Geology Reference
In-Depth Information
Fig. 7. Views of tufa deposited on tablets 7 and 8 placed in stepped waterfalls from August 1999 to 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 lines of measurement. Several erosion phases can be seen
(e.g. the cool period of 2000 - 2001).
(2002 - 2003) with thicker-than-normal sedimen-
tation, the slightly higher mean temperature during
the first part of the cool period favoured the high
sedimentation rates recorded in all cases devoid of
erosion. In addition, this explains the lack of a
sharp textural change with respect to the preceeding
warm period sediment.
The most suitable sedimentary record from
tablets for the seasonal study of textural features
and stable isotope composition was that of dense,
laminated deposits of fast flowing water areas
(facies 1), and in some cases, of spongy, crudely
laminated,
that caused periodic changes in sedimentation
characteristics (i.e. sedimentation rates) are close
to six-month periods. Variations can be expected
according to climate and hydrological conditions
of some months at the beginning or end of the six-
monthly periods. For instance, when warmer-than-
normal conditions extend through most of autumn.
Tufa facies
The structural and textural characteristics of the
sediments deposited in the several subenvironments
allowed five main types of fluvial facies to be distin-
guished, based on field observations and the sedi-
mentary record on tablets. The difficulty to
identify seasonal (close to six-month) intervals in
tablet cross-sections from slow flowing, spray and
cave areas led the seasonal study of texture to be
mostly focused on the facies formed in two distinct
subenvironments: fast flowing water areas on the
river bed; and stepped waterfalls (see section of
Sedimentary subenvironments of the fluvial system).
(1) Dense laminated tufa (Figs 5 & 8), that is,
stromatolitic tufa, formed in fast flowing water
areas
mainly
moss
and
alga
deposits
of
stepped waterfalls (facies 3).
On the other hand, it is important to note that
despite the duration of the warm and cool periods
chosen for monitoring may not exactly coincide
with the duration of the natural processes/cycles
that caused periodic variations in sedimentation
rates, in most cases the thickness values of six-
monthly measurements with the MEM and of the
intervals recognized on tablet sections are very
close. So are measurements of warm intervals of
seven months and of cool intervals of five months,
as monitored during the first year of this study. It
must then be concluded that the cycles of processes
(subenvironment
a)
without
growth
of
Search WWH ::
Custom Search