Jökull - 01.12.1974, Blaðsíða 58

Fng. 10. Location map ancl section of rocks correlating in time to the lowest part of the Gauss
epoch below the Mammoth event. This section borders the geological map Pl. 1 immediately on
the west. Numbers on the inset map refer to flows in the section.
Mynd 10. Jarðlagasnið af Rauðsgili. Sniðið er tekið í gegnurn jarðlagastafla, sem liggur undir
Giljafoss-jökulberginu. Þetta jökulbergslag kemur fram efst i sniðinu ofan á 50 rétt segulmögnuð-
um hraunlögum, sem eru ialin heyra til fyrsta hluta Gauss-segultimabilsins.
fluvial conglomerates and hyaloclastites. On ap-
proacliing the Mammotli event such layers be-
come increasingly important in the section (Fig.
10). We identify the lowest tillite just above the
base of the Mammoth event with the onset o£
glaciations in the region of western Iceland.
Altogether the Húsafell section contains evi-
dence of 11 glaciations. Only that part of it,
however, which represents an unbroken suces-
sion allows an estimate of the frequency of
glaciations. This is the section beginning with
the Mammoth event of the Gauss and ending
witli the tliird acid phase in early Matuyama
times. Here we find a total of 8 glacial hori-
zons in a period of about 0.7 m. years. The first
three glacial events appear to have been separ-
ated in time by about 120,000 years, whereas
the last five occurred every 90,000 years. The
56 JÖKULL 24. ÁR
presence of extensive hyaloclasdtes among only
two glacial horizons suggests that most were of
hort duration, perhaps no more than the aver-
age time interval between two successive flows,
i. e. they were of the order af only 5,000—
10,000 years.
4.4 UNCONFORMITIES AND THE SUB-
SEQUENT MORPHOLOGIC HISTORY
The Iv/Ar datings indicate that the liyalo-
clastites and basalt lavas of Bæjarfell and Kaldi-
dalur were formed during the Gilsá event. The
hiatus separating them from the third phase
lavas thus involves some 500,000 to 600,000
years. It is possible to outline the succession o£
events during this period with some confidence.
The clue to this is the evidence yielded by tlie
secondary mineral zoning of the lava pile. It is