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individual vertisol cycles although they are often
grouped together within the sequence (e.g. Fig. 2
at c. 800 m) and represent generally wetter intervals.
Lateral continuity of the sandstones is often inter-
rupted as individual channel complexes become
tinted green and fade out laterally (Fig. 4g). This
is the result of intense soil turbation mixing the
channel sandstones with the floodplain silts and
providing confirmation of the dominance of arid
climate processes.
A good analogue for the Britta Dal Formation is
Cooper Creek, Australia, where a very large multi-
channel ephemeral river system runs to internal
drainage (Rust 1981; Nanson et al. 1986; Fagan &
Nanson 2004; Kingsford 2006). Water flow in
Cooper Creek is irregular with rare (i.e. decadal)
but very intense flooding events when the water
volumes become sufficient to sustain temporary
lakes in the terminal fan area (Lake Eyre) for over
a year (i.e. at least through the following dry
season). During these flooding events, the local
fauna expands (Kingsford et al. 1999) to occupy
the system before becoming restricted to waterhole
refugia that remain in the deeper sections of the
channels. These waterholes (billabongs) remain as
independent ecosystems through the intervening
dry season, although most have dried out after two
seasons (Hamilton et al. 2005; Bunn et al. 2006a).
They form important habitats for fish, amphibians
and reptiles (Kingsford et al. 2006) which can
survive through successive dry seasons. Plants
(Brock et al. 2006) are present within these
systems and include aquatic benthic algae (Bunn
et al. 2006b) which form an important part of the
waterhole food chain.
In normal seasons the extensive downstream fan
area remains dry with water only present in the low
water anastomosing channels in the upper reaches of
the system. These low level channels can fill with
water during flow pulses events, when the water-
holes can be recharged but the floodplain is not
inundated (Bunn et al. 2006a).
The Cooper Creek system contains an extensive
silt/clay floodplain that is dominated by vertisols
with characteristic deep cracking and surface undu-
lations known as gilgai (e.g. Mermut et al. 1996).
Developed over much of the floodplain are a
pattern of surface channels that are only active
during flooding. In the modern (i.e. more recently
than 40 ka, Maroulis et al. 2007) Cooper Creek
system sediment movement includes transport as
mud aggregate particles that behave as sand grains
(Maroulis & Nanson 1996) which are deposited
within channels that are present on the floodplain
surface. The most common pattern to these flood-
plain surface channels is an anastomosing network
where channels isolate and define small braided
islands or braid bars. Vertisol processes are active
and, following soil turbation, these mud-within-mud
features will have minimal preservation within older
sediments. In Cooper Creek it was prior to 40 ka
(Maroulis et al. 2007) that the channels within the
system were sand dominated, demonstrating a
much more active phase of sand sheet deposition.
In the Britta Dal Formation the dominant sedi-
ments are the silt vertisols. The Cooper Creek
analogy would suggest an origin from rare flood
events that moved silt particle aggregates through
the system in a braided network of channels. These
would not necessarily originate from a major low-
water sand-filled channel cut into the floodplain
but, more distantly, via a network of silt-within-silt
floodplain surface channels. However, for much of
the times vertisol or intermittently active vertisol
processes would have dominated the floodplain
environment. The intensive cracking and re-wetting
would have caused total loss of original depositional
fabrics.
The presence of the much rarer but grouped
sandstones parallels the pre-40 ka development of
sand sheets in Cooper Creek. In the Britta Dal For-
mation, these were deposited during the very major
flood events that brought sand into the system.
These sand-sheets would have spread over the flood-
plain surface of silt in an anastomosing pattern
but with only parts of the system active at any one
time in a channel and braid bar pattern. The same
sand sheet would have periodically re-flooded
with the production of multiple scoured surfaces,
migrating the channels and bars. At these times,
the system would have flooded across much of the
floodplain and a lake formed at the terminal fan.
Again, analogies with both Cooper Creek and
Lake Eyre suggest that this system lasted more
than a single season and would have become occu-
pied by both fish and tetrapods with the distinct
possibility of ephemeral plants growing along the
watercourses. The tetrapods could therefore have
actively moved through the system post-flooding
to inhabit the dry season waterholes, been swept
along during the flood as dead or dying animals
(like domesticated cattle and other tetrapods in
Cooper Creek) or been sourced from an upstream
accumulation of bone debris.
Figure 7 is a cartoon reconstruction of the Britta
Dal Formation fluvial system. The reconstruction is
during a wetter interval following a major flood
event. The palaeoflow direction is from south to
north (Olsen 1993) with a sand-dominated sequence
in northern Traill Ø as seen in the Rebild Bakker
section (fig. 7 in Vigran et al. 1999; the sandstone
sequence beneath the first occurrence of Retispora
lepidophyta in sample 334804). To the north on
southern Ymer Ø, the sandstones have become
interbedded with red vertisols. Further to the north
on Gauss Halvø the channels become more
