in many respects to those formed at mid-ocean ridges. However, the incipient rifting failed and the magma chambers died with the formation of the fractionated basalts in the uppermost 'reverse' unit.
The long-term effects of crustal contamination appear to have gradually worn off during the build-up of the Geikie Formation, such that basalts changed their character from quartz to olivine normative ( fig. 3).

Second volcanic episode
After a hiatus and local sediment accumulation, especially in the eastern areas, vo1canic activity started anew. The activity was displaced to the south-east relative to that of the first episode, and both the following two formations thin inland from the Atlantic coast, and probably never reached north-west of Gåsefjord ( fig. 1).

Rømer Fjord Formation
This formation is up to 2S0"Ih thick and often forms a recessive bench on top of the Geikie Formation ( fig. 5). It is characteristically mixed and contains sequences of thin, grey compound flows of aphyric Mg-rich basalts intercalated with sequences of strongly differentiated and highly porphyritic 'big feldspar' basalts. The Mg-rich basalts in particular are rich in zeolites which contribute to the grey colour of the lavas.
Individuallava volumes in this formation are small and some of the sequences form shieldlike structures which were probably erupted in different local areas. Judged from thickness variations in individual sequences, the major eruption sites appear to have been around the present Atlantic coast, with minor eruption sites scattered in inland areas around the Geikie Plateau. At this time magma chambers were apparently small and widespread, and there are no signs of crustal contamination of any lavas.
Of local origin is a nephelinitic ash layer found in the northern part of the Geikie Plateau, where it covers an area of around 20 km 2 and is up to 12 m thick. The vent for this violently the original volulllc af this formatioll has subsequclllly been removed by the orening af the North Allanlic Ocean because the oCe<tn-to-colltinent transition zone is situated 10-25 km east ol' the presenl eoast (Larsen, 11)84), Tllc Ihrcc units ol' the Skra~lllcrne Fonnalion are dcfined cOlllpositionally and are inler-pTl'lCU as signifying incrcflsing. cu!minaling. and dccrcasing vo\canic <lctivity lfig, 3: Larsen & W~ltL JLJS)). Tile low-Ti basalts produced during tlle pcak period constilute d larger proportion ol' this episode than af the firsl. Thc activity of the seeond episode was thus more intcnsiw lhan thc fic·,t. hUL thl' se<:ond episode <llso failed ,lIld lhl' aetivity died.

Iglerlirå Formatioll and com'lal dyke ,nvarm
Anu Ihe t\VO volcanic episodes which led to Ihe formation af the two regional basalt scquences. ;-1 third episode took place which W<lS narrowl)' confined to the present Atlantic  Larsen (unpublished manuscript, 1985).
coast. Rifting and faulting fragmented the coastal zone and an intense coast-parallel dyke swarm was intruded ( fig. 2). The Igtertivå Formation lavas, now only preserved in a 400 m thick down-faulted sequence at Kap Dalton, were apparently fed by this dyke swarm. The lavas are not cut by these dykes and are compositionally very similar to them. The lavas were probably always confined to the faulted zone. They are overlain by fossiliferous marine sediments of Lower Eocene age, and some of the faulting is younger than this (Soper & Costa, 1976).
The lavas and dykes of this episode exhibit a contrast in some important element ratios (Ti/P, Zr/Nb) compared to those of the earlier lavas and dykes. Their overall composition and their element ratios are similar to those of basalts formed at actively spreading ridges on Iceland and at the Mid-Atlantic Ridge (e.g. Jakobsson, 1979;Schilling et al., 1983;Larsen & Watt, 1985), and judged from the chemical compositions it appears that the third rifting episode, in contrast to the two preceding ones, did produce spreading.

Relations to sea·t1oor spreading
It is generally agreed that the East Greenland Tertiary lavas were erupted during the period of 3 million years between geomagnetic anomalies 25 and 24, approximately 53-56 m.y. agn (Faller, 1975;Soper et al., 1976). At the same time, sea-floor spreading with opening of the North Atlantic Ocean was initiated in the region. The spreading process was very complicated in this region, and the first spreading axis had a winding course with considerable eastwards displacement in the region considered here ( fig. 6; Larsen, unpublished manuscript, 1985). The two first magmatic episodes deciphered in the Scoresby Sund basalts apparently represent failed attempts to short-cut the eastwards displacement of the spreading axis, while the third episode is connected with the short-Iived East Greenland extinct axis ( fig. 6c) and was successful in so far as it produced a narrow strip of oceanic crust (Larsen, unpublished manuscript, 1985) and lavas with the geochemical imprint of active spreading.