a) Profiles computed for the north cliff
Polynomial regression curves have been computed to
illustrate the variation in element concentrations in the 100 m
thick sequence of mudstones, shales and cherts which is exposed in
the north cliff at Hartfell. The analysis of variance for the
computed curves is given in Table 3-7. Out of a total of 17
elements considered, only Cu, Mn, Mg and Sr yield profiles which
are significant at the 99 per cent confidence level. The profiles
for Al, K and Pb are significant at the 95 per cent level.
All those profiles which are significant at the 95 per
cent level have been superimposed upon the scatter plots (Figs
59a - 76a). The curves for K and Pb are similar in that both
display trends which increase for the first 15 m from the base of
the cliff and then decrease for the next 10 m. Values then
increase once more and both elements have their greatest
concentrations in the P,linearis Zone, 87 m from the base of the
cliff.
The profiles for Al and Cu are essentially the same as
for K and Pb for the first 85 m up the cliff. Thereafter, values
increase more rapidly. This observation is also true for the
trends computed for Mn and Mg. These, although not varying much
for the first 85 m, begin to change rapidly around the 85 m mark.
The profile for Sr also exhibits inflexions at around 15 m and
- 51
b) Restoration profiles
The true stratigraphic position of the samples before
faulting and folding have been calculated (Chapter I, 3d) and the
results are given in Table 1-2. Polynomial regression curves were
then computed to illustrate the variation of element concentration
through the structurally restored succession. The analysis of
variance for the restored curves is given in Table 3-8.
Out of a total of 17 elements considered, only Al, Cu and
C yield restoration profiles which are significant at the 99 per
cent confidence level. The profiles for Fe and Na are significant
at the 95 per cent level. The six profiles which are significant
at the 95 per cent level have been superimposed upon scatter plots
of the data (Figs 6'lb, 62b, 65b, 68b, 69b, 76b). These profiles
exhibit the same trends, although with greater detail, as the
lower parts of the profiles which have been computed for the total
suite of the samples (cf. Figs 49, 51, 52, 53 and 57).
The significance levels of the vertical profiles for the
sequence as exposed, and for the sequence of samples after
structural restoration are compared in Table 3-9. It is found
that the profiles for Mn, Mg, Sr, K and Pb are more significant
when computed in the in-situ position, whereas structural
restoration improves the fit of the profiles computed for Al, Fe,
Na and C. Although both profiles for Ca lie outside the chosen
confidence limits it is noteworthy that the F-value is greater for
the in-situ profile. This may mean that the in-situ profile of
Ca is more significant than the restoration profile. These results
stratigraphic variations whereas the profiles for Mn, Mg, Ca, Sr,
K and Pb represent variations attributable to effects associated
with faulting.
c) Interpretation
The element assemblage involved with the effects associated
with faulting indicates that a carbonate phase may have been
introduced into the sediments at Hartfell. This phase was not
detected in the mineralogical examination of the samples but as the
mean values of Ca, Mn and Mg at this locality are only 0.02, 0.01
and 0.48 per cent respectively, only small quantities of a carbonate
mineral could possibly be present. Two possibilities can be
considered for the introduction of a carbonate into the sediments:
firstly, metasomatism related to an igneous source, and secondly,
diagenesis related to carbonate-bearing groundwaters. In the first
connexion, Weir (1974) has related the presence of carbonate
minerals in the Silurian rocks of Gatehouse to granitic emanations.
In the Gatehouse area there is ample evidence for surface and sub
surface igneous activity. However, there is little evidence for
such activity in the vicinity of Hartfell. The nearest proven
igneous body lies below the surface at Leadhills about 25 km to the
north-west of Hartfell. A recent geophysical survey has outlined
the presence of a strong magnetic anomaly in the vicinity of Moffat
(Powell, pers comm.). This anomaly follows the Caledonoid trend
and may indicate the presence of a concealed igneous body.
Carbonate metasomatism may therefore be related to this body.
53
replacement is a widespread feature of the Lower Palaeozoic
greywackes (Weir 1974). Such replacement has been observed in
inliers in S.W.Ireland which are well removed from any exposed centre
of igneous activity (Weir 1962). In these instances the outcrops
occur in Carboniferous Limestone terrain and it is likely that the
sediments have been exposed for a considerable period of time to the
circulation of carbonate-rich groundwaters. Calcite is observed to
replace quartz at outcrop in sandstones exposed to semi-desert
conditions (Dapples 1967). As the Hartfell Shale sequence is
predominantly quartz rich it is possible that calcite replacement
could be related to circulating carbonate-saturated groundwater
following the Carboniferous episode of limestone deposition. However,
in contrast to Ireland, carbonate rocks are not prevalent components
of the Scottish Carboniferous. Moreover, deposition of Carboniferous
rocks in the Southern Uplands was almost certainly confined
largely to the basins in which they now occur.
On balance therefore, it would appear that the small
amount of carbonate introduced into the sequence at Hartfell is
metasomatic in origin.
Other than the visual similarity between the profiles for
K and Pb, there is no evidence to support an inter-relationship
between these two elements. Indeed, as will be shown later, K is
considered to be associated with the clay factor and Pb with the
metal factor. Lead is known to be a common substitute for potassium,
especially in igneous rocks (Fairbridge 1972). This substitution is
most frequently observed in potassium feldspars and micas both of