Geology Reference
In-Depth Information
(Fig. 6b, d). The deposits preserved on the two
tablets were thin and had a rather massive or very
poor laminated character. The profiles of thickness
data obtained seasonally with the MEM did not
match any particular surface of the cross-sections.
Indeed, some of these lines crossed among them
indicating frequent erosion processes, although
only in one case (cool period) negative values of
accumulation were obtained with the MEM
(Table 3). As a result, these deposits were not con-
sidered suitable for seasonal isotopic studies.
Sedimentary record from tablets in
spray areas and caves
The very thin thickness of sediment made imposs-
ible recognize distinct sedimentation intervals on
cross-sections. These tablets were not used for
seasonal isotopic studies.
Conclusion on sedimentary records
In conclusion, sedimentation rates could be ident-
ified on cross-sections of tablets thanks to data
obtained from seasonal measurements with the
MEM, although with variable degree of accuracy
mostly depending on subenvironments and erosive
processes.
In some cases, the lack of accurate or fine coinci-
dence between thickness values from the MEM and
thickness of distinct intervals in cross-sections was
attributed to the fact that the profiles of five-point
rows (four straight lines) did not reflect the real,
complete morphology of each interval, and, hence,
to the lateral variations in thickness caused by the
geometrical features of each facies (e.g. smooth to
conspicuous domes; e.g. see Fig. 5b). In other
cases, however, the lack of coincidence (cf.
Figs 4b & 7b) was greater and a clear indicator of
erosion in some intervals. The MEM measurements
revealed that the pattern of alternating thin dark
and thick light intervals was not always present.
The reversals in thickness data can be associated
either with erosive processes or particular climate
conditions. In the case of the last cool period
Sedimentary record from tablets in stepped
waterfalls
The identification of the cool and warm intervals
was difficult in the cross-sections from tablets in
stepped waterfalls (Figs 4b & 7b, d), in part
because lamination was absent or very poor. The
generally irregular, mostly domed bed morphology
of this facies (Fig. 7a, c) favoured the profiles of
measurement lines and the intervals seen in cross-
sections not to be fully coincident. The identifi-
cation of the several periods was only possible
thanks to the thickness data taken seasonally with
the MEM, that revealed the existence of several
erosion phases (Table 3, Figs 4b & 7b, d). In
tablet 7, these erosion processes occurred almost
through all cool and warm periods, although some
record was preserved from each period. In contrast,
in tablet 8, erosion during the last cool period caused
the sediment record from October 2001 - March
2003 to be totally eliminated (Table 3, Fig. 7d).
Fig. 6. Views of tufa deposited on tablets 1 and 6 placed in slow flowing water areas from August 1999 - March 2003.
(a) and (c): overhead views with indication of the cross-section cuttings shown in (b) and (d). The colonization
phase at the base corresponds to August - November 1999. The sediment thickness of the cool and warm periods shown
in (b) and (d) is that obtained with the MEM at the corresponding rows of measurement. Notice the lack of
lamination and the loose appearance of the deposit.
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