Geoscience Reference
In-Depth Information
Clay mineral assemblage 3 (CMA3)
Clay mineral assemblage 5 (CMA5)
Clay mineral assemblage CMA3, comprising the
interval L03 to L01, is again marked by some char-
acteristic trend patterns. The content of smectite
varies a lot up through CMA3, from zero in some
mudstone units up to about 80% of the total clay
mineral content in some other floodplain mud-
stones. The overall smectite trend, though, is
upward-decreasing, like that in the underlying
CMA2 (Fig. 14). The content of swelling ML min-
erals shows generally an increasing trend from
close to zero in the lower part of unit L03 to a
maximum in the lower part of L01.
The chlorite content varies from zero to about
25% of clays + iron oxides (Fig. 14). The relative
content of illite decreases from about 80% of
clays + iron oxides in the upper part of L05 down
to about 15% in L03 and L02. A slight increase
may be present in L01 at the top of CMA3. The
relative content of kaolinite varies considerably,
with peaks in the upper parts of L03 and L02,
separated by a low in the lower part of L02. There
is a slight increase in goethite in part of CMA3,
compared to the underlying and overlying strata.
A goethite peak is recorded in a floodplain mud-
stone in the middle part of unit L02. The hematite
content also varies within clay mineral assem-
blage 3 (Fig. 14).
The uppermost clay mineral assemblage CMA5
comprises the upper part of the Statfjord Group
(USG), S2 and S1, also including FSA6, see above.
The grey and greenish-grey mudstone and palae-
osols of S1 are characterised by a low smectite
content. The kaolinite content is generally high,
varying between about 60% and 100% of all clay
minerals. Swelling ML minerals, chlorite and illite
are scarce or absent and goethite is the only iron
oxide recorded (Table 4, Fig. 14).
Vertical trends and regional correlation
The Lunde-Statfjord succession can be subdivided
into two major parts, each defined by characteristic
vertical grain-size trends. The lower part, compris-
ing the allostratigraphic units L10 to L01/S5, is
fining-upwards, whereas the upper part, consisting
of the allostratigraphic units S4 to S1, is coarsen-
ing-upwards. As mentioned above, the turnaround
from fining-upward to coarsening-upward, located
in the L01-S5 interval, was applied as the lithostrati-
graphic boundary between the Lunde Formation
and the Statfjord Group by Deegan and Scull (1977).
The Lunde-Statfjord succession reveals several
vertical trends in alluvial features, pedogenic
features and mudrock mineralogy (Fig. 15). The
fluvial sandstones in the Lunde Formation are
interpreted as ephemeral streams in LLF and MLF,
increasingly large meandering streams from the
middle part of MLF leading to dominance of this
stream type in ULF and then small, single storey
meandering or/and straight streams in the flood-
plain-dominated uppermost part of the ULF. In the
lower part of the Statfjord Group (Raude Formation)
the fluvial style of small floodplain streams con-
tinues to dominate, whereas the fluvial style of
the upper part of the Statfjord Group (Eiriksson
Formation) is characterised by laterally extensive
braided stream sandstone sheets, deduced to have
been more perennial than ephemeral.
The main change in palaeosol features is from
entisols/inceptisols and compound pedocom-
plexes in LLF and MLF to high chroma vertisols
and cumulative pedocomplexes in the upper part
of the Lunde Formation and lower part of the
Statfjord Group and finally to inceptisols, entisols
and low chroma vertisols, but decreased frequency
of cumulative pedocomplexes in the Eiriksson
Formation in the upper part of the Statfjord
Group. Horizons with coalesced carbonate nodules
Clay mineral assemblage 4 (CMA4)
Clay mineral assemblage CMA4 occurs in the
allostratigraphic units S5 to S3, corresponding to
LSG and the lower part of USG (Figs 3 and 14).
CMA4 reveals the most dramatic change in clay
mineralogy within the whole Lunde-Statfjord
succession (Table 4, Fig. 14). The change takes
place very abruptly from L01 to S5 and is
reflected by all the clay minerals. Smectite domi-
nates totally with a content that varies between
80% and 100% of all clays + iron oxides; how-
ever, the smectite content drops to zero at the top
of CMA4. The swelling ML minerals disappear
(except in one sample), chlorite is absent and the
relative content of illite drops well below 10%.
The content of kaolinite is generally low but
increases abruptly in the upper part of S3, here
accompanying an increasing frequency in chan-
nel sandstone bodies that coincides with the dra-
matic drop in the smectite content. Goethite and
hematite are both practically absent within clay
mineral assemblage 4.
Search WWH ::
Custom Search