Náttúrufræðingurinn - 2007, Qupperneq 33

Tímarit Hins íslenska náttúrufræðifélags
miðju lægðarinnar. Að lokum
runnu hraun yfir lægðina og jöfn-
uðu hana út og stöðuvatnið hvarf
með öllu (11. mynd E).
SUMMARY
Late Miocene sediments
at Lake Hreðavatn
There are several sedimentary
formations present in the north-
western and western part of
Iceland. These sedimentary forma-
tions are separated by thick lava
formations. The sediments around
Lake Hreðavatn were found to be
part of an extensive sedimentary
formation traceable over considera-
ble distance in western Iceland. The
sediments described here compos-
ing the formation were found to
vary between geographical areas.
The sediments around Lake Hreða-
vatn accumulated in a lake basin.
The basin is believed to have for-
med in relation to rift relocation
when the Snæfellsnes rift zone
gradually "jumped" to its present
location. This caused the stratum to
break up and severe tilting took
place. This resulted in sudden eros-
ion in elevated areas and accumula-
tion in areas of subsidence.
The sediment types of the basin
investigated were found to reapp-
ear and some were clearly associ-
ated with each other. On that basis
the sediments were classified into
several facies (15), dependent on
their characteristic appearance,
composition and origin. The facies
were controlled by intrabasinal fact-
ors as well as external basinal fact-
ors, often associated with cata-
strophic events like volcanic erup-
tions or earthquakes. The facies
were defined into facies associ-
ations dependent on their intra-
relation and deposit type. The
associations reflect river channel
deposits, organic lacustrine basin
plain deposits, clastic basin plain
deposits, pyroclastic event deposits,
clastic delta deposits and organic
delta area deposits. The facies
associations were often found to
interfinger with each other where
the environment changed from
deep water to shallower and the
volcanic-dominated associations
were found to interfinger with
several other associations due to
their relative wide distribution
caused by aerial dispersal before
entering the lake.
The sediments reflect the opening
phase of a small lake, its develop-
ment and final closure. The sedi-
mentary fill of this ancient lake
basin can be divided into three
main categories: clastic-dominated,
volcanic-dominated and organic-
dominated.
Þakicir
Leiðbeinendum mínum við kandídatsritgerðina, þeim Leifi A. Símonar-
syni við Háskóla íslands og Eckart Hákansson við Kaupmannahafnarhá-
skóla, þakka ég kærlega fyrir alla ráðgjöf og hjálp meðan rannsókn stóð
yfir.10 Snorri Gíslason á mikið inni hjá mér og þakka ég honum kærlega
fyrir þá hjálp sem hann veitti mér við vettvangsvinnu sumrin 2000 og
2001 og fyrir alla þá miklu vinnu sem hann lagði í að koma teikningum
mínum yfir á tölvutækt form. Jón Eiríksson og Björn S. Harðarson fá
þakkir fyrir uppbyggjandi samræður um rannsóknarsvæðið. Birgir
Hauksson fær þakkir fyrir veittar upplýsingar um staðhætti við
Hreðavatn og gistingu. Grímur Rúnar Friðgeirsson, Halldóra Björnsdótt-
ir, Þorbjörg Kristín Jónsdóttir og Jón Már Halldórsson fá þakkir fyrir þá
miklu vinnu og tíma sem þau lögðu í að aðstoða mig þegar mest lá við.
Heimildir
1. Schwarzbach, M. 1956. Das Vulkangebiet von Hredavatn (West-
Island). Neues Jb. Geol. U. Paláontol., Abh. 104,1. Bls. 1-29.
2. Haukur Jóhannesson 1972. Tertíeri jarðlagastaflinn frá Norðurárdal
inn Hvítársíðu í Borgarfirði. Óbirt BS-ritgerð. Háskóli íslands,
Reykjavík. 57 bls.
3. Haukur Jóhannesson 1975. Structure and petrochemistry of the
Reykjadalur central volcano and the surrounding areas, Midwest
Iceland. Doktorsritgerð. Háskólinn í Durham.
4. McDougall, I., Kristján Sæmundsson, Haukur Jóhannesson, Watk-
ins, N.D. & Leó Kristjánsson 1977. Extension of the geomagnetic
polarity time scale to 6.5 m.y.: K-Ar dating, geological and
paleomagnetic study of a 3,500-m lava succession in western
Iceland. Geological Society of America Bulletin 88.1-15.
5. Khodayar, M. 1999. On the pattern of faults and dykes in Borgar-
fjörður, W-Iceland. Jökull 47. 21-44.
6. Khodayar, M. og Páll Einarsson 2004. Reverse-slip structures at
oceanic diverging plate boundaries and their kinematic origin: data
from Tertiary crust of west and south Iceland. Journal of Structural
Geology 26.1945-1960.
7. Kristján Sæmundsson 1967. An outline of the structure of
SW-Iceland. Iceland and Mid-Ocean Ridges 38.151-161.
8. Haukur Jóhannesson 1980. Jarðlagaskipan og þróun rekbelta á
Vesturlandi. Náttúrufræðingurinn 50. 13-31
9. Moorbath, S., Haraldur Sigurðsson & Goodwin, R. 1968. K-Ar ages
of the oldest exposed rocks in Iceland. Earth and Planetary Science
Letters 4.197-205.
10. Friðgeir Grímsson 2002. The Hreðavatn Member of the Hreða-
vatn-Stafholt Formation and its fossil flora. Óbirt kandídatsritgerð.
Háskólinn í Kaupmannahöfn. 219 bls.
11. Haukur Jóhannesson 1982. Yfirlit um jarðfræði Snæfellsness. Árbók
Ferðafélags íslands 1982. 151-174.
12. Army Map Service 1949. A.M.S. series C762. Sheet 5521 III. Hjarðar-
holt, Iceland, Mýrasýsla, 1:50,000. Washington, D.C.
13. Haukur Jóhannesson & Einar Gunnlaugsson 1979. Jarðhitakönnun
fyrir Bifröst í Norðurárdal, Mýrasýslu. Jarðfræðikort af utanverðum
Norðurárdal, Mýrasýslu. Orkustofnun Reykjavík. OS79027/JHD11.
14. Königer, S. & Stollhofen, H. 2001. Environmental and tectonic
controls on preservation potential of distal fallout ashes in flu-
vio-lacustrine settings: the Carboniferous-Permian Saar-Nahe
Basin, south-west Germany. í: Volcanoclastic sedimentation in
lacustrine settings. White, J.D.I & Riggs, N.R. (ritstj.). Bls. 263-284.
15. Reading, G.H. (ritstj.) 1996. Sedimentary Environments: Process,
Facies and Stratigraphy. Blackwell Science. 3 útgáfa. 688. bls.
UM HÖFUNDINN
■ Friðgeir Grímsson (f. 1976) lauk BS-prófi í jarðfræði frá
Háskóla íslands vorið 1999, kandídatsprófi frá Kaup-
mannahafnarháskóla sumarið 2002 og doktorsprófi frá
Háskóla íslands í febrúar 2007. Friðgeir hefur aðallega
fengist við rannsóknir á míósensetlögum og steingerðum
plöntum frá Vestfjörðum og Vesturlandi, einkum með
tilliti til þróunar flóru og loftslagsbreytinga.
PÓSTFANG HÖFUNDAR/AUTHORS’ ADDRESSES
Friðgeir Grímsson
fossil@hi.is
Raunvísindastofnun Háskólans
Öskju, Sturlugötu 7
IS-101 Reykjavík
33