Geoscience Reference
In-Depth Information
Fig. 7. Cartoon reconstruction of the Britta Dal Formation palaeoenvironment. The system is reconstructed as a
large-scale ephemeral fluvial system flowing northwards. The braid system is only partially active during major floods.
On Gauss Halvø the braids have become widely separated and the system is dominated by a silt-rich floodplain with
extensive development of vertisols.
dispersed both laterally and vertically within
the section. They are also laterally equivalent to
the green-coloured vertisols. Further to the north
there is no reported Britta Dal Formation, but the
facies should be represented by shallow perennial-
to-ephemeral lake sediments.
require greater annual precipitation as both the
source for the Ca
2þ
and to move it through
the soil so that it becomes concentrated within the
nodular layer.
The East Greenland Devonian Basin is at an
estimated palaeolatitude of c.158S within the
southern hemisphere arid zone. The mechanism by
which annual precipitation can increase within this
system is by a strengthening of the seasonal
monsoon (e.g. Olsen 1990, 1993; Marshall et al.
2007). The seasonal monsoon is driven by the
summer insolation maximum which, if strong
enough, can draw in moist oceanic air that then
produces intense rainfall. The simplest mechanism
to increase summer insolation is to have a greater
seasonal contrast with hotter winters and colder
summers.
Spore assemblages occur just beneath the base of
the Stensi¨ Bjerg Formation and their appearance is
coincident with the ending of the sustained aridity
that characterizes the Britta Dal Formation. The
spore assemblage is characterized by locally abun-
dant Retispora lepidophyta (sometimes in excess
of 60%) which are clearly latest Famennian in age
(e.g. Streel et al. 1987; Maziane et al. 2002).
Stensi ¨ Bjerg Formation
The Stensi¨ Bjerg Formation is defined by an end to
the thick monotonous sequence of vertisols. Instead
there are now palaeosols (aridisols) with calcrete
nodules and more rarely individual calcrete beds.
There are also numerous fluvial sandstone beds
that represent wetter intervals. Also present are
dark-coloured mudstones rich in organic matter
that were deposited in stratified permanent lakes
when the system became flooded sufficiently to
establish deep perennial lakes.
The prevailing climate of the Stensi¨ Bjerg For-
mation was therefore more variable including
periods varying from aridity through to humidity,
in marked contrast to the monotonous vertisols of
the Britta Dal Formation. It is now been shown
(Royer 1999) that it is not possible to make simple
quantitative interpretations of palaeoprecipitation
directly from palaeosols. However, it is most
likely that the Britta Dal vertisols represent more
sustained aridity with an average annual palaeopre-
cipitation of ,100 mm. The calcretes in the Stensi¨
Bjerg Formation, although still an indicator of
aridity (,760 mm annual rainfall; Royer 1999),
Obrutschew Bjerg Formation
The thickest of the black mudstone and calcrete
pairs defines the overlying Obrutschrew Bjerg For-
mation where 4-6 m of laminated organic-rich
sediments with limestones was deposited.
It
