WEST KIMBERLEY PLACE REPORT - Department of Sustainability ...

ECOLOGY, BIOGEOGRAPHY AND EVOLUTION
The Devonian reef complexes preserved in the Oscar, Napier, Emmanuel and Pillara
ranges show continuous deposition from 390 million years ago (the Givetian stage of
the middle Devonian period) to 370 million years ago (the Famennian stage of the late
Devonian) (Playford 1980; Tyler 2000). The extent and duration of this deposit, as
well as the time period it represents is unique on this continent. It spans the Frasnian-
Famennian mass extinction and is among only four sites in the world where there is
evidence that reef environments survived this event, although the major reef-building
organisms changed (Wood 2002). Natural sections through the reef occur in several
places, revealing the reef structure in spectacular cross section: the most remarkable
are Windjana and Geikie Gorges, through which run the Lennard and Fitzroy rivers
(Playford et al. 2009).
The Devonian was the largest pre-Holocene reef-building period; Devonian reefs are
common throughout the world and in Australia. However, the majority of these reefs
disappear from the record at the Frasnian-Famennian boundary, 375 million years
ago. This coincides with a mass extinction in which 57 per cent of genera were lost
worldwide, making this as significant a biological event as the better known
Cretaceous–Tertiary extinction that brought the 'Age of Dinosaurs' to a close
(Sepkoski 1996 quoted in Wood 2000). The exposed Kimberley Devonian reef
complexes are relatively undisturbed by burial, taphonomic or tectonic processes and
offer an in situ sequence through this time (Playford et al. 2009). This sequence
demonstrates how reef communities reacted to climate and sea level changes at the
Frasnian-Famennian boundary (Playford and Lowry 1966; Playford 1980).
Another outcrop of the same Devonian reef occurs in the Ningbing range, indicating
the extraordinary geographical extent of this reef complex at times, but this outcrop is
significantly smaller than those of the west Kimberley and much less well studied.
Other areas in Australia that preserve significant Devonian reef assemblages are
Buchan, in Victoria, and Taemas and Wee Jasper, in New South Wales. These
outcrops date from the Early Devonian, preserving reefs that are tens of millions of
years older than the Kimberley reefs, and do not span a global mass extinction event
(Basden et al. 2000). The most fossiliferous outcrops at Buchan have been removed
through mining (Inan et al. 1992). Wee Jasper and Taemas in New South Wales
preserve the Early Devonian reef limestones of the Murrumbidgee Group. The
outcrops at Taemas are equivalent in age to the Buchan group, while those at Wee
Jasper continue for a few million years longer (Basden et al. 2000). Both the
Murrumbidgee and Buchan groups have been deformed through tectonic movement,
folded by the middle Devonian Tabberabberan orogeny and often subject to extensive
dolomitisation (Yeates 2001). Neither of these formed at the right time to provide any
evidence relating to the Frasnian-Famennian extinction or any other recognised
significant extinction events.
The Devonian Reef sequence preserved in the Kimberley region is a continuous
record from the Frasnian to the Famennian stage of the Late Devonian. Famennian
reefs are rare throughout the world and none are present elsewhere in Australia. In
addition, valleys that run through the reef at Windjana and Geike Gorges provide
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