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 92

basins in northern Pangaea during the Late Triassic-
Early Jurassic. The rhythmites can equally well
be explained by cyclic changes in climate, like the
allostratigraphic valley fill successions described
from the Pleistocene (Blum, 1993), without invok-
ing any change in base level.

Milankovitch-scale changes in temperature (Ruhl
et al ., 2010; Bonis et al ., 2010), may thus also have
influenced runoff during the Rhaetian-Hettangian
in the northern North Sea region, modified by
local catchment morphology. However, studies of
modern runoff in Europe have shown that low-
frequency variance (long-term variations) in runoff
on a continental scale appears to be positively
correlated with mean climatic conditions (temper-
ature and precipitation), more than with catchment
properties (size, elevation and geomorphology)
(Gudmundsson et al ., 2011). This may also have
been the case in North-west-Europe during the Late
Triassic-Early Jurassic climatic change.
A climatic depositional model for the Lunde
Formation and lower part of the Statfjord Group
in the northern North Sea region is summarised in
Fig. 17. A depositional model of the upper part of
the Statfjord Group (S3 to S1) is comparable with
that of L12 to L08, but in a humid climate, with
monsoonal precipitation and high-energy flood-
ing in mobile braided streams instead of in an arid
or semi-arid climate characterised by ephemeral
floods.

Cause and wider regional aspects of the
Triassic-Jurassic climate change

The Hettangian-Sinemurian climatic change from
arid/semi-arid to humid conditions has been
recorded further south in the North Sea (Frostick
et al ., 1992; Goldsmith et al ., 2003), in the Mid-
Norwegian shelf (Müller et al ., 2005), in East
Greenland (Clemmensen et al ., 1998) and in south-
ern Scandinavia (Arndorff, 1993; Ahlberg et al .,
2002). McKie & Williams (2009) suggested that
arid basins in the northern North Sea region dur-
ing Late Triassic were supplied by sediments from
Fennoscandia and Greenland by winter runoff and
from the Variscan Mountains in the south by sum-
mer monsoon flooding. The Shetland Platform
area was also a hinterland furnishing the Tampen
Spur area with clastic material, in addition to a
hinterland to the north-west of the northern North
sea area, as indicated by samarium-neodymium
isotope studies (Mearns et al ., 1989). Bedrock geol-
ogy, relief, drainage morphology and local climatic
conditions in the hinterland areas were of crucial
significance for sediment production and sedi-
ment transport to the alluvial basin.
In Late Triassic times the central northern North
Sea region was located about 33 o N to 35 o N; and in
Early Jurassic about 42 o N to 43 o N (Torsvik et al .,
2002; Goldsmith et al . 2003). During this north-
ward drift the North Sea region moved out of the
high pressure belts and into temperate regions
with humid climate. The change in climate might
also have been influenced by the Rhaetian marine
transgression from the Tethys (Ahlberg et al .,
2002). Superimposed on this regional climatic
change, induced by plate tectonic movements,
there appears to have been a global temperature
increase at the Triassic-Jurassic boundary, due to
the release of large volumes of CO 2 to the atmos-
phere brought about by the igneous activity in the
Central Atlantic Magmatic Province (Hesselbo
et al ., 2002; Ruhl & Kürschner; 2011 and refer-
ences therein). Cycles of temperature change and
associated monsoonal precipitation, triggered by
variation in the CO 2 content of the atmosphere
(Hesselbo et al ., 2002; Bonis et al ., 2012) and

Impact on alluvial architecture and
consequences for hydrocarbon
exploration

The change in fluvial style in response to climatic
changes impacted the sand : mud ratio and the
alluvial architecture and palaeosol development
in the Lunde-Statfjord succession. Variation in
channel belt width/thickness and avulsion fre-
quency on the geometric properties of fluvial
sandstone bodies has been modelled in several
studies (Leeder, 1978; Bridge & Leeder, 1979;
Bridge & Mackey, 1993; Mackey & Bridge, 1995)
and also tested in outcrops (Zaleha, 1997; Bridge
et al ., 2000; Kjemperud et al . 2008). In these stud-
ies, variation in channel belt proportion (CDP)
has been primarily related to different avulsion
frequencies and channel belt dimensions (width
and thickness). The decrease in CDP upward
through the Lunde Formation and the Statfjord
Group (Fig. 5) may be ascribed to reduced avul-
sion frequencies caused by an overall change
from ephemeral, braided rivers to more peren-
nial, meandering rivers. The return to braided
streams during deposition of the upper part of the
Statfjord Group is here considered to have been
forced by high periodic influx of coarse-clastic
debris and high runoff caused by increased