Geology Reference
In-Depth Information
and upper parts of the stratigraphic record, and relatively
drier conditions in the central part of the stratigraphic
record, corresponding to intervals of 4,500
beach sand (composed of carbonate grains of aragonite, Mg-
calcite, and calcite) are agents for selective dissolution of
carbonate grains. With dissolution, there can be loss of
carbonate content from 40 % carbonate to 25 % carbonate,
or even 40 % carbonate to 5 % carbonate and, as such, with
loss of carbonate volume, the
-
3,000 years
1,500 years BP, and 1,500 years BP to pres-
ent, respectively;
4. the record for the two species of
Chara
show that
charophytes have been in the wetlands from their
inception;
5. the record for the two species of
Chara
is broadly similar
to each other, with peaks in numbers near the base and
near the top of the stratigraphic record, with relatively
lower numbers in the central part of the stratigraphic
record;
6.
Chara
is most abundant at the times that
Baumea
is most
abundant, and near the top of the stratigraphic column,
when
Typha
is most abundant, signalling that they reflect
wet times in the basin.
These patterns partly re
BP, 3,000
-
oor of the wetland subsides
into the dissolving underlying sheet of beach sand (Seme-
niuk
2007
). Asymmetry in the hydrochemical plumes, due to
groundwater
fl
ow from east to west results in asymmetric
dissolution and asymmetrical wetland basin subsidence
(Semeniuk
2007
).
The principle that is evident here is that the microscale
geological processes have a major effect on determining the
evolution (deepening) of the wetlands basins.
fl
3.5
Calcrete and Reconstruction of Climate
History
ect regional climate changes, and
also intra-basin hydrologic and hydrochemical evolution
(Semeniuk
2007
).
fl
Calcrete and tuart trees are closely linked. Tuarts are a
phreatophytic eucalypt tree (
Eucalyptus gomphocephala
DC) that, with extraction of water at the water table, causes
precipitation of
3.3
Sponge Record and Taphonomy
ne-grained calcite around their root mats.
This
(Semeniuk and
Meagher
1981
). Depending on climate setting and proximity
of the water table, tuarts develop different types of calcrete,
viz., in wetter climates where there are abundant tuarts, the
calcrete is a sheet parallel to the water table; in drier climates
with less abundant tuarts forming isolated copses, the cal-
crete is a series of lenses parallel to the water table; in ter-
rains located high above a water table, calcrete is in the form
of rhizocretions (Semeniuk
1986
). The use of calcrete as a
climate indicator allows reconstruction of climate history
and shows the climate in southwestern Australia became
humid only some 2,000 years ago (Semeniuk
1986
).
In the Becher Point area, calcrete occurs in isolated lenses
along the water table. Its occurrence as a small-scale geo-
logical feature is an indicator of the former occurrence of
tuarts, and the fact that the calcrete is thin scattered in
occurrence and lensoid is an indicator of past climate.
ne grained calcite is termed
'
calcrete
'
Spicules of the sponge
Heterorotula muliformis
(Weltner)
occur in the stratigraphic pro
le of wetland basin 161. In
contact with carbonate mud and pellicular water that is
alkaline (Semeniuk
2007
), the biogenic silica of the sponge
spicules is in various stages of dissolution. Over a length of
*
100 cm of core extracted from wetland basin 161, which
represents the thickness of carbonate mud
lling the wetland
basin and representing
4,500 years of stratigraphic record,
sponges spicules occur in the upper third of the stratigraphic
pro
*
le, they are corroded (etched) over the central parts of
the pro
le. Dis-
solution clearly has taken place, and the older parts of the
pro
le, and absent in the lower part of the pro
le, with etched and corroded spicules show that the
longer the spicules have been in contact with alkaline
groundwaters the more they are corroded, until eventually
they have been dissolved away.
The history of the sponge spicules shows that while
sponge remains were originally present in the wetland, with
burial and diagenesis they have become etched and selec-
tively dissolved away, essentially removing a biotic com-
ponent from the biostratigraphic record.
4
The Becher Point Area as a Geosite
and a Geopark—and Its Microscale
Features
Often geoparks are selected for their splendour or scenic
grandeur, which attracts tourism, with macroscale geological
features that can be described as to their age or physical
characteristics. In this context, the Becher Point area pro-
vides a plethora of inter-related geological features that can
be used in a geopark. These features include the landscape
evolved from coastal processes,
3.4
Carbonate Grain Dissolution
The base and margins of wetland basins in the Point Becher
area may be weakly acidic. There are hydrochemical plumes
discharging from the
oors (Semeniuk
2007
). These hyd-
rochemical plumes coming into contact with the underlying
fl
the development of the
