Jökull

Issue

Jökull - 01.12.1991, Page 55

Jökull - 01.12.1991, Page 55
22 to correlate with the lower and niiddle parts of our Mosfellssveit section. There are indications of three glaciations (no. 9,10 and 11) in this part of Esja. Five glacial horizons are found in the Mosfellssveit sec- tion, two within the parts which we correlate to Kristjánsson et al.'s (1980) Esja section and three in the topmost part which is younger than the Esja suc- cession. The glacial horizons may either occur as hyalo- clastite series of subglacial volcanic origin and/or as tillite beds. We correlate the Gufunes hyaloclastite with the glacial horizon 9 of Kristjánsson et al. (1980) and Esja Unit 18 which forms the base of the Stardalur central volcano (Friðleifsson 1973). The base of the series is not exposed and thus no tillite bed is observed. The tillite bed between lavas GB 8 and 9 corresponds presumably to glacial horizon 10 in Esja. The tillite bed resting on flow UE 8B and the Lágafell hyaloclastite seems to have no counterpart in Esja other than two conglomerate beds (SB 46/49 and SB 61/62) which Kristjánsson et al. (1980) sug- gested may represent separate glaciations. The tillite bed seen in profiles UL and UA at the base of the upper normal magnetic event corresponds to glacial horizon 11 in Esja. Rhyolite boulders in the tillite are thought to correspond to rhyolite hyaloclastites erupt- ed subglacially in Móskarðshnúkar and Grím- mannsfell during glaciation 11 of Esja (Friðleifsson 1973, Kristjánsson et al. 1980, Friðleifsson 1985). Two apparently separate glacial horizons occur with- in the Stórihnúkur series, one at the base of the lay- ered silt beds and the other as a hyaloclastite unit at its top. These two horizons may represent one glacial advance with a shorter readvance. The uppermost glacial horizon is the Hafrahlíð hyaloclastite and underlying tillite. PALEOMAGNETIC METHODS 2.5-cm core samples were collected using portable drills and oriented as described by Kristjánsson et al. (1980). All profiles are numbered upwards from the base; the suffixes A, B etc. indicate outcrops which were located after the initial mapping and which may represent separate units. Some thin, inaccessible, poorly exposed, or crumbly flows were not sampled. In the first profile (UL), two cores were generally taken from each unit and AF-demagnetized at a sin- gle peak field of 150 Oe. In subsequent profiles, three cores were taken from most units and demagnetized either at 100 and 200 Oe (profiles UA,UB,UC,UD, UH 6-15) or at 100,150 and 200 Oe, in a two-axis tumbler system. Laboratory measurements, mostly done at the University of Iceland, employed a static four-probe fluxgate arrangement. A brief description of the sampling locations (Figs. 2, 3) and procedures is given in the Appendix. PALEOMAGNETIC RESULTS In most of the samples it was easy to isolate a prima- ry direction of remanence; the mean intensity of this remanence is similar to the averages for other collec- tions published by Kristjánsson (1984). In cases of within-unit directional inconsistencies, samples were occasionally demagnetized at 250 or 300 Oe, or the units were resampled. All lava-mean paleomagnetic results, listed in Table 1, have been corrected for tec- tonic tilt. The amount of tilt which is assumed to be due southeast, is also given in Table 1. Stratigraphy. For stratigraphic work, the most useful paleomagnetic parameter in this case is the polarity, shown for each flow in Fig. 3. We have attempted to set up a composite stratigraphic column for the area (Figs. 4, 5). An N to R reversal takes place near the bottom of the composite section, at GB 5/6 and UE 1/2. The outcrop of UE 1 east of Koipa gave ambiguous paleomagnetic results, so we later resampled the flow on the beach west of the stream, obtaining consistent directions. The bearing between the two outcrops of the N to R transition is ENE (cf. Fig. 4) rather than NE, suggesting the presence of complications such as an unconformity or a set of large faults. We propose that the normal-polarity lavas corre- late with Einarsson's (1957) N3 zone which is of variable thickness in the Hvalfjörður area, but Einarsson gives a mean thickness of 50 m. In the Kistufell profile SB of Kristjánsson et al. (1980) flows no. 24-36 belong to this zone. The flow UK 7, which has a transitional magnetic direction (Table 1), JÖKULL, No. 41, 1991 53
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x

Jökull

Undirtitill:
Ársrit Jöklarannsóknarfélags Íslands og Jarðfræðafélags Íslands
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Flokkur:
Gegnir:
ISSN:
0449-0576
Tungumál:
Árgangar:
72
Fjöldi tölublaða/hefta:
73
Skráðar greinar:
Gefið út:
1951-í dag
Myndað til:
2023
Útgáfustaðir:
Ritstjóri:
Jón Eyþórsson (1952-1967)
Sigurður Þórarinsson (1957-1982)
Guðmundur Pálmason (1965-1976)
Sveinbjörn Björnsson (1967-1976)
Helgi Björnsson (1983-1985)
Leó Kristjánsson (1983-1985)
Ólafur G. Flóvenz (1986-1987)
Tómas Jóhannesson (1988-1989)
Helgi Björnsson (1990-1993)
Leó Kristjánsson (1990-1993)
Áslaug Geirsdóttir (1994-2007)
Tómas Jóhannesson (1998-1998)
Bryndís Brandsdóttir (1998-2007)
Halldór Gíslason (2002-2003)
Snævarr Guðmundsson (2006-2007)
Freysteinn Sigmundsson (2008-2008)
Leifur A. Símonarson (2008-2008)
Olgeir Sigmarsson (2008-2008)
Ívar Örn Benediktsson (2012-2012)
Helgi Björnsson (2012-2012)
Guðrún Larsen (2012-2012)
Olgeir Sigmarsson (2012-2012)
Bryndís Brandsdóttir (2013-2016)
Snævarr Guðmundsson (2013-2016)
Þorsteinn Þorsteinsson (2013-2016)
Efnisorð:
Lýsing:
Reykjavík Jöklarannsóknafélag Íslands Jarðfræðafélag Íslands 1951-.

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1. tölublað (01.12.1991)

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