Jökull - 01.12.1957, Blaðsíða 12

TABLE (II)
HYPOTHESIS (A)
Location Observed Gradient °C/km Gradient corrected for climatic changes and sedimentation °C/km Assumed pre-Quaternary Erosion km Assumed erosion during the stages km Reduced
I II 8c III IV Gradient °C/km Heat flow microcal/ cm2 sec
í 93 108 1.5 0.15 0.075 0.075 88 3.1
2 161 164 1.5 0.450 0.225 0.225 109 4.4
3 110 113 1.5 0.30 0.15 0.15 82 3.3
Mean 3.6
TABLE (III)
HYPOTHESIS (B)
Loca- tion Assumed erosion during the stages km Reduced
I II & III IV Gradient °C/km Heat flow microcal/ cm2 sec
í 0 0.25 0.25 76 2.7
2 0 0.60 0.60 86 3.4
3 0 0.40 0.40 69 2.8
Mean 3.0
of 2.6 to 3.6 microcal/cm2 sec ranges fronr the
double to the triple of the global average
which is generally assumed to be 1.2 (Birch, 9).
RELEVANT DATA
Before entering a discussion of the above
results a number of relevant data will be
listcd.
Subsurface temperatures in Iceland as in-
dicated by lignite horizons. Walker (14) has
studied a 43 km long inclined block of Terti-
ary basalts in the Reydarfjördur area in east-
ern Iceland. He concludes that the total thick-
ness of the visible basalts may be of the order
of 8 km. Einarsson (3) reports that there is a
thin horizon of lignites at the bottom of this
section. The flora of the lignites has been
dated as Eocene.
Furthermore, Einarsson (3) finds a similar
horizon of lower Tertiary lignites at the
TABLE (IV)
HYPOTHESIS (C)
Loca- tion Assumed erosion during the stages km Reduced
I II & III IV Gradient °C/km Heat flow microcal/ cm2 sec
i 0 0 0.5 71 2.5
2 0 0 1.2 74 2.9
3 0 0 0.8 62 2.5
1 Mean| 1 2.6
bottom of a 5 km thick section of Tertiary
basalts in western Iceland.
There is no doubt that the lignites have
been buried at the depth of 8 and 5 km re-
spectively. In fact, these figures may be
minimum figures as it is not known how much
of the Tertiary basalts has been totally eroded
away.
Lignites are generally recognized as the lowest
rank of the coal series, that is, representing
the lowest degree of alteration. Time, tempera-
ture, pressure and chemical environment are re-
garded as the main factors affecting the geo-
chemical changes of organic sediments. Un-
fortunately, there are no reliable quantitative
data available on the importance of the various
factors.
At this juncture we are mainly interested in
the possibility that the presence of the lignites
may be indicative of a maximum temperature
at the depth of burial. Twenhofel (15) reports
10