Tectonic influence on the Jurassic sedimentary architecture in the northern North Sea with focus on the Brent Group - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 395

Table 1. Facies associations of the Rannoch, Etive, Ness and Tarbert formations within the study area: description and interpretation of the facies associations.

Formation Description Interpretation Comments

Tarbert Fm Coarsening upward units (2 to 7 m) from mudstones to fine-

grained sandstones with micro-hummocks, wave and current
ripples, planar lamination, trough cross-bedding, lenticular-
bedding and flaser -bedding capped by palaeosols. Bioturbation,
syneresis cracks and coal clasts occur.

These units are probably bay-head
deltas building into bays or lagoons.

See Figs 9A and 12B,C.

Fining-upward units from medium to very-fine grained sandstones
with cross-bedding, sharp/erosive bases, current ripples and
coal-clasts.

Crevasse splays. See Figs 9B and 12D.

Well-sorted and planar laminated units of fine-grained sandstones
generally with bioturbation, wave and current ripples.

Back barrier deposits and
washover fans.

See Figs 9A, 12C.

Up to 20 m thick units of medium-grained to coarse-grained
sandstones with erosive lower boundary and cross-bedding with
even-spaced mud or organic drapes.

Channel complex with fluvial and tidal
interaction in an estuarine setting.

See Fig. 12B. Also recorded
by Davies et al ., (2000) in
block 34/7.

Medium to very fine-grained sandstones constitute the upper part
of the Tarbert Formation with trough cross-stratification, high
and low-angle lamination, swaley cross-bedding, wave and
current ripples (Fig. 3A).

Beach barrier complex with shoreface
at the top.

See Figs 9A and 12A,D.

Ness Fm Interbedded coal-layers and mudstones (Fig. 4D) with minor

amounts of thin silt and very fine-grained sandstones with
root-horizons.

Delta plain with overbank deposits and
stagnant swamp forming coal-beds.

See Figs 9A,B,C and 12B,C.

Coarse to fine grained sandstones, 1 to 5 m thick fining upward
units with erosive or sharp-based, cross-bedding and current
ripples.

Fluvial channels. See Figs 9A,C and 12C.

1 to 8 m thick siltstones coarsening-upward to fine-grained
sandstones with wave ripples, syneresis cracks, mud-drapes,
micro-hummocks and laminated beds capped by palaesols.

Bay head deltas. See Figs 9A and 12B.

Tens of metres (~20 to 40 m) of medium to fine-grained (bimodal)
cross-bedded sandstones, mud-drapes (Fig. 4J) and sporadically
bioturbation.

Stacked tidal dune deposits in an
estuarine setting.

See Figs 9A, C. See also Mjøs (2009)
who has similar interpretation of
the depositional environment.

Etive Fm Moderate sorted medium sandstones (coarse to fine) with massive,

cross-bedded and planar-laminated units with some coal-clasts
and mud-clasts. Sharp or erosive bases, stacked in a general
fining-upward trend. Palaeosols are common at the top of these
sandstones.

Mouthbars to wave-reworked
mouthbars.

See Figs 9A,C and 12D.

Rannoch
Fm

Very fine-grained to fine-grained and well-sorted sandstones with
horizontal, low-angle lamination, hummocky and swaley cross
bedding, wave ripples. Some muddrapes and stylolites.

Shoreface environment in a delta front
setting.

See Figs 9A, C and 12D.

Alternating mudstones, siltstones and very-fine grained
sandstones with lenticular and flaser bedding, horizontal
lamination, micro-hummocks, wave and current ripples,
muddrapes, syneresis-cracks, coal-fragments and clasts.
Abundant Botryococcus (Batten & Grenfell, 1996).

Pro-delta deposits. The occurrence of
abundant Botryococcus , syneresis
cracks, current ripples and coal-
material indicates strong fresh water
influx. The bioturbation, wave-ripples
and micro-hummocks indicates

Fig. 12A, (well 35/4-1) show this
facies as a 90 m thick aggrading
unit. Palynological data indicate
that the lower part was deposited
under freshwater condition that
became progressively more
brackish in the upper part.