Late Triassic to Early Jurassic climatic change, northern North Sea region: impact on alluvial architecture, palaeosols and clay mineralogy - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 83

to varying degrees in the mudstone beds. Sand-
filled desiccation cracks are rare and only recorded
in the lowermost part of the USG. Carbonate nod-
ules, rhizocretions and soil peds are absent.
The palaeosols of PA5 can be characterised as
inceptisols, entisols and low-chroma vertisols all
of which form pedocomplexes. Most pedocom-
plexes are compound and composite. The dark
greenish-grey palaeosols with high organic content
are inferred to contain iron in the reduced state
(Fe 2+ ), formed by gleying processes (Table 6). This
palaeosol type occurs only in S2 and S1.

also low. Goethite and hematite have not been
detected in these samples.

Clay mineral assemblage 1 (CMA1)

The general distribution pattern of clay minerals
in the Alke Formation continues through the
mudrock intervals in the allostratigraphic units
L12 and L11 in the lowermost part of the Lunde
Formation (LLF). This is clay mineral assemblage
CMA1 (Fig. 14). There appears to be a peak in the
content of swelling mixed-layer (ML) minerals
within L12. The combined content of illite and
chlorite, though fluctuating, is higher than the
kaolinite content. Illite is prominent in CMA1
with an average of 41% of the total clay mineral
content, whereas kaolinite occupies on average
26% of all clay minerals. Hematite is the domi-
nant iron oxide in this clay mineral assemblage
(Table 4, Fig. 14).

Clay mineralogy and iron oxides

The mineralogy of mudrocks changes through the
Lunde Formation and the Statfjord Group. This is
reflected in both the non-clay and clay mineral
contents (Tables 2 and 3, Fig. 14). The quartz and
feldspar content of the mudrocks (Table 3) reflects
a similar stratigraphic trend to that in the fluvial
channel sandstones (Table 2), with a moderate
content of quartz in the Lunde Formation and a
high content in the Statfjord Group, whereas the
opposite trend is shown by the feldspars. As in
the sandstones (Sørlie, 1996; Khanna et al ., 1997),
albitic plagioclase feldspar dominates relative
to potassium feldspar. No significant trend with
burial depth has been shown for the mudrock
minerals, whereas the stratigraphic trends are dis-
tinct (see below); for this reason it is concluded
that the recorded variation in the mineral content
of mudstones is primarily due to factors related to
sediment source and depositional environment
and not to burial diagenesis. The Lunde-Statfjord
succession can be subdivided into five 'clay min-
eral assemblages' (CMA1-5, Fig. 3), according
to the total amounts and relative occurrence
of the minerals smectite, swelling mixed-layer
clay minerals (ML), chlorite, illite and kaolinite
(Table 4). Units L04 and S2 were not sampled as
the mudstones of these units were anticipated to
be very similar to those in L05 and S1, respec-
tively. In addition, the relative amounts of the iron
oxides goethite and hematite contribute to the
characteristics of the clay mineral assemblages
(Table 4, Fig. 14).
Samples from the upper part of the Alke
Formation contain a clay mineral assemblage
comprised of illite and kaolinite in practically
equal amounts and a slight lesser amount of
chlorite. Smectite is present only in negligible
amounts and the swelling ML mineral content is

Clay mineral assemblage 2 (CMA2)

CMA2 occurs within the allostratigraphic units
L10 to L04, from the lower to the upper part of the
Lunde Formation. The boundary between L11 and
L10 shows a marked change in the relative pro-
portion of the clay minerals. Most noticeable is
the abrupt increase in smectite content, from prac-
tically zero in L12 and L11 to about 90% of all
clay minerals in the lower part of L10 (Table 4,
Fig. 14). Upwards from this maximum the smec-
tite fluctuates, but reveals a distinct decreasing
trend towards zero at the top of the assemblage,
uppermost in unit L05. The relative content of
swelling ML minerals is low in units L10 to L05
and does not reveal any trend through CMA2.
The relative amount of chlorite varies greatly
through CMA2; there may be a slight upward-
decreasing trend. The change in the smectite
content at the lower boundary of CMA2 is accom-
panied by a drop in the illite content in the lower
part of L10, succeeded by a general increase
upwards to a maximum of about 80% of the clays
within unit L05. The kaolinite content is gener-
ally low through the whole L10 to L05 interval,
showing no specific trend through CMA2. Hematite
is the totally dominant iron oxide in L05, showing
a vertical increase to approximately 40% of clays
and iron oxides in the uppermost part of L05,
though followed by a distinct decrease in the upper-
most part of CMA2, at the transition to CMA3
(Fig. 14).