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 77

in the sedimentological model of the Snorre
Field (Diesen et al ., 1995), has been confirmed by
the production history since 1992. The mudstone
beds that occur between the fluvial channel
sandstone bodies are attributed to mud settling
on floodplains and in shallow lakes (Nystuen &
Fält, 1995; Ryseth, 2001). Many mudstone beds
have been affected by pedogenic alteration to a
greater or lesser extent (see below).

sandstones formed as channel levées and crevasse
splays. The palaeosol assemblages are defined on
their content of mudrock facies, types of pedo-
genic structures, content and types of calcrete and
colour and thickness of palaeosol profile. Five
mudrock facies (MF1 to 5), including palaeosol
types, are defined on the basis of relative propor-
tion of clay, silt and sand, colour and mottling,
degree of preserved primary lamination, occur-
rence of reworked mudrocks, pedogenic mud
aggregates (peds), pedogenic slickensided surfaces
and desiccation cracks (Table 6). Further informa-
tion on palaeosol types is given by Müller et al .
(2004). Most attention is given to the 'redbed facies'
in the Lunde Formation and lower part of the
Statfjord Group (allostratigraphic units S5 to 4).

Palaeosols and mudrock facies

Five palaeosol assemblages (PA1 to 5) have been
defined (Fig. 3). Most of the palaeosols are devel-
oped in floodplain mudstones and only to a minor
extent in mudrocks and associated fine-grained

Table 6. Mudrock facies and palaeosols recorded in the Lunde Formation and lower part of the Statfjord Group
(Raude Formation).

Mudrock facies Mudrock characteristics

MF1
Laminated mudrock

Variable content of clay, silt, sand and mud aggregates. Well preserved
plane-parallel lamination. In LF and LSG: Red-brown colour, small
isolated rhizocretions, up to 10 cm-deep desiccation sand-filled cracks.
In USG: Greenish-grey, some few coalified root structures and some
gleyed palaeosols. Interpretation: Deposited in small temporal ponds
or shallow lakes on floodplain. Inceptisols.

MF2
Unstratified and structureless

Clay/sand ratio relative low and lower than in MF3 and MF4. In LF and
LSG: brownish red colour, discrete and isolated carbonate nodules,
isolated rhizocretions, burrows, 10 to 60 cm-deep sand-filled desiccation
cracks. No mud aggregates, peds or pedogenic slickensided surfaces.
In USG: Greenish-grey, some coalified root structures. Interpretation:
Mud altered by pedogenic processes to entisols/inceptisols.

MF3
Mudrock with slickensided fault surfaces

Clay/sand ratio higher than in MF2. In LF and LSG: Brownish-red
colour, frequent arcuate and smooth randomly orientated slickensided
surfaces, spheroidal and angular peds, ptygmatic folds, rhizocretions,
isolated carbonate nodules. In USG: Greenish-grey colour, some few
slickensided surfaces, coalified root structures. Interpretation: Mud
altered by pedogenic processes to high chroma vertisols in LF and LSG
and low chroma vertisols in USG.

MF4
Greyish-green and mottled mudrocks

Generally high content of clays, some mudrocks with high contents of
fine-grained to medium-grained sand. Greenish-grey and mottling
red-greenish grey colour. Slickensided horizons, spheroidal peds,
mud aggregates, 0.1 m to 1.5 m-deep, sand-filled desiccation cracks,
complex mottling patterns. Interpretation: Mud with colour formed
by reduction processes of iron oxides due to fluctuating groundwater
level and periods of waterlogging. Altered high and low chroma
vertisols.

MF5
Reworked mudrock

Texturally varied reworked pedogenic mud aggregates (0.1 mm to 5 mm)
and carbonate rip-up clasts (1 cm to 6 cm). Brownish-red colour, some
places mottled green-red. Interpretation: Reworked mud and calcrete
nodules deposited as infill of shallow channels and in crevasse splay
and sheet floods. Reworked palaeosol horizons.
Definition of palaeosols: Gleyed soil and palaeosol: Soil or palaeosol with bluish grey and greenish grey colour, accumulation of organic
matter common, root structures may be preserved in top of gleyed beds. Vertisol: Clay-rich soils with wide desiccation cracks and peds
formed by repeated seasonal dry-wet cycles, calcrete nodules common, no remains of primary sedimentary features. Inceptisol: Relicts of
primary sedimentary features may be present. May have calcrete nodules. Entisol: Slight degree of soil formation. Primary sedimentary
features little altered. (Modified from Retallack, 2001.)