Channel development in the chalk of the Tor Formation, North Sea: evidence of bottom current activity - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 570

and winnowing of an unconsolidated to partly
lithified chalk by a bottom current. This deposit
might represent an initial erosive phase of the
channel development and might be similar in
character to the chalk overlying the base of
channels in the Etretat cliffs, France, where
conglomeratic, packstone and hardground chalk
facies pass abruptly upwards into chalk wacke-
stone and marly chalk (Quine & Bosence, 1991).
The thick overlying succession of facies BHC/LC
in the lower channel unit reflects vertical aggrada-
tion of pelagic chalk within the channel (Table 2).
The decimetre-scale to metre-scale variations in
density and porosity values observed in the log
curves can originate from various mechanisms
(see Niebuhr & Prokoph, 1997; Scholle et al ., 1998;
Bramwell et al ., 1999; Stage 1999, 2001; Niebuhr
et al ., 2001; Damholt & Surlyk, 2004 for detailed
discussions). However, in the absence of core
data it is impossible to determine the mechanism
that caused the cyclicity or indeed to identify
the lithological changes that are occurring. The
uppermost interval of the lower channel-fill unit
is characterised in core by bioturbated argillaceous
chalks (facies BAC, Table 2), which suggests sea
floor colonisation by infauna and probably
represents a period of sediment starvation with
relative enrichment of the terrigenous vs. the
carbonatic fraction (cf. Bramwell et al . 1999) in
the channel preceding the deposition of the upper
channel-fill unit.

(A)

(B) (C)

Sh

Ch

C

C

Th

C

(D)

C

(E)

C

(G)

(F)

C

C

Sh

(H)

(I)

(J)

Facies association of the upper channel-fill unit

Description: The upper channel-fill unit sharply
overlies the lower channel-fill unit and is domi-
nated by the pebble floatstone chalk of facies PFC
(Table 2, Fig. 12C). The chalk here is generally
more porous than any of the facies in the lower
channel unit and also has lower clay content, as
indicated by the gamma ray log (Table 1). Facies
PFC forms ~ 80% of this unit and contains well-
rounded chalk clasts ranging from coarse sand to
pebble in size (Fig. 13C). Shell fragments and
foraminifera tests occur throughout this strati-
graphic interval, but bioturbation is rare (Fig. 13A).
Bedding is sometimes recognisable due to inclined
parallel to crenulated primary laminations
(Figs 13B, 13C, 13D and 13E) rarely disturbed by
bioturbations. However most of samples appear
with a homogeneous matrix (Figs 12G and 13 F).
Generally, the PFC facies (Table 2) are divided
into metre-thick beds showing coarse-tail grading,

Fig. 13. Photographs of allochthonous chalk facies in cores
4 and 5 from well 2/4-12 (Figs 6 and 8). (A) Facies PFC,
pebble floatstone chalk with deformed Thalassinoides and
small dark Chondrites burrows. (B − D) Facies PFC, pebble
floatstone chalk with crenulated lamination. (E) Facies
PFC, pebble floatstone chalk with high-angle lamination.
(F) Facies PFC, pebble floatstone chalk with isolated pebbles
in fine-grained matrix. (G) Facies DC, deformed chalk with
sub-vertical lamination. (H) Facies HC, homogeneous chalk
with stylolites and fractures. (I) Facies DC, deformed chalk
with high-angle crenulated lamination. (J) Facies DC,
deformed chalk with water-escape structures. Letter
symbols: C = chalk clast; Sh = shell fragment; Ch = Chondrites
burrow; Th = Thalassinoides burrow. Well location in Figs 1
and 4. For abbreviations see Table 2.